METAMORPHIC STUDIES GROUP

Online, 27–28 May 2020

Joining Details circulated to registered delegates on Tuesday 26th May 2020

METAMORPHIC STUDIES GROUP - Research in Progress 2020

PROGRAMME AND ABSTRACTS

Wednesday 27th May 2020 Oral Presentations Time Title, authors

13.45–14.00 Speakers' Briefing

14.00–14.05 Introduction by Chair of Metamorphic Studies Group, Catherine Mottram

14.05-14.20 The controls on the thermal evolution of continental mountain ranges Alex Copley and Owen Weller: p. 4

14.20–14.35 How did the Archaean crust evolve? Insights from the structure and petrology of the Lewisian of Scotland S.R. Miocevich (STUDENT), A. Copley, O.M. Weller: p. 5

14.35–14.50 Interactions of stress and chemical processes in the Earth John Wheeler: p. 6

14.50–15.05 Pre-orogenic upper crustal softening by lower greenschist facies metamorphic reactions in crystalline basement units (example of central Pyrenees) Laura Airaghi, Nicolas Bellahsen, Benoît Dubacq, David Chew, Claudio Rosenberg, Emilie Janots, Maxime Waldner and Valérie Magnin: p. 7

15.05–15.20 Drainage of subduction interface fluids into the fore-arc mantle evidenced by a pristine jadeitite network (Polar Urals) Samuel Angiboust, Johannes Glodny, Aitor Cambeses, Tom Raimondo, Patrick Monié, Michael Popov, Antonio Garcia-Casco: p. 8

15.20–15.35 Metamorphism on the Moon Recorded by the Granulite Suite J. F. Pernet-Fisher and K. H Joy: p. 9

15.35–15.50 Break

Poster Presentations 15.50–15.55 Introduction to Poster Session – B. Kunz

15.55–16.05 Behaviour of critical metals in micas during metamorphism Barbara E. Kunz, Clare J. Warren, Nigel B.W. Harris, Tom W. Argles, Frances E. Jenner: p. 10

16.05–16.15 Origin of metasomatic fluids in subducted continental crust and implications for volatile recycling in subduction zones B.J.R. Harris (STUDENT), J.C.M. De Hoog, S.L. Harley, R. Halama and H.-P. Schertl: p.11

16.15–16.25 From local dewatering to large-scale flushing during burial in the blueschist facies: Insights from a pervasively fractured metamorphic terrane. Jesús Munoz-Montecinos (STUDENT), Samuel Angiboust, Antonio García-Casco, Johannes Glodny and Gray Bebout: p. 12

16.25–16.35 On the origin and geodynamic significance of eclogite in the Brunovistulian terrane at the eastern margin of the Bohemian Massif Stephen Collett, Pavla Štípská, Karel Schulmann, Jitka Míková and John Hora: p.13

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Poster Presentations (Wednesday, continued) 16.35–16.45 Halogens in metasomatic garnet websterite and eclogite from the Western Gneiss

Region, Norway: implications for subcontinental mantle metasomatism R. Burgess, L. Hughes, S.J. Cuthbert, A. Quas-Cohen, A. Pawley and G.T.R. Droop: p. 14

16.45–16.55 Retrograde Tectonics of the Stadlandet Peninsula, Western Gneiss Region, Norway Isabel S. M. Carter (STUDENT), Phillip Gopon, David J. Waters and Andrew Parsons: p. 15

16.55–17.05 Spatial variation of deformation in the Western Gneiss Region: implications on UHP exhumation Joanna Male (STUDENT), Andrew Parsons, Rellie Goddard, Phillip Gopon and David J. Waters: p. 16

17.05–17.15 Reflectance spectral features of Koan hornfelses, at Dikeos Massif contact metamorphic aureole, Aegean Sea M. Kokkaliari (STUDENT), C. Kanellopoulos and I. Iliopoulos: p. 18

17.15–17.25 Lawsonite eclogite and blue schist from Syros, Greece: Insights into the fluid pathways in subducted crust and the eclogite-blueschist transition Thomas Lamont and Richard White: p. 19

17.30 Social

Thursday 28th May 2020

Oral Presentations Time Title, authors

13.45–14.05 Speakers' Briefing + Introduction14.05-14.20 Not all kyanite is created equal – The petrogenesis of kyanite migmatites in Eastern

Bhutan S.E. Phillips (STUDENT), T.W Argles, N.B.W Harris, C.J. Warren, N.M.W. Roberts and B. Kunz: p. 20

14.20–14.35 Quantitative automated mineralogy to constrain metamorphic processes using ZEISS Mineralogic Richard J.M. Taylor: p. 20

14.35–14.50 Reaction Affinity Patterns in Rocks with Porphyroblasts C. T. (Tom) Foster, Jr.: p. 21

14.50–15.05 LA-ICP-MS mapping of granulite-facies monazite: textural insights and implications for petrochronology Owen Weller, Simon Jackson, William Miller, Marc St-Onge and Nicole Rayner: p. 22

15.05–15.20 Microtectonic analysis of the Teziutlan metamorphic complex, Puebla State, Mexico: Mylonitic deformation in the late Paleozoic Luis Javier Gutiérrez-Trejo (STUDENT), Mario A. Ramos-Arias, Edgar Ángeles-Moreno, José Martín Yáñez-Limón, Adair Jiménez-Nieto and Teresa Pi-Puig: p. 23

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Poster presentations 15.55–16.05 Microstructural Controls on the Crystallization of Garnet: An Example from the Meguma

Terrane, Nova Scotia Allie Nagurney (STUDENT) and Mark Caddick: p. 23

16.05–16.15 P-T path determination using phase equilibria modelling with multiple fractionation of garnet in a metapelite Carlos Iván Lembo Wuest (STUDENT), Samanta Serra-Varela, Sebastián O. Verdecchia, Juan A. Murra, Manuela E. Benitez, Gladis Palacio Balderramo: p. 24

16.15–16.25 Post-kinematic and matrix-dependent garnet nucleation and growth in the inverted Barrovian metamorphic sequence of the Sikkim Himalaya Freya R. George and Fred Gaidies: p. 25

16.25–16.35 Prograde UHP metamorphism in felsic and mafic lithologies and clues to pre-Himalayan metamorphism in the Tso Morari dome, Ladakh, Himalaya A.K. Bidgood (STUDENT), D.J. Waters and M.P. Searle: p. 26

16.35–16.45 Leucogranites in the Garhwal Himalaya: The story according to zircon Charlie J. Oldman (STUDENT), Clare J. Warren, Christopher J. Spencer, Tom W. Argles: p. 27

16.45–16.55 Provenance and geochemistry of metasedimentary rocks in the basement of the Sierra Madre terrane, Mexico: Implications of deposition along the western margin of Pangea Sonia Alejandra Torres Sánchez , Carita Augustsson, Uwe Jenchen, José Rafael Barboza Gudiño, Andreas Gärtner, Mandy Hofmann, Ulf Linnemann, Carlos Mario Morales Bautista: p. 28

16.55–17.05 Juxtaposed high- and low-pressure metamorphic field gradients rocks and its tectonic implications, a study case of Turvo-Cajati Formation, Ribeira Belt, Brazil B.S. Ricardo (STUDENT), R. Moraes, F. M. Faleiros, O. Siga Júnior, G. Campanha, C.M. Mottram: p. 29

17.05–17.15 New insights from the Fammatinian-Quetame-Caparonensis orogenic event in the Northern Andes (NW South America) Diego O. Ruiz-Portilla (STUDENT), Carlos A. García-Ramírez and Vaness Rey-Leon: p. 30

17.15–17.25 A detailed petrological study of the metamorphic retrogression textures of the granulitic Oaxacan Complex, México M.M. Almazán-López (STUDENT), F. Ortega-Gutiérrez and V. Colás: p. 31

17.25–17.35 The Raspas Complex (Ecuador): A fragment of a J-K melange belt in the NW corner of the South American plate M.C. Arrieta-Prieto (STUDENT), C.A. Zuluaga-Castrillón, O.M. Castellanos-Alarcón and J.D. Hernández-Montenegro: p. 32

17.35 Student Social

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The controls on the thermal evolution of continental mountain ranges

Alex Copley* and Owen Weller

Department of Earth Sciences, University of Cambridge)

*E-mail: acc41@cam.ac.uk

We have examined the controls on the thermal evolution of continental mountain ranges. We find

that Barrovian metamorphic conditions can most easily be achieved by thickening of the mid- to

upper-crust above a rigid lower crustal substrate, and that homogeneous thickening of the entire

lithosphere rarely results in Barrovian conditions. This result implies that thickening above rigid

lower crust has been the dominant mode of mountain-building preserved in the rock record. Modest

rates of radiogenic heating (1+/-0.5 micro-W/m3) are able to produce Barrovian metamorphic

conditions and timescales, without the requirement for an external source of heat. In a given model,

peak temperature conditions for rocks originally positioned closely in depth (e.g. 5km apart) occur at

times that can be separated by over 10Myr, highlighting the spectrum of ages and conditions that

can be produced by a single geometrically-simple mountain-building event. The direction of

approach in P-T space towards the highest-T conditions experienced by a rock (i.e. the curvature of

the P-T loop) is the most diagnostic feature that allows the thermal and tectonic history of a

mountain belt to be estimated. Many configurations of continental mountain-building result in rocks

transiently passing through regions with low average thermal gradients (e.g. 10--15 oC/km),

suggesting that such gradients should not be used to infer the presence of subduction. Removal of

the lower mantle lithosphere results in a characteristic change in the gradient of the decompression

part of P-T loops, thereby providing a test for the existence of this process in past mountain ranges.

Analysis of existing P-T loops from the 3.2Ga Barberton metamorphism suggests that mountain-

building processes at that time and place were similar to those in present-day mountain ranges,

implying that large regions of rigid continental lithosphere were already in existence, and were

underthrusting the margins of mountain ranges.

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How did the Archaean crust evolve? Insights from the structure and petrology of the Lewisian of

Scotland

S.R. Miocevich*, A. Copley, O.M. Weller

Department of Earth Sciences, University of Cambridge

*E-mail: srm81@cam.ac.uk

High-grade Archean gneiss terranes expose mid to lower crustal rocks. Occurrences of mafic-

ultramafic bodies and potentially supracrustal rocks within these mid-crustal environments have

been used to provide evidence for tectonic processes operating in the Archean that involved mass

transfer from the surface to the mid-crust. However, there is significant uncertainty regarding the

nature of such tectonic processes, with suggestions including a range of uniformitarian and non-

uniformitarian scenarios. One non-uniformitarian scenario, ‘sagduction’, has been proposed as a

possible mechanism (Johnson et al., 2016, and references therein), although the dynamics of

sagduction are still relatively unexplored.

This study focuses on the ‘Hill of Faeries’ locality in the Lewisian Gneiss Complex of northwest

Scotland as a test case to investigate the behaviour of possibly supracrustal rocks in a mid-crustal

environment. The Hill of Faeries dominantly comprises garnet-biotite felsic gneiss assemblages and

an associated two-pyroxene mafic gneiss. Field mapping was undertaken to collect samples

representative of the observed heterogeneity of the suite, and to assess the field associations

between the possible supracrustals and the surrounding tonalite-trondhjemite-granodiorite (TTG)

gneisses. Phase equilibria modelling was then conducted on the garnet-biotite gneisses, the mafic

gneiss and the surrounding TTGs to ascertain peak pressure-temperature (P-T) conditions, and to

calculate the density of the modelled rocks at peak conditions.

The results indicate peak metamorphic conditions of 950 ± 50 °C and 9 ± 1 kbar, consistent with

thermobarometric data from the rest of the Lewisian Complex (Feisel et al., 2018). The calculated

density contrasts between the Hill of Faeries suite and the surrounding TTGs were used to estimate

the buoyancy force that drives density-driven segregation. This allows us to investigate the rates of

vertical motion that result from density contrasts, as a function of the effective viscosity during

metamorphism. Independent viscosity estimates can be attained using mineral flow-laws and our

estimated P-T conditions, and from examination of modern-day regions of crustal flow. We are

therefore able to estimate the conditions under which sagduction could have been a viable?

mechanism for crustal evolution in the Lewisian and similar high-grade metamorphic terranes.

Johnson, T.E., Brown, M., Goodenough, K.M., Clark, C., Kinny, P.D. & White, R.W., 2016. Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland. Precambrian Research, 283, 89-105.

Feisel, Y., White, R.W., Palin, R.M., Johnson, T.E., 2018. New constraints on granulite facies metamorphism and melt production in the Lewisian Complex, northwest Scotland. Journal of Metamorphic Geology. 36, 799-819

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Interactions of stress and chemical processes in the Earth

John Wheeler

Dept. Earth, Ocean and Ecological Sciences, Liverpool University, Liverpool L69 3GP, U.K.

*E-mail: johnwh@liverpool.ac.uk

There is no doubt that metamorphism often occurs under stress, so it is important to have a

theoretical description of the effects that stress has on chemistry (and vice versa). Such a description

is not agreed upon, but in my approach some large effects of stress are predicted, so there is a need

for further investigation. There are also practical needs since interactions of stress and chemistry

affect, for example, serpentinisation (with consequences for the viability of CO2 sequestration) and

hydration of anhydrite to gypsum (a reaction which has damaged whole towns as stress builds up).

In this talk I present support for a key relevant equation from diverse experiments. These include

experiments on pressure solution, force of crystallisation and polymorphic transformations under

stress. Given this foundation, and additional support from theory in other branches of science, the

equation can be used in predictions – one of which is that there may be surprisingly large effects of

stress on metamorphism. It also shows that there is no thermodynamic equilibrium in a stressed

system, so we must focus instead on the kinetics of change to better understand metamorphism

coupled to stress.

Wheeler, J. 2014. Dramatic effects of stress on metamorphic reactions. Geology 42(8), 647-650. Wheeler, J. 2018. The effects of stress on reactions in the Earth: sometimes rather mean, usually normal, always important. Journal Of Metamorphic Geology 36, 439-461.

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Pre-orogenic upper crustal softening by lower greenschist facies metamorphic reactions in

crystalline basement units (example of central Pyrenees)

Laura Airaghi1,*, Nicolas Bellahsen2, Benoît Dubacq2, David Chew3, Claudio Rosenberg2, Emilie

Janots4, Maxime Waldner2 and Valérie Magnin4

1ISTO, University of Orléans, 2ISTeP, Sorbonne University, Paris 3Trinity College, Dublin 4ISTerre, University Grenoble Alpes,

*E-mail: laura.airaghi@univ-orleans.fr

Pre-kinematic greenschist facies metamorphism is often observed in granitic basement of mountain

belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre-kinematic

alteration is challenging because of its usual obliteration by subsequent syn-kinematic

metamorphism often occurring at the brittle–ductile transition conditions. It is, however, to be

expected that pre-kinematic alteration has major implications for the rheology of the upper crust. In

the 305 Ma-old Variscan basement of the Bielsa massif (Axial Zone of the Pyrenees), successive

fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and

petrographic analysis, low-T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and

monazite show extensive pre-orogenic (pre-Alpine) and pre-kinematic alteration related to feldspar

sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300

Ma. This event is followed by a second fluid–rock interaction event marked by new crystallization of

phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and

fractures parallel to the shear planes. U/Pb anatase and monazite ages as well as the microtextural

relationships of accessory minerals suggest an age for this event at 40–70 Ma. The Variscan

basement was therefore softened at late to post-Variscan time, at least 150–200 Ma before the

main Alpine shortening. Alpine-age compression (c. 35–50 Ma) leads to the formation of a dense net

of mylonites resulting in a distributed deformation at the massif scale that precedes the strain

localization along the major thrusts of the Axial Zone. A similar metamorphic and deformation

calendar is observed in other geological contexts (e.g. the basement of eastern Tibet), showing that

inherited, pre-orogenic upper crustal softening by low-grade metamorphic reactions controls the

strain distribution in granitic basement and play a crucial role in the structuration of orogens.

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Drainage of subduction interface fluids into the fore-arc mantle evidenced by a pristine jadeitite network (Polar Urals)

Samuel Angiboust1,*, Johannes Glodny 2, Aitor Cambeses 1,3, Tom Raimondo 4, Patrick Monié5, Michael Popov 6,7, Antonio Garcia-Casco 3,8

1 Université de Paris, Institut de Physique du Globe de Paris, F-75005 Paris, France *E-mail: angiboust@ipgp.fr 2 GFZ German Research Centre for Geosciences, D-14473 Potsdam, Germany 3 Department of Mineralogy and Petrology, Faculty of Sciences, University of Granada, Campus Fuentenueva s/n, 18002 Granada, Spain, 4 UniSA STEM, University of South Australia, GPO Box 2471, Adelaide SA 5001, Australia 5 Géosciences Montpellier, Université de Montpellier, UMR-CNRS 5243, Place E. Bataillon, 34090 Montpellier, France 6 Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences, Pochtovy per. 7, Yekaterinburg, Russia

7 Russia Ural State Mining University, ul. Kuibysheva, 30, Yekaterinburg, Russia 8 Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, 18100 Armilla, Granada, Spain.

The physical and mechanical processes rooted in the hydrated, serpentinized mantle above subduction zones remain insufficiently explored despite fundamental implications on our understanding of rheology and fluid recycling along subduction interfaces. Through a field-based investigation, serpentinized peridotites and jadeitite samples from a fossil fore-arc mantle in the Polar Urals (Russia) are studied here to document fluid-rock interaction processes in the high-pressure field, as well as the long-term evolution of the base of the mantle wedge. Petrographic, geochemical and microstructural observations reveal a complex, protracted evolution of the jadeitite-bearing fluid pathway throughout the gradual cooling of the forearc mantle and increasing serpentinization of the host. It is shown that the jadeitite lenses in the studied locality (i) derive for a large part from a trondhjemitic dyke earlier emplaced in a warm subduction environment, and (ii) record the cooling of the subduction hanging wall under high-pressure conditions associated with increasing host serpentinization. In the studied locality, the majority of the jadeitites formed at relatively high temperatures (>600°C) by the influx of Na-Al rich, slab-derived metamorphic fluids that were drained along the base of the mantle wedge, parallel to the subduction interface. Changes in bulk rock geochemical signatures and in paragenetic sequences also constrain the compositional evolution of the fluid channelized along this drainage, with an increasing sedimentary component. The phlogopite-bearing walls of the dyke exhibit Rb-Sr and Ar-Ar ages ranging between c. 405 and c. 390 Ma, a range partly overlapping within uncertainty with the previously dated zircons from the jadeitite core (410-400 Ma; U-Pb). This study opens a unique window on the pristine structures formed above the plate interface by melting and fluid-rock interaction in the early subduction stages, as well as their evolution during secular cooling of the base of the mantle wedge.

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Metamorphism on the Moon Recorded by the Granulite Suite

J. F. Pernet-Fisher* and K. H Joy

Department of Earth and Environmental Sciences, University of Manchester, M13 9PL, UK

*E-mail: john.pernet-fisher@manchester.ac.uk

Metamorphic rocks on the Moon are an important yet under-studied suite of lunar lithologies that

have been identified in the Apollo and lunar meteorite collections [1]. These rocks, with granoblastic

textures, are generally referred to as granulites, however, unlike their terrestrial counterparts, they

are considered to represent the products of only high-temperature (> 1000 oC) thermal

metamorphism that completely re-crystallizalised their protoliths.

Lunar granulites are commonly sub-divided into two main compositional groups related to their

protolith lithologies: The Fe-granulites, found at most Apollo landing sites, are generally accepted to

derive from metamorphosed plagioclase-rich igneous cumulates, termed the ferroan anorthosite

(FAN) suite. The FAN suite are important lithologies as they represent products of the primary lunar

crust. The Mg-granulites, found mostly at the Apollo 16 landing site and within lunar meteorite

samples. The protolith of this group is not so well understood. Early studies have linked the protolith

to secondary magmatic intrusions into the primary anorthositic crust (termed the Mg-suite),

however, recent studies have tentatively connected the protoliths to a Mg-rich variation of the

primary crustal plagioclase cumulates (termed the MAN suite). The occurrence of MANs is

controversial, indeed, it is unclear how the MAN suite fits into canonical lunar crustal formation

models. In the absence of sample return missions, by identifying the protolith lithologies of the Mg-

granulite suite, we greatly enhance our understanding of the range of lithologies that make up the

primary crust. In turn, this enables lunar crustal formation models to be better constrained.

We present preliminary plagioclase trace-element data, in combination with published peak

metamorphic temperatures, to assess the protoliths of two Apollo granulites. Furthermore, we aim

to investigate the extent to which thermal metamorphism affects the diagnostic trace-element

signatures that are commonly used to classify lunar highland lithologies.

Lindstrom & Lindstrom, 1986, JGR, 91(B4), 263-276.

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Behaviour of critical metals in micas during metamorphism

Barbara E. Kunz*, Clare J. Warren, Nigel B.W. Harris, Tom W. Argles, Frances E. Jenner

School of Environment, Earth and Ecosystem Sciences, The Open University, Walton Hall, Milton

Keynes, MK7 6AA

*E-mail: barbara.kunz@open.ac.uk

The process(es) that aid enrichment of critical metal (e.g. Li, Cs, Ta, Sn & W) in granitic magmas is

still a matter of discussion: fractional crystallisation from typical parental magmas is incapable of

producing the observed enrichment on its own. Potential alternative enrichment processes include

metamorphic reactions, partial melting, assimilation and fractionation of (pre-enriched) protoliths.

The relative importance of the original protolith enrichment versus the concentrating potential of

sub- and supra-solidus metamorphic reactions is under debate as well. Comprehensive datasets of

economically interesting element concentrations in a range of metamorphic minerals are also

lacking. We present the first large-scale dataset of in-situ LA-ICP-MS trace element data including Li,

Cs, Ta, Sn & W in metapelite-hosted muscovite and biotite from three different metamorphic cross-

sections covering sub-greenschist to granulite facies conditions. Our data show distinctive

concentration changes with metamorphic grade, seemingly independent of geological/tectonic

setting (continental collision vs. crustal thinning). These trends provide a preliminary framework in

which we can study and predict systematic changes in critical metal concentration of micas from

solid-state metamorphism through partial melting. Our dataset confirms the previously predicted

pre-enrichment potential of metamorphic reactions and the importance of studying such processes

to constrain the formation of crustal type critical metal deposits.

Metamorphic Studies Group – Research in Progress 2020

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Origin of metasomatic fluids in subducted continental crust and implications for volatile recycling in subduction zones B.J.R. Harris 1,*, J.C.M. De Hoog1, S.L. Harley1, R. Halama2 and H.-P. Schertl3

1 University of Edinburgh, James Hutton Road, Edinburgh EH9 3FE, UK 2 Keele University, William Smith Building, Keele, ST5 5BG, UK 3 Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Germany *E-mail: b.j.r.harris@sms.ed.ac.uk The Dora Maira Massif in the Western Alps exposes ultra-high pressure (UHP) continental rocks which have experienced subduction to >100 km depth, as implied by the presence of coesite. Lenses of UHP pyrope-phengite-kyanite bearing whiteschist occur within Permian metagranitoids and orthogneiss, which are partially overprinted by the Alpine UHP metamorphism. The whiteschists are characterized by decimentric pyrope megablasts (Xpy = 0.92-0.98) set in a matrix of phengite (7.06-7.21 Si a.f.u., XMg = 0.97-0.98), kyanite, quartz (after coesite), rutile and smaller pyrope porphyroblasts (Xpy = 0.84-0.90). Phengite also occurs as inclusions in pyrope porphyroblasts, and as reaction rims on pyrope megablasts. Whiteschists are the primary recorders of UHP metamorphism (~730 °C, 43 kbar peak conditions [Hermann, 2003]), but there has been extensive debate as to their genesis and relationship to the host orthogneiss. Whilst most authors now agree that whiteschists are metasomatic alteration products of the country rock, the timing and source of metasomatic fluids is debated. The fluids must be rich in Mg, Al and Si in order to produce the unusual, highly magnesian whiteschist bulk composition. Serpentinites have often been invoked as a source of such fluids within a subduction zone. Boron (B) and B isotopes are useful tracers of fluid-rock interaction processes in subduction zones (e.g. Konrad-Schmolke and Halama, 2014, De Hoog and Savov, 2018). Models of mineral dehydration during subduction predict loss of B from rocks into dehydrating fluids, with associated decrease in δ11B in the residual solid. Sea floor serpentinites and altered oceanic crust have positive δ11B values, compared to continental crust, which has δ11B < -5 ‰. In situ analyses of B isotopes in phengite by SIMS show that the whiteschists have negative δ11B values (-15.6 to -3.8 ‰), which are similar to those of the protolith orthogneiss (-15.7 to -6.0 ‰). These are not consistent with a fluid sourced from slab serpentinites. Mantle wedge serpentinites are a possible source of fluids with δ11B values down to around -10 ‰, and have been previously suggested as a metasomatic fluid source (e.g. Chen et al., 2016). B and other fluid mobile elements were also analysed by SIMS and provide additional constraints on the metasomatic fluid composition. Further work, combining phase equilibrium modelling and trace element partitioning is underway to understand the fluxes of fluids involved in metasomatism, their composition, and the evolution of the metasomatic fluids between different generations of phengite. The results demonstrate that white mica can retain significant quantities of volatile elements to sub-arc depths. Further work on white mica in subduction zone rocks is required to understand its role in recycling volatile elements to the mantle and in metasomatic processes in the slab. Chen, Y.-X., Schertl, H.-P., Zheng, Y.-F., Huang, F., Zhou, K., and Gong, Y.-Z. (2016). Mg–O isotopes trace the origin of Mg-

rich fluids in the deeply subducted continental crust of Western Alps. Earth and Planetary Science Letters 456, 157–167

De Hoog, J. C. M. and Savov, I. P. (2018). Boron Isotopes as a Tracer of Subduction Zone Processes. Boron Isotopes: The Fifth Element. Springer International Publishing, 217–247

Hermann, J., 2003. Experimental evidence for diamond-facies metamorphism in the Dora-Maira massif. Lithos. 6th International Eclogite Conference: Eclogites and Related High and Ultrahigh Pressure Metamorphic Rocks 70.3, 163–182.

Konrad-Schmolke, M. and Halama, R. (2014). Combined thermodynamic–geochemical modelling in metamorphic geology: Boron as tracer of fluid–rock interaction. Lithos 208-209, 393–414

Metamorphic Studies Group – Research in Progress 2020

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From local dewatering to large-scale flushing during burial in the blueschist facies: Insights from a pervasively fractured metamorphic terrane.

Jesús Munoz-Montecinos1,2, Samuel Angiboust1, Antonio García-Casco2,3, Johannes Glodny4, Gray

Bebout5

1Université de Paris, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France. 2Department of Mineralogy and Petrology, Faculty of Sciences, University of Granada, Campus

Fuentenueva s/n, 18002 Granada, Spain. 3Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, 18100 Armilla, Granada,

Spain. 4GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany. 5Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015, USA.

*E-mail: jesus.munozmontecinos@gmail.com

Release of volatiles along the subduction interface, critically controlling seismicity and arc

magmatism, is clearly evidenced in exhumed metamorphic terranes by the occurrence of veins that

provide information about fluid-rock interaction, in particular at depths where episodic tremor and

slip events are inferred. Here, we investigate the blueschist-facies (500°C-1.8GPa; late Cretaceous)

Seghin unit (Zagros suture zone), a well-preserved block-in-matrix paleo-subduction channel

composed of blueschists, serpentinites and minor carbonate-bearing metasediments. This field-

based work aims to determine the relative chronology and the potential fluid source(s) associated

with the formation of Lws+Cpx+Gln veins observed along the main foliation and crosscut by

carbonate-filled hydraulic breccias. Petrographic and petrological analyses suggest that silicate-rich

vein systems began precipitating during early burial, and evolved with ongoing metamorphism,

while most carbonate-rich veins and hydrofractures formed at near peak P-T conditions. In situ LA-

ICPMS trace element analyses reveal that: (i) individual silicate phases in veins have trace element

signatures similar to those of the same phases in metamafic host-rocks, reflecting local-scale fluid-

mediated element redistribution (ii) carbonate-bearing veins and metasediments also have similar

trace element signatures and (iii) lawsonite in blueschist veins exhibit REE enrichments along their

rims, suggesting an increasing contribution of metasedimentary-derived fluids approaching peak P-T.

Carbonate O-C isotope compositions of the veins and metasedimentary rocks range from +13.6 to

+19.6‰ (ẟ18OVSMOW) and -1.0 to +3.1‰ (ẟ13CVPDB). These values together with LA-ICPMS results,

indicate a metasedimentary-derived fluid source associated with hydrofracturing. Thermodynamic

modelling (using the DEW database) predicts that decarbonation via fluid-assisted reactions is

inefficient at blueschist-facies and that C release occurs deeper in the subduction channel (i.e., at

eclogite-facies). The CO2 released by devolatilization and dissolved in H2O-rich fluids could travel

upward in highly pressurized “pulses” over scales of several tens of km parallel to the subduction

interface before precipitating carbonate and to produce the observed vein-sets.

Metamorphic Studies Group – Research in Progress 2020

13

On the origin and geodynamic significance of eclogite in the Brunovistulian terrane at the eastern

margin of the Bohemian Massif

Stephen Collett1,*, Pavla Štípská1,2, Karel Schulmann1,2, Jitka Míková3 and John Hora3

1Centre for Lithospheric Research, Czech Geological Survey, Klarov 131/3, Prague, Czech Republic 2EOST, IPGS, CNRS UMR7516, Université de Strasbourg, 1 Rue Blessig, F-67084, France 3Laboratories of the Czech Geological Survey, Geologická 577/6, Prague, Czech Republic

*E-mail: scollett311@gmail.com

Whole-rock geochemical analyses of metabasite from the Velke Vrbno Dome (NE Bohemian Massif)

indicate that protoliths are represented by two groups. The first group, characterised by deep

niobium anomalies (Nb*=0.19–0.65) and flat REE profiles (LaN/YbN=1.2–1.9), are interpreted to

have been formed in a volcanic-arc setting. The second group have relatively small niobium

anomalies (Nb*=0.78–0.89) and steeper REE profiles (LaN/YbN=5.4–6.0) consistent with a within-

plate tectonic setting.

A c.535 Ma emplacement age of the within-plate type is confirmed by new U-Pb dating of zircon;

Ediacaran inheritance suggests a continental-rift rather than ocean-island setting. Ediacaran age of

arc-type metabasites is inferred from associated intermediate-felsic volcanics. All studied samples

have moderately-positive age-corrected εNd values (+3.9–+5.9) and depleted-mantle model-ages

range from c.880 Ma for within-plate type to c.1200–2400 Ma for volcanic-arc type, indicating

variable interaction with an old continental source.

Identification of omphacite in garnet and clinopyroxene-plagioclase-amphibole symplectite in both

type of metabasite confirms eclogite-facies metamorphism during the Variscan Orogeny. Peak

conditions of 18–20 kbar and 625–675°C, estimated from pseudosection modelling, are in

agreement with previous studies. P-T-M(H2O) modelling indicates that observed reaction textures in

least retrogressed samples are achievable in a closed system. Increasing retrogression is

characterised by chloritisation and resorption of garnet and growth of a medium-grained amphibole-

plagioclase matrix following localised fluid infiltration at crustal levels. Strong correlation between

retrogression and 87Sr/86Sr ratios suggests that this fluid was enriched in radiogenic strontium.

Results are consistent with a Brunovistulian-affinity for the Velke Vrbno Dome and that the

protoliths of eclogite were emplaced in a continental-arc to rift setting during the Late-Ediacaran to

Early-Cambrian. Importantly, no evidence of oceanic rocks was identified and combined with a

relatively warm subduction geotherm this supports models of Brunovistulia as a continental

backstop during Paleozoic amalgamation of the Bohemian Massif.

Metamorphic Studies Group – Research in Progress 2020

14

Halogens in metasomatic garnet websterite and eclogite from the Western Gneiss Region, Norway: implications for subcontinental mantle metasomatism

R. Burgess.1, L. Hughes1, S.J. Cuthbert2, A. Quas-Cohen1, A. Pawley1, G.T.R. Droop1

1SEES, University of Manchester, Oxford Road, M13 9PL, UK 2AGH-UST, Faculty of Geology, Geophysics and Environmental Protection, al. Mickiewicza 30, 30-059 Krakow, Poland *E-mail: simon.cuthbert@agh.edu.pl

Fluid flow and fluid-rock interaction under ultra-high-pressure (UHP) eclogite facies conditions during subduction is evidenced by vein-associated, ultra-coarse-grained garnet websterites and eclogites in the Western Gneiss Region (WGR), Norway. These rocks offer useful analogues for metasomatism of mantle peridotite in contact with subducted continental crust. This study sought to determine the signature, source and evidence for recycling of halogens at the interface between these mafic-ultramafic massifs and their host granitoid gneisses. Anhydrous minerals (garnet, quartz, olivine) contain halogens at levels exceeding depleted MORB mantle concentrations, with the major fraction of halogens hosted in multi-phase inclusions. These results are consistent with a previous study of halogens in eclogite facies fluid inclusions in the more common metabasaltic eclogitesa of the WGR. The halogen compositions show similarities to brine inclusions in diamondsb and some mantle peridotite xenolithsc. Subcontinental mantle metasomatism may be associated with UHP, supercritical fluids derived from subducted eclogite-facies continental crust, which may be a greater source of the halogens and water that characterise mantle metasomatism than oceanic crust.

a Svensen, H., Jamtveit, B., Banks, D.A., Austrheim, H. (2001) Halogen contents of eclogite facies fluid inclusions and minerals. J. Met. Geol. 19:165-178.

b Johnson, L.H., Burgess, R., Turner, G., Milledge, H.J., Harris, J.W. (2000) Noble gas and halogen geochemistry of mantle fluids: comparison of African and Canadian diamonds. Geochimica et Cosmochimica Acta 64:717-732.

c Broadley, M.W., Barry, P.H., Ballentine, C.J., Taylor, L.A., Burgess, R. (2018) End-Permian extinction amplified by plume-induced release of recycled lithospheric volatiles. Nature Geoscience 11: 682–687.

Metamorphic Studies Group – Research in Progress 2020

15

Retrograde Tectonics of the Stadlandet Peninsula, Western Gneiss Region, Norway

Isabel S. M. Carter1, Phillip Gopon1, David J. Waters1,2 and Andrew Parsons1

1 Department of Earth Science, University of Oxford, South Parks Road, Oxford, UK OX1 3AN 2 Oxford University Museum of Natural History, Parks Road, Oxford, UK, OX1 3PW

*E-mail: isabel.carter@st-annes.ox.ac.uk

The Western Gneiss Region (WGR) of Norway is one of the largest and best studied examples of exhumed ultra-high pressure (UHP) continental terrains in the world. It is part of the Caledonian Orogenic Belt, which formed during the collision of Baltica with Laurentia after the closure of the Iapetus Ocean in the Cambro-Ordovician. The WGR is situated in the lowermost basement unit of the Scandinavian Caledonides, and contains numerous mafic enclaves which preserve eclogite facies mineral assemblages. Peak assemblages grade from 3.2 GPa/800°C in the northwest to 1.8 GPa/600°C in the southeast (Griffin, 1987; Wain, 1997), with the isograds being roughly aligned NE-SW. Due to the steady decrease in metamorphic grade towards the southeast, the WGR has generally been treated as having been exhumed as a single coherent UHP-HP slab. A record of the exhumation-related pressure-temperature conditions and trajectories is contained in the breakdown products of eclogite-facies minerals, for example the breakdown of omphacite to a diopside–plagioclase (± amphibole) symplectite (Van Roermund and Boland, 1983; Joanny et al; 1991). These provide constraints on the large-scale mechanisms of exhumation. Previous work by Gopon, McKenzie, and Waters (McKenzie, 2018) showed systematic changes in retrograde P-T paths for the eclogites in the northern part of the WGR between southeastern mainland samples and the northwestern island samples. This supports the hypothesis of a tectonic break (i.e. shear zone) separating the mainland and the islands which was originally proposed by Wain (1997).

This projects aims to see if the same trend in

retrograde P-T path discontinuities holds in the

southwestern part of the WGR. Detailed SEM-

EDS and EPMA analysis was conducted on a

suite of mafic eclogite samples from the

Stadlandet Peninsula of Western Norway,

collected in July 2019. Retrograde P-T

pathways were then constructed using the

plagioclase-hornblende thermometer and

clinopyroxene-plagioclase-hornblende

barometer.

Griffin, W. L. (1987) ‘“On the eclogites of Norway”—65 years later’, Mineralogical Magazine, 51(361), pp. 333–343. Joanny, V., van Roermund, H. and Lardeaux, J. M. (1991) ‘The clinopyroxene/plagioclase symplectite in retrograde eclogites:

A potential geothermobarometer’, Geologische Rundschau, 80(2), pp. 303–320. McKenzie, W. F. (2018) Reliving Norway’s stressful past: Determining the Retrograde History of the Western Gneiss Region.

University of Oxford. Van Roermund, H. L. M. and Boland, J. N. (1983) ‘Retrograde P-T trajectories of high temperature eclogites deduced from

omphacite exsolution microstructures.’, Bulletin de Mineralogie, 106(6), pp. 723–726. Wain, A. (1997) ‘New evidence for coesite in eclogite and gneisses: Defining an ultrahigh-pressure province in the Western

Gneiss region of Norway’, Geology, 25(10), pp. 927–930.

Figure 1 - BSE image of a plagioclase-clinopyroxene symplectite, within parent omphacite. Taken on FEI Quanta 650 Scanning Electron Microscope.

Metamorphic Studies Group – Research in Progress 2020

16

Spatial variation of deformation in the Western Gneiss Region: implications on UHP exhumation

Joanna Male1,*, Andrew Parsons1, Rellie Goddard1, Phillip Gopon1 and David J. Waters1,2 1Department of Earth Science, University of Oxford, South Parks Road, Oxford, UK, OX1 3AN 2Oxford University Museum of Natural History, Parks Road, Oxford, UK, OX1 3PW

*E-mail: joanna.male@seh.ox.ac.uk

The Western Gneiss Region (WGR) of Norway is one of the largest and most studied high pressure

(HP) and ultrahigh pressure (UHP) continental terranes on the planet. During the Caledonian

orogeny, the WGR formed as a result of the collision of Baltica and Laurentia on closure of the

Iapetus Ocean (Terry & Robinson, 2003). Baltica subducted below Laurentia taking the WGR to

depths of 100 km (Andersen et al., 1991). Temperature and pressure measurements from eclogites

rocks in the WGR give peak conditions of 800°C and 3.5 GPa (Hacker, 2006). However, current

research and models are inconsistent on their interpretation of the present HP/UHP boundary

patterns and structure and their evolution during the exhumation of the WGR from UHP conditions.

Recent work has demonstrated a distinct difference between island and mainland eclogites facies

rocks; the former are highly foliated and often show evidence of partial melting, whereas the latter

are unfoliated and show no evidence of melting (Young, 2018; McKenzie et al. 2018). In addition,

previous work by McKenzie et al., (2018) found the retrograde pathways for the northern part of the

WGR showed a metamorphic discontinuity between the mainland and the island. The apparent

change in retrograde path is in agreement with Wain’s (1997) suggestion of a tectonic break

between the islands and the mainland.

This project investigates the widely debated processes responsible for subduction and exhumation

of the WGR. The Stadlandet peninsular offers an unbroken transect between the mainland and the

islands across which the hypothesized tectonic break is thought to be exposed on land. Regional

deformation fabrics and gradients were identified through fieldwork, carried out in July 2019.

Samples transecting the proposed metamorphic discontinuity were collected for analysis of grain-

scale deformation. Electron backscatter diffraction was used to measure crystallographic preferred

orientations to identify any areas of significant deformation that may be responsible for the

reported spatial variations in eclogite metamorphism.

McKenzie, W. F. (2018) Relieving Norway’s stressful past: Determining the Retrograde History of the Western Gneiss Region. University of Oxford.

Andersen, T. B., Jamtveit, B., Dewey, J. F., & Swensson, E. (1991). Subduction and eduction of continental crust: major mechanisms during continent-continent collision and orogenic extensional collapse, a model based on the south Norwegian Caledonides. Terra Nova, 303–310. Retrieved from http://folk.uio.no/torgeir/pdfpapers/eduction.pdf

Hacker, B. R. (2006). Pressures and temperatures of ultrahigh-pressure metamorphism: Implications for UHP tectonics and H2O in subducting slabs. International Geology Review, 48(12), 1053–1066. https://doi.org/10.2747/0020-6814.48.12.1053

Terry, M. P., & Robinson, P. (2003). Evolution of amphibolite-facies structural features and boundary conditions for deformation during exhumation of high- and ultrahigh-pressure rocks, Nordøyane, Western Gneiss Region, Norway. Tectonics, 22(4), n/a-n/a. https://doi.org/10.1029/2001tc001349

Wain, A. (1997). New evidence for coesite in eclogite and gneisses: Defining an ultrahigh-pressure province in the Western Gneiss region of Norway. Geology, 25(10), 927–930. https://doi.org/10.1130/0091-7613(1997)025<0927:NEFCIE>2.3.CO;2

Young, D. J. (2018). Structure of the (ultra)high-pressure Western Gneiss Region, Norway: Imbrication during Caledonian continental margin subduction. Bulletin of the Geological Society of America, 130(5–6), 926–940. https://doi.org/10.1130/B31764.1

Metamorphic Studies Group – Research in Progress 2020

17

Reflectance spectral features of Koan hornfelses, at Dikeos Massif contact metamorphic aureole,

Aegean Sea

M. Kokkaliari1,*, C. Kanellopoulos1 and I. Iliopoulos1

1Department of Geology, University of Patras, 26504 Patras, Greece

*E-mail: kokkaliari_m@upnet.gr

The application of Near-Infrared Spectroscopy (NIR) on hornfelses from Kos Island, Aegean Sea,

Greece, provided new information about the spectroscopic properties of these rocks, considering

the occurring mineralogical assemblages. The oldest lithological units of the island occur at Dikeos

Massif and date to Permocarboniferous. These suffered contact metamorphism due to the intrusion

of a large Miocene I-type quartz monzonite. Laboratory reflectance spectra were acquired with a

portable spectrometer (SM-3500 Spectral Evolution spectrometer) and a range of NIR-active

minerals were identified, based on the United States Geological Survey spectral library (USGS). The

presence of specific absorption features is attributed mostly to water and hydroxyl groups vibrations

such as Al-OH, Fe-OH, Mg-OH. The corresponding wavelength position and the depth of the

absorption feature as well as the overall shape of the continuum are influenced by many factors,

such as the mineralogical assemblages, the abundance of the minerals, the texture of the sample,

the color, etc. The minerals that were spectrally identified focus mainly on amphibole, vezuvianite,

cordierite and mica. Optical microscopy (OM) as well as cathodoluminescence (CL) imagery were

used complementary in order to gain additional information about their mineralogical paragenesis,

textural characteristics and metamorphic grade. X-Ray Powder Diffraction analysis (XRPD) helped us

to cross-check the results obtained, whereas the geochemical analysis provided additional

information about the bulk-rock chemistry and in combination to the mineralogy, suggests a variety

in the protolith of the hornfelses. Near Infrared Spectroscopy proved an economical, promising

methodology for a rapid in situ evaluation and screening of the studied lithologies, since no sample

preparation is needed.

Metamorphic Studies Group – Research in Progress 2020

18

Lawsonite eclogite and blue schist from Syros, Greece: Insights into the fluid pathways in

subducted crust and the eclogite-blueschist transition

Thomas Lamont* and Richard White

School of Earth and Environmental Science, University of St Andrews

*E-mail: tnl1@st-andrews.ac.uk

Lawsonite is a key mineral for understanding the migration of water within subducted crust as it acts as a sponge, soaking up available water released during prograde metamorphic dehydration reactions (Clarke et al., 2006). Lawsonite equilibria have traditionally presented a paradox to petrologists, as experimental work and modelling suggest that they occur across a broad P–T spectrum (Newton & Kennedy, 1963; Liou, 1971; Chatterjee et al., 1984; Heinrich & Althaus, 1988, Clarke et al., 2006), however natural lawsonite‐bearing assemblages are rare. The high-water contents required to form lawsonite‐bearing assemblages suggests external H2O must be added at elevated pressure to produce significant proportions of lawsonite. Breakdown (de-hydration) of lawsonite upon exhumation subsequently re-hydrates the rock facilitating retrograde reactions. On Syros, Greece, lawsonite bearing blueschists lie adjacent to dry eclogite facies rocks suggesting the two metamorphic facies occur at the same P-T conditions of ca. 22 kbar and 550 C but record very different hydration states. In these rocks, there is a slight difference in bulk composition with more Ca and Na in the lawsonite bearing blueschists compared to the eclogite which do not display matrix lawsonite. In some of the eclogites, lawsonite occurs as small pseudomorphs preserved as inclusions within garnet, but not the external matrix, suggesting lawsonite was present at peak pressures and was followed by complete breakdown to clinozoisite and paragonite during decompression. The preserved total assemblage therefore records a lower hydration state compared to when at peak conditions. By contrast, the blueschists with matrix lawsonite do not contain garnet, suggesting either: i) the presence of lawsonite inhibited garnet growth during prograde metamorphism either due to a consuming the components required for garnet stability or ii) lawsonite grew on the retrograde path due to the input of external water during exhumation. Equilibrium phase diagrams from all rocks consistently show that between 3–6% molar H2O became available upon crossing the lawsonite-out reaction during the initial stages of exhumation following peak pressures. The adjacent nature of eclogite and lawsonite blueschists therefore suggest the presence of lawsonite is primarily controlled by the initial bulk rock composition, which in turn controls the subsequent fluid pathways during prograde and retrograde metamorphism in subducted crust. In either case, lawsonite breakdown must play a significant role in hydrating the adjacent rocks, enhancing weakening, shear zone development and consequent exhumation and facilitating the growth of secondary zoned amphiboles associated with lower values of NaB and increasing barroisite composition from core to rim, indicative of continuous fluid release during exhumation. References:

Chatterjee, N. D., Johannes, W. & Leistner, H., 1984. The sys- tem CaO-Al2O3-SiO2-H2O: new phase equilibria data, some calculated phase relations, and their petrological applications. Contributions to Mineralogy and Petrology, 88, 1–13.

Clarke, G.L., Powell, R. and Fitzherbert, J.A., 2006. The lawsonite paradox: a comparison of field evidence and mineral equilibria modelling. Journal of Metamorphic Geology, 24(8), pp.715-725.

Heinrich, W. & Althaus, E., 1988. Experimental determination of the reaction 4lawsonite + 1albite 1⁄4 1paragaonite + 2zo- site + quartz + 6H2O and 4lawsonite + 1albite 1⁄4 1parag- onite + 2zoisite + quartz + 6H2O. Neus Jahrbuch fu ̈r Mineralogie Monatshefe, 11, 516–528.

Liou, J. G., 1971. P–T stabilities of laumontite, wairakite, lawsonite and related minerals in system CaAl2-Si2O8-SiO2- H2O. Journal of Petrology, 12, 378–411.

Newton, R. C. & Kennedy, G. C., 1963. Some equilibrium re- actions in the join CaAl2Si2O6-H2O. Journal of Geophysical Research, 68, 2967–2983.

Metamorphic Studies Group – Research in Progress 2020

19

Not all kyanite is created equal – The petrogenesis of kyanite migmatites in Eastern Bhutan

S.E. Phillips1, T.W Argles1, N.B.W. Harris1, C.J. Warren1, N.M.W. Roberts2, B. Kunz1

1 The Open University, Milton Keynes, United Kingdom 2NERC Isotope Geosciences Laboratory, British Geological Survey, Nottingham, UK *E-mail: stacy.phillips@open.ac.uk The presence of aluminosilicate minerals in metamorphic and igneous rocks provides good constraints on P-T conditions; in granites however, these minerals could be xenocrysts, peritectic phases or have directly crystallised from melt. Each of these occurrences provides different implications for the formation of their host. Kyanite-bearing migmatites in the Himalaya are commonly interpreted to form by early (Oligocene) melting during prograde burial (Iaccarino et al., 2015; Groppo et al., 2010); in contrast, sillimanite-bearing leucogranites are interpreted as forming during later (Miocene) decompression melting (Weinberg, 2016; Harris and Massey, 1994). The petrogenesis of these aluminosilicate-bearing melts therefore carries implications for the timing of mid-crustal weakening of the Himalaya. Investigation of kyanite from small-scale (cm-dm), ‘in source’ migmatites and their host rocks from eastern Bhutan has revealed different populations of kyanite, based on variations in morphology, cathodoluminescence patterns, and trace element characteristics. The majority of kyanite grains found in the leucogranites studied are inherited from the host rocks and thus are xenocrystic. Only a small number of grains in a few studied samples are interpreted to have formed directly as a result of the crystallisation of melt. Peritectic kyanite, formed during fluid-fluxed muscovite dehydration, are also observed in some E Bhutan samples. LA-ICP-MS analyses of zircons from these migmatites yield a range of ages from ~32 - 12 Ma with peaks at ~21 and 15 Ma. This Miocene-aged melting in the lower Greater Himalayan Sequence (GHS) is consistent with more recent tectonic models that suggest the base of the GHS was the last part of the channel to be exhumed (Carosi et al., 2018). Investigating the petrogenesis and reaction history of these migmatites in detail is therefore crucial for understanding their significance in Himalayan tectonics.

Metamorphic Studies Group – Research in Progress 2020

20

Quantitative automated mineralogy to constrain metamorphic processes using ZEISS Mineralogic

Richard J M Taylor1 , Chris Clark2 and Tim Johnson2

1Carl Zeiss Microscopy Ltd, ZEISS Group, Cambourne, Cambridge CB23 6DW, UK 2School of Earth and Planetary Sciences, Curtin University, Bentley 6102, Perth, Western Australia

*E-mail: richard.taylor@zeiss.com

The ZEISS Mineralogic automated quantitative mineralogy takes a new approach to automated

mineralogy. Mineralogic uses a Scanning Electron Microscope (SEM) equipped with Energy

Dispersive Spectroscopy (EDS) detectors, but following an analytical protocol more closely aligned

with the quantitative Electron Probe Micro Analysis (EMPA). A combination of matrix corrections,

peak deconvolution, and standard calibration means that peak intensities are converted directly into

wt% element directly at the time of analysis. The result is a data output that can be immediately

interpreted by both new and experienced users, even for minerals not previously in the database.

Here the ZEISS Mineralogic system is demonstrated as a tool for automated mapping of thin sections

from metamorphic terrains. The bulk chemistry of the entire thin section, as well as individual

mineral compositions can be used to constrain P-T conditions directly from the SEM, without the

need for an additional step of obtaining mineral chemistry from an EPMA. With quantitative analysis

at every pixel, major element profiles can be obtained at any point in the thin section, and P-T can

therefore be determined from any domain within the mapped section. This approach makes the use

of P-T pseudosections possible with greater speed and flexibility than has previously been possible.

Metamorphic Studies Group – Research in Progress 2020

21

Reaction Affinity Patterns in Rocks with Porphyroblasts

C. T. (Tom) Foster, Jr.

University of Iowa, Iowa City, Iowa USA 52240

*E-mail: tom-foster@uiowa.edu

The affinity of a reaction is an important control on the nucleation and growth of new minerals when a isograd is overstepped during metamorphism. Thermodynamic calculations based on the overall bulk composition of the rock may provide a reliable estimate of the affinity in rocks with a uniform distribution of reactant minerals that maintain metastable equilibrium among the entire reactant assemblage as an isograd is overstepped. However, rocks with porphyroblasts are not compositionally uniform, and reaction affinity is dependent upon the scale of equilibration. If rock-wide equilibrium is not maintained, the affinity of the new mineral in sub-domains of the rock will be buffered by different reactions, depending upon which type of porphyroblasts and matrix are in local equilibrium. For example, the chemical potential of Al2SiO5 in a rock with Bt+Ms+Plg+Ilm+Qz+water (matrix) and porphyroblasts of garnet and staurolite that oversteps the sillimanite isograd is controlled by three reactions, one involving staurolite plus matrix minerals, one involving garnet plus matrix minerals and a third involving equilibrium among matrix minerals not involving either type of porphyroblast. If the isograd is overstepped fast enough so that domains of equilibrium are sufficiently small, each of these reactions will buffer the chemical potential of Al2SiO5 in different ways so that the affinity of reaction for sillimanite is overstepped by different amounts in subdomains buffered by the three different reactions (Fig. 1). As the sillimanite isograd (583.5 oC, 4kb) is over-stepped, the reaction

affinity (-G) increases most rapidly in domains without St because St buffers

Al2SiO5 most effectively. Provided preferential nucleation sites (e.g. Bt) are equally distributed in all three domains, this means that sillimanite should nucleate most readily in domains away from both types of porphyroblasts, followed closely by domains near garnet, and lastly by domains near staurolite. This pattern will also be influenced by the fact that as time progresses, the domains in local equilibrium with garnet and/or staurolite will grow larger according to (~Dt)0.5, so the regions in equilibrium with only matrix minerals will grow smaller, gradually decreasing the volume of rock

with the largest affinity for sillimanite nucleation.

Fig. 1. G sillimanite vs T @4kb for domains in

equilibrium with St+Grt+matrix (the wholerock

assemblage), St+matrix, Grt+matrix and matrix

without St or Grt. Calculated using PD2015-HP1

Metamorphic Studies Group – Research in Progress 2020

22

LA-ICP-MS mapping of granulite-facies monazite: textural insights and implications for

petrochronology

Owen Weller* (University of Cambridge), Simon Jackson (Geological Survey of Canada), William

Miller (University of Cambridge), Marc St-Onge (Geological Survey of Canada), Nicole Rayner

(Geological Survey of Canada)

*E-mail: ow212@cam.ac.uk

Texturally complex monazite grains within two granulite-facies pelitic migmatites from southern

Baffin Island, Arctic Canada, were mapped by laser ablation-inductively coupled plasma-mass

spectrometry to quantitatively determine the spatial variation in trace element chemistry using

nominal beam diameters of 4-5 microns resolution (with up to 1883 analyses per grain). The maps

highlight the 3-D chemical complexity of monazite grains that have experienced multiple episodes of

growth, resorption and chemical modification by dissolution-precipitation during high-grade

metamorphism. Following detailed chemical characterisation of monazite compositional zones, a

related U-Pb dataset is re-interpreted, allowing petrologically-significant ages to be extracted from a

continuum of concordant data. Synthesis of these data with pseudosection modelling of prograde

and peak conditions allows for the temporal evolution of monazite trace element chemistry to be

placed in the context of the evolving P-T conditions and major phase assemblage. This approach

enables a critical evaluation of three commonly used petrochronological indicators: linking Y to

garnet abundance, the Eu anomaly to feldspar content, and Th/U to anatectic processes. Europium

anomalies and Th/U behave in a relatively systematic fashion, suggesting that they are reliable

petrochronological witnesses. However, Y systematics are variable, both within domains interpreted

to have grown in a single event, between grains interpreted to be part of the same age population,

and between samples that experienced similar metamorphic conditions and mineral assemblages.

These observations caution against generalised petrological interpretations on the basis of Y

content, as it suggests Y concentrations in monazite are controlled by domainal equilibria. The

results reveal a ~45 Myr interval between prograde metamorphism and retrograde melt

crystallisation in the study area, emphasising the long-lived nature of heat flow in high-grade

metamorphic terranes. Careful characterisation of monazite grains suggests that continuum-style U-

Pb datasets can be decoded to provide insights into the rates of metamorphic processes.

Metamorphic Studies Group – Research in Progress 2020

23

Microtectonic analysis of the Teziutlan Metamorphic Complex, Puebla State, Mexico: Mylonitic

deformations in the Late Paleozoic

Luis Javier Gutiérrez-Trejo1,*, Mario A. Ramos-Arias2, Edgar Ángeles-Moreno3, José Martín Yáñez-Limón4, Adair Jiménez-Nieto4, Teresa Pi-Puig2 1Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, México 2Instituto de Geología, Universidad Nacional Autónoma de México, México 3Depto. Ingeniería en Minas, Metalurgia y Geología, División de Ingenierías de la Universidad de

Guanajuato, México 4Centro de Investigación y Estudios Avanzados del IPN, Unidad Querétaro, México

*E-mail: luisja.gtz.t@gmail.com

The Teziutlan Metamorphic Complex (TMC) is located in the eastern portion of Mexico, it is a

polydeformed assamblage of metasedimentary, metavolcaniclastic and metaigneous rocks with

schistose, gneissic and mylonitic textures. The TMC is defined by four lithodemes: (i) La Soledad,

dominated by plagioclase+K-feldspar+quartz orthogneisses; (i) Chicuaco, mainly composed by

quartz+muscovite+albite±biotite±chlorite schist; (iii) Cozolexco formed by

actinolite+chlorite+epidote schists and greenstones; and (iv) El Mirador, constituted by

muscovite+quartz schist interbedded with meta-volcanosedimentary rocks.

These assemblages of rocks experienced crystal-plastic deformation under greenschist facies

conditions during the Permian, developing two progressive generations of ductile structures:

isoclinal folding F1 with S1 foliation and a subsequent superimposed S2 foliation related to closed

and recumbent folding F2, resulting in a penetrative transposition of foliation S1/2 (222°/23°). Field

observations and structural data suggest an arrangement of several ductile subhorizontal high-strain

shear zones in a compressive regime. The microtextures developed in the orthogneisses and schists

exhibit a dynamic recrystallization of quartz mainly, identified by subgrains, grain boundary

migration and bulging textures, which are strongly penetrative at the mylonitic zones. The EBSD data

of quartz CPO suggests that the crystal-plastic deformation took place at 427±50 °C.

The U-Pb ages obtained from magmatic zircons of tonalite, and 40Ar/39Ar muscovite cooling ages

from muscovite+quartz schist suggest that the ductile deformation probably occurs between 312

and 287 Ma. Such ages are in good agreement with the last phases of the amalgamation of Pangea

as well as with the first stages of the paleo-Pacific subduction in Mexico, shown by the formation of

a regional scale dextral-fault system (i.e., the Caltepec Fault). Thus, we suggest that the ductile

deformation of the TMC was influenced by the Caltepec Fault as it was also originated during the

first stages of subduction along the paleo-Pacific margin of Pangea.

Metamorphic Studies Group – Research in Progress 2020

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Microstructural Controls on the Crystallization of Garnet: An Example from the Meguma Terrane,

Nova Scotia

Allie Nagurney1 and Mark Caddick1

1Department of Geosciences, Virginia Tech, Blacksburg, VA

*E-mail: nagurney@vt.edu

Despite the success of equilibrium thermodynamic methods for calculating the stability of minerals

during progressive metamorphism, the crystallization processes that control the size and distribution

of metamorphic minerals are incompletely understood. We integrate petrographic, geochemical,

and microstructural data to elucidate the controls on metamorphic re-crystallization of garnet in an

amphibolite facies pelitic rock from the Mosher’s Island Formation in the Meguma Terrane, Nova

Scotia. We study one sample, which contains biotite, chlorite, garnet, ilmenite, muscovite,

plagioclase, quartz, staurolite, and sillimanite. This sample presents an opportunity to study

crystallization processes because loss of the reactant minerals involved in the formation of garnet

was incomplete. Garnet chemical zoning is typical of prograde metamorphism, with a rim-wards

decrease in manganese and calcium and increase in iron and magnesium contents. Phase equilibria

suggest that garnet started to crystallize at ~475ºC and 3.8 kbar after a minimal (< 20ºC) overstep.

Petrographic and geochemical observations indicate that garnet ceased growing once staurolite was

present, with initial staurolite growth calculated at 520ºC and 4.1 kbar. Peak P-T conditions are

~560ºC and 4.2 kbar.

The spatial organization of garnet porphyroblasts was obtained by High-Resolution X-Ray Computed

Tomography. The population of garnet crystals has a unimodal crystal size distribution with an

average crystal radius of 126 µm. 15 garnet crystals were analyzed by Electron Back Scatter

Diffraction, with all crystals sharing one of three crystallographic orientation relationships with

phyllosilicates defining the foliation of the rock. We interpret that garnet crystals (i) nucleated over a

short interval after minimal overstepping, (ii) grew epitaxially on chlorite and muscovite crystal

structures, and (iii) grew to a similar size that does not suggest intense competition for nutrients.

This work highlights the utility of combining thermodynamic modelling with microstructural

techniques to elucidate some of the controls on mineral crystallization in metamorphic rocks.

Metamorphic Studies Group – Research in Progress 2020

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P-T path determination using phase equilibria modelling with multiple fractionation of garnet in a metapelite

Carlos Iván Lembo Wuest1,*, Samanta Serra-Varela2, Sebastián O. Verdecchia1,3, Juan A. Murra1,3, Manuela E. Benítez4,5 and Gladis Palacio Baderramo6

1Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICET), Centro de investigaciones en Ciencias de la Tierra (CICTERRA), Av. Vélez Sarsfield 1611, X5016CGA Córdoba, Argentina. 2Universidad Nacional de Río Negro, Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICET), Instituto de Investigaciones en Paleobiología y Geología. Av. Roca 1242, 8332, General Roca, Río Negro, Argentina. 3Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Av. Vélez Sarsfield 1611, X5016CGA. Córdoba, Argentina 4Instituto de Recursos Minerales (INREMI), La Plata, provincia de Buenos Aires, Argentina 5Comisión de Investigaciones Científicas de la provincia de Buenos Aires (CICPBA) 6Centro de Investigación de la Geosfera y Biosfera (Consejo Nacional de Investigaciones Científicas y Tecnológicas, CONICET), Gabinete de Mineralogía y Petrología, Facultad de Ciencias Exactas Físicas y Naturales. Universidad Nacional de San Juan (San Juan – Argentina). *E-mail: civanlw@gmail.com

Zoned garnets are considered useful to determine prograde metamorphic evolution, in which a fractionation of chemical components from whole-rock composition occurs. Thus, effective bulk composition (EBC) should change while garnet is growing to remove the essential components used for its nucleation. Therefore, a step-by-step estimate of the equilibrium volume is necessary to reconstruct the P-T path toward the metamorphic peak. In this work, we analyze a metamorphic P-T path for a garnet mica-schist from Sierra El Gigante (Argentina) using multiple fractionations of garnet in conjunction with P-T pseudosections and compositional isopleths based on chemical mineral analysis.

Garnet zoning is characterized by a core with Sps21-Grs23-Alm54-Prp4 that progressively changes toward a Sps6-Grs22-Alm65-Prp6 rim. The chemical fractionation was conducted using Theriak-Domino software and successive new bulk compositions were obtained. Five stages of garnet fractionation from core to rim were defined from the compositional profile of garnet, for each of which the EBC was calculated. For each stage, P-T conditions were estimated from Sps (XMn), Alm (XFe), and Sps (XMn) isopleths. Thus, P-T condition for the initial nucleation of garnet was calculated at 545° C and 7.2 kbars, whereas the rim (P-T peak) is constrained at 595° C and 9.5 kbar, inside of stability P-T field of plagioclase - garnet - muscovite - paragonite - biotite - albite - quartz - rutile.

The last garnet fractionation after the estimation of the P-T peak was performed for retrograde stage analysis. In this step, a simplified system without Ca and Mn was selected because the matrix is composed of albite, muscovite, chlorite, biotite, and rutile. Thus, 390° C and 6 kbars were calculated from Si (p.f.u.) of muscovite and Mg/(Mg + Fe) of chlorite.

The results suggest that the metapelites from Sierra El Gigante evolved over a high P/T gradient in the prograde segment.

Metamorphic Studies Group – Research in Progress 2020

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Post-kinematic and matrix-dependent garnet nucleation and growth in the inverted Barrovian metamorphic sequence of the Sikkim Himalaya

Freya R George1 and Fred Gaidies2

1Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, USA 2Department of Earth Sciences, Carleton University, Ottawa, Canada

*E-mail: fgeorge7@jhu.edu

In metapelites from the inverted Barrovian sequence of the Sikkim Himalaya, x-ray µ-computed tomography reveals garnet populations that exhibit a general increase in porphyroblast size with increasing metamorphic grade, coupled with a decrease in their population density. However, the upper garnet–staurolite zone demarcates a region containing microstructurally anomalous and post-kinematic garnet populations, hosted in strongly segregated matrices. Phase equilibria modelling precludes the possibility that these differences arise from contrasting thermobarometric histories or bulk chemical compositions. Given the lack of syn and post-crystallisation deformation that distorts the primary crystallisation record in these samples—as is the case in other samples from Sikkim—these populations provide insight into the controls on heterogeneous nucleation and the importance of the grain boundary during metamorphic crystallisation.

Garnet polycrystals in these heterogeneous samples are strongly clustered and exhibit crystallographically-controlled and morphologically irregular interfaces adjacent to micaceous and quartzo-feldspathic domains, respectively. Electron backscatter diffraction indicates that these structures contain numerous coalesced porphyroblasts that are structurally uncorrelated across grain boundaries. However, a crystallographically preferred orientation at the polycrystal-scale is interpreted to derive from epitaxial crystallisation of early-formed garnet on precursor micas. Later-nucleated porphyroblasts within polycrystals preferentially concentrated towards quartzo-feldspathic domains, with the overall nucleation distribution potentially controlled by a complex interplay between chemical heterogeneities, strain partitioning, and epitaxial crystallisation. The growth of these polycrystals was also spatially heterogeneous; it was moderated by differences in the efficiency of grain boundary transfer between quartzo-feldspathic and micaceous domains that prevented thin section-scale chemical equilibration. In contrast to samples from Sikkim containing more typical porphyroblastic populations in continuous and disseminated matrices, heterogeneous availability of garnet-forming components within this strongly layered matrix is shown to have resulted in grain-scale variations in growth rates and the spatial juxtapositioning of interface-controlled microstructures and locally equilibrated chemical compositions with those that were transport-controlled.

Metamorphic Studies Group – Research in Progress 2020

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Prograde UHP metamorphism in felsic and mafic lithologies and clues to pre-Himalayan

metamorphism in the Tso Morari dome, Ladakh, Himalaya

A.K. Bidgood1,*, D.J. Waters1,2 and M.P. Searle1

1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, UK 2Natural History Museum, Oxford, Parks Road, OX1 3PW

*E-mail: anna.bidgood@univ.ox.ac.uk

Ultra-high pressure (UHP) metamorphism has long been associated with deep subduction. UHP minerals, such as coesite and microdiamond, are often found in discrete mafic units, hosted in larger scale continental terrains, such as in the mafic eclogites of the Tso Morari dome, Ladakh, Himalaya. Terranes such as these have commonly been overprinted by amphibolite facies, Barrovian metamorphism, so their early history is difficult to decipher. Samples of mafic eclogite dykes hosted in low-strain metagranite were collected from the centre of the Tso Morari dome in Ladakh, Himalaya, where the granite has been observed to transition into granite gneiss towards the high-strain mafic–felsic lithological boundary. All three of these lithologies are shown to preserve high pressure (HP) metamorphism indicating that the entire unit must also have been subducted to UHP conditions. Low-strain metagranites provide a window into the early history of the rocks, where feldspar, white mica and pseudomorphs after cordierite preserve evidence of hydrothermal alteration prior to HP metamorphism. Kyanite inclusions within garnet also indicate that the high-Al bulk composition of the granite gneiss must have been achieved prior to garnet growth, also indicating that hydrothermal alteration prior to garnet growth may have been significant. Elemental zonation within garnets from mafic eclogite and granite gneiss protoliths show a similar zoning pattern in rocks of different bulk compositions, indicating that both rock types share a similar prograde high-pressure metamorphic evolution, where highest pressure conditions are preserved in the garnet rims (cf. St-Onge et al., 2013). Previous modelling of mafic eclogites has shown that there are two possible isopleth intersections for garnet core growth at ca. 20-23kbar, 520-550°C and 10-13kbar, 575-625°C (Palin 2013). However, alteration of inclusion assemblages has made it difficult to determine the actual conditions of core growth. Combined modelling of mafic and felsic protoliths, shows that the lower pressure intercept of 10-14kbar, 410-460°C is common in both bulk compositions in water undersaturated conditions. Iterative modelling by fractionation of garnet and H2O with respect to the observed garnet growth zones shows that successive garnet growth zones record higher pressure. Both protoliths record a discontinuity in garnets at their core-rim boundary, between abundant and few inclusions, which is also marked by an annulus of higher Mn attributable to garnet resorption and reincorporation of released Mn. Inclusions of pseudomorphed lawsonite are found at this boundary in mafic samples. All three features may reflect an influx of aqueous fluid at this stage.

Metamorphic Studies Group – Research in Progress 2020

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Leucogranites in the Garhwal Himalaya: The story according to zircon

Charlie J. Oldman1,*, Clare J. Warren1, Christopher J. Spencer2, Tom W. Argles1, Nigel B. W. Harris1

and Sam J. Hammond1

1School of Environment, Earth and Ecosystem Sciences, The Open University, Milton Keynes MK7

6AA, UK 2Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston,

Ontario KL7 3N6, Canada

*E-mail: charlie.oldman@open.ac.uk

Prograde metamorphism during orogenic evolution results in the melting of high-grade metamorphic rocks, which dramatically decreases mechanical strength, activates shear zones, and may consequently initiate exhumation. Granitic bodies and partially melted source migmatites, are found across the Himalaya in the Greater Himalayan Sequence (GHS). Many of these granites formed during decompression in the Miocene. However, exact timings and reaction pathways appear to vary laterally across the orogen. The timescales of anatexis, amalgamation, migration, and emplacement are the focus of active research and have vital implications for orogenic tectonic development. It is, therefore, important to recognise and distinguish between the geochemical signatures that these processes create and to delineate more precisely the relevant mechanisms and timescales leading to magma genesis. We present a preliminary dataset that constrains the source, melt reactions, and timescales of melting episodes that form the migmatites and leucogranites of the upper GHS. We sampled leucogranites, migmatites, and their host metasediments along the Rishi Ganga (Badrinath) and Alaknanda valleys in the Garhwal region of the Indian Himalaya. Zircon from these samples were analysed for their crystallisation age (U-Pb), Hf-isotopic ratios, oxygen isotope and trace element composition using SIMS and LA-ICPMS. Rim domains identified using cathodoluminescence (CL) imaging were preferentially targeted to collect data that related to Himalayan melting processes. Results suggest that the leucogranites crystallised from 22 Ma to ~13 Ma, with punctuated zircon crystallisation occurring throughout this timespan. Zircon rim ages from migmatites are generally older, ranging from 34 Ma to ~15 Ma. Oxygen isotopes (δ18O) of both the granites and migmatites fall within the expected range for a sedimentary origin, with values between 8.5 and 11.5‰. Integration of Hf-isotopic and trace elemental data, combined with petrographic observations, allow mineral age data to be linked to changes in geological processes.

Metamorphic Studies Group – Research in Progress 2020

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Provenance and geochemistry of metasedimentary rocks in the basement of the Sierra Madre terrane, Mexico: Implications of deposition along the western margin of Pangea

S.A. Torres Sánchez1, C. Augustsson2, U. Jenchen3, J.R. Barboza Gudiño4, A. Gärtner5, M. Hofmann5, U. Linnemann5 and C.M. Morales Bautista6

1 Área de Ciencias de la Tierra, Facultad de Ingeniería, Universidad Autónoma de San Luis Potosí, San Luis Potosí, S.L.P, Mexico 2Institutt for Petroleumsteknologi, Universitetet i Stavanger, Stavanger, Norway 3Facultad de Ciencias de la Tierra, Universidad Autónoma de Nuevo León, Linares, Nuevo Leon, Mexico 4Instituto de Geología, Universidad Autónoma de San Luis Potosí, San Luis Potosí, S.L.P, Mexico 5Senckenberg Naturhistorische Sammlungen Dresden, Königsbrücker Landstraße 159, 01109 Dresden, Germany 6Cuerpo Académico Química Aplicada a la Gestión Ambiental. División Académica de Ciencias Básicas. Universidad Juárez Autónoma de Tabasco. Cunduacán, Tabasco, México. *E-mail: soniatorressan@hotmail.com

The basement complex of the Sierra Madre terrane in northeastern Mexico is classified into four domains a) granulite facies rocks of Grenville age (ca. 1 Ga) known as Novillo Gneiss, b) the Granjeno Schist, c) an unmetamorphosed succession of Paleozoic marine siliciclastic and volcanic rocks and d) an Ordovician plagiogranite intrusive body. We present a comprehensive geochemical and provenance study of the Paleozoic metasedimentary rocks component of the Granjeno Schist, which are associated with metavolcanic and meta-ultramafic rocks. In Granjeno Schist ratios of major elements (Al2O3/TiO2), trace (Th/Sc, La/Th, La/Sc, Zr/Sc), and REE patterns for the metasedimentary rocks are consistent with derivation of detritus from basement gneisses. The chemical index of alteration indicates low to moderate weathering of the source. The clastic metasedimentary rocks have moderately radiogenic Nd-isotopic compositions whose initial εNd(t) values range from -9 to -

6, and with DM*model ages between 1.5 and 1.7 Ga. Ages of detrital zircon grains from the metasedimentary rocks are mostly 1590 to1070 Ma, but with some in the 530 to 410 Ma range. Such a Novillo Gneiss-like, age spectrum allows a short transport distance for the majority of zircon grains. High-P, Ordovician, subduction-related metamorphism of the Granjeno Schist was followed by a Pennsylvanian/Permian sub-greenschist to greenschist metamorphic overprint in an accretionary prism developed during the configuration of western Pangea

Metamorphic Studies Group – Research in Progress 2020

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Juxtaposed high- and low-pressure metamorphic field gradients rocks and its tectonic implications, a study case of Turvo-Cajati Formation, Ribeira Belt, Brazil B.S. Ricardo1, R. Moraes1, F. M. Faleiros1, O. Siga Júnior1, G. Campanha1 and C. M. Mottram2 1Institute of Geosciences, University of São Paulo (Brazil) 2School of Earth and Environmental Sciences, University of Portsmouth (UK) *E-mail: bruna.ricardo@usp.br

The Turvo-Cajati Formation (TCF) is a metasedimentary unit forming the Curitiba Terrane, a major segment of the southern Ribeira Belt, SE Brazil. It is composed of rocks of distinct metamorphic degree such as greenschist (Low-TCF), amphibolite (Medium-TCF) and granulite (High-TCF) facies. Previous studies in High-TCF indicate that the subunit underwent extensive partial melting within the kyanite stability field, under high-pressure conditions (670-810 °C and 9.5-12 kbar). New data on the metamorphic zoning within Low and Medium-TCF were collected using pseudosection modelling in the NCKFMASHTO and MnNCKFMASHTO systems. Four sillimanite zones, and sillimanite zone is predominant. Metamorphic pressure is below 8 kbar, as staurolite breaks down straight to sillimanite. Pseudosections yielded metamorphic peak conditions of ~530-560 °C and ~6-7 kbar (garnet zone) and ~660-690 °C and ~6-7 kbar (sillimanite zone). The metamorphic field gradient is flat and just below the typical barrovian-type baric regime with low to medium pressure. It is inferred that Low and Medium-TCF were metamorphosed in a tectonic setting different from the High-TCF. Available petrological data suggest that the TCF comprises a paired of metamorphic zones were recognized for Low-TCF and Medium-TCF: biotite, garnet, staurolite and low-P and high-P belt which is associated with a major Ediacaran suture zone in the southern Ribeira Belt. Probability density plots from detrital zircon indicate sources from late-Cryogenian-Ediacaran arc-related and Rhyacian for all TCF sub-units. This detrital scenario suggests that the TCF is made up of a collisional juxtaposition of an accretionary wedge (High-TCF) and a back-arc basin (Medium-TCF and Low-TCF) on the border of a microplate that includes a Rhyacian basement microcontinent (Atuba Complex). The high metamorphic gradient recorded in the Medium-TCF and Low-TCF was related to asthenospheric upwelling in the back-arc region. This also produced extensive partial melting in the Atuba Complex basement. Previous studies in High-TCF points to metamorphic ages of 589 ± 12 Ma (Faleiros et al., 2011) and 584 ± 4 Ma (Faleiros et al., 2016). The petrochronology now will be used to obtain the age of metamorphic events of Low and Medium-TCF, using monazite and apatite grains and compare them to available data in the literature to understand and adjust the proposed model.

Metamorphic Studies Group – Research in Progress 2020

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New insights from the Fammatinian-Quetame-Caparonensis orogenic event in the Northern Andes

(NW South America)

Diego O. Ruiz-Portilla*, Carlos A. García-Ramírez and Vaness Rey-Leon

Industrial University of Santander

*E-mail: diegoruizp55@gmail.com

The Santander Massif is characterized as the Northern part of the Eastern Cordillera of Colombia

(north-westhern South America). The metamorphic units from this massif are the oldest rocks in the

area and are known as: Bucaramanga Gneiss, Silgará Schists, Berlin Orthoneis and the Floresta

Formation (Ward et al., 1973; Royero, 1994; Moreno et al., 2005; Mantilla et al., 2016a,b).

The oldest metamorphic units (i.e. Bucaramanga Gneiss and Silgara Schists) had been thought to

represent a Grenvillian-age Orogeny and a Fammatinian-age Orogeny, respectively (Cardona, 2003;

Cordani et al., 2005; García et al., 2005; Urueña y Zuluaga, 2011; Mantilla et al 2016a,b).

Nevertheless, recent geochronological studies have challenged this hypothesis and have

demonstrated that both lithostratigraphic units belong to a unique event of Famatinian age (~465

Ma), and therefore no record has been found for an event of Grenvillian-age (~1000 Ma) (Van der

Lelij et al., 2016; Amaya, 2017; Zuluaga et al., 2017).

The current studied area is located in the western side of the Santander Massif, wherein both

Bucaramanga Gneiss and Silgara Schists behave as parts of a single strip, in which the former one is

the inner and hotter record, and the latter one is the outer and colder record of a single tectono-

metamorphic event. Through petrography and Scanning Electron Microscopy (SEM), this study

presents the results of mapping three metamorphic zones of a Buchan-type series for the whole

metamorphic strip. Moreover, geochemical analyzes from each litostratigraphic unit were carried

out in order to correlate or differentiate the composition of their protoliths.

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A detailed petrological study of the metamorphic retrogression textures of the granulitic Oaxacan Complex, México M.M. Almazán-López1,*, F. Ortega-Gutiérrez 1 and V. Colás 1

1Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Ciudad de México, México *E-mail: mar_Alopez@ciencias.unam.mx The Oaxacan Complex is the largest inlier of Precambrian rocks in Mexico, comprising an area of about 7,000 km2 in southern Mexico. The complex is constituted by mafic, pelitic, quartzofeldespathic and calcisilicate rocks of igneous and sedimentary origin that were metamorphosed under granulitic facies 990±10 Ma ago, peaking at 750-909°C and 7.2-8.2 Kbar. Here, we present a detailed petrological study specifically dedicated to understanding the Oaxacan Complex metamorphic retrogression, which involved the application of conventional geothermometry. These data characterized the petrological and chemical evolution of the retrograde textures preserved in the studied samples from (ultra) high temperature granulite facies to lower greenschist facies. We propose that the associated mineral and textural diversity studied here illustrates, at least, four stages of metamorphic retrogression (MR), defined as follows:

1) MR1: occurred under high temperature (HT)-no fluid present conditions, which produced different types of exsolved textures such as lamellaes, mesoperthites and symplectites, observed in quartz, ortho and clinopyroxenes, feldspars, carbonates and ilmenites.

2) MR2: took place under HT-fluid present conditions, and defined by the development of HT amphibole crystals and monomineralic coronae surrounding clino- and orthopyroxene.

3) MR3: started at lower temperature-fluid present conditions, and characterized by the development of symplectitic coronae of two amphiboles (cummingtonite or anthophyllite) surrounding MR2 textures and partial pseudomorphs with relict pyroxene or amphiboles crystals in their cores.

4) MR4: occurred under low temperature-fluid present conditions and characterized by the formation of complete pseudomorphs of amphibole, biotite or chlorite and a chlorite corona that surrounds and replaces MR3 textures.

This retrograde history occurred mainly under isotropic stress conditions indicating a rather slow cooling and uplift history essentially driven by isostatic processes along a protracted path from the lower to the upper crust.

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The Raspas Complex (Ecuador): A fragment of a J-K melange Belt in the NW corner of the South American Plate M.C. Arrieta-Prieto1,*, C.A. Zuluaga-Castrillón1, O.M. Castellanos-Alarcón1,2 and J.D. Hernández-Montenegro1 1Universidad Nacional de Colombia 2Universidad de Pamplona *E-mail: macarrietapr@unal.edu.co High-pressure complexes along the Earth's surface provide evidence of the processes involved in both the crystallization of rocks in the subduction channel and its exhumation. Such processes are key to understand the dynamics and evolution of subduction zones (e.g., through reconstruction of P-T trajectories). The Raspas complex (southern Ecuador) contains high pressure rocks such as blueschists and eclogites with the mineral assemblages glaucophane + garnet + epidote + omphacite + white mica + rutile ± quartz ± apatite ± pyrite ± calcite stabilized. This complex has been genetically related to accretion and subduction processes of seamounts, which occurred in South America during Late Jurassic - Early Cretaceous, and the exhumation of the complex was probably related to subduction channels. However, the evidence presented in the existing literature makes little emphasis on the reconstruction of P-T trajectories for the rocks of this complex. We present here a combination of new petrographic observations, whole-rock chemistry data, and mineral chemistry data used to determine peak metamorphic conditions for the high pressure rocks of the complex and to reconstruct P-T trajectories. Thermodynamic modelling shows that after peak metamorphism (ca. 21 Kbar and 600 °C) the rocks recorded a retrograde event suffered by the complex during its exhumation. The interpretation of the retrograde process is consistent with results from zircon in rutile thermometry and the presence of retrograde microstructures such as amphibole replacing pyroxene, garnet chloritization, plagioclase crystallization and rutile replacement by titanite. Also, Yttrium zoning patterns suggest possible additional processes that could occur during prograde, peak, and retrograde metamorphism. Results obtained here compared to published data from the Arquía complex in Colombia (age and P-T conditions) may suggest a genetic relationship between these two complexes as a result of the tectonic processes associated with an active subduction margin that affected the NW margin of the South American plate at the end of the Jurassic and the beginning of the Cretaceous.

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AUTHOR INDEX A Airaghi, L., 7 Almazán-López, M.M., 32 Ángeles-Moreno, E., 23 Angiboust, S., 8, 12 Argles, T.W., 10, 19, 28 Arrieta-Prieto, M.C., 33 Augustsson, C., 29 B Barboza Gudiño, J.R., 29 Bebout, G., 12 Bellahsen, N., 7 Benítez, M.E., 25 Bidgood, A.K., 27 Burgess, R., 14 C Caddick, M., 24

Cambeses, A., 8 Campanha, G., 30 Carter, I.S.M., 15 Castellanos-Alarcón, O.M., 33 Chew, D., 7 Clark, C., 20 Colás, V., 32

Collett, S. 13 Copley, A. 4, 5 Cuthbert, S.J., 14 D De Hoog, J.C.M., 11 Droop, G.T.R., 14 Dubacq, B., F Faleiros, F. M., 30 Foster, C.T., 21 G Gaidies, F., 26 García-Casco, A., 8, 12 García-Ramírez, C.A., 31 Gärtner, A., 29 George, F.Y., 26 Glodny, J., 8, 12 Goddard, R., 16 Gopon, P., 15, 16

Gutiérrez-Trejo, L.J., 23 H Halama, R., 11 Hammond, S.J., 28 Harley, S.L., 11 Harris, B.J.R. , 11 Harris, N.B.W., 10, 19, 28 Hernández-Montenegro, J.D., 33 Hofmann, M., 29 Hora, J., 13 Hughes, L., 14 I Iliopoulos, I., 17 J Janots, E., 7 Jenchen, U., 29 Jenner, F.E., 10 Johnson, T., 20 Joy, K. H, 9 K Kanellopoulos, C., 17 Kokkaliari, M., 17 Kunz, B., 10, 19 L Lamont, T., 18 Lembo Wuest, C.I., 25 Linnemann, U., 29 M Magnin, V., 7 Male, J., 16 Míková, J. 13 Miocevich, S.R., 5 Monié, P., 8 Moraes, R., 30 Morales Bautista, C.M., 29 Mottram, C. M., 30 Munoz-Montecinos, J., 12 Murra, J.A., 25 N Nagurney, A., 24

O Oldman, C.J., 28 Ortega-Gutiérrez, F., 32 P Palacio Baderramo, G., 25 Parsons, A., 15, 16 Pawley, A., 14 Pernet-Fisher, J. F., 9 Phillips, S.E., 19 Popov, M., 8 Q Quas-Cohen, A., 14 R Raimondo, T., 8 Ramos-Arias, M.A., 23 Rey-Leon, V., 31 Ricardo, B.S., 30 Roberts, N.M.W., 19 Rosenberg, C., 7 Ruiz-Portilla, D.O., 31 S Schertl, H.-P., 11

Schulmann, K., 13 Searle, M.P., 27 Serra-Varela, S., 25 Siga Júnior, O., 30 Spencer, C.J., 28 Štípská, P., 13, T Taylor, R.J.M., 20 Torres Sánchez, S.A., 29

V Verdecchia, S.O., 25 Waldner, M., 7 Warren, C.J., 10, 19, 28 Waters, D.J., 15, 16 Weller, O., 4, 5, 22 Wheeler, J., 6 White, R., 18 Z Zuluaga-Castrillón, C.A., 33