Journal of Structural Geology 24 72002) 1179±1193 www.elsevier.com/locate/jstrugeo

Dissolution and replacement creep: a signi®cant deformation mechanism in mid-crustal rocks

R.P. Wintscha,*, Keewook Yia,b

aDepartment of Geological Sciences, Indiana University, Bloomington, IN 47405, USA bSchool of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea Received 4 December 2000; revised 2 June 2001; accepted 26 June 2001

Abstract Zoning patterns and zoning truncations in metamorphic minerals in a granodioritic orthogneiss indicate that strain and S±C fabrics in these rocks were produced by dissolution, precipitation, and replacement processes, even at epidote± facies metamorphic conditions. The metamorphic fabric is de®ned by alternating layers and folia dominated by , feldspars, and 1 epidote. Zoning patterns in most metamorphic , , epidote, and sphene are truncated at boundaries normal to the shortening direction, suggesting dissolution. Interfaces of relict igneous orthoclase phenocrysts that face the shortening direction are embayed and replaced by biotite, epidote, and myrmekitic intergrowths of plagioclase and quartz. Metamorphic plagioclase grains are also replaced by epidote. We interpret these microstructures to re¯ect strain-enhanced dissolution. The cores of many grains show asymmetric overgrowths with at least two generations of beards, all oriented on the ends of grains that face the extension direction. We interpret these textures to re¯ect precipitation of components dissolved by deformation-enhanced dissolution. While biotite and quartz probably deformed by dislocation creep, the overall deformation was accommodated by dissolution perpendicular to the shortening direction, and precipitation parallel to it. These chemical processes must have been activated at lower stresses than the dislocation creep predicted from extrapolations of data from experiments in dry rocks. Thus wet crust is likely to be weaker than calculated from these experimental studies. q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Dissolution; Precipitation; Replacement creep; Pressure solution; Metamorphic reaction mechanism

1. Introduction shortening direction, as has been recognized in sandstones 7e.g. Blatt, 1992), metaconglomerates 7Mosher, 1976), and Evidence from naturally deformed rocks for the operation augen gneisses 7Simpson and Wintsch, 1989), commonly by of solution transfer is now well documented in sedimentary the concentration of less soluble minerals, `insoluble and low-grade metamorphic rocks 7e.g. Passchier and residue'. The dissolved components may precipitate as Trouw, 1996). Most signi®cantly, the development of quartz, feldspar, and even monazite, forming beards on and metamorphic fabric is now identi®ed to be a host grains 7Wintsch and Knipe, 1983; Williams et al., chemical process, albeit mechanically induced. The 1999). differentiation is thought to be caused by the dissolution In rocks under suf®ciently high-grade metamorphic of quartz and feldspar from developing mica-rich domains, conditions, dislocation glide and climbare activated and precipitation into growing quartz±feldspar domains. progressively in phyllosilicates, then in quartz, and ®nally Bulk chemical analytical studies do not support the proposi- in feldspars and pyroxenes 7Passchier and Trouw, 1996). tion that silica is removed from the developing slate at the Thus in dry rocks dislocation creep may be the most kilometer scale 7e.g. Wright and Platt, 1982). On the important deformation mechanism in the ductile deforma- contrary, micas and feldspars as well as quartz appear to tion of higher temperature 7.3008C), lower crustal rocks dissolve and reprecipitate locally 7centimeter scale), with 7Fig. 1). Where ¯uids are present, however, a ®eld of dis- only minor and trace elements that are differentially mobile solution and precipitation displaces the dislocation creep moving .100 m 7e.g. Wintsch et al., 1991). The sites of ®eld. This is best documented in low-grade rocks of dissolution are the surfaces normal to the principal ,200±3008C and is typically called `pressure solution' 7Fig. 1; Snoke et al., 1998). * Corresponding author. Tel.: 11-812-855-4018; fax: 11-812-855-7899. In this contribution we describe an orthogneiss meta- E-mail address: [email protected] 7R.P. Wintsch). morphosed to wet epidote±amphibolite facies conditions.

0191-8141/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0191-8141701)00100-6 1180 R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193 mechanism as suggested by the extrapolation of experimen- tal results from dry rocks 7see Fig. 1).

2. Geologic setting

The samples analysed in this study were collected in the Glastonbury gneiss in north-central Connecticut 7Fig. 2). The Glastonbury gneiss is a complex of Ordovician volcanic and plutonic rocks that comprise the Bronson Hill terrane of central New England. It includes metamorphosed granitic, granodioritic, trondjemitic, and dioritic orthogneisses and . U±Pbgeochronology on zircon and sphene in this gneiss 7Leo et al., 1984; Aleinikoff et al., 2002) and on the Bronson Hill rocks in general 7Zartman and Leo, 1985; Tucker and Robinson, 1990) con®rm a Late Ordovician Fig. 1. Diagram showing the strength envelope of rocks in which frictional igneous crystallization age for these rocks. sliding ®rst in quartz-rich rocks, followed by feldspar-rich and ®nally by diopside-rich rocks gives way to plastic ¯ow at higher temperatures Apparently all Bronson Hill rocks were metamorphosed 7compiled from Kohlstedt et al. 71995) and Rybacki and Dresen 72000)). twice. In New Hampshire and Massachusetts, U±Pb The addition of a `pressure solution creep' ®eld is from Snoke et al. 71998). geochronology and 40Ar/39Ar thermochronology con®rms amphibolite facies metamorphism and cooling The foliation development, metamorphic differentiation, from the early Devonian Acadian orogeny. However, and grain shape in these rocks are all most easily explained Carboniferous to Permian 40Ar/39Ar mica and feldspar in terms of dissolution, precipitation, and replacement, with cooling ages document an Alleghanian greenschist facies relatively little evidence for dislocation creep. If these ¯uid- metamorphic overprint 7Harrison et al., 1989). In dependant deformation mechanisms are representative of Connecticut, the intensity of Alleghanian overprinting is processes operating in wet rocks at high grades, then dis- suf®ciently high that U±Pbages of all metamorphic sphene location creep may not be the prevailing deformation and all 40Ar/39Ar hornblende and mica cooling ages yield

Fig. 2. Map of New England showing the distribution of lithotectonic terranes or zones, and the location of our study area within the Glastonbury gneiss of the Bronson Hill terrane of northern Connecticut. R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193 1181

Fig. 3. Plane light and crossed polarized light images of augen gneiss from the Glastonbury Complex. The images show 1 cm diameter grains of orthoclase 7Or) with Carlsbad twinning 7Cs Tw in B), locally embayed by myrmekite 7Myr). Darker layers in A, de®ned by biotite and epidote layers locally intergrown with 7Amp), outline an S±C fabric. These folia are separated by layers of clear quartz 7Qtz) and dusty plagioclase 7Pl). The locations of Figs. 4 and 5D are indicated. only Pennsylvanian to Permian ages 7Wintsch et al., 1993, these minerals are Ordovician igneous minerals, and some 1998; Coleman et al., 1997; Aleinikoff et al., 2002). are metamorphic minerals, produced by metamorphic reactions. Some participate directly in de®ning the foliation, and some do not. Consequently, each mineral is described 3. Observations separately below.

The gneisses examined for this study are granodioritic, 3.1. Plagioclase containing centimeter±diameter K-feldspar crystals in a medium- to coarse-grained matrix of plagioclase, quartz, Plagioclase, the most abundant mineral in the gneiss, biotite, epidote, hornblende, K-feldspar, and sphene 7Fig. typically occurs as polygonal untwinned grains 100± 3). Biotite de®nes an inconspicuous foliation, and a very 300 mm in length 7Fig. 4), but some grains may be twice conspicuous NNW-plunging lineation parallel to prolate as large 7Fig. 5A). It tends to occur in layers and domains ma®c enclaves, such that the rocks are locally L-tectonites. from 1 to 5 mm wide and 10±15 mm long where it consti- S±C fabric relationships show that top moved to the SSE tutes .80% of the minerals present 7Figs. 3 and 4). Some 7Wintsch et al., 1998). Biotite is the most prominent mineral grains are equant 7Fig. 5B and C), but most are elongated de®ning the lineation, but elongate grains of plagioclase, parallel to the mineral lineation 7Fig. 5A and D). Aspect epidote and quartz also de®ne the lineation in their aspect ratios of 2:1 are typical, but they may be up to 4:1. These ratios and overgrowth patterns 7Figs. 3 and 4). Some of grains are universally zoned, with compositions ¯uctuating 1182 R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193

Fig. 4. A colorized backscattered electron image across the foliation shown in Fig. 3. The images were constructed by assembling and separately colorizing four images of the same regions, each adjusted to maximize compositional contrasts in plagioclase 7blue), orthoclase 7yellow), biotite, epidote, amphibole 7green) and sphene 7red). The low contrast of quartz and albite 7exsolution lamellae in orthoclase only) leave them black. The image shows the strong mineralogical layering parallel to the C foliation 7Fig. 3). Also visible is compositional zoning in plagioclase 7darker blue is more sodic), epidote 7pale green is richer in REE), and sphene 7pale pinks are richer in REE). The green line identi®es the location of the microprobe traverse of Fig. 6C. between An20 and An30 7Table 1). Zoning patterns de®ne to C foliations where epidote is intergrown with biotite up to three zones. Compositions of core regions are ,An20, 7Fig. 5C). surrounded by normally and reversely zoned mantles and Most epidote grains are zoned, with compositions ranging ®nally relatively albitic but still zoned rims 7e.g. Fig. 6A). from pistacite 17 to 25%. Cores tend to be less rich in ferric Some grains have rims with an abrupt compositional iron, while rims are richer in pistacite. They may also increase to An30, decaying again to ,An20 or less at the contain 5±20-mm-wide bands relatively rich in Ce2O3 7up grain boundary. Cores tend to be relatively equant, while to 1.1 wt%), other rare earth elements, and yttrium 7Figs. 5E mantles, overgrowths and rims with aspect ratios of 6±7 to 1 and F and 6B; Table 2). Small grains tend to be zoned, with contribute greatly to the overall elongate shape preferred less pistacitic 7Pi 17±20%) cores and more pistacitic 7Pi 20± orientation of the grains 7Figs. 4 and 5A±D). Along some 24%) rims. Up to three generations of zoned overgrowths grain boundaries, especially along surfaces normal to the are common in larger grains, their shapes de®ned by regions inferred extension direction, plagioclase grains are capped of similar pistacite content, and their boundaries outlined by by albitic beards 7An15, Fig. 6A), which may be associated REE-rich bands 7e.g. Figs. 5E and 6B). with epidote 7region Ab, Fig. 5D).

3.3. Amphibole 3.2. Epidote Amphibole grains are relatively large, up to 1.5 mm long, Epidote grains are much smaller than plagioclase, from with aspect ratios of .3:1. Compositions 7Table 3) lie in the about 50 to 200 mm in diameter, and their shape is variable. magnesiohastingsite ®eld of Leake et al. 71997) when Many grains are polygonal and equant, while others are reduced following the scheme of Holland and Blundy elongated, with aspect ratios up to ,2:1. Many grains are 71994) which maximizes the ferric iron estimate. Large associated with biotite and hornblende 7Fig. 4) where they grains are weakly zoned, with cores only very slightly help produce a conspicuous dark layering in the rock 7Fig. enriched in Fe, Al, and Ti 7Fig. 6C). They tend to be aligned 3). Other, smaller grains occur with less preferred grain parallel to the foliation de®ned by biotite, which is generally shape orientation, and where included in large plagioclase associated with it, along with plagioclase and epidote 7Fig. grains or dispersed in plagioclase-rich domains, they are in 4). Larger grains of magnesiohastingsite are embayed by near random orientation and show less zoning 7Figs. 4 and rounded epidote grains with continuous zoning patterns, 5A). Orientation tends to be more strongly aligned parallel suggesting that epidote partially replaces the amphibole. R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193 1183 Magnesiohastingsite grains are commonly in contact with .4:1. Zoning patterns in metamorphic sphene show all minerals except orthoclase. increases in Al from core to rim 7Fig. 6D). Concentrations are at a minimum adjacent to the igneous core, and 3.4. Biotite climb7typically with three oscillations) to values near the rims that are almost twice those of igneous grains Biotite occurs as subhedral ¯akes typically 300±600 mm 7Fig. 6D). long and 50±300 mm wide. Flakes with aspect ratios of 1:2 to 1:5 are concentrated into folia of subparallel grains that de®ne a conspicuous S±C fabric 7Fig. 3). Biotite grain 3.6. K-feldspar contiguity typically exceeds 1±2 cm but is commonly inter- rupted by subhedral epidote grains. The compositions of Orthoclase occurs in several associations. The regional biotite grains de®ne a cluster in the iron-rich portion of augen structure of the Glastonbury gneiss is de®ned by the phlogopite ®eld 7Fig. 6F). Within this cluster grains pervasive megacrysts of orthoclase about 1 cm in diameter, associated with sphene or epidote are relatively rich in that universally display Carlsbad twinning. Foliation tends MgO, while those intergrown with quartz and feldspars to wrap around these grains, de®ning the S±C fabric 7Fig. contain higher concentrations of Al7VI) 7Table 3). Most 3). The sides of the crystals that are subparallel to S 7i.e. that grains are unzoned but some larger grains that extend face the shortening direction) tend to host epidote, biotite, length-wise from feldspar-dominated domains to sphene- and myrmekitic intergrowths of plagioclase and quartz dominated domains show signi®cant zoning in Ti content. 7Figs. 3 and 5C). Amphibole is universally separated from This correlation of biotite composition with mineral assem- orthoclase by the above minerals 7eg. Fig. 4). blage re¯ects the local differences in the activities of FeO These phenocrysts are strongly perthitic, with albite and Al O de®ned by these assemblages. 2 3 exsolution lamellae common in all grains 7Figs. 4 and 5C; 3.5. Sphene Table 1). Deformation twinning is locally present in some grains, especially adjacent to embayments of myrmekite One of the minerals that can be proven to be both igneous and epidote. These and locally bent exsolution lamellae and metamorphic is sphene. Grains up to 2 mm long host suggest local crystal plastic deformation of these grains. both igneous cores and metamorphic overgrowths. Ion The composition of the orthoclase in these perthites microprobe analysis and U±Pb dating of these grains is relatively uniform, about Or94-Ab05-An00-Ce01. 7Aleinikoff et al., 2001) show that the core regions are However, the integrated bulk composition of the original Late Ordovician to Silurian, consistent with zircon U±Pb grain, as determined by averaging the compositions of ages, and thus relict igneous grains. However, the over- many broad beam 720 mm) microprobe analyses along growths are Permian, and therefore Alleghanian and meta- long traverses, is ,Or82 7Table 1). BaO is a common morphic in origin. The compositions of the two ages of minor constituent, with concentrations of ,0.4 wt%, except sphene are distinct. Igneous sphene crystals always contain adjacent to myrmekitic replacements, where the concentra- relatively high REE contents, making them relatively bright tion may be three times this amount 7Table 1). Two compo- in backscattered electron 7BSE) images. Metamorphic sitions of plagioclase lamellae exist. Fine lamellae sphene contains relatively low concentrations of REEs, 75 £ 50 mm) have a composition of Or02-Ab93-An05, but much higher Al contents 7Table 2), giving it a darker while coarser lamellae with more irregular shapes are less shade of red in Fig. 4. sodic 7Table 1). 7Recognizing that the analyzed concentra-

The structures within these grains, as de®ned by chemical tions of Na2O may be low, we rely more heavily on the K, patterns are also distinctive. Igneous cores show euhedral Ca, and Ba concentrations to de®ne formulae.) The lamellae oscillatory zoning in the concentrations of REEs, as is visi- in most grains are straight but some are curved, de®ning ble in BSE 7lower center, Fig. 4). They also commonly sigmoidal shapes 7Fig. 5C). display resorption and overgrowth structures where the A second occurrence of orthoclase is as interstitial grains overgrowths show anhedral oscillatory zoning 7Aleinikoff within a patchwork of plagioclase grains. These grains are et al., 2002). These igneous grains universally show anhe- not perthitic at the optical or SEM scale, show no twinning dral outlines and are overgrown by metamorphic sphene that or undulosity optically, and are much less sodic than the always isolates the igneous sphene from the matrix mineral larger grains 7Table 1). They may be polygonal or wedge- assemblage. These rinds range from a few tens up to shaped 7Fig. 3), or they may form crescent-shaped caps or 100 mm wide, and are themselves zoned, with Al and Nb beards on plagioclase or quartz grains 7Fig. 5B and D). In all showing the largest relative percent changes 7Wintsch et al., cases, these grains are thickest parallel to the lineation, and 1999). Subhedral metamorphic sphenes also occur with no taper to zero thickness facing the shortening direction. This igneous cores, and both varieties occur with biotite and geometry is especially common in a third occurrence as epidote 7e.g. extreme right, Figs. 4 and 5C). Aspect ratios beards around orthoclase phenocrysts. Most grains do not are commonly .2:1, but regions de®ned by internal display systematic chemical zoning, but some grains that zoning of high and low Nbcontents have aspect ratios of form necks do show U-shaped zoning 7Fig. 6E). 1184 R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193 R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193 1185

Table 1 Representative electron microprobe analysis of feldspar 7n.d. ˆ not detected, n.a. ˆ not analyzed. Operating conditions of 15 kV, 10 nA, 5 mm beam size, and natural mineral standards in all tables except as indicated otherwise)

Orthoclase phenocryst Orthoclase neck Plagioclase

Host Abexsolution a Averageb Pl inlcusionc Low Ba High Ba Core Core Rim

SiO2 64.42 67.03 64.96 61.35 64.12 63.90 60.19 63.03 67.34 Al2O3 18.69 20.92 19.09 24.50 18.73 18.94 25.54 24.00 20.34 CaO n.d. 0.99 0.08 5.97 n.d. n.d. 6.01 4.61 1.02 FeO n.d. 0.10 0.03 0.04 n.d. n.d. 0.08 0.09 0.13 BaO 0.50 0.01 0.39 0.03 0.60 1.16 n.a. n.a. n.d.

Na2O 0.57 10.51 1.71 8.02 0.26 0.38 7.82 8.48 10.47 K2O 15.89 0.42 13.90 0.17 15.79 15.38 0.13 0.17 0.10 Total 100.06 99.97 100.16 100.08 99.49 99.75 99.77 100.38 99.39 Number of ions based on eight oxygen atoms Si 2.98 2.94 2.98 2.72 2.98 2.97 2.68 2.77 2.96 Al 1.02 1.08 1.03 1.28 1.03 1.04 1.34 1.25 1.05 Ca 0.00 0.05 0.00 0.28 0.00 0.00 0.29 0.22 0.05 Fe 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 Ba 0.01 0.00 0.01 0.00 0.01 0.02 ± ± 0.00 Na 0.05 0.89 0.15 0.69 0.02 0.03 0.68 0.72 0.89 K 0.94 0.02 0.82 0.01 0.94 0.91 0.01 0.01 0.01 Total 5.00 4.98 4.99 4.99 4.98 4.98 4.99 4.97 4.96

a . 10 mm exsolution lamellae. b n ˆ 61, 5 mm beam size and n ˆ 17, 20 mm beam size: identical results. c Inclusions of plagioclase that may be coarse exsolution lamellae.

3.7. Quartz 4. Discussion

Quartz is not uniformly distributed in these rocks. It In the above section we have shown that virtually all makes up only a small part of feldspar-rich domains, metamorphic minerals in the granodioritic orthogneiss are especially in regions adjacent to orthoclase porphyroblasts zoned, and that the zoning patterns show elongation of the at grain boundaries normal to the shortening direction 7Figs. grains. In this section we bring these observations together 3 and 4). It is common in beards and pressure shadows to interpret deformational processes. First, however, we around orthoclase phenocrysts, but it also de®nes pure must establish which minerals are metamorphic and which quartz layers parallel to C 7Fig. 4). Grains are typically are relict igneous grains. slightly undulose and equant, 200±400 mm in diameter, with smooth interfaces with feldspars, but moderately 4.1. Metamorphic petrology sutured boundaries with adjacent quartz grains. Larger undulose grains up to 3±5 mm long de®ne quartz-rich 4.1.1. Metamorphic versus igneous minerals folia 7Fig. 3). The Glastonbury gneiss complex is a large body of

Fig. 5. Colorized backscattered electron images of small regions of Glastonbury orthogneiss, colored to emphasize mineralogical and compositional differences. Colors as in Fig. 4 except in 7E) and 7F) where biotite is brown. Shades of dark 7sodic) and pale 7calcic) blue show that zoning in plagioclase is nearly universal. Dark and pale 7REE-rich) green show zoning in epidote is common. 7A) A typical plagioclase-rich domain, showing a relatively large plagioclase grain with overgrowths at grain boundaries that are nearly normal to the stretching direction 7red arrows) in a cluster of smaller grains. 7B) The selective occurrence of epidote grains at plagioclase triple junctions and penetrating plagioclase grains 7curved red arrows) in a feldspar-rich layer. Over- growths on plagioclase grains extend the dimensions parallel to 7C) 7straight red arrows). Orthoclase necks 7yellow) between plagioclase, quartz, and epidote grains 7yellow arrows) allow extension of the aggregate 7see also 7D)). 7C) The local embayment of an orthoclase phenocryst by biotite, epidote, and myrmekite. Biotite, epidote, and sphene de®ne the dark folia typical of Fig. 3. The curved exsolution lamellae of albite in the orthoclase, parallel to the external S±C fabric may re¯ect plastic deformation of this grain. 7D) Image of quartz-rich and feldspar-rich layers, showing the occurrence of orthoclase as necks between plagioclase grains, and as beards on quartz grains. The positions of microprobe traverses of plagioclase and orthoclase 7Fig. 6A and E) are indicated with the red lines. The truncation of zoning along the upper boundary of the analyzed plagioclase grain probably re¯ects the dissolution of the upper part of that grain. 7E) An intergrowth of metamorphic sphene, biotite, and subhedral zoned epidote. Igneous core of sphene 7pale pink) is visible in lower left corner. Concentric bands of REE in epidote monitor the subhedral growth of the grain. 7F) An intergrowth of biotite 7brown) and epidote 7green) where quartz as inclusions and a layer 7left) is black. REE-rich bands in the epidote 7white) outline core, mantle, and rim growth stages. The truncation of these bands along grain boundary X±X0 is evidence for the dissolution of both epidotes along this boundary. 1186 R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193

Fig. 6. Typical compositional variations of minerals in this study. 7A) Compositional pro®le of a typical plagioclase grain, here from Fig. 5D, showing multiple overgrowths. 7B) Compositional pro®le of the epidote grain shown in Fig. 6F. The core±mantle±rim structure is marked by increases in the concentration of

Ce2O3 7and other REE), while the change in the Fe/Fe 1 Al varies continuously. 7C) Compositional pro®les of amphibole showing the lack of chemical zoning at the scale of .1 mm. 7D) Compositional pro®le of sphene crossing the igneous core and metamorphic rim of the grain. Fluctuations in composition of the igneous core correlate with euhedral oscillatory zoning, while ¯uctuations in Fe, Al, and other trace elements de®ne core, mantle and rim regions 7I, II, III, respectively) in the metamorphic overgrowth. 7E) Compositional pro®le of the orthoclase neck between the two plagioclase grains of Fig. 5D. The concentration of BaO in the rims of the orthoclase neck is twice that of the core, and is interpreted as growth zoning in this extensional site. The concentration of albite in the orthoclase grain is nearly constant at ,7 mol%, while it shows normal zoning in the adjacent plagioclase grains. The concentrations of Or and BaO in the plagioclase are uniformly low. 7F) The compositional range of biotite solid solutions as a function of host assemblage, plotted as a function of Mg/ 7Mg 1 Fe) 7atomic) and octahedral Al. Phlogopites associated with feldspars are richer in Fe and Al7VI), while those associated with biotite are richer in Mg and poor in Al7VI). orthogneisses of variable composition. A southern phase of integrated composition of the grains is more much sodic this complex, 15 km long and 3 km wide, contains an augen than the second textural variety that is intergrown with gneiss with conspicuous crystals of orthoclase with plagioclase. These features plus the trial thermometry Carlsbad twins. This twinning is common in igneous 7Section 4.1.2) suggest that the megacrysts are phenocrysts rocks, and unknown to us in metamorphic rocks. The that crystallized in the original Ordovician intrusion. R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193 1187

Table 2 Representative electron microprobe analysis of sphene and epidote 7U, Th, Pb, Sn, La, Zr, K, Mg, Na analyzed; all ,0.01 wt% oxide)

Sphene Epidote

Igneous core Metamorphic rim Core Core Rim

High Ce Low Ce High Y Low Y High Ce Low Ce

SiO2 29.38 29.14 29.18 28.60 35.64 36.27 36.45 TiO2 36.18 35.45 35.66 35.98 0.06 0.06 0.07 P2O5 0.31 0.25 0.27 0.25 0.23 0.22 n.a. Nb2O5 0.09 0.05 0.28 0.30 n.d. n.d. n.a. Ta2O5 n.d. n.d. n.d. n.d. 0.05 n.d. n.a. Al2O3 1.11 1.36 1.51 1.53 23.58 25.04 23.61 Fe2O3 1.06 1.66 0.74 0.79 10.96 10.22 12.09 Y2O3 0.16 0.22 0.15 0.02 0.27 0.01 0.01 Ce2O3 0.65 0.56 0.17 0.18 0.77 0.10 0.05 Pr2O3 0.18 0.18 n.d. n.d. 0.03 0.08 n.a. Nd2O3 0.03 0.13 0.02 0.03 0.09 0.03 0.00 Sm2O3 n.d. 0.04 n.d. 0.02 0.01 n.d. n.a. Gd2O3 0.05 0.14 n.d. n.d. 0.11 0.05 n.a. Dy2O3 n.d. n.d. n.d. n.d. 0.09 n.d. n.a. Yb2O3 0.09 n.d. n.d. 0.03 0.01 0.03 n.a. MnO 0.17 0.14 0.10 0.10 0.30 0.28 0.35 CaO 27.63 27.80 27.98 27.94 21.14 22.49 22.54 F 0.31 0.49 0.38 0.50 0.09 0.11 0.10 O ˆ F 2 0.13 2 0.21 2 0.16 2 0.21 2 0.04 2 0.05 2 0.04 Total 97.41 97.46 96.34 96.38 93.63 94.94 95.23 Number of ions based on 5 oxygen atoms for sphene and 25 for epidote Si 0.991 0.985 0.991 0.975 5.939 5.901 5.954 Ti 0.918 0.901 0.911 0.922 0.007 0.007 0.009 P 0.009 0.007 0.008 0.007 0.033 0.030 ± Nb0.001 0.001 0.004 0.005 ± ± ± Ta ± ± ± ± 0.002 ± ± Al 0.044 0.054 0.060 0.061 4.630 4.802 4.544 Fe 0.027 0.042 0.019 0.020 1.375 1.252 1.486 Ca 0.999 1.007 1.018 1.020 3.775 3.920 3.945 Mn 0.005 0.004 0.003 0.003 0.042 0.039 0.048 Y 0.003 0.004 0.003 ± 0.024 0.001 0.001 Ce 0.008 0.007 0.002 0.002 0.047 0.006 0.003 Pr 0.002 0.002 ± ± 0.002 0.005 ± Nd ± 0.002 ± ± 0.005 0.002 ± Sm±±±±±±± Gd 0.001 0.002 ± ± 0.006 0.003 ± Dy ± ± ± ± 0.005 ± ± Yb0.001 ± ± ± ± 0.002 ± F 0.033 0.053 0.041 0.054 0.046 0.056 0.050 Total cations 3.009 3.017 3.019 3.015 15.895 15.967 15.991

Matrix feldspars are probably all metamorphic. Plagio- oclase crystallized in feldspar-rich domains during the clase never shows albite twinning nor the oscillatory development of the foliation. zoning typical of igneous rocks. The lack of twinning Magnesiohastingsite is probably also magmatic. Its together with its coarse-scale, ¯uctuating zoning, inclu- composition 7Table 3) cannot be used to discriminate sions of epidote and biotite, and participation in the meta- between crystallization at high-grade metamorphic and morphic layering 7Figs. 3±5) indicates that the plagioclase igneous conditions. However, unlike many metamorphic is metamorphic. Orthoclase is intergrown with this plagi- , it is only weakly zoned, and this at the scale oclase. It also occurs as beards on orthoclase phenocrysts, of 2 mm 7Fig. 6C), and trial thermometry 7Section 4.1.2) is and as beards and necks on metamorphic plagioclase and not consistent with the epidote±amphibolite facies condi- epidote grains in the feldspar domains. Some grains even tions of metamorphism. show zoning in BaO content that correlates with the Beyond orthoclase phenocrysts and amphibole, the cores microstructure 7Fig. 6E). Thus both orthoclase and plagi- of sphenes and zircons, both dated as Ordovician by U±Pb 1188 R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193

Table 3 librium. For example, orthoclase textures are complicated, Representative electron microprobe analysis of amphibole and biotite probably by exsolution and deformation but have an inte- Amphibolea Biotiteb grated composition of ,Or82. Trial feldspar thermometry 7Elkins and Groves, 1990) with a plagioclase of An40 and Core Rim With sphene With feldspar With epidote 5 kbpressure gives 670±720 8C, consistent with igneous crystallization temperatures, but not with epidote± SiO 42.06 43.08 36.32 36.39 37.15 2 amphibolite facies metamorphic conditions. Exploratory TiO2 0.68 0.57 1.34 1.86 2.35 Al2O3 11.81 11.70 15.81 16.23 16.84 hornblende±plagioclase thermometry 7Holland and Blundy, FeO 18.19 18.42 19.84 18.62 17.30 1994) gives a similar temperature of ,7008C. MnO 0.68 0.57 0.36 0.48 0.35 The amphibole Al barometer is tempting to apply, but the MgO 9.26 9.47 12.07 11.50 11.42 buffer assemblage is incomplete: no Fe±Ti oxide is present. If CaO 11.01 11.01 n.d. n.d. n.d. one had been present in the igneous rock, the pressure Na2O 1.28 1.35 0.05 0.09 0.05 K2O 1.42 1.29 9.19 9.71 9.47 estimated would be between 5.5 7Johnson and Rutherford, F 0.23 0.26 0.41 0.38 0.34 1989) and 7 kb7Schmidt, 1992). These P±T conditions are O ˆ F 2 0.10 2 0.11 2 0.17 2 0.16 2 0.14 consistent with near solidus magmatic crystallization condi- Total 96.52 97.60 95.21 95.09 95.11 tions 7Schmidt, 1993), and support the above conclusions that the orthoclase and amphibole are relict igneous minerals. Number of ions based on 23 oxygen atoms for amphibole and 22 for biotite 4.1.3. Metamorphic crystallization conditions Si 6.38 6.44 5.55 5.55 5.60 The high variance of the metamorphic assemblage and Ti 0.08 0.06 0.15 0.21 0.27 the poor approach to chemical equilibrium among the zoned Al 2.11 2.06 2.85 2.92 2.99 Fe31 0.67 0.64 ± ± ± metamorphic minerals in these rocks preclude rigorous ther- Fe21 1.64 1.66 2.53 2.37 2.18 mobarometric calculations. However, estimates can be Mn 0.09 0.07 0.05 0.06 0.04 made if local equilibrium between adjacent grains is Mg 2.09 2.11 2.75 2.61 2.57 assumed. For example, the rim compositions of orthoclase Ca 1.79 1.77 0.00 0.00 0.00 and plagioclase in feldspar domains may closely approach Na 0.38 0.39 0.01 0.03 0.01 K 0.28 0.25 1.79 1.89 1.82 equilibrium. Using the thermometer of Elkins and Groves F 0.11 0.12 0.20 0.18 0.16 71990) and the compositions of coexisting feldspars from regions like those shown in Fig. 5B and D yield nearly Total 15.62 15.57 15.68 15.65 15.48 pressure-independent temperatures of ,4258C; this is prob- a Ferric iron of amphibole calculated following Holland and Blundy ably a minimum estimate considering the undeformed state 71994) yield near maximun Fe31 estimates and a magnesiohastingsite clas- and likely late stage of development of these microstruc- si®cation of Leake et al. 71997). tures. Another minimum estimate comes from reset b Ba analyzed; all ,0.01 wt% oxide. All iron treated as FeO. 40Ar/39Ar 7Permian) amphibole ages that re¯ect at least epidote±amphibolite facies metamorphic conditions in the methods 7Aleinikoff et al., 2001) are the only other minerals Alleghanian 7Wintsch et al., 1998) or temperatures of of reliably igneous origin. The other minerals are all meta- . , 5008C 7McDougall and Harrison, 1988). Model P± morphic. Permian U±PbSHRIMP ages 7Aleinikoff et al., T±t paths similar to Wintsch et al. 71993) suggest peak 2001) of sphene overgrowths on igneous sphene 7Figs. 4 and temperatures of ,6008C at 6 kb. This is consistent with 5E) are relatively high in Al and F, consistent with the the staurolite±kyanite grade assemblages just east of these compositions of metamorphic sphenes 7Kowallis et al., rocks 7e.g. Snyder, 1970; Hickey and Bell, 1999). Finally 1997). Biotite and epidote are intergrown with metamorphic the assemblages of reaction 1 are typical of the epidote± sphene 7Figs. 4 and 5C and E), and together de®ne the amphibolite facies of metamorphism 7Spear, 1993). These metamorphic foliation. The composition of biotite is sensi- observations converge to yield an estimate of metamorphic tive to the local metamorphic mineral assemblage 7Fig. 6F), conditions of these rocks of 500±6008C and 4±6 kb, as and must have grown at the same time. Quartz forms pres- shown in Fig. 7. sure shadows around orthoclase phenocrysts 7Fig. 3) and participates in de®ning the metamorphic foliation 7Figs. 3 4.2. Origin of foliation and 4), so it is also metamorphic in origin. 4.2.1. Reaction softening 4.1.2. Igneous crystallization conditions From the above analysis, it is clear that a relict igneous In principle the conditions of igneous crystallization can assemblage including orthoclase 7,Or82) and magnesio- be estimated from the compositions of igneous minerals. hastingsite was being converted to two metamorphic Although igneous grains are present in these rocks, there assemblages: 71) Or93 1 epidote 1 biotite 1 plagioclase 1 is abundant evidence in mineral textures and assemblages quartz, and 72) magnesiohastingsite 1 biotite 1 epidote 1 for the very poor preservation of igneous chemical equi- quartz 1 plagioclase. Although magnesiohastingsite and R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193 1189

Pressure (MPa) that quartz is not texturally associated with the other 0 200400 600 800 1000 reaction products. It is likely that its relatively high solu- 0 bility frees it to precipitate in its own layer, as shown in Figs. 3±5D. Some of the quartz must have moved at least Frictional Sliding 5 mm, presumably as aqueous silica, but local quartz veins 200 up to 20 cm long suggest that some silica moved several decimeters of more.

Creep Dislocation Creep in dry Qtz-rich Rocks C) 400 4.2.2. Growth during extension: dissolution-precipitation o creep T( in dry Fsp-rich Rocks The concentration of biotite, quartz, and plagioclase into ssolutionin wet rocks Metam. Cond. folia parallel to the C foliation 7Fig. 4) produces a highly 600 Di layered rock in which different deformation mechanisms Dislocation Creep may dominate in each layer. Biotite and quartz can deform and in dry Mafic Rocks ky sill sill relatively easily by dislocation creep. However, the 800 10 20 30 pervasive chemical zoning and especially truncated zoning Depth (km) of the other minerals suggest that they did not deform by dislocation creep. Fig. 7. Deformation mechanisms as a function of temperature, based on the Many grains display overgrowths around relatively strength envelopes extrapolated from experimental data and geothermal gradient of Kohlstedt et al. 71995) and Rybacki and Dresen 72000) from equant cores. Zoning in plagioclase is universal 7Figs. 4 Fig. 1. The diagram shows the regions of frictional sliding below 2008C and 5), and core regions of plagioclase grains commonly 7white), dislocation creep in dry quartz-rich rocks 7pale gray), in dry feld- show up to three stages of growth and overgrowth spar-rich rocks 7medium gray), and dry ma®c rocks 7dark gray) with 7Fig. 6A). In microstructures where the precipitation of increasing temperature. The diagram thus predicts dislocation creep in all plagioclase did not keep pace with extension, orthoclase dry quartzo±feldspathic rocks at temperatures above 4508C. However, the operation of dissolution creep at the epidote±amphibolite facies conditions 7,Or92) precipitated as beards and necks 7Figs. 4 and 5B of the rocks described here 7box) extends the ®eld of operation of this and C). deformation mechanism to these temperatures 7white diagonal lines). The epidote±sphene overgrowth pair is an interesting Aluminosilicate triple point from Holdaway and Mukhopadhyay 71993). coupled reaction. Epidote grains, especially those inter- grown with biotite appear as overgrowths on relatively Or93 are both stable at the epidote±amphibolite facies of equant cores. Stages of overgrowth of epidote are marked these rocks, they do not occur in contact. Rather, they are by 30±50-mm-wide rings of epidote enriched in REEs and always separated by biotite and epidote or by quartz. Biotite Y 7Table 2). The only local source for REEs is the igneous and especially epidote embay, and apparently replace sphene. Thus, pulses of extension in the rock, which localize magnesiohastingsite 7Fig. 4). Biotite intergrown with dissolution and overgrowth in epidote, also lead to pulses of epidote also embays, and apparently replaces Or94 7Fig. dissolution of igneous sphene and precipitation of meta- 5C). These relationships re¯ect the incompatibility of morphic sphene. Because the metamorphic sphene accepts amphibole 1 orthoclase via the reaction: only about a third of the REEs in solid solution that igneous sphene does 7Table 2), the REEs released by the dissolution magnesiohastingsite 1 orthoclase ˆ biotite 1 epidote 1† of igneous sphene are concentrated into metamorphic Using simpli®ed analyses from Tables 1±3, the balanced epidote. At the same time, the pulses of overgrowth of reaction is: metamorphic sphene are documented by the ¯uctuating but increasing Al concentrations 7Fig. 6D). Thus we surmise 31 56KAlSi3O8 1 52Ca2 Mg; Fe†3:5Fe0:46Al1:04† that the three stages of growth recognized in plagioclase are also present in epidote and sphene. ‰Si6:5Al1:5ŠO22 OH†2 1 30H2O 1 7O2 While the evidence for precipitation in the form of over- 2† growths is abundant in these rocks, evidence for dissolution ˆ 56K Mg; Fe† Al †‰Si Al ŠO OH† 2:75 0:25 2:75 1:25 10 2 is less common but still present in the form of truncated 1 52Ca FeAl Si O OH† 1 196SiO zoning. Zoning in plagioclase is commonly truncated but 2 2 3 12 2 in some textures it is dif®cult to prove that this is not due This is a discontinuous reaction re¯ecting the incompati- to incomplete crystallization. The case with epidote is more bility of magnesiohastingsite 1 orthoclase under tempera- compelling. Fig. 5F shows a cluster of epidote grains 7green) tures considerably less than those of the original igneous in a biotite-rich 7brown) layer. The two grains on the left crystallization 7Fig. 7). This reaction explains well the inter- show the REE-rich bands 7white) typical of most epidote growth of biotite and epidote that de®nes the S±C fabric, grains. The biotite host formed one of the weakest and the apparent replacement relationships of these minerals aggregates in this rock 7Shea and Kronenberg, 1993) and against magnesiohastingsite and orthoclase. It is signi®cant probably deformed by dislocation creep. The smooth zoning 1190 R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193 in the epidote grains shows no substantial ¯ow in these mize the calculated volume loss, the reaction is: grains, and slip seems to have been localized along the epidote±epidote surface 7X±X0, Fig. 5F). Here the trunca- 11 13 8Plag An25† 1 8Ca 1 15H2O 1 5Fe tion of the REE bands provides strong evidence for the 1 1 dissolution of both epidote grains along this unusually ˆ 5epidote 1 6Na 1 7SiO2 1 25H 4† straight boundary. Evidence for elongation comes from the asymmetric overgrowths on the longer grain 7red with a 17% volume loss, in addition to the release of silica. arrows). Thus evidence for dissolution as well as precipita- This silica could easily ®ll the extension sites in Fig. 5, or tion in these rocks is strong, and much of the strain in these react with aqueous K1 to produce necks of Or90. rocks can be attributed to processes involving dissolution and precipitation. 4.2.4. Reaction mechanisms Carmichael 71969) identi®ed the local metasomatism that accompanied reactions in pelitic rocks, and noted that in 4.2.3. Replacement creep general the aqueous ionic reactants and products of these Several reactions occur in those rocks that indicate that reactions balance, such that rocks behave as closed chemical ¯ow occurs, in part, by mass transport, and that volumetric systems 7centimeter scale) except for volatiles. The typical strains are non-zero. Reaction 1 above, while producing igneous compositions of these granodioritic rocks 7Leo et phases of lower intrinsic strength and thus leading to reac- al., 1984) suggests that this is the case here as well. The tion softening, constitutes a volume reduction. As such its metastability of the orthoclase 1 magnesiohastingsite operation will contribute to the shortening of the rock assemblage probably contributed greatly to the dissolution perpendicular to the S surfaces. Moreover, the redistribution of both the orthoclase and the magnesiohastingsite pheno- of the quartz makes the shortening effect even greater 7up to crysts. The K1 liberated by the dissolution of orthoclase 27%). Using representative molar volumes, the change in reacted with magnesiohastingsite to produce biotite. This volume for reaction 1, ignoring gases, is 23.5%. It is driven liberated Ca11 and some Na1. The Ca11 reacted with the to the right by an increase in fO2,fH2O, and pressure. The amphibole and plagioclase to produce epidote. Most of reaction produces approximately equal volumes of biotite these reactions constitute a volume loss, and so would and epidote, and is signi®cant in that some of the weakest have been driven by a relatively high normal stress perpen- minerals in metamorphic rocks, biotite, and quartz replace dicular to the S fabric. The same high normal stress would some of the stronger, feldspar and especially hornblende. have increased the dislocation density in weaker minerals at The segregation of these weak minerals into layers rich in strong±weak mineral interfaces, and this strain energy biotite or quartz contributes greatly to the overall weakening would have further driven these dissolution reactions of the rock 7Shea and Kronenberg, 1993), especially con- 7Wintsch, 1985). The ultimate distribution of reaction sidering the likely random orientation of grains and inter- products was controlled in part by normal stress producing locking textures in the igneous protolith. the composite foliation and by extension parallel to it. Another reaction causing a signi®cant volume reduction is the replacement of orthoclase by the myrmekitic inter- growth of quartz and plagioclase 7Figs. 3 and 5C): 5. Conceptual model of fabric development

1:25orthoclase 1 0:25Ca11 1 0:75Na1 The texture of the protolith of the augen gneiss is not available, but it was undoubtedly granodioritic porphyry. ˆ plagioclase An25† 1 1:25K1 3† Under epidote±amphibolite facies conditions of meta- morphism, the orthoclase phenocrysts reacted with relict The change in volume of the solids for this reaction is magnesiohastingsite to produce biotite, epidote, and quartz. 3 1 215 cm /mol of plagioclase, corresponding to an 11% The differential mobility of K and SiO2 led to the transport volume reduction. In these rocks, as in those described by of K1 from the dissolving orthoclase 7Or82) to the reacting Simpson and Wintsch 71989), myrmekite is also concen- magnesiohastingsite. The product biotite de®ned a new trated at interfaces that are perpendicular to the shortening C-foliation, and the strong preferred orientation and direction 7Fig. 4). contiguity of these biotite grains is interpreted to have facili- It also appears that epidote replaced only plagioclase in tated dislocation creep under the applied stress in these new some plagioclase domains. Many epidote grains embay folia. Because none of the other minerals in the rock could plagioclase and truncate plagioclase zoning, especially at ¯ow by dislocation creep as fast as biotite, many micro- 1 interfaces parallel to the foliation 7red arrows, Fig. 5B). In extensional sites were produced. SiO2 and K were drawn contrast, orthoclase 1 quartz ®ll spaces between grains at to these sites, producing quartz and orthoclase beards on interfaces normal to the foliation. Where truncation by phenocrysts and on extending plagioclase grains. Where epidote is most conspicuous 7e.g. Fig. 5B), only plagioclase quartz layers developed, deformation may also have and epidote are involved. Simplifying the formula to mini- occurred by dislocation creep. In this particular example, R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193 1191 the common occurrence of two overgrowths on cores of 7. Driving mechanism plagioclase, epidote, and sphene shows that at least three pulses of extension occurred. The excess energy driving mineral dissolution may come from three sources: inhomogeneous stress, recoverable elastic strain energy, and permanent `plastic' strain energy associated 6. Dissolution and precipitation creep with dislocation and twin boundaries 7Wintsch, 1985). We have been an advocate of the latter plastic strain as a source With the extrapolation of experimental rheological of energy driving dissolution 7Wintsch and Dunning, 1985), studies to the slower strain rates of natural crustal rocks, particularly in feldspars 7Knipe and Wintsch, 1985; Wintsch the depth domains of frictional sliding and dislocation and Andrews, 1988; Simpson and Wintsch, 1989). This is also creep in dry crust have been predicted. The yield strengths probably the case here, as the density of deformation twins and in the brittle ®eld are a strong function of con®ning pressure undulosity in orthoclase tends to be higher in regions adjacent 7e.g. Kohlstedt et al., 1995), but deformation by dislocation to plagioclase 7myrmekite) and epidote replacements. creep is thought to be more sensitive to temperature, strain However, because the sites of high normal stress are also the rate, and the fugacity of H2O rather than to con®ning sites of high dislocation density, these thermodynamically pressure 7Rybacki and Dresen, 2000). These extrapolations distinct sources of energy should lead to dissolution at the to a strain rate of 10214 have predicted that in quartz-rich same sites, and without speci®c textural evidence 7Bosworth, rocks, brittle deformation dominated by fracture mechan- 1981; Engelder, 1982), they cannot be discriminated. isms gives way to ductile deformation by dislocation However, the commonly negative volume changes for the creep at about 2008C 7Fig. 1). This brittle-ductile transition reactions identi®ed above points toward stress as a component occurs at higher temperatures in rocks containing minerals of the driving force for dissolution. in which dislocation migration mechanisms are not acti- Whatever the speci®c source of excess internal energy, vated until higher temperature. Plasticity is activated in this energy rendered the strained regions of those grains plagioclase at ,4508C 7Rybacki and Dresen, 2000), while metastable. In the absence of ¯uids this energy would plasticity in clinopyroxene, one of the strongest rock- drive recovery and recrystallization processes 7e.g. forming minerals at crustal conditions, is not predicted Passchier and Trouw, 1996). However, with ¯uids present, until .5008C 7Fig. 1). If the presence of H2O is considered, the dissolution of grains or parts of grains with high normal then the frictional limits drop to lower differential stresses stresses or with high internal energy was more easily 7Chester, 1995; Kohlstedt et al., 1995) and a ®eld of activated than dislocation creep, and dissolution and `pressure-solution' displaces dislocation creep in the replacement creep became the dominant deformation temperature range of 200 to at least 3008C 7Snoke et al., mechanism. Excess internal energy increased the solubility 1998). of the deformed regions of grains 7Wintsch and Dunning, To place the strength information of Fig. 1 in more petro- 1985) and drove a complicated series of dissolution reac- logic terms, we transfer it to the pressure±temperature tions similar to those caused by the metastability of the diagram of Fig. 7. Here the P±T conditions of the magnesiohastingsite 1 orthoclase. Because the dissolution Glastonbury orthogneiss 7Section 4.1.3) lie well within the occurred selectively at sites of high normal stress, it con- dislocation creep ®elds of quartz and feldspar, as extra- tributed to changing the shapes of the grains. Where polated from dry experimental studies. However, this extra- dissolution and precipitation occur at the same site, polation may not be valid where ¯uids are present during volume-loss replacements occurred. Where precipitation deformation. Fluids were present in the Glastonbury gneiss of the dissolved components produced beards and over- as demonstrated by the operation of reaction 1, which growths, the rock extended in the stretching direction. The requires the introduction of H2O and O2. Indeed, most overgrowths appear to be undeformed, and the precipitation minerals in these rocks deformed primarily by dissolution- of minerals in extensional sites probably limited the rate of precipitation processes that require the presence of an extension, similar to displacement-controlled ®bre growth aqueous solution to accommodate the local mass transfer. 7Passchier and Trouw, 1996). Thus the dissolution-precipitation creep ®eld in wet rocks must extend at least to the temperatures 7and depths) of the Glastonbury gneiss 7Fig. 7). The signi®cance of these results 8. Conclusions may lie in the recognition that at relatively high meta- morphic grades dissolution-precipitation processes are acti- We have found abundant evidence for overgrowth of vated at lower stresses than is dislocation creep 7see also orthoclase, plagioclase, epidote, and sphene leading to Stoeckert et al., 1999). If this is true, then the strength of the elongation in the extension direction in a developing S±C crust estimated from dry experiments may produce a maxi- fabric in an orthogneiss. There is strong evidence for mum estimate of crustal strength, while consideration of dissolution of igneous orthoclase and amphibole and meta- ¯uid-assisted deformation mechanisms may give more morphic epidote on surfaces facing the shortening direction. realistic estimates 7e.g. Chester, 1995). There is also evidence for the replacement of orthoclase and 1192 R.P. Wintsch, K. Yi / Journal of Structural Geology 24 /2002) 1179±1193 plagioclase by biotite and epidote. These features lead to the relations of andalusite: thermochemical data and phase diagram for the conclusion that under epidote±amphibolite facies con- aluminum silicates. American Mineralogist 78, 298±315. ditions, creep in these rocks was accomplished by dissolu- Holland, T., Blundy, J., 1994. Non-ideal interactions in calcic amphiboles and their bearing on amphibole±plagioclase thermometry. Contribu- tion-precipitation, reaction, and replacement processes. tions to Mineralogy and Petrology 116, 433±447. Some dislocation creep at these conditions undoubtedly Johnson, M.C., Rutherford, M.J., 1989. Experimental calibration of the occurred in evolving biotite folia and quartz layers, but aluminum-in-hornblende geobarometer with application to Long Valley the bulk strain in the large volume percentage of the rock caldera 7California) volcanic rocks. Geology 17, 837±841. appears to have occurred mostly by chemical process and Knipe, R.J., Wintsch, R.P., 1985. Heterogeneous deformation, foliation development and metamorphic processes in a polyphase mylonite. In: mass transfer, even if these chemical processes were Thompson, A.B., Rubie, D.C. 7Eds.). Advances in Physical Geo- mechanically driven. It is important to note that relict chemistry, vol. IV. Springer, New York, pp. 180±210 Chapter 7. igneous minerals remain, and all metamorphic minerals Kohlstedt, D.L., Evans, B., Mackwell, S.J., 1995. Strength of the litho- are zoned, so that even at these epidote±amphibolite facies sphere: constraints imposed by laboratory experiments. Journal of conditions, chemical equilibrium among the metamorphic Geophysical Research 100, 17587±17602. Kowallis, B.J., Christiansen, E.H., Griffen, D.T., 1997. Compositional minerals has not occurred. Estimates of the strength of wet variations in titanite. Geological Society of America Abstracts with crust that disregard these solution transfer processes are thus Programs 29 76), 402. likely to be too high. Leake, B.L., Woolley, A.R., et al., 1997. Nomenclature of Amphiboles: report of the subcommittee on amphiboles of the international mineralogical association, commission on new minerals and mineral Acknowledgements names. American Mineralogist 82, 1019±1037. Leo, G.W., Zartman, R.E., Brookins, D.G., 1984. Glastonbury gneiss and mantling rocks 7a modi®ed Oliverian dome) in south-central This work has bene®ted from discussions with J. Massachusetts and north central Connecticut: geochemistry, petro- Aleinikoff, T. Bell, T. Byrne, M. Coleman, D.M. genesis, and radiometric age. U.S. Geological Survey Professional Carmichael, G. Hirth, S. Johnson and M. Kunk. We Paper 1295, 45. especially thank Mike Williams for editorial handling, and McDougall, I., Harrison, T.M., 1988. Geochronology and Thermo- chronology by the 40Ar/39Ar Method. Oxford University Press, New B. Bayly and A. Kronenberg for very thorough reviews of York. an earlier draft. K. 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