Controls on solid‑phase inclusion during porphyroblast growth ...

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Contrib Mineral Petrol (2014) 168:1089DOI 10.1007/s00410-014-1089-0

ORIGINAL PAPER

Controls on solid‑phase inclusion during porphyroblast growth: insights from the Barrovian sequence (Scottish Dalradian)

Katja Farber · Mark J. Caddick · Timm John

Received: 27 March 2014 / Accepted: 12 November 2014 / Published online: 25 November 2014 © Springer-Verlag Berlin Heidelberg 2014

formation, with chloritoid breakdown supplying sufficient material to form the large amounts (c. 25 vol%) of stauro-lite found in the rock. This reaction produces an excess of SiO2, which leaves the crystal domain as SiO2aq and thus caused the formation of the inclusion-free areas in the stau-rolite and precipitation of quartz in the matrix. In the sil-limanite zone, staurolite is consumed forming new garnet. The newly formed garnet has less quartz inclusions than its core due to a proportionally greater consumption of quartz by the second garnet-forming reaction than by the initial, garnet-grade reactions. Textural and thermodynamic data both suggest that inclusions in these porphyroblasts repre-sent leftovers of a preferentially overgrown matrix than co-products of the porphyroblast-forming reaction.

Keywords Barrovian sequence · Metamorphic mineral reactions · Porphyroblast growth · Garnet · Staurolite · Quartz inclusions

Introduction

Porphyroblasts often provide the key information for any petrological interpretation of metamorphic rocks. Whereas compositions of porphyroblasts and their inclusions are often used to give insights into the pressure–temperature (P–T) evolution of the host rock, the textural context of porphyroblast and matrix minerals, and the distribution of mineral inclusions throughout the porphyroblast, may be interpreted as a detailed but often complex record of defor-mational history. In combination, these chemical and tex-tural records may allow deciphering of the underlying geo-dynamic processes of petrogenesis. Despite much recent work (e.g. Pattison and Tinkham 2009; Gaidies et al. 2011; Gieré et al. 2011), the mechanisms controlling the growth

Abstract A series of Barrovian sequence samples rang-ing from garnet to sillimanite zones were investigated to infer their porphyroblast-forming reactions and mineral inclusion histories. Quartz is overgrown and partly con-sumed during garnet formation and remains as inclusion-rich layers in porphyroblasts of the garnet zone. Stauro-lite crystals in the staurolite zone display sharp transitions between inclusion-rich and inclusion-free areas, suggest-ing two stages of growth with a different role of quartz in each. The inclusion-rich domains formed similarly to those in garnet by simple overgrowth and resorption of matrix minerals, with thermodynamic constraints suggesting that this staurolite-forming reaction required the presence of chloritoid that is now absent from the examined sam-ples. The participation of garnet was limited in staurolite

Communicated by O. Müntener.

Electronic supplementary material The online version of this article (doi:10.1007/s00410-014-1089-0) contains supplementary material, which is available to authorized users.

K. Farber · T. John Institute of Mineralogy, Westfälische Wilhelms-Universität Münster, Corrensstraße 24, 48419 Münster, Germany

K. Farber (*) Institute of Mineralogy and Economic Geology, RWTH Aachen, Wüllnerstraße 2, 52062 Aachen, Germanye-mail: [email protected]

M. J. Caddick Department of Geosciences, Virginia Tech, 5060 Derring Hall, Blacksburg, VA 24061, USA

T. John Institute of Geological Sciences, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany

http://dx.doi.org/10.1007/s00410-014-1089-0

Contrib Mineral Petrol (2014) 168:1089

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of porphyroblasts from matrix minerals are still not fully understood. Carmichael (1969) showed that the process of mineral formation can be subdivided into a series of com-ponent reaction sequences, generally requiring more min-erals than needed by the simple, cumulative reaction. For example, the polymorphic mineral transition kyanite = sil-limanite can be considered as a reaction cascade involving white mica, feldspar and biotite. These local metasomatic reactions effectively yield an open system at small scales, particularly in the presence of fluid (e.g. Wintsch et al. 2005; Putnis and John 2010; Ferry et al. 2013). It is gen-erally interpreted that porphyroblasts growing from these multi-stage processes record parts of the rock’s metamor-phic evolution in both their composition as well as in the sequence of phases that they preserve as inclusions. The link between macroscopic (rock-wide) thermodynamics and local (grain-scale) processes is, however, poorly con-strained, and specific mechanisms that result in inclusion of minerals are often uncertain. We address this issue here, based primarily upon observations of mineral inclusion pat-terns in porphyroblasts from the classic Barrovian sequence of the Scottish Highlands. Pelitic rocks are widespread in this sequence and show a rapid textural equilibration of their mineral assemblage, apparently controlled by chang-ing ambient pressure–temperature (P–T) conditions (e.g. Spear and Cheney 1989). Equilibration during metamor-phism is recorded by multiple mineral reactions, which resulted in a distinct regional pattern of mineral assemblage occurrences. This growth of porphyroblast-forming index minerals in pelites was first described by Barrow (1893, 1912), leading to the metamorphic concept of the Barro-vian zones.

Mineral inclusions are often thought to record parts of a former matrix that was overgrown by the porphyroblast (e.g. Vernon 1977; Vernon et al. 2008). They may, however, also represent a reactant phase that was incompletely con-sumed or a sister product that was co-precipitated by the porphyroblast-forming reaction. Accordingly, the inclu-sions can be interpreted to record information about the P–T evolution of the rock, if it is assumed that they contain rel-icts of its changing mineral assemblage. Information about the specific porphyroblast-forming reaction may also be recorded, but this is generally harder to decipher from indi-vidual inclusion phases (needing instead a more complete parent assemblage). In this study, the index minerals gar-net and staurolite were investigated in several samples that reached different metamorphic grades. Each sample shows characteristically different morphologies and distributions of both the host porphyroblasts and their inclusions. In gar-net crystals, areas rich in inclusions are separated rather diffusely from areas that are almost devoid of inclusions, whereas in staurolite, a similar transition is present but is sharply developed. In particular, single staurolite crystals

contain both areas with highly abundant quartz inclusions and areas with very few inclusions. These observations are elaborated on below and coupled with thermodynamic con-straints to test Carmichael’s (1969) proposition that quartz inclusions in staurolite do not represent pre-existing grains that were subsequently included, but rather precipitated as a co-product during staurolite formation.

An important factor controlling metamorphic reactions is the requirement of fluid availability, and the role of fluids in progressive equilibration following variation of the sys-tem P, T and composition (X) has been increasingly recog-nised in recent decades (e.g. Rubie 1986; Austrheim 1987; Ague 1994; Wintsch et al. 2005; Putnis and John 2010; Jamtveit and Austrheim 2010; Ferry et al. 2013). Fluid may act as a catalyst for the reaction or a chemical driving force by mobilising key reaction components between reaction domains. It may be actively involved in dissolution, trans-port and precipitation of key minerals. Importantly in this context, a difference between metamorphism and metaso-matism is not specified at a small scale, where the equilib-rium volume involved in any set of equilibria is often dif-ficult to define (Putnis and Austrheim 2010). In the case of prograde metamorphism, fluids released during dehydra-tion reactions are available for subsequent use in the por-phyroblast-forming reactions either as a necessary chemi-cal constituent or a means of cation mobilisation (Ague 1994; Putnis and John 2010). For example, intergranular Al mobility can vary by more than four orders of magnitude between anhydrous and fluid saturated rocks at otherwise equivalent conditions (e.g. Carlson 2010).

Here, we use the simplest assumption of no major influx of material during the metamorphic history, considering just element mobility and mineral reactions on the thin sec-tion scale. The textures that we observed can generally be explained by isochemical evolution at this scale, so large metasomatic changes and fluid infiltration are not included, although they have been proposed for some areas in the Barrovian zones (Ague 1994; Breeding et al. 2004). We identify metamorphic reactions and mechanisms respon-sible for garnet and staurolite porphyroblast growth, and for the variable frequency with which quartz is found as an inclusion within porphyroblasts. This leads us to propose that the occurrence and formation of quartz can be directly related to the porphyroblast-forming reaction and is partly controlled by fluid-mediated material transport during inclusion.

Geological setting

The samples studied here are pelitic metasediments from the Dalradian Supergroup of Scotland. These consist of Neoproterozoic to Cambrian sediments deposited on the


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margin of Laurentia during break-up of Rodinia and open-ing of the Iapetus Ocean (Anderton 1985; Harris et al. 1994). The Dalradian in Scotland is limited to the north by the Great Glen Fault and to the south by the High-land Boundary Fault (Fig. 1). It displays a wide variety of marine facies controlled by basin depth and sediment supply. Sediments were overprinted by regional metamor-phism during an early phase of the Caledonian Orogeny, named the Grampian Orogeny for Scotland (Dewey 2005). Harte et al. (1984) determined four phases of deformation (D1–D4) for which McLellan (1989), based on textural evidence, constructed a relative timing of mineral growth. Garnet growth was syn-D2 with a second growth phase syn- or post-D3. Staurolite growth was syn- to post-D2, while kyanite grew pre- to syn-D3 and sillimanite syn- to post-D3. Peak metamorphism is assumed to be around D3 (McLellan 1989). The area of Clen Clova is the type local-ity of Barrovian metamorphism (Barrow 1893).

The Caledonian Orogeny began with the obduction of the Ballantrae ophiolite complex at c. 480 Ma (D1; Oliver et al. 2000). Sm–Nd dating implies a garnet growth interval of 8 Ma from c. 473–465 Ma starting during D2 deforma-tion phase with probable extension into D3 (Oliver et al. 2000; Baxter et al. 2002). The age difference between peak metamorphism in the garnet zone and the kyanite–silliman-ite zone is short (3 ± 4 Ma). This effective contemporane-ity has been explained as the result of an additional heat source, such as local syn-metamorphic igneous intrusions (Baxter et al. 2002). Strontium diffusion modelling in apa-tite and multiple-component diffusion modelling in garnet

suggest that peak metamorphic conditions were reached in rapid thermal pulse(s) lasting for only a few hundred thou-sand years that were superimposed on (Ague and Baxter 2007) or cumulatively formed (Viete et al. 2011) the longer timescale ‘background’ heating. These pulses have been associated with intruding magmas and associated fluid flow (Ague and Baxter 2007; Vorhies and Ague 2011). The entire Grampian Orogeny lasted only 15 ± 3 Myr from overthrusting at c. 480 Ma to termination upon exhumation at c. 465 Ma. This exhumation was characterised by high rates and rapid cooling (Oliver et al. 2000; Baxter et al. 2002).

A significant body of detailed thermobarometric work exists on rocks from this region (e.g. Vorhies and Ague 2011; McLellan 1985; Baker 1985; Ague et al. 2001). Vorhies and Ague (2011) determined conditions of 500 °C and 5 kbar for the lowermost garnet zone and peak temper-atures of 660 °C and 6 kbar for the sillimanite zone. Tem-peratures of 650–700 °C and pressures of 6–6.5 kbar for peak metamorphic conditions were determined by McLel-lan (1985) and Baker (1985). In Glen Muick, temperature and pressure increase to 750–800 °C and c. 9.5 kbar in the sillimanite-K-feldspar zone (Vorhies and Ague 2011; Baker 1985).

Methods

Four samples were investigated in detail (Table 1; Fig. 1) and represented the garnet zone (two samples, Fort

Fig. 1 Geological map of the Scottish highlands between the highland boundary fault and the great glen fault (modified from Dempster et al. 2002). Sample locations Spean Bridge (36020, I) and Glen Clova (35084, 35104) are shown


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William), the staurolite zone (one sample, Glen Clova) and the sillimanite zone (one sample, Glen Clova). The bulk composition of each sample was measured using quantitative X-ray fluorescence spectroscopy (XRF) with lithiumtetraborate (Li2B4O7) as a flux at the University of Hamburg.

Major element compositions of minerals in each sample were determined with a JEOL JXA 8900 Superprobe at the Institute of Mineralogy, Münster University. This used a 15-kV accelerating voltage, a 15-nA beam current, and a 1- to 5-µm beam spot diameter (for different mineral phases). The quality of analyses was determined by frequent meas-urement of standards and reference minerals. Corrections of raw data utilised the Phi-Rho-Z procedure (Armstrong 1988).

Electron backscatter diffraction (EBSD) was carried out at the Institute of Mineralogy, Münster University, using a JEOL 6610-LV SEM. For EBSD, the sample was tilted at 70° and coated with carbon to avoid charging. Working dis-tance was set at 20 mm and acceleration voltage at 20 kV.

Petrology and geochemistry of samples

In garnet zone samples, garnet, biotite and chlorite are pre-sent as small porphyroblasts in a matrix of quartz and mus-covite. Muscovite is fabric-forming and increases in grain size with increasing metamorphic grade. Chlorite is gen-erally clinochlore-rich, with an XFe of 0.48–0.51, whereas chlorite present as an alteration product in the staurolite zone has an XFe of ~0.6. Biotite shows an increase in Ti and a slight decrease in XFe (0.57–0.51) with increasing meta-morphic grade.

Garnet textures and inclusions

Garnet forms euhedral porphyroblasts with abundant inclu-sions. Its grain size increases with increasing metamorphic grade from 0.5 to 5.0 mm. In the garnet zone, garnet crys-tals contain inclusions of quartz, plagioclase, ilmenite and zircon (Fig. 2). The abundance of inclusions in any one

garnet crystal varies from 5 to 15 vol%, with quartz being the dominant inclusion phase. The presence of inclusions defines a foliation, separated by areas devoid of inclu-sions (presumably reflection of a pre-existing composi-tional heterogeneity). In sample 36020, quartz inclusions can be directly correlated to the matrix layering next to the porphyroblast. The shape of quartz inclusions is irregular rounded.

Garnet in the staurolite zone contains fewer quartz and ilmenite grains (5 vol%, Fig. 2c). These are rounded and elongated, with no clear spatial ordering throughout the host crystal. Garnet itself is often partly included in stau-rolite (Fig. 2c). In the sillimanite zone, garnet typically contains an inclusion-rich core (with rounded quartz and ilmenite grains totalling ~15–25 vol%) and an inclusion-poor rim (Fig. 2d). A zone of staurolite and/or kyanite inclusions marks the transition between core and rim, with the rim containing very few inclusions (almost exclusively of quartz).

Staurolite textures and inclusions

Staurolite contains very abundant quartz inclusions in stau-rolite zone rocks (20–30 vol%, Fig. 2c), with sharp con-tacts between inclusion-rich and inclusion-free domains. These quartz inclusions show undulose extinction and are aligned and partly sigmoidally folded. Matrix ilmenite has the same shape and orientation as ilmenite inclusions in staurolite. Staurolite in the sillimanite zone is inclusion free (Fig. 2d). EBSD measurements of staurolite grains from both staurolite and sillimanite zones showed no measure-able difference within each grain, i.e. whole crystals have a homogeneous orientation, which probably reflects growth from a single nucleus. Analysed quartz inclusions within staurolite, however, do not share an orientation axis with the surrounding staurolite or with each other. This orienta-tion is thus thought to be a pre-staurolite growth feature; it might reflect the original orientation of quartz crystals in the matrix crystals or may record deformation that sub-sequently modified the arrangement of atom layers in the quartz crystals.

Mineral and rock composition

Major element analyses of garnet exhibit a ‘bell-shaped’ zoning for crystals from the garnet and staurolite zones (Fig. 3). This probably represents partially diffusionally modified prograde zoning (Florence and Spear 1991; Cad-dick et al. 2010). In the sillimanite zone, a distinct over-growth formed around a similarly zoned crystal. This overgrowth is Ca rich with a sharp but scalloped bound-ary towards the crystal core (Fig. 3c) and probably reflects partial resorption followed by rim overgrowth (e.g. Kohn

Table 1 Classification and description of samples. Mineral abbrevia-tions used throughout this text are recommended by the IUGS Sub-commission on the Systematics of Metamorphic Rocks following Kretz (1983)

Sample Strat. unit, group Met. zone

Mineral assemblage

36020 Appin grt grt + bt +qtz + ms + pl + chl

I Appin grt grt + bt +qtz + ms + pl

35084 Southern highland st grt + bt + ms + qtz + st

35104 Southern highland sil grt + bt + ms + qtz + st + AlS


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et al. 1997). All analysed garnet is Fe rich, with Xalm (Fe/Fe + Mg + Ca + Mn) varying from 0.6 to 0.8. Grossular zoning is oscillatory in some crystals, but Xgrs contents are characteristically low, ranging from 0.1 to 0.2 in the garnet zone and 0.02–0.04 in the sillimanite zone.

Staurolite crystals are generally Fe rich (XFe ≈ 0.8) and show no systematic zoning (although a little patchy variation in Al and Fe is occasionally evident). In the sil-limanite zone, staurolite developed small Al-rich rims (c.f. Dempster 1985). An increase in the Zn content of staurolite

with decreasing modal abundance probably reflects partial resorption and redistribution of Zn through the residual recrystallized staurolite.

Bulk compositions of samples from the two localities are significantly different. The garnet zone samples fit into a field of typically low-Al pelites of the Barrovian (Fig. 4; Atherton and Brotherton 1982). The higher metamorphic grade samples contain more Al and plot above the Grt–Chl tie line (Fig. 4), as described by Atherton and Brotherton (1982).

Fig. 2 Photomicrograph of garnet and staurolite crystals showing their abundant inclusions. a Garnet zone sample 36020 and b sample I showing layers of quartz inclusions c St. zone sample 35084 show-ing garnet is included in staurolite which has an inclusion-free area

on the right crystal side. d Sill zone sample 35104B showing inter-grown inclusion-free staurolite and kyanite, both occurring as inclu-sions in garnet at the transition from a quartz inclusion-rich core to an inclusion-poor rim


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Fig. 3 BSE pictures, com-positional maps and line scan analyses of garnet crystals rep-resentative of their metamorphic zone. Scale bars are 500 µm


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Thermodynamic constraints on porphyroblast growth and quartz stability

Perple_X version 6.6.6_5 (Connolly 1990; Connolly 2005) was used to calculate pseudosections, mineral modal abun-dances and compositions for the bulk composition of each sample. Thermodynamic data were from Holland and

Powell (1998; THERMOCALC v3.33), and the solid–solu-tion models used are listed in the appendix.

Thermodynamic modelling and its limiting conditions

P–T pseudosections, phase abundance and phase composi-tion diagrams (Figs. 5, 6, 7) were calculated for the rep-resentative bulk compositions for each zone with H2O in excess (Table 2). The pseudosection for staurolite zone sample 35084 was also taken to be representative of the compositionally similar studied sillimanite zone sample. Sample 36020 was used to represent both garnet zone sam-ples. Garnet and staurolite zone samples were collected from areas that have been interpreted as experiencing dif-ferent metamorphic histories, with a distinct thermal pulse associated with magma emplacement postulated for higher-grade rocks (Vorhies and Ague 2011). We emphasise here that although the evidence of this pulse may be found by comparing the apparent P–T histories of garnet-grade and sillimanite-grade rocks (e.g. Fig. 6), our discussion of the evolution of mineral assemblages neither requires nor pre-cludes that these rocks experienced ‘Barrovian metamor-phism’ sensu stricto rather than a combination of ‘regional’ and ‘regional contact’ metamorphism. A more important assumption in the context of this work is that the composi-tion of each hand specimen crushed for XRF is representa-tive of the rock volume of equilibration during metamor-phism, and that no significant changes in the bulk chemistry occurred during or since prograde conditions. Such bulk chemistry changes may be induced by fluid infiltration or loss, or by fractionation of phases into subsequently ‘inert’ mineral phases (e.g. garnet porphyroblasts) that may then sporadically release material upon partial resorption. Frac-tionation of material into new grown phases and loss of fluid during progressive metamorphism are easily modelled (e.g. Konrad-Schmolke et al. 2008; Baxter and Caddick 2013), but we find here that an assumed fixed composition is appropriate for explaining mineral reaction sequences associated with the key textural observations.

Constraints on porphyroblast growth and quartz stability

The main difference between the pseudosections of garnet zone and the higher-grade samples is the predicted occur-rence of low-T chloritoid in the latter (Fig. 5, with pseudo-section for garnet zone samples available in supplementary material). Chloritoid stability is not predicted in calcula-tions for lower Al garnet zone samples, consistent with their position below the chlorite–garnet tie line in AFM space (Fig. 4). Chloritoid stability for rocks of the staurolite and sillimanite zone is terminated by the appearance of gar-net and staurolite (c. 530–500 °C in Fig. 5).

Fig. 4 AFM diagram showing bulk composition of samples. Shaded area displays bulk rock analyses of Atherton and Botherton (1982)

Fig. 5 Pseudosection for staurolite and sillimanite zone samples. Geothermobarometry results from two samples described in text are shown


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The approximate P–T equilibration of staurolite and sillimanite zone samples, calculated with the geobarom-eter calibrations of Hodges and Spear (1982) and Ghent and Stout (1981) and with inverse modelling with Ther-mocalc (Powell and Holland 1994; White et al. 2008), is highlighted on Figs. 5, 6 and 7. At these peak conditions, the pseudosection (Fig. 5) represents well the mineral assemblage observed in each of the samples. Sillimanite zone sample 35104B also contains two metastable miner-als (kyanite and staurolite) that the pseudosection predicts should breakdown at lower temperature. The sample (rep-resented by a grey circle in Fig. 5) is calculated to have equilibrated close to the kyanite–sillimanite transition and less than ten degrees C above the limit of staurolite stabil-ity. Both mineral reactions are incomplete as observed in thin section.

Vorhies and Ague (2011) constructed P–T paths for regions of Scotland. Their inferred P–T history for rocks from a similar locality to the higher-grade samples (35084

and 35104B) is in good agreement with our own calcula-tions (black dashed curves on Fig. 6) and was used for the discussion that follows. Mineral evolution along this spe-cific P–T path was calculated for the bulk rock composition of sample 35084, and progressive changes in the modal abundance of each phase reflect reaction of the system dur-ing these evolving P–T conditions (Fig. 6).

Several key mineral reactions occur along the P–T path constrained by Vorhies and Ague (2011). For example, Fig. 6 shows the amount of garnet (a), quartz (b), chlori-toid (c), and staurolite (d) that are calculated to be present in rock 35084 during its metamorphic evolution (assuming a fixed bulk composition for simplification). The observed proportion of all mineral phases agrees well with the mod-elled proportions at the peak temperature achieved, with the maximum difference between the observed and calculated abundance of any phase being 5 vol% for quartz in the sil-limanite zone calculation. Figures 5 and 6 identify the rela-tionships between each phase during each metamorphic

(a) (b)

(c) (d)

Fig. 6 Phase diagrams with modelled mineral abundance for the staurolite and sillimanite zone samples. a garnet, b quartz, c chlori-toid and d staurolite abundance. Geothermobarometry results from

two samples described in text are shown: yellow dot staurolite zone sample 35084, blue dot sillimanite zone sample 35104B. P–T path from Vorhies and Ague (2011)


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reaction, revealing that chloritoid is consumed during garnet and initial staurolite growth along the P–T path of Vorhies and Ague (2011), with absence of chloritoid above 550 °C corresponding with the onset of staurolite stability. Clearly, the growth of staurolite also has an influence on the abundance of other accompanying minerals. Of most inter-est are that garnet growth is halted and garnet is partially consumed (as described by Florence and Spear 1993), and that the abundance of quartz increases accordingly (e.g. during the decompression and heating stage between ~570 and 620 °C in Fig. 6a, b). A second stage of garnet growth begins with the breakdown of staurolite. Quartz partici-pates in these reactions as both a product (during stauro-lite growth) and as a reactant (during the 2nd garnet growth phase).

Tracking evolving garnet composition along the P–T path (Fig. 7) reveals consistency with analysed microprobe

(a) (b)

(c) (d)

Fig. 7 Modelled garnet composition for the staurolite and sillimanite zone samples. a Xalm b Xprp c Xgrs d Xsps. P–T path from Vorhies and Ague (2011)

Table 2 Bulk rock composition of samples used for thermodynamic modelling

Grt zone Sil zone

wt% wt%

SiO2 61.74 54.11

TiO2 0.29 0.92

Al2O3 17.03 25.29

Fetotal 6.08 8.98

MnO 0.30 0.10

MgO 2.18 1.74

CaO 0.73 0.86

Na2O 1.17 2.1

K2O 4.16 2.26

P2O5 0.09 0.51

H2O 3.61 2.66

Sum 97.39 99.67


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traverses (Fig. 3) up until peak temperature is reached. This can broadly be characterised as increasing Xalm and Xprp and decreasing Xgrs and Xsps from core to rim of garnet (Fig. 3) during growth due to progress along the P–T path (Fig. 7). Despite this general agreement, some garnets show slight differences of up to 0.05 for Xalm and 0.03 for Xgrs. The evolution of garnet composition along the retrograde path is effectively opposite to that of the prograde core but is not recorded because of the lack of garnet growth during this phase. The stability field of staurolite is reflected in the calculated equilibrium composition of garnet by decreased Xalm and Xsps contents (Fig. 7). Xgrs is relatively insensitive to the stability of staurolite, but outermost parts of garnet crystals may resorb to expose higher Xgrs contents at this point (Florence and Spear 1993).

Discussion

Combining textural observations with the thermodynamic modelling allows us to infer the porphyroblast-forming reactions (Fig. 8a). Broadly, this can be summarised as initial garnet growth in the garnet zone, with a hiatus in growth and then partial dissolution as staurolite becomes stable, and then regrowth as staurolite breaks down (Fig. 8b). The reactions indicated in Fig. 8 also allow us to make a more detailed estimate of the role of quartz in the evolution of these reactions. Post-entrapment alteration of the quartz inclusions is excluded as quartz inclusions in porphyroblasts show undulose extinction and random ori-entations in EBSD analyses. Random orientation may also be caused by static recrystallization but the internal texture of quartz inclusions, such as the undulose extinction, sug-gests that they record a pre- to syn-trapping deformation.

Initial garnet growth

Although garnet growth may begin at or below 450 °C if stabilized by MnO and CaO (e.g. Theye et al. 1996), the pseudosection calculated for an appropriate rock composi-tion predicts initial garnet stability at approx. 525 °C. The calculated garnet growth reaction is:

During this reaction, both the porphyroblastic phases observed in garnet zone samples, garnet and biotite, grow while matrix minerals such as chlorite and white mica are consumed. The abundance and grain size of garnet and biotite porphyroblasts increase considerably from sample 36020 to sample I (grt increases from 0.5–1.0 mm in 36020 to 4.0–5.0 mm in sample I). Sample 36020 still contains primary chlorite, whereas chlorite is absent from sample I. Matrix quartz is consumed by reaction 1 but occurs as a common inclusion phase in the garnet that is interpreted to have grown by it, where its rounded shape is indicative of partial resorption prior to inclusion. Several possible reasons may explain the presence of an inclusion in a por-phyroblast: (a) precipitation of the included phase as a co-product during host crystal growth, (b) rapid porphyroblast growth, overgrowing ‘passive’ matrix minerals, (c) trapping of a remnant of the porphyroblast-forming reaction, i.e. as a reactant that was not completely consumed. The first pos-sibility can be excluded in the case of garnet growth though reaction 1, which consumed quartz. Options (b) and (c) are not necessarily mutually exclusive, i.e. a reactant may be only partially consumed due to rapid occlusion by the por-phyroblast if the reaction stoichiometry produces volumet-rically more product (in this case garnet) than it consumes

(1)

chl + ms + qtz = grt + bt + H2O

3.0 chl + 1.0 ms + 2.0 qtz = 4.0 grt + 1.0 bt + 3.0 H2O

(a)

(b)

Fig. 8 Modelled mineral reactions and influence on the modal min-eral abundance for the staurolite and sillimanite zone samples. a Inferred mineral reactions with emphasis on the main mineral phases of qtz, grt and st. b Qtz, grt and st modal abundance and their depend-ence on each other during the reactions shown in a. Each node equals a point on the p–T path from Vorhies and Ague (2011)


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reactant (quartz). Processes (b) and/or (c) may, however, be discerned from inclusion textures. In the examined sam-ples, inclusion grain alignment suggests retention of the foliation of a previous matrix. Moreover, garnets in both garnet zone samples have alternate quartz-rich and quartz-absent layers (Figs. 2, 9), probably reflecting a quartz-rich and phyllosilicate-rich foliated protolith. Each matrix layer thus left a fingerprint during overgrowth by the por-pyhyroblastic garnet (Fig. 9c; Passchier and Trouw 1996). Phyllosilicate-rich layers lead to fewer inclusions because reaction 1 consumes volumetrically more white mica and chlorite than quartz. Quartz inclusions are thus interpreted as overgrown and partly consumed matrix quartz.

Staurolite growth

The stability of staurolite terminates the growth of garnet (Fig. 6). Two different domains identified texturally in stau-rolite crystals (inclusion rich and inclusion poor: Figs. 3, 10) are consistent with thermodynamic modelling showing two growth stages, each with different staurolite-forming reactions. The first staurolite-forming reaction for this bulk composition represents the terminal breakdown of chlori-toid, which also consumes quartz (Fig. 8b, reaction 2):

Chloritoid is required by this reaction but is not found in the samples examined here or elsewhere in the gen-eral sample location. Indeed, in the central Highlands, the occurrence of chloritoid is restricted to the coastal area north and to the south-west of Stonehaven (Fig. 1; Bar-row 1912; Chinner 1967). The bulk compositions of the staurolite and sillimanite zone samples, however, strongly suggest former stability of chloritoid (Figs. 5, 6), and we infer that its absence is due to complete replacement during reaction 2.

Reaction 2 also consumes quartz to form staurolite and garnet, but as in the example above, quartz inclusions are abundant in staurolite crystal cores. Again, the origin of this quartz may be either overgrown matrix quartz that did not participate in staurolite growth or a reactant that per-sisted after completion of the reaction. Textural alignment of inclusions in inclusion-rich staurolite domains resembles a former matrix foliation and the rounded elongated shape may additionally indicate partial resorption (Fig. 10). Simi-lar examples from other settings (e.g. described by Yardley

(2)

cld + qtz = chl + st + grt + H2O

23.0 cld + 5.2 qtz = 1.0 chl + 2.1 st + 3.3 grt + 14.9 H2O

Fig. 9 BSE photomicrograph of garnets from the garnet zone showing inclusion trails which reflect parent minerals in the protolith matrix. Scale bars are 500 µm. a Sample 36020 b Sample I c from Passchier and Trouw (1996)

Fig. 10 Staurolite crystal from the staurolite zone sample showing abundant aligned quartz inclusions and an inclusion-free domain that was formed by a different porphyroblast-forming reaction


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et al. 1990) preserve micro-crenulated quartz inclusions that clearly indicate that they were part of a former, deformed matrix. Quartz inclusions in the inclusion-rich domains of staurolite are thus interpreted to record partially resorbed and overgrown matrix crystals, similar to inclu-sions in low-grade garnet.

The second staurolite-forming reaction occurs at slightly higher temperatures than reaction 2 and consumes garnet along with white mica and chlorite while producing a con-siderable amount of quartz and biotite (Fig. 8b, reaction 3):

Staurolite growth of the expense of garnet is supported texturally by anhedral garnet crystals in the staurolite zone sample and the fact that garnet is generally found as inclu-sions in or near staurolite, indicating their genetic relation-ship. However, garnet shows only weak chemical evidence for dissolution (e.g. with no significant resorption-related Xsps increase towards crystal rims). Minimal chemical evi-dence for participation of garnet in the staurolite-forming reaction seems to be a general feature and was previously used to exclude garnet from participation in this reaction (Pattison and Tinkham 2009). It is unlikely at the temper-atures and inferred durations of metamorphism that com-positional evidence of garnet growth following its partial resorption would be lost through intra-crystalline diffu-sion. We thus conclude that reaction 3 was terminated upon exhaustion of chlorite, before a significant proportion of the garnet formed by reactions 1 and 2 was lost.

Reaction 3 also produces significant quantities of quartz and water and could be interpreted to be responsible for the inclusion-poor domains in staurolite. The presence of fluid is essential for mass transfer during any such dissolution–precipitation reaction (Putnis 2002). As such, while the constituent components of the reacting minerals are within solution, components such as SiO2aq may be significantly more mobile than in the absence of fluid. If this results in extraction from the domain of the growing staurolite crystal, an area devoid of inclusions will result and quartz will precipitate elsewhere. This scenario is in accordance with the general observation of element exchange between various subdomains during fluid-induced replacement pro-cesses in crustal rocks (Putnis and John 2010) and may explain the absence of quartz in domains of crystals that grew while quartz was a reaction co-product and its pres-ence as inclusions while quartz was a reactant phase.

The two growth stages of staurolite that have been sug-gested by the thermodynamic modelling are consistent with microstructures and differences in inclusion density. No

(3)

chl + ms + grt = st + bt + qtz + H2O

4.6 chl + 13.0 ms + 6.0 grt = 2.0 st + 13.0 bt + 23.0 qtz

+ 14.8 H2O

difference in the crystal orientation was found by EBSD for the two different domains (inclusion rich and inclusion poor), implying that the areas represent either one crystal formed during a single growth event or that a new stauro-lite grew epitaxial to the previous crystal. Despite the dis-cussion above, additional processes (such as the influence of overgrowth of a heterogeneous matrix or a change in growth rate) could be inferred to result in the formation of two contrasting domains. We now briefly review the evi-dence for and consequences of these processes in relation to the option described above that two different staurolite-forming reactions operated, with excess fluid and SiO2aq leaving the crystal domain.

The first mechanism for forming the observed inclusion pattern is similar to the proposed scenario for the quartz-rich layers in garnet from the garnet zone. Therefore, quartz did not co-precipitate during the staurolite-forming reaction like it could be inferred from reaction 3 but prob-ably reflects overgrown matrix quartz (Carmichael 1969). Therefore, the inclusion-free areas could be areas where no quartz was present in the matrix during staurolite growth, i.e. overgrown mica or chlorite grains (Fig. 9c).

Secondly, a change in the growth rate of the staurolite porphyroblast may have modified the abundance of inclu-sions within it. During slower growth rates, the crystal has a higher potential to efficiently resorb previously existent minerals, thus leaving fewer remnants. An increase in the growth rate is likely to be caused by brief pulses of high temperature, as previously proposed for the area (Bax-ter et al. 2002; Ague and Baxter 2007; Vorhies and Ague 2011). The short-term high temperature either may have led to a fast growth rate and thus a high inclusion density, or it might have led to increased recrystallization of a previous existing staurolite crystal forming areas without inclusions.

A third possibility for the formation of the two different domains considers two different staurolite-forming reac-tions (reactions 2, 3) and excess fluid for transport of SiO2 out of the crystal as shortly discussed before. Reaction num-ber 2 requires chloritoid breakdown, and the evidence for the presence of chloritoid can only be inferred from the mas-sive abundance of staurolite in the rock (≥25 vol%). If no chloritoid was initially present, the only possible staurolite-forming reaction involves garnet and chlorite breakdown (reaction 3) implying that the rock must have once con-sisted of ~15 vol% garnet. This amount is modelled for the high-grade samples just before staurolite enters the mineral assemblage (Fig. 6). This would further imply that ~60 % of the garnet that was formed in the garnet zone must have resorbed to form staurolite (Fig. 7). Pattinson and Tinkam (2009) calculated similar values for samples from the Nelson Aureole of British Columbia (70 % resorption of previously formed garnet to form staurolite), but an absence of textural and chemical evidence for prograde garnet resorption led


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them to exclude significant participation of garnet during staurolite formation (Pattinson and Tinkham 2009; Moyni-han and Pattinson 2013). Performing the calculation again without using garnet suggests that the rock must initially have contained 35 % chlorite, which is neither observed in the sample nor predicted by our modelling (which predicts a maximum of 17 vol% chlorite at low temperatures). Near the staurolite-in reaction, the modelled amount of chlorite is only ~6 vol%, and chlorite is now absent from the sample. Therefore, the production of ~25 vol% staurolite in the rock would require the former presence of chloritioid. Absence of chloritoid would require a large metasomatic influx of Fe, Mg and/or Al before or during staurolite growth. For such a reaction, a fluid phase has to be present (Putnis and John 2010) and is evident from the balanced reaction (2, 3), yet in contrast to the energetic valuable transport of SiO2aq out of the crystal, the high influx fluid immobile components into the rock seems not plausible.

Staurolite breakdown and 2nd garnet growth

The next major reaction is the breakdown of staurolite with a coupled re-growth of garnet (Fig. 8). Two reactions can be balanced (e.g. Dempster 1985; McLellan 1985), but only the latter (reaction 5) is consistent with the pseudosec-tion calculation (Fig. 5), which suggests that chlorite had previously been lost by reaction 3:

and

In the examined sample from the ky–sil zone, staurolite occurs as inclusion-free relicts. This is in contrast to the abundant quartz inclusions seen in staurolite from the stau-rolite zone. The absence of inclusions may simply reflect their lack of formation during staurolite growth, though this is unlikely given the prevalence of inclusions in stau-rolite the lower metamorphic grade samples. Alternatively, higher temperatures of the sillimanite zone in combination with the partial consumption of staurolite may have led to recrystallization of much of the remaining staurolite crys-tals. This would have redistributed components and helped to remove the inclusions and is considered likely due to the commonly high Zn content of sillimanite zone staurolite relicts, with little evidence of zoning of this Zn.

Textural evidence for reaction 5 includes the increase in grain size of garnet and the modal decrease in stauro-lite in the sillimanite zone sample. Kyanite is the first

(4)

Chl + St + Qtz = AlS + Grt + H2O

0.1 Chl + 1 St + 4.3 Qtz = 7.8 Ky + 1.5 Grt + 2.4 H2O

(5)

Ms + St + Qtz = AlS + Grt + Bt + H2O

0.2 Ms + 3.5 St + 10.7 Qtz = 18.8 ky + 3.5 Grt + 0.2 Bt + 5.0 H2O

aluminosilicate formed during this second phase of garnet growth, replacing staurolite. Both staurolite and kyanite can be found as inclusions in garnet, predominantly at the transition between the quartz inclusion-rich core and the inclusion-poor rim (Fig. 2). Upon further heating, silliman-ite is produced by reaction 5 and by polymorphic transfor-mation from kyanite, nucleating as fibrolite needles in bio-tite rather than directly consuming the parent kyanite. This transition occurs through a series of subreactions involving fibrolite growing at the expense of biotite and white mica consuming kyanite (Carmichael 1969; Chinner 1961). Fol-lowing Chinner (1961), it is likely that the nucleation of sil-limanite needles in biotite is energetically more favourable than growth of a prismatic porphyroblast.

Two possibilities may explain why the second garnet generation contains significantly less quartz inclusions than the first. Slower growth rates during the second phase may have allowed sufficient time to fully resorb matrix miner-als from the interface of the growing garnet, but relatively abundant staurolite and kyanite inclusions in garnet crys-tals and apparently high heating rates (e.g. Ague and Bax-ter 2007) do not support this. A simpler and more plausible explanation is that far more quartz is needed for reaction 5 (~3 mol per mole of garnet) than for reaction 1 (~0, 5 mol per mole of garnet). As the dominant reactant phase in each reaction, loss of quartz from the local vicinity around each growing garnet crystal face was thus much more likely than in reaction 1.

Garnet crystals have rims in which Xgrs increases from 0.02 to 0.05 (Table 3; Fig. 3). The composition of garnet is represented relatively well by the thermodynamic models, although a sharp increase in Xgrs upon initiation of the sec-ond garnet growth phase is not predicted (Fig. 7). During the essentially isobaric heating to peak T, calculated Xgrs decreases to ~0.03, increasing back to ~0.04 at the end of the estimated p–T path, at which point garnet resorption is predicted. This heating–cooling at 6–7 kbar may reflect the short-lived thermal pulse of advective heating superimposed upon a long duration regional metamorphism described pre-viously (Baxter et al. 2002; Ague and Baxter 2007; Viete et al. 2011; Vorhies and Ague 2011) and is likely responsi-ble for the new, high-Ca garnet overgrowth seen only in the high-grade samples (Fig. 3). With the exception of this over-growth, differences in garnet composition between the natu-ral crystal and the thermodynamic model probably record the partial diffusional relaxation of garnet growth zoning (e.g. Florence and Spear 1991; Caddick et al. 2010). For the crystal sizes observed here, the duration of heating–cooling was sufficient to partly modify crystal zoning, consistent with calculated characteristic Mn diffusion length scales of ~1 mm for sillimanite zone garnets (Viete et al. 2011). Complete loss of crystal-scale zoning, however, requires a far greater degree of ‘diffusion completeness’, so retention


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of zoning patterns broadly consistent with prograde zoning is unsurprising (Caddick et al. 2010).

The question of an open versus closed system cannot be answered here, but substantial evidence points to meta-somatic fluids during metamorphism. Chinner (1967) and Tilley (1925) argued that the Barrovian sequence cannot be isochemical and that for the lower grade assemblage to form staurolite or kyanite-rich rocks, a change in alkali–alumina ratio is needed. Ague (1994) stated that the stau-rolite and kyanite isograd in a Barrovian sequence may reflect partial metasomatic changes in the bulk composi-tion caused by massive fluid infiltration during amphibolite facies metamorphism. The high Al content found in high-grade samples effectively reflects lower SiO2-content than the low-grade rocks (Atherton and Brotherton 1982), which may simply record preferential dissolution and transport of silica during fluid infiltration. In this case, the mineral evolution of this type locality Barrovian sequence may not simply be a feature of increasing p and T, but may also reflect the local importance of fluid infiltration during met-amorphism. We have shown here, however, that inferred

hand specimen-scale changes can be explained simply by isochemical metamorphism.

Concluding remarks

A series of mineral growth and breakdown reactions occur during prograde Barrovian metamorphism (Fig. 7). Tex-tural observations were combined with thermodynamic modelling and quantification of reactants and products in key reactions. The observed reactions involve (1) growth of garnet, (2) breakdown of chloritoid and garnet in addition to the growth of staurolite and (3) breakdown of staurolite and re-growth of garnet. These reactions were connected to porphyroblast formation and their inclusion history (sum-marised in Fig. 11), which suggest that several distinct mechanisms of formation were required.

1. The first garnet grew from matrix minerals. Abundant quartz inclusions found within garnet porphyroblasts are interpreted to represent former matrix quartz,

Table 3 Representative garnet analyses and structural formulae on basis of 12 oxygens

Zone Grt Grt St St ky-sil ky-sil ky-sil

Rim Core Rim Core Rim Mid Core

SiO2 37.78 37.69 37.18 37.46 37.54 37.49 37.18

TiO2 0.00 0.10 0.02 0.00 0.02 0.01 0.02

Al2O3 21.01 20.51 20.96 21.08 20.99 21.13 21.08

Cr2O3 0.01 0.00 0.03 0.02 0.00 0.00 0.02

FeO 33.79 28.00 35.80 34.00 36.25 36.80 35.72

MgO 2.11 1.24 3.31 2.82 3.14 3.64 3.51

MnO 2.03 6.59 1.56 2.79 0.43 0.64 0.86

CaO 4.22 6.47 1.18 1.87 1.80 0.70 1.35

Total 100.95 100.60 100.03 100.04 100.17 100.41 99.73

Structural formulae on a basis of 12 oxygens

Si 3.02 3.02 3.00 3.01 3.01 3.00 3.00

Al 0.00 0.01 1.99 2.00 1.99 2.00 2.00

Ti 1.98 1.94 0.00 0.00 0.00 0.00 0.00

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00

∑ 1.98 1.95 1.99 2.00 1.99 2.00 2.00

Fe 2.26 1.88 2.41 2.29 2.43 2.47 2.41

Mg 0.25 0.15 0.40 0.34 0.38 0.44 0.42

Mn 0.14 0.45 0.11 0.19 0.03 0.04 0.06

Ca 0.36 0.56 0.10 0.16 0.16 0.06 0.12

∑ 3.00 3.03 3.02 2.98 2.99 3.00 3.00

Total 8.00 8.00 8.01 7.99 7.99 8.00 8.00

XAlm 0.75 0.62 0.80 0.77 0.81 0.82 0.80

XPrp 0.08 0.05 0.13 0.11 0.13 0.14 0.14

XSps 0.05 0.15 0.04 0.06 0.01 0.01 0.02

XGrs 0.12 0.18 0.03 0.05 0.05 0.02 0.04

XFe 0.90 0.93 0.86 0.87 0.87 0.85 0.85


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which was overgrown and partly resorbed during the formation of the porphyroblast.

2. Quartz inclusions in staurolite also represent previ-ously matrix quartz, similar to that trapped in garnet. An initial growth reaction consumed chloritoid and quartz, with the inclusion-rich areas formed due to only limited resorption of matrix quartz. Inclusion-poor domains of staurolite suggest a second growth phase in which the consumption of chlorite and gar-net produced excess SiO2, which may have been transported as SiO2aq dissolved in the contemporane-ously produced fluid. Only a small amount of garnet, the outermost crystal edges, was dissolved by this reaction.

3. The partial consumption of staurolite produced a sec-ond garnet generation. This new garnet encloses both staurolite and kyanite, which are reactants during its growth. Fewer quartz inclusions are observed within the newly grown garnet than in the older core. This again reflects differences in the garnet-forming reac-tion compared with its initial growth at lower grade, with proportionally more quartz consumption leaving less quartz available to be included.

As suggested by Carmichael (1969), porphyroblast growth can be directly linked to a sequence of mineral reac-tions by considering the supply of material to the growing crystal. It seems, however, that inclusions in porphyroblasts generally represent preferentially overgrown matrix lefto-vers rather than co-products of the porphyroblast-forming reactions. This might be a consequence of the underlying reaction mechanism: incompletely consumed (i.e. dis-solved) reactants are easily overgrown, whereas product phases precipitate from reaction-mediating fluid (e.g. Put-nis 2002; Putnis and John 2010), and their components are

thus relative easily transported outside of the growing por-phyroblasts. This allows the formation of a thermodynami-cally favourable inclusion-poor crystal. Accordingly, and opposed to Carmichael’s suggestion (1969), if the growth rate is not too high, it is to be expected that inclusions pre-dominately reflect reactants and only rarely products of porphyroblast-forming reactions. The consequence of this for mineral inclusion thermobarometry may be profound, opening the possibility that many inclusion phases may record a composition from a stage before porphyroblast host growth, rather than a composition that reflects equilib-rium with the overgrowing mineral.

Acknowledgments Thanks to S. Harley and T. Dempster for sam-ple distribution, J. Berndt for assistance with EMPA, C. Vollmer for assistance with EBSD analysis, and A. Putnis and E. Baxter for help-ful discussions. We also want to thank two anonymous reviewers and our editor for helpful and insightful comments.

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chl + ms +

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Fig. 11 Growth history of the porphyroblasts garnet and staurolite and the ambient evolution of inclusions in the metamorphic sequence. (1) Garnet grows overgrowing matrix quartz. (2) Staurolite and garnet

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