Lithos 105 (2008) 225–238

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Geochronological and mineralogical constraints on depth of emplacement andascencion rates of epidote-bearing magmas from northeastern Brazil

Alcides N. Sial a,⁎, Paulo M. Vasconcelos b, Valderez P. Ferreira a, Ricardo R. Pessoa a,Roberta G. Brasilino a, João M. Morais Neto b

a NEG-LABISE, Geology Department, UFPE, Recife, 50670-000, Brazilb Department of Geology, University of Queensland, Brisbane, Australia

⁎ Corresponding author. NEG-LABISE, Department of G50670-000, Brazil. Tel.: +55 81 2126 8243; fax: +55 81 2

E-mail address: sial@ufpe.br (A.N. Sial).

0024-4937/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.lithos.2008.04.002

A B S T R A C T

A R T I C L E I N F O

Article history:

Calc-alkalic to high-K calc- Received 21 March 2007Accepted 8 April 2008Available online 22 April 2008

Keywords:Magmatic epidote40Ar/39Ar ageDepth of crystallizationMagma transport rate

alkalic granitoid plutons in the Borborema province, northeastern Brazil, havebeen studied to constrain depth of emplacement by mineralogical and geological methods and to estimateupward magma transport rate based on partial dissolution of magmatic epidote.Laser-probe incremental heating 40Ar/39Ar dating of biotite and hornblende single crystals from theNeoproterozoic Tavares and Brejinho high-K calc-alkalic magmatic epidote (mEp)-bearing plutons revealsage differences of around 60 M.y. between these two minerals in each of these two intrusions. These datasuggest solidification at relatively great depth followed by prolonged cooling interval between the closuretemperatures of biotite and hornblende.Al-in-hornblende barometry indicates that hornblende in several mEp-bearing plutons in the TransversalDomain of the Borborema province solidified at 5 to 7 kbar, whereas in the Seridó and Macururé terranes,solidification pressures range from 3 to 4 kbar.Partial dissolution of epidote indicates very rapid upward transport. Partial corrosion occurred during 15–35 years (Cachoerinha–Salgueiro terrane), 10–30 years (Alto Pajeú), 15 years (Seridó), and 10 years(Macururé) corresponding to upward transport rates of 450–1300, 650–1050, 1200, and 1800 m/yearrespectively in these four terranes. Rapid upward magma migration in most cases was probably facilitated bydiking simultaneous with regional shearing.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Characteristics of magma transport in the Earth are of first-orderimportance to unravel thephysical and chemical evolution of our planet.Transport properties of magma have been intensively studied in recentdecades (e.g. Dingwell, 2006, and references therein); however, the rateof magma upward transport has received much less attention withrelatively few contributions directed towards this topic (e.g., Petfordet al.,1993; Fernández and Castro,1999, among others). Experiments onepidote dissolution kinetics (Brandon et al., 1996) and on epidotestability in granitic melts (Schmidt and Thompson, 1996) suggest thatepidote textures offer a powerful means to estimate crystallizationdepth, oxygen fugacity and rate of melt upward transport.

The occurrence of magmatic epidote (mEp) is still debated. Plutonsof similar chemical composition, crystallized at similar pressure, mayor may not carry magmatic epidote. However, there appears to begeneral consensus that occurrence of mEp in granitic rocks, atmoderate to high pressure (6 to 8 kbar), depends partly on magma

eology, UFPE, C.P. 7852, Recife,126 8242.

l rights reserved.

composition and partly on depth of emplacement. Brandon et al.(1996) reacted epidote with natural granodioritic glass at pressuresabove and below the stability limit of mEp. They observed that at highpressure (11.5 kbar, 780 °C) therewas no evidence of reaction betweenepidote and the granitic melt, whereas at lower pressure (4.5 kbar,750 °C) epidote developed irregular rims due to dissolution. Theseauthors regarded epidote dissolution as a relatively fast process, andthus the presence of mEp in calc-alkalic granitoids implies a fastupward transport, probably occurring via diking rather than bydiapirism.

Schmidt and Thompson (1996) studied the stability of epidote incalc-alkalic magmas and demonstrated that at water-saturatedconditions and fO2 buffered by NNO, epidote has a wide stabilityfield in tonalite, with a minimum pressure of about 5 kbar. Experi-ments show that if fO2 is buffered by HM, the stability field of epidoteis enlarged down to 3 kbar.

Knowledge of flow of magma and its upward migration rate isfundamental to understanding of pluton emplacement and growthand crustal differentiation. The main objective of this study is toestimate depth of emplacement and rates of magma upwardmigration.For this purpose, eleven mEp-bearing granitoid plutons from fourterranes in northeastern Brazil were selected for detailed investigation

Fig. 1. Simplified geological map of northeastern Brazil, indicating locations of Neoproterozoic mEp-bearing granitoids, distributed in five tectonostratigraphic terranes (SD— Seridó,CS — Cachoeirinha–Salgueiro, RP — Riacho do Pontal, AP — Alto Pajeú and MC — Macururé terranes). Numbered 1 through 26 are the mEp-plutons in this study, eleven from whichhave been used for estimating upward magma transport rate.

226 A.N. Sial et al. / Lithos 105 (2008) 225-238

(Fig. 1). We used Al-in-hornblende barometry and 40Ar/39Ar dating ofcoexisting hornblende–biotite pairs to constrain relative depth ofemplacement. Our estimates of rates of magma upward migration arebased upon partial dissolution of magmatic epidote.

2. Geologic setting and petrography

Magmatic epidote-bearing granitoids are widespread in north-eastern Brazil, having been identified in five Neoproterozoic terranes:Seridó (SD), Cachoeirinha–Salgueiro (CS), Riacho do Pontal (RP), AltoPajeú (AP) and the Macururé (MC). They belong to the calc-alkalic,high-K calc-alkalic, shoshonitic and trondhjemitic series, mostlyemplaced at 630–650 Ma (Ferreira et al., 1998, Fig. 1). Whole-rockchemical data for the epidote-bearing plutons are discussed in Sial(1986, 1990), Sial and Ferreira (1988), Ferreira et al. (1998), Galindoet al. (1995) and Long et al. (2005).

In these rocks, mEp exhibits four textural relationships: (1) embayedor in vermicular contact with unaltered plagioclase; (2) rimmed bybiotite, with zoned allanite core, (3) enclosing patches of hornblende,and (4) partially enclosed by biotite, in the interstices of K-feldsparaggregates. All of these textural types are present in all of the above-mentioned granitoids except for type 4,which is restricted to the high-Kcalc-alkalic granitoids.

Calc alkalic mEp-bearing granitoids occur in the SD, CS, RP and MCterranes. The mEp-bearing plutons in the CS terrane intrude low-grade meta-(marine) turbidites. The plutons are round to elongate,metaluminous tonalites to granodiorites containing calcic amphiboleand biotite as main mafic phases. Magmatic epidote occurs as largecrystals as much as 2 mm long, and is substantially less abundant toabsent where clinopyroxene (diopside–salite) is present. This obser-vation is consistent with experiments by Schmidt and Thompson(1996, p. 470) on water-saturated tonalitic melts, which demonstratethat themodes of epidote and clinopyroxene are linked via the epidotemelting reaction epidote+hornblende+H2O=clinopyroxene+liquid.

Angular to rounded amphibole-rich clots (ARC) are present in themajority of the calc-alkalic plutons in CS terrane, as two types:

(a) deep-green calcic amphibole aggregates (interpreted to be frac-tionated from the host magma), and (b) with fabric, fine-grained,angular to rounded, up to 15 cm long clots composed of actinoliticamphibole with margins of Mg-hornblende (Fig. 2A–D), that has beenonce regarded as fragments from the source transported upward bythe granodiorite/tonalite magmas (Sial et al., 1998). For this secondtype, an armor layer of biotite and amphibole commonly hasprevented further interaction with the host magma.

Black-spotted kyanite-bearing thermal aureoles developed aroundtwo of these calc-alkalic plutons in the CS terrane are characterized byfine-grained mica foliation (Caby and Sial, 1996). According to thepetrogenetic grid of Xu et al. (1994), the assemblage garnet, kyanite,staurolite, muscovite with Sib3.1, biotite, plagioclase, and quartz (asobserved in these aureoles) suggests T around 670 °C and P=7.5±0.5 kbar. Quartz and rutile inclusions in garnet attest to peak P≅9 kbarduring garnet growth (Bohlen et al., 1983).

Magmatic epidote-bearing calc alkalic plutons in the SD terrane(Fig. 1) intruded intermediate to high-grade metasedimentary rocks.They occur as tonalitic dikes and sheets (modal epidote up to 5% pervolume) and as elongate granodioritic plutons.

In the MC terrane (Fig. 1), calc-alkalic granodiorites to tonalitesintruded intermediate-grade metasedimentary rocks locally generat-ing thermal aureoles with staurolite+cordierite+garnet porphyro-blasts (McReath et al.,1998). The calc-alkalic plutons of this terrane arelate to post-kinematic according to Davison and Santos (1989) andsimilar in texture, mineralogy and geochemical characteristics tothose of the CS terrane. The metaluminous Gloria Norte and CoronelJoão Sá plutons are better known among them. In these two plutons,amphibole-rich clots are similar in size, mineralogy and textures tothose described in mEp-bearing plutons of the CS terrane (Sial et al.,1998).

High-K calc-alkalic metaluminous mEp-bearing granitoids aremainly found in the AP terrane, forming the Brejinho, Tavares,Caldeirão Encantado, and Conceição das Creoulas plutons (Fig. 1);and one pluton of this type is found in the SD terrane (São Rafaelpluton). They intruded gneisses to migmatites in the AP terrane, and

Fig. 2. Amphibole-rich clots (ARC) in mEp-bearing plutons in the Cachoeirinha–Salgueiro terrane. (A) and (B): angular-shaped amphibole-rich clot in quartz diorite inclusions that inturn are hosted by mEp-bearing granodiorite; (C): angular amphibole-rich clot hosted by granodiorite and (D) rounded actinolite-rich clot armored by a narrow hornblende plusbiotite fringe.

227A.N. Sial et al. / Lithos 105 (2008) 225–238

mica schists and gneisses in the SD terrane, and consist of coarse-grained porphyritic (K-feldspar megacrysts, up to 10 cm long)granodiorite and granite, with subordinate quartz monzodiorite toquartz monzonite. Magmatic epidote occupies up to 1.5% per volume,and biotite and amphibole comprise the main mafic phases. Locally,quartz diorite syn-plutonic dikes are observed where comminglingand partial mixing of granodiorite and quartz diorite magmas tookplace.

Magmatic epidote is found in only one shoshonitic monzogranitein NE Brazil, an intrusion at the eastern portion of the Teixeirabatholith next to the northernmargin of the AP terrane (Fig.1). Amongthe mafic minerals, ferro-edenite is the main phase which, in places,forms agglomerates. Primary epidote is found as euhedral tosubhedral crystals included in biotite or, less commonly at the bordersof amphibole, in a textural relationship similar to that described byZen and Hammarstrom (1984) as typical of magmatic epidote. Someepidote grains exhibit allanite cores.

Epidote is also observed in two leucocratic trondhjemitic tonaliteto granodiorite plutons: the Palmeira pluton, which intrudesgneisses of the Alto Pajeú terrane, and the Serrita pluton thatintrudes medium-grained metapelites in the Cachoeirinha–Salgueiroterrane (Fig. 1). Mafic minerals occupy less than 10% per volume andepidote is present in low amounts (b1%) as both primary andsecondary phases.

2.1. Magmatic structures and regional tectonic features

In the eastern Borborema province, emplacement of Neoproter-ozoic granite was contemporaneous with establishment of sinuoussystem of conjugate dextral E–W shear zones and left-lateral NE-trending shear zones. The convergence of shear zones or releasingbends generated spaces that allowed emplacement of magma from

different depths, and facilitated partial melting of lower crust as theintrusion of mafic hotter mantle-derived magmas led to magmacommingling and mixing.

Syn-kinematically emplaced, high-K calc-alkalic plutons areelongate, some of them horn-shaped, with major axes parallel toregional tectonic foliation. In some of these plutons, intermediate(monzodiorite) andmafic (diorite) magmas had coexisted for a certainperiod of time, and locally experienced mechanical and/or chemicalmixing. Presence of mafic magma gave rise to uncommon magmaticand flow structures that include: (a) meter-scale tear- or mushroom-shaped blobs (Fig. 3A) that suggest magma convection in the chamber;(b) biotite+K-feldspar+epidote meter-scale ellipsoids (Fig. 3B);(c) ladder dikes (Fig. 3C) and snail structure (Fig. 3D) that representcross sections of several superposed cylindrical magma channels; and(d) S–C magmatic fabrics (e.g. Tavares pluton), which exhibitkinematics in conformity with those of nearby regional shear zones.Weinberg et al. (2001) have described some of these structures thatare common in the Tavares pluton.

In some of the high-K calc-alkalic (e.g. Tavares and Brejinho andSão Rafael) plutons well-delineated mafic Schlieren are abundant atactive channel margins. Field evidence suggests that some of thesemagmatic structures had consisted of semi liquid-crystal mush withrelatively low viscosity.

3. Mineral chemistry

3.1. Amphibole chemistry and barometry

Chemical analyses of hornblende have been performed at theelectron microprobe laboratory of the University at São Paulo, Brazil,using a JEOL JXA-8600 microprobe. Representative analyses ofhornblende rims are found in Table 1, and a Table with all hornblende

228 A.N. Sial et al. / Lithos 105 (2008) 225-238

analyses, including cores and rims, is available in a repository with theLithos editorial office. The accuracy of the analytical technique usedwas checked by analyzing an amphibole of known composition as astandard (Arenal hornblende; Table 1).

Cores and rims of hornblende grains have been analyzed lookingfor zonation and total chemical variation of this mineral in the samepluton. Part of the chemical variation observed is due to disequili-brium as magma ascended, decompressed and cooled. As one of themain goals was to use the chemistry of hornblende for barometry,whenever possible, hornblende rims in contact with quartz have beenchosen for analysis to minimize the effect of chemical re-equilibrationwith other Al-rich phases.

Hammarstrom and Zen (1986) demonstrated that the total Alcontent of hornblende in intermediate calc-alkalic rocks varieslinearly with crystallization pressure provided that the barometricassemblage consisting of quartz, K-feldspar, plagioclase, biotite,hornblende, titanite, and iron titanium oxide is present. These authorsproposed an empirical barometric equation that is essentiallyidentical to that of Hollister et al. (1987) who enlarged the calibrationdata set and reduced the ±3 kbar margin of uncertainty to ±1 kbar.Both empirical calibrations are based on pressures derived fromcontact-aureole barometry and the given errors represent the scatterof hornblende data.

Johnson and Rutherford (1989) and Thomas and Ernst (1990)added experimental calibrations to this barometer. Results slightlydiffer from the empirical calibrations but importantly, uncertaintieshave been reduced to approximately ±0.5 kbar. Schmidt (1992)recalibrated this barometer using epidote-bearing tonalite and madeit applicable up to 13 kbar. The experiments of Johnson and Rutherford(op. cit.) used a CO2–H2O fluid, whereas those of Schmidt used anH2O–saturated fluid. As the presence of mEp in calc-alkalic plutons isproposed to be indicative of low CO2 activity (Ghent et al., 1991) the

Fig. 3. Some structures observed in high K calc-alkalic plutons in this study: (A) meter-scaleepidote meter-scale ellipsoids; (C) ladder dikes (mafic minerals are mainly biotite and epidmagma channels.

calibration by Schmidt should be expected to be the most appropriatefor mEp-bearing plutons.

However, Anderson and Smith (1995) and Anderson (1996)pointed out that other factors controlling the chemistry of hornblendeshould be taken into account. According to these studies, temperature,fH2O and total pressure bear important influence on mafic silicatemineral chemistry but fO2 is themain controlling factor. These authorsdemonstrated that this barometer fails by indicating unrealisticelevated pressures for low-fO2 plutons in which iron-rich hornblendecoexists with the full barometric assemblage. With increasing fO2, theFe/(Fe+Mg) ratio for hornblende and biotite markedly decreases,independent of the Fe/Mg ratio of the whole rock (Anderson andSmith, 1995; Anderson, 1996). The present study uses the Al-in-hornblende barometer by Anderson and Smith (1995) for samples thathave the appropriate bufferingmineral assemblage, and when Fe/(Fe+Mg) in hornblende is in the range 0.40–0.65, indicating high fO2.

Total Al-in-hornblende varies from 1.81 to 2.48 in the CS granitoidsincluding clinopyroxene-bearing (i.e., epidote-poor) plutons, implyingpaleopressures ranging from 5 to 8.5 kbar. No regional P–T data areavailable for host metasedimentary rocks near mEp-bearing plutonsin the CS terrane. However, the presence of kyanite–staurolite–garnetassemblage in contact aureoles of two plutons is compatible withthese high pressures.

Al-in-hornblende yields pressure estimates of 5 and 6.5 kbar incalc-alkalic granitoids in the MC terrane, in agreement with themetamorphic assemblages in host metagreywackes for which poorly-constrained estimates suggest maximum pressures around 5.5 kbar(McReath et al., 1998).

In the AP terrane, Al-in-hornblende pressure estimates are within5.5 to 8 kbar range and for individual plutons, pressure variation isusually of less than 1 kbar. In all these plutons, liquidus temperatureestimates by the Zr saturation method are in the 785–850 °C range,

tear- or mushroom-shaped blobs (meter-scale convection cell); (B) biotite+K-feldspar+ote), (D) snail structures representing cross sections of several superposed cylindrical

Table 1Representative electron microprobe analyses of amphibole rims from magmatic epidote-bearing granitoids in this study

Terrane Seridó Cachoeirinha–Salgueiro Alto Pajeú Macururé Arenal HornblendeStandard(1)

Pluton São Rafael Boa Ventura St. AntonioCreek

Penaforte Pedra Branca Emas Tavares Conceiçãodas Crioulas

Brejinho CaldeirãoEncantado

Murici Gloria Norte Coronel JoãoSá

Sample KSR-4

KSR-16

MBV-23

MBV-20

SER-45

SER-47

SER-77

SER-86

PB-33 E-10 E-11 TV-2 TV-58.2

RCC-04 ITIM-22 Calen-S

Calen-S

PM2 PM2 GN4 HJCS

Point 2 1B 18 15 An An 1b 2b 2 3 1b 1b 1R 2B 24R 33R 2b 3b 1 2 1C 1B R-1 R-2 05-R1

05-R2

1 2 3 E

SiO2 44.33 45.43 43.29 47.56 44.00 42.00 43.38 42.71 42.67 42.33 42.71 42.94 40.98 43.84 42.01 40.95 39.57 39.42 41.28 41.96 42.62 41.20 44.25 44.92 43.95 43.18 41.40 41.15 41.55 41.46TiO2 0.66 0.79 1.00 0.52 0.80 1.25 0.84 1.01 1.06 0.66 1.91 1.88 0.58 0.60 0.80 0.65 0.37 0.39 0.68 1.02 0.51 0.50 1.33 1.58 0.91 1.14 1.39 1.40 1.38 1.41Al2O3 8.44 7.56 10.39 9.72 12.90 12.55 11.23 10.76 11.55 12.02 10.91 10.51 11.29 10.16 12.22 12.81 11.47 11.71 11.59 11.63 11.50 11.14 9.80 9.62 10.8 11.17 15.50 15.65 15.40 15.47FeOt 18.15 16.81 16.83 13.32 18.60 18.20 19.02 18.32 17.29 16.86 17.76 17.01 21.29 17.37 21.03 20.51 23.57 24.50 20.01 20.37 19.56 19.82 16.89 17.25 14.61 17.61 11.32 11.40 11.24 11.48MnO 0.37 0.400 0.37 0.33 0.30 0.01 0.36 0.36 0.35 0.41 n.d. n.d. 0.46 0.36 0.41 0.44 0.28 0.27 0.50 0.49 0.49 0.46 0.35 0.38 0.44 0.38 0.14 0.13 0.14 0.15MgO 11.25 11.20 9.78 12.30 9.00 8.90 8.57 9.04 8.46 8.13 10.30 10.84 7.54 9.53 7.49 7.07 5.72 5.71 8.08 7.68 8.84 8.52 11.24 10.61 9.59 9.33 14.45 14.30 14.41 14.24CaO 11.65 11.58 12.91 11.51 11.40 12.74 11.43 11.64 12.62 12.69 11.60 12.14 11.53 11.72 11.58 11.56 11.37 11.54 10.97 10.88 11.78 11.51 11.50 11.54 11.48 11.51 11.43 11.50 11.35 11.55Na2O 1.32 0.92 1.21 1.40 1.70 1.00 1.51 1.48 1.44 1.45 1.45 2.15 1.26 0.99 1.23 1.18 1.08 1.10 1.67 1.63 1.08 1.00 1.64 1.61 1.37 1.16 1.85 1.87 1.89 1.91K2O 0.95 0.85 1.59 1.11 1.40 1.45 1.31 1.39 1.71 1.69 1.32 1.11 1.59 1.20 1.75 1.44 1.43 1.57 1.56 1.67 1.26 1.40 1.29 1.14 1.38 1.51 0.20 0.20 0.20 0.21Total 97.12 95.55 96.87 96.66 100.17 97.99 97.64 96.71 97.15 96.24 98.4 98.84 96.58 95.77 96.77 96.61 94.86 96.20 96.34 97.33 97.64 95.55 98.29 98.65 97.53 96.99 97.68 97.6 97.56 96.67Si 6.894 6.947 6.618 6.350 6.460 6.530 6.58 6.552 6.860 6.740 6.443 6.435 6.420 6.725 6.400 6.349 6.382 6.305 6.431 6.468 6.502 6.457 6.590 6.665 6.634 6.568 6.042 6.017 6.067 6.046AlIV 1.006 1.053 1.382 1.650 1.540 1.470 1.420 1.448 1.140 1.260 1.557 1.565 1.580 1.275 1.600 1.651 1.618 1.695 1.569 1.532 1.498 1.543 1.410 1.335 1.366 1.435 1.958 1.983 1.933 1.954SUM 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000Ti 0.065 0.090 0.115 0.140 0.150 0.110 0.095 0.117 0.140 0.150 0.217 0.212 0.068 0.07 0.092 0.076 0.045 0.047 0.080 0.118 0.058 0.059 0.149 0.176 0.103 0.130 0.153 0.154 0.152 0.155AlVI 0.375 0.310 0.490 0.590 0.940 0.880 0.588 0.497 0.350 0.250 0.383 0.291 0.505 0.561 0.593 0.687 0.562 0.512 0.559 0.581 0.570 0.515 0.311 0.348 0.554 0.565 0.708 0.714 0.717 0.705Fe 2.261 2.150 2.152 2.290 1.760 2.350 2.413 2.35 2.280 2.200 2.241 2.132 2.789 2.229 2.714 2.541 3.179 3.277 2.607 2.626 2.495 2.597 2.113 2.148 2.207 2.240 1.382 1.394 1.373 1.400Mn 0.052 0.052 0.048 0.000 0.040 0.030 0.046 0.047 0.050 0.050 0.000 0.000 0.061 0.046 0.053 0.058 0.038 0.036 0.066 0.064 0.064 0.061 0.044 0.048 0.056 0.049 0.017 0.016 0.017 0.019Mg 2.487 2.554 2.225 2.000 2.290 1.830 1.937 2.067 2.510 2.550 2.316 2.422 1.761 2.179 1.701 1.634 1.375 1.36 1.876 1.765 2.011 1.991 2.495 2.346 2.158 2.115 3.144 3.117 3.136 3.096SUM 5.240 5.156 5.030 5.020 5.180 5.200 4.984 5,078 5.330 5.200 5.157 5.057 5.184 5.085 5.153 4.996 5.199 5.232 5.188 5.154 5.198 5.223 4.963 5.066 5.078 5.099 5.404 5.395 5.395 5.375Ca 1.792 1.898 1.923 2.060 1.630 1.800 1.857 1.913 1.830 1.760 1.875 1.949 1.935 1.926 1.890 1.920 1.965 1.977 1.831 1.797 1.925 1.933 1.835 1.835 1.857 1.875 1.787 1.802 1.776 1.805Na 0.351 0.272 0.504 0.290 0.440 0.460 0.443 0.44 0.330 0.390 0.424 0.625 0.383 0.294 0.393 0.355 0.338 0.341 0.504 0.487 0.318 0.304 0.474 0.463 0.362 0.322 0.524 0.530 0.535 0.540K 0.187 0.166 0.246 0.280 0.170 0.300 0.254 0.272 0.170 0.180 0.254 0.212 0.318 0.234 0.340 0.382 0.294 0.320 0.310 0.328 0.246 0.279 0.245 0.216 0.266 0.293 0.037 0.037 0.037 0.039SUM 2.330 2.336 2.673 2.630 2.240 2.560 2.554 2.625 2.330 2.330 2.553 2.786 2.636 2.454 2.623 2.657 2.597 2.638 2.645 2.612 2.489 2.516 2.554 2.514 2.485 2.490 2.348 2.369 2.348 2.384P kbar 4.1 3.5 5.9 5.0 7.6 7.4 6.6 6.3 6.9 7.4 6.2 5.8 7.0 5.7 7.0 8.0 7.4 7.5 7.0 7.0 6.8 6.8 5.2 5.0 6.3 6.4

Analyses in wt.%. Number of cations on the basis of 23 oxygens. (1)=Standard; 1, 2 and 3: values measured in this study; E: expected value (Jarosewich et al., 1980. Geostandard Newsletter, 4: 43–47); n.d.=not determined.A complete data set of analyses of hornblende is available from a repository.

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Table 2Representative electron microprobe analyses of epidote in this study

Terrane Seridó Cachoeirinha–Salgueiro Alto Pajeú Macururé

Pluton São Rafael Boa Ventura Emas SantoAntônioCreek

Serrita Palmeira Teixeira Conceiçãodas Crioulas

CaldeirãoEncantado

Tavares Murici Brejinho Gloria Norte Cel. João Sá

Sample SR-4 MBV-23 E-57 SA-17B Ser-10 P-6 TX-12 RCC-16-A Calen-2 TV3.2/2 PM-2 HT105/2 GN-4 HCJS11

Point Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim Core Rim

SiO2 36.93 37.30 38.44 38.12 37.97 38.22 38.22 38.07 37.46 38.24 38.15 38.11 38.34 37.73 38.92 37.49 38.29 38.35 37.94 38.10 37.77 37.90 37.91 37.62 38.29 38.35 38.47 38.23TiO2 0.00 0.02 0.20 0.15 0.10 0.12 0.1 0.2 0.21 0.21 0.09 0.02 0.11 0.01 0.00 0.13 0.12 0.12 0.13 0.12 0.18 0.21 0.07 0.12 0.12 0.12 0.14 0.22Al2O3 22.18 22.20 24.33 23.80 23.82 23.91 25.77 25.48 24.31 24.90 23.26 24.2 23.99 24.30 25.02 25.01 24.97 24.73 23.41 23.24 24.47 24.84 24.26 24.04 24.97 24.73 8,55 26.68FeO 13.42 13.23 10.38 10.70 11.45 11.13 8.82 9.00 9.92 9.54 12.98 12.23 11.43 11.56 11.22 11.76 10.55 10.49 10.95 11.76 10.06 10.43 11.23 10.77 10.55 10.49 8.55 8,45MnO 0.13 0.15 0.22 0.19 0.09 0.07 0.14 0.16 0.10 0.22 0.47 0.25 0.34 0.00 0.00 0.00 0.00 0.04 0.00 0.12 0.23 0.16 0.12 0.18 0.00 0.04 0.18 0.12MgO 0.00 0.00 0.05 0.04 0.07 0.15 0.05 0.06 0.08 0.06 0.00 0.00 0.01 0.00 0.14 0.00 0.09 0.12 0.04 0.12 0.07 0.07 0.02 0.02 0.09 0.12 0.03 0.03CaO 22.83 23.01 23.87 23.90 23.84 24.08 24.00 23.91 23.90 24.13 22.42 23.47 23.19 23.27 24.17 23.45 2.53 23.66 23.01 23.34 23.19 23.54 23.28 22.86 23.53 23.66 23.71 23.32Na2O 0.00 0.00 0.03 0.00 0.00 0.00 0.03 0.02 0.01 0.00 0.00 0.01 0.00 0.04 0.00 0.00 0.03 0.03 0.05 0.00 0.02 0.00 0.01 0.03 0.03 0.03 0.00 0.01K2O 0.00 0.00 0.01 0.01 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.04 0.04 0.02 0.02 0.01 0.00 0.00 0.00 0.00 0.01Total 95.50 95.92 97.60 96.96 97.36 97.80 97.14 96.9 95.99 97.30 97.42 98.32 97.44 96.87 98.37 98.03 97.58 97.59 95.57 96.84 95.98 97.17 96.90 95.64 97.58 97.59 97.48 97.07Si 2.977 2.989 2.981 2.983 2.968 2.973 2.950 2.950 2.951 2.962 3.011 2.981 2.988 2.958 2.964 2.915 3.026 3.031 3.007 2.996 2.969 2.949 2.968 2.977 2.962 2.970 2.948 2.937Ti 0.000 0.001 0.012 0.009 0.006 0.007 0.006 0.012 0.012 0.012 0.005 0.003 .006 0.001 0.000 0.008 0.007 0.007 0.008 0.007 0.011 0.012 0.004 0.007 0.007 0.007 0.008 0.013Al 2.107 2.097 2.224 2.196 2.195 2.192 2.344 2.327 2.257 2.273 2.162 2.229 2.204 2.245 2.246 2.292 2.324 2.302 2.187 2.154 2.267 2.278 2.238 2.242 2.277 2.257 2.385 2.416Fe 0.905 0.887 0.673 0.700 0.749 0.724 0.569 0.583 0.654 0.618 0.856 0.799 0.745 0.758 0.715 0.765 0.630 0.620 0.726 0.773 0.661 0.679 0.735 0.713 0.683 0.679 0.548 0.543Mn 0.009 0.010 0.014 0.013 0.006 0.005 0.009 0.011 0.007 0.014 0.014 0.012 0.022 0.000 0.000 0.000 0.000 0.003 0.000 0.008 0.015 0.011 0.008 0.012 0.000 0.003 0.012 0.008Mg 0.000 0.000 0.006 0.005 0.008 0.017 0.006 0.007 0.009 0.007 0.000 0.000 .001 0.000 0.016 0.000 0.011 0.014 0.005 0.014 0.008 0.008 0.002 0.002 0.010 0.014 0.003 0.003Ca 1.972 1.976 1.983 2.004 1.997 2.007 1.985 1.985 2.017 2.003 1.895 1.967 1.973 1.955 1.972 1.953 1.992 2.004 1.954 1.966 1.953 1.962 1.953 1.939 1.951 1.963 1.947 1.920Na 0.000 0.000 0.005 0.000 0.000 0.000 0.004 0.003 0.002 0.000 0.000 0.000 0.000 0.006 0.000 0.000 0.000 0.000 0.008 0.000 0.003 0.000 0.001 0.005 0.005 0.005 0.000 0.001K 0.000 0.000 0.001 0.001 0.000 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.004 0.004 0.002 0.002 0.001 0.000 0.000 0.000 0.000 0.001Ps 30 30 23 24 25 25 20 20 22 21 28 27 25 25 24 24 21 21 25 26 23 23 25 24 23 23 19 18

Analyses in wt.%. Number of cations on the basis of 12 oxygens.

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similar to the temperature range estimated using this method formEp-bearing plutons in the CS and MC terranes (Sial et al., 1999).

Al-in-hornblende from four calc-alkalic and one high-K calc-alkalicmEp-bearing plutons in the SD terrane yields pressures in the 3.5–4.5 kbar range. Pressures obtained from hornblendes of the São Rafaelpluton, one of the largest mEp-bearing granitoids in this terrane are inagreement with pressure estimates of 3–4 kbar for nearby meta-morphic country rocks of the Seridó Group (Lima, 1987).

3.2. Epidote chemistry

We used the textural criteria described by Zen and Hammarstrom(1984) to distinguish magmatic from secondary epidote in granitoids.Such criteria include presence of chemical zonation in epidote, allanite-rich core, embayed contacts with plagioclase and quartz, and wormycontacts (almost myrmekitic). We also used the chemical criteria citedby Tulloch (1979) based on the pistacite (Ps) content of epidote(Ps=molar [Fe3+/(Fe3++Al)]×100)N25%, and the criteria by Evans andVance (1987) who show that magmatic epidote typically has b0.2 wt.%TiO2, whereas secondary epidote replacing biotite has N0.6 wt.% TiO2. Inall of the studied plutons, modal abundances of mEp are low (b5 vol.%).

Sial et al. (1999) reported more than seventy microprobe chemicalanalyses of epidote with a minimum of cores and rims of at least threegrains per pluton. Table 2 includes representative core and rim analyses,and pistacite mol% composition for each point-analyzed grain from Sialet al. (1999) and from this study. Mol% pistacite of euhedral mEp in thehigh-K calc-alkalic São Rafael batholiths (SD terrane) lies in a narrowrange from Ps27 to Ps30; there is a slight variation of Al and Fe contentsfrom core to margin (Table 2). These pistacite contents are within therange reported to be typical for mEp (Ps25–29) by Tulloch (1979) andVyhnal et al. (1991). Galindo et al. (1995) reported a similar narrowcompositional range (Ps28–29) for epidote in the Pradopluton, a granitoidto the northwestern of the São Rafael pluton.

Pistacite contents of mEp in CS terrane calc-alkalic plutons rangebetween 20 and 24, similar to Ps19–24 in unequivocally magmaticepidote phenocrysts in high-K calc-alkalic dikes in the Front Range,Colorado, USA (Dawes and Evans, 1991). Pistacite contents describedby Farrow and Barr (1992), Rogers (1988), and Owen (1991) also lie inthis range. Typically, the CS epidote has lower pistacite, higher Si, Al,Ca, Ti and lower Fe than epidote of the SD terrane.

Magmatic epidote from the shoshonitic Teixeira pluton andtrondhjemitic Palmeira pluton (AP terrane) has a narrow composi-tional variation (Ps25–28). Pistacite content in the trondhjemitic Serritapluton (CS terrane) is lower (around Ps21).

Pistacite contents in AP high-K calc-alkalic plutons vary morebroadly (Ps21–29). Most mEp grains in the Conceição das Creoulaspluton are zoned, and Fe+3 increases from core to rim. Pistacitecontent varies with epidote textural types: (a) for epidote included infeldspars, around Ps21 at their rims; (b) for epidote surroundingallanite, Ps25–27, and (c) for epidote rimmed by biotite, Ps21–23 at therim. During crystallization of epidote, oxygen fugacity in the SD and CSplutons differed. Pistacite contents in SD plutons lie between Ps25 andPs33 (these extreme values correspond, respectively, to the NNO andHM buffer curves; Liou, 1973). CS epidote crystallized under fO2 nearthe NNO buffer curve. In the MC terrane, mEp in the Gloria Nortepluton has Ps20–22, and in the Coronel João Sá pluton, display Ps19–25.

Table 3 lists rock type, magma series, mol% Ps, Al-In-hornblendebarometry isotope data, magnetic susceptibility, and host meta-morphic grade as a summary of common features of epidote in thediverse plutons in this study.

4. Upward magma migration

According to the analytical procedure of Brandon et al. (1996),epidote textural relationships provide a clue to understanding upwardmagma transport. To test this hypothesis, we selected eleven mEp-

bearing plutons from the SD, CS, AP and MC, terranes (Fig. 1) fordetailed analysis of epidote textures. Fig. 4 exhibits epidote relictswith simultaneous extinction within unaltered plagioclase (a),containing allanite cores and partially rimmed by biotite (b),subhedral grains totally rimmed by biotite and partially resorbed bythe host magma (c, e, h), and subhedral grains with relict ofhornblende that reacted with the host magma to generate epidote.

Magmatic epidote pictured in Fig. 4C, E, G and H seems to havesurvived dissolution attack by the host magma because it was totallyarmored by biotite. In high-K calc-alkalic plutons the aggregates arecommonly contained within interstices occupied by K-feldsparaggregates. These examples suggest that that upward transport wassufficiently rapid for epidote to “survive” dissolution, accompanied byrapid near-solidus growth of K-feldspar.

For epidote crystals shown in Fig. 4B and F, the magma upwardtransport rate was probably fast enough to guarantee epidote survivalagainst completedissolution. This suggests a dikingmechanismthat alsooperated in plutonswheremEp textural relationships are less definitive.

4.1. Rate estimates

Schmidt and Thompson (1966, Fig. 3, p. 467) demonstratedexperimentally that epidote and plagioclase can coexist around10 kbar in tonalitic magmas. This situation permits one to estimaterates of magma upward movement where partially dissolved epidoteis armored by plagioclase, or epidote has grown with near-solidus K-feldspar at pressures estimated from Al-in-hornblende barometry.

We estimate magma transport rates using the following approach:

(1) Select mEp on the basis of their mol.% Ps, consisting of highlycorroded subhedral grains that are partially shielded byplagioclase, biotite or K-feldspar;

(2) Infer the original shapes of corroded grains and measure themaximum dissolution zone width (see examples in Fig. 4);

(3) Estimate the duration of corrosion by using the minimumapparent diffusion coefficient of 5×10−17 m2 s−1 for Si, Al, Caand Fe between tonalitic magma and epidote at 750 °C(Brandon et al., 1996) as follows:

dz¼ Dapp � t� �

1=2� �

where

dz width of dissolution zone (m);

Dapp apparent diffusion coefficient (5×10−17 m2 s−1); and t time for partial dissolution of epidote (s)

Accordingly,

t ¼ d2z= 5� 10�17� �

(4) Depth of host magma emplacement is inferred from Al-in-hornblende barometry;

(5) The rate of magma transport is the ratio of the route length(difference between the emplacement depth and the sourcedepth) to the average time of corrosion of epidote exposed tothe host melt. For a tonalite melting, at water-saturatedconditions and fO2 buffered by NNO, plagioclase and epidotemay coexist from a depth ~10 kbar (Schmidt and Thompson,1996, Fig. 2). We infer the route length as the differencebetween 10 kbar and the emplacement depth. Accordingly,

Tr ¼ Lr=t

where

Tr transport rate (m/year)Lr (10−Pe) ·104/3 (m)

Table 3Geological and geochemical features of representative epidote-bearing granitoid plutons in Northeastern Brazil

Northeastern Brazil

Terrane /geologicalsystem

Pluton Age (Ma) Rock type Magmaseries

Sr i ɛ Nd TDM(Ga) δ18Osmow

(‰)Magneticsusceptibility(×10−3 SI)

Host metamorphic grade Al-in-hblA & SP (kbar)

H & BT (°C)

Ps epidote(mol%)

Tectonicsetting

Seridó São RafaelBatholith

575 (U–Pb) (1) Porphyritic qz monzoniteto granite

High Kcalc-alkalic

0.7130 −23.0 to−18.0 (1)

2.73 (1) +7.5 to+8.5

1.0 to 4.0 Amphibolite facies(mica schists and limestones)and gneisses

3.5 to 4.5 650 to700

27 to 30 Latecollisional

Cachoeirinha–Salgueiro

Boa Ventura 633±0.9 (Rb–Sr) (2) Granodiorite to tonalite Calc-alkalic 0.70598 −2.0 to−1.0 (3)

1.20 to1.40 (3)

+11.0 to+13.0

0.15 to 0.40 Greenschist facies(marine turbidites);kyanite-bearingthermal aureoles.

4.5 to 6.5 650 to720

20 to 25

Emas 4.5 to 5.0 780 to810

Pedra Branca 5.5 to 6.5 730 to740

Penaforte 6.5 to 8 725 to730

St. AntônioCreek

6.0 to 7.0 730 to765

Alto Pajeú Brejinho 638±29 (Rb–Sr) (4) Porphyritic granodioriteto monzogranite

High Kcalc-alkalic

0.70933(4)

−3.6 to−3.5

1.32 to1.42

+10.0 to+12.0

0.20 to 0.50 High-grade gneisses,quartizites, and schists

6.5 to 7.5 740 to760

20 to 25

Tavares Porphyritic granodiorite 0.20 to 1.20 5.0 to 7.0 660 to680

21 to 27

Conceição dasCrioulas

Porphyritic granodioriteto monzogranite

0.15 6.5 to 8.5 650 to715

21 to 24

CaldeirãoEncantado

Porphyritic granodiorite 0.20 6.5 to 7.5 650 to720

21 to 29

Palmeira Leucocratic granodioriteto tonalite

Trondjhe-mitic

−14.6 to−14.1

2.15 0.10 5.0 to 6.0 680 to700

27 to 28

Teixeira Quartz monzonite toquartz syenite

Shoshonitic +8.5 to+9.5

5.5 to 6.0 675 to700

24 to 26

Macururé Gloria Norte 600 Granodiorite to tonalite Calc-alkalic −4.2 1.46 +10.0 0.20 Amphibolite facies 5.0 to 5.5 655 to680

18 to 24

Cel. João Sá 627 (U–Pb) 618±9.5(Rb – Sr) (5)

0.70837(5)

−7.4 to−4.8 (5)

1.3 to 1.7 6.0 to 6.5 660 to670

Notes: A & S=Anderson and Smith (1995); H & B=Holland and Blundy (1994); Italicized age is from regional geologic consideration. (1) Ketcham et al. (1995); (2) Sial (1993); (3) Van Schmus et al. (1995); (4) Brasilino et al. (1997); (5) Long et al.(2005).

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Fig. 4. Examples of magmatic epidote textural relationships observed in most of the studied mEp-bearing plutons in the Borborema province: (A) epidote within unalteredplagioclase; (B) epidote with allanite core, partially resorbed by the magma; (C) partially resorbed epidote included in biotite; (D) epidote partially resorbed with hornblende andbiotite patches; (E) euhedral epidote included in biotite; (F) partially resorbed epidote included in biotite and plagioclase; (G) and (H) epidote totally rimmed by biotite. Abbreviationsare: All = allanite, Bi = biotite, Ep = epidote, Plag = plagioclase and Qz = quartz. Dashed lines are an attempt to reconstruct original shape of epidote crystals indicating howmuch someof them have been dissolved by host magma. Nicols crossed except for photos B, G and H.

233A.N. Sial et al. / Lithos 105 (2008) 225–238

Lr length route (m)Pe pressure of emplacement (kbar)t time of partial dissolution of epidote (year)

Using the cited diffusion coefficient, inward dissolution of 0.11–0.23 mm of epidote crystal margins was completed in less than35 years, equivalent to a transport rate from a depth of 10 kbar to a

234 A.N. Sial et al. / Lithos 105 (2008) 225-238

depth of 6 kbar (~12.5 km route length) that happened at ratesbetween 450 and 1850 m year−1 (Table 4).

4.2. Examining the assumptions

Some limiting assumptions of this approach include: (a) determi-nation of the route length of magma transport; consequence of themagnitude of the error brackets on the Al-in-hornblende barometry,typically in the order of ±0.6 kbar, as well as of the appropriateness ofthe selection of the correct hornblende generation, (b) use ofappropriate diffusion coefficient for Ca, Al, Si and Fe for each magmacomposition, (c) bias in measuring dissolution zone widths of mEpgrains (only subhedral grains are used and anhedral grains, ignored),(d) dependency of the apparentwidth of the dissolution zone upon theorientation of the examined section, and (e) decreasing rate of epidotedissolution (Brandon et al.,1996, Fig. 2), leading to an underestimate ofdigestion time. Despite these uncertainties, the method clearlyprovides a good general indication of rapid magma ascent. This isprimarily due to the very short calculated dissolution times, i.e. theycan be increased by two orders of magnitude and still be short. Therapid rate of upward magma movement suggests diking as the ascentmechanisms not diapirism.

5. Geochronology

Granitic magmatism in the Transversal zone domain (area boundedto the north by the Patos and to the south by the Pernambuco shearzones) in the Borborema province (Fig. 1), occurred in three main timeintervals: (i) 650–620 Ma, (ii) 590–570 Ma, and (iii) 545–520 Ma(Ferreira et al., 2004). Interval (i) is characterized by intrusion of syn-kinematic mEp-bearing calc-alkalic, high-K calc-alkalic and shoshoniticgranitoids. Interval (ii) is marked by abundant intrusions of mEp-absenthigh-K calc-alkalic magmas, and by peralkalic, metaluminous high-Ksyenitic, unique ultrapotassic, and rare shoshonitic magmas. Peralkalicand rare A-type magmas mark interval (iii). The mEp-bearing, high-Kcalc-alkalic interval (i) plutons share textural, chemical, and miner-alogical similarities (Ferreira et al., 2004).

5.1. 40Ar/39Ar dating

Coexisting hornblende and biotite from the mEp-bearing high-Kcalc-alkalic Brejinho and Tavares batholiths (Fig. 1) have been dated in

Table 4Estimated rates of granitic magma transport for some magmatic epidote-bearing granitoids

Pluton Ps Epidote (mol %) Average measured dissolutionzone width (mm) of epidote —

number of measurements (n)

Time takdissolutio

A — Seridó TerraneSão Rafael 27 to 30 0.156 (10) 15

B — Cachoeirinha–Salgueiro TerraneBoa Ventura 20 to 24 0.228 (7) 33Emas 20 to 25 0.145 (15) 13St. Antonio Creek 20 to 24 0.184 (6) 22

C — Alto Pajeú TerraneBrejinho 20 to 24 0.143 (6) 13Tavares 20 to 27 0.200 (40) 30Conceição das Creoulas 21 to 24 0.143 (10) 13CaldeirãoEncantado

27 to 29 0.113 (10) 8

Boqueirão 27 to 29 0.114 (12) 8.3Murici 27 to 29 0.143 (7) 13

D — Macururé TerraneGlória Norte 20 to 24 0.117 (2) 9

(1) Apparent diffusion coefficient=510−17 m2 s−1; A & S=Anderson and Smith (1995); H & B

this study by the 40Ar–39Ar method at the UQ-AGES Laboratory,University of Queensland, Brisbane, Australia. A 2×3×5 cm slab wascut from each sample and crushed down to 0.1–0.2 mm, ultrasonicallycleaned for 30 min in distilled water, and then with ethanol. Five orsix hand-picked, pure grains were placed in Al disks with FC sanidinestandards following the geometry illustrated in Vasconcelos et al.(2002) and irradiated for 14 h in the C Triga reactor at the Universityof Oregon (USA). After a seven-month cooling period, each samplewas stepwise heated up to N1600 °C under an Ar-laser beam. Thereleased gas fraction was purified through a cryocooled cold-trap (T=−140 °C) and two Zr–V–Fe getters, and Ar isotopes analyzed in a MAP-215-50 mass spectrometer as described in Vasconcelos (1999) andVasconcelos et al. (2002).

Apparent ages were corrected for mass discrimination, nucleo-genic interference and atmospheric contamination, and plateau andideograms were plotted for all analyzed grains. Apparent ages werecalculated using 40Ktotal=5.543×10−10 a−1. Values for the reactorcorrection factors are: (2.64±0.02)×10−4 for (36Ar/37Ar)Ca, (7.04±0.06)×10−4 for (39Ar/37Ar)Ca and (8.00±0.3)×10−4 for (40Ar/39Ar)K. TheJ factor for the irradiation disk, J=0.003612±0.000021, was obtainedthrough the analyses of at least 15 individual grains of the FC sanidinestandard positioned as shown in Vasconcelos et al. (2002). A full set ofanalytical data are available as a Supplemental electronic data table.

5.2. 40Ar/39Ar analytical results

5.2.1. Brejinho biotiteGrain 3950-01 yields an age of 510±5 Ma, with a well-defined

plateau corresponding to more than 70% of the released 39Ar (Fig. 5A).For grain 3950-02, a plateau forced between steps B and I providessame age (510±5 Ma). A probability density plot for the two grainsdefines a peakwith amaximum at 509Ma and aweighted average ageof 509.6±1.1 Ma (MSWD=1.59) (Fig. 5B).

5.2.2. Brejinho amphiboleGrain 3951-01 defines a plateau (75% of total 39Ar released) with an

age of 568±6 Ma (Fig. 5C). Grain 3951-02 defines a plateau age (morethan 60% release of total 39Ar) of 585±7 Ma. A probability density plotfor the two grains shows a relatively large spread of apparent ages forindividual steps but with a central tendency at ca. 572±2 Ma (Fig. 5D),which we consider to be the best age estimate. The amphibole age issignificantly greater than the biotite age, suggesting a prolonged

in Northeast Brazil

en for partialn (years) (1)

Pressure (kbar)Al-in-hbl A & S

Temperature (°C)H & B

Magneticsusceptibility(×10−3 SI)

Rate of upwardtransport(m year−1)

3.5 to 4.5 650 to 700 1.0 to 4.0 1200

5.0 to 6.5 650 to 720 0.15 to 0.40 4555.2 to 6.2 789 to 810 12826.0 to 7.5 730 to 765 530

6.5 to 7.5 740 to 760 0.20 to 0.50 7695.7 to 7.0 660 to 680 0.20 to 1.20 7246.5 to 8.5 650 to 715 0.15 641

650 to 715 0.20 1042

650 to 715 0.10 to 0.60 1004650 to 715 0.10 to 1.90 641

5.0 to 5.5 655 to 680 0.20 1851

=Holland and Blundy (1994).

Fig. 5. 40Ar/39Ar age dating for biotite and amphibole from the high-K calc-alkalic Brejinho pluton, state of Pernambuco.

235A.N. Sial et al. / Lithos 105 (2008) 225–238

cooling interval between 480 °C (amphibole closure ca. 572 Ma) and300±50 °C (biotite closure ca. 510 Ma).

5.2.3. Tavares biotiteGrain 3953-01 defines a four-step plateau (60% of 39Ar released)

with an age of 538±6 Ma (Fig. 6A), and grain 3953-02 yields astatistically compatible age 543±6 Ma plateau (N70% of the 39Ar

released). An ideogram for the two grains defines a most probable ageof 538.1±1.6 Ma, with MSWD=2.46 (Fig. 6B).

5.2.4. Tavares amphiboleThe two analyzed amphibole grains do not define plateaus. For

grain 3954-01, a forced plateau (steps A–C, ca. 80% of the 39Arreleased) yields an age of 593±7 Ma. A forced plateau for grain 3954-

Fig. 6. 40Ar/39Ar age dating for biotite and amphibole from the high-K calc-alkalic Tavares pluton, state of Paraiba.

236 A.N. Sial et al. / Lithos 105 (2008) 225-238

02 (steps B–C, ca. 75% of the 39Ar released) yields 618±7 Ma (Fig. 6C).Amphibole crystals in this sample may have undergone significantalteration that had partially reset the isotope system consistent withthe broad distribution of apparent ages between 617 and 587 Ma (Fig.6D), and with the petrographic observation that amphibole is partiallyaltered to biotite. Notwithstanding the uncertainties, the amphibolecrystals must have cooled below the closure temperature at least by587 million years ago, significantly earlier than 538 Ma when biotitecrystals reached Ar closure (Fig. 6E). Both the Brejinho and Tavaresplutons cooled slowly.

6. Discussion and conclusions

Presence of fresh plagioclase in most of the studied plutonssuggests that surface weathering and subsolidus alteration werelimited, in support for an igneous origin of most of the epidote.

The observed overall compositional variation of epidote (20 to30mol% Ps) is in agreement with the values proposed by Johnston andWyllie (1988) and by Tulloch (1979) for magmatic epidote. Theepidote compositional range tends toward 20–24 mol% Ps for plutonsemplaced at or above 5 kbar pressure (as determined by Al-in-

237A.N. Sial et al. / Lithos 105 (2008) 225–238

hornblende paleobarometry) and 27–29 mol% Ps for plutonsemplaced at lower pressure (Table 3) with one exception among the11 plutons examined (Palmeira pluton). Epidote in the Ps20–24 groupcrystallized buffered by the NNO or in the QFM to NNO range atP≈5 kbar or above. Epidote in the Ps27–29 group crystallized at Pbetween 3 to 5 kbar and fO2 between NNO and HM.

Ubiquituous partially resorbed magmatic epidote crystals suggestthat this mineral, although unstable, survived complete dissolution intothehostmeltdue to relatively rapidupwardmelt transport. Armoringbybiotite or (in high-K calc-alkalic granitoids) near-solidus K-feldsparassisted survival. K-feldspar must have crystallized much faster thanepidote dissolved (b100 years, according to Brandon et al., 1996).

It is important to consider that as crystallization progresses, thecombined physical and chemical conditions of the magmamay evolveout of epidote stability field, as melt composition might changethroughmixing, crystallization or crystal segregation to become out ofequilibrium with epidote. So, the instability of epidote may not onlybe tied tomagma upward transport, but also to compositional changesof the magma. In mEp-bearing plutons in this study, in whichcommingling of tonalite/granodiorite with more mafic magma hasbeen recorded (e.g. Tavares pluton), magmamixing apparently has notplayed an extensive role in their evolution.

In this study, 40Ar/39Ar age dating of coexisting biotite andhornblende grains of magmatic epidote-bearing plutons has beenused to confirm solidification age of these plutons, at intermediate tohigh depth in the continental crust, as anticipated from experimentalgrounds and shown by Al-in hornblende barometry. 40Ar/39Ar ages ofbiotite and hornblende from high-K calc-alkalic mEp-bearing plutonsare highly discordant. Slow cooling suggests that these plutonssolidified at relatively great depth and remained there for a longtime. Dallmeyer et al. (1987) reported discordant 40Ar–39Ar ages ofbiotite and hornblende (about 20 M.y. difference) from the Conceiçãocalc-alkalic pluton in the CS terrane.

Al-in-hornblende barometry, in some cases, yielded pressureestimates whose variation roughly correlates with the correspondingvariation in chemistry of coexisting epidote. The highest Ps contentsare in epidotes of plutons in which hornblende solidified underPb5 kbar, under slightly higher oxygen fugacity allowing forexpansion of the stability field of epidote.

Our calculations yield relatively consistent transport rates for eachpluton. Altogether, mEp in the studied plutons was transportedupward at rates between 450 and 1850 m year−1 and with dissolutiontime between 10 and 35 years. In any event, the time estimated forepidote transport is slightly shorter than the actual time becauseduring the dissolution process, the dissolution rate tends to decreaseas pointed out by Brandon (op. cit.).

The intricate step in themodel to estimatemagma transport rate inthis study is determining the route length of magma upwardtransport. Unless tonalitic magmas were generated at depths of lessthan 10 kbar (rather improbable, simply for heat budget reasons), theonly way to infer a route length seems to be by using the differencebetween depth where epidote starts coexisting with plagioclase intonalitic magma (10 kbar) and pluton emplacement depth.

Armoring of epidote may prevent its complete dissolution but maypose a significant problem in the method discussed here. An armoredepidote canbe transported,metastably, out of its stabilityfield andpersistas a metastable phase for a considerable time. If the armoring is thenbroken and dissolution begins, the time that epidote dissolutionmeasures is the timeperiodbetweencrystallizationandepidoteexposureand, bynomeans, ameasureofmagma transfer time.One solution for thisproblem, perhaps, is tomake a large number ofmeasurements of epidotedissolution widths and check for consistency of results.

Absence of magmatic epidote in 590–570 Ma high-K calc-alkalicgranitoids in the Granjeiro terrane (north of the northern boundary ofthe CS terrane) that otherwise are mineralogically/chemically similarto mEp-bearing high-K calc-alkalic granitoids in the AP terrane (to the

south of the CS terrane) is justified by: (a) in a thinner and hotterterrane, the magmas migrated upward so slowly that epidotecompletely dissolved (Ferreira et al., 2004) ; or (b) growth of near-solidus K-feldspar or biotite occurred too slowly to armor the epidote,or (c) compositions or fO2 conditions in magmas were not suitable tocrystallize epidote.

Acknowledgments

We are thankful to Leon E. Long and Gary Stevens whosecomments and suggestions helped improving the original manuscript.All statements and conclusions in this paper, however, are of the entireresponsibility of the authors. This research was partially supported bythe Program of Support to the Scientific and Technological Develop-ment (PADCT/FINEP, grant no. 65.930.619-00) and by the VITAEFoundation (B-11487/3B001). Construction of the argon laboratory atthe University of Queensland was partially funded by ARC EquipmentGrant A39531815; analyses were funded by UQ-AGES. This is thecontribution no. 245 of the Nucleus of Geochemical Studies (NEG),Dept. of Geology, Federal University of Pernambuco, Brazil.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.lithos.2008.04.002.

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