Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina

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1 J. metamorphic Geol., 2005, 23, doi: /j x Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina S. J. WHITMEYER 1 * AND R. P. WINTSCH 2 1 Department of Earth Sciences, Boston University, Boston, MA 02215, USA (swhitmey@utk.edu) 2 Department of Geological Sciences, Indiana University, Bloomington, IN 47305, USA ABSTRACT The Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine-grained biotite + ioclase, that lacks a well-defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of ioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1-related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + ioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine-grained folia of muscovite + chlorite ± biotite truncate earlier biotite + ioclase textures, and coarser-grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) ioclase is c. An30, distinct from D1 and D2 ioclase (c. An35). The anorthite content of D3 ioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of ioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H + and/or H 2 O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system. Key words: Argentina; reaction localization; reactivation; textural softening; ultramylonite. INTRODUCTION The characterization of fault zones through the full range of crustal depths is an important goal for understanding the rheology of the crust and, in particular, the brittle to ductile transition. Fault zones are typically depicted as narrow in the upper crust where softening processes such as cataclasis (Sibson, 1977; Passchier & Trouw, 1998) or foliation strengthening (Shea & Kronenberg, 1993; Wintsch et al., 1995) localize deformation. In the deeper crust, below the brittle ductile transition, fault zones are thought to widen (e.g. Sibson, 1977; Sibson 1986; Scholz, 1990), perhaps because ductile processes, especially crystal plasticity, but also solution creep, may strengthen fault zones in relation to the surrounding host rocks. *Present address: Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN , USA. At crustal depths near the brittle ductile (frictional viscous) transition, locally brittle (microseismic) processes may initiate deformation and enable fluid infiltration prior to a switch to aseismic, creepdominated (ductile) processes (Imber et al., 2001; Holdsworth, 2004). Thus, knowledge of the specific deformation mechanisms defining the brittle ductile transition is critical to this issue. Brittle deformation is dominated by fracture (cataclastic) processes, which are variably preserved and incompletely understood because of the uncertainty of fluid pressures. Ductile processes are typically dominated by dislocation creep, dissolution creep, or both. These processes may produce rocks with strong crystal lattice-preferred orientations (e.g. Lister & Williams, 1979; Etchecopar & Vasseur, 1987), or with strongly zoned mineral grains (e.g. Wintsch et al., 2005). However, such processes by themselves cannot adequately explain the initiation of ductile deformation at the brittle ductile transition, 411

2 412 S. J. WHITMEYER & R. P. WINTSCH nor do they completely account for the widening of ductile fault zones with increasing depths. In this contribution we report on an ultramylonite zone in central Argentina that reaches a thickness of at least 15 km and is locally cut by narrow, centimetres to metres thick phyllonites (Simpson et al., 2003; Whitmeyer & Simpson, 2003). Mineralogy of the ultramylonites indicates that the predominant deformation fabric developed at temperatures above 500 C, certainly warm enough to be in the field of crystal plasticity for most crustal minerals including the ioclase and biotite that dominate the zone. Therefore, a fundamental question that we address is how deformation in this zone expanded from what are presumed to have been initially localized shear zones into a kilometres-wide ultramylonite zone. A textural hardening process is proposed that induced migration of the high-strain zone into adjacent, well-foliated host gneisses. Reactivation of the texturally hardened ultramylonites along discrete, phyllonitic shear bands is attributed to strain localization because of both reaction and textural softening. Microseismicity may have allowed the introduction of fluids that initiated the dissolution-precipitation processes that, in turn, produced discrete mylonitic shear bands. Finally, a conceptual model is presented for reaction-enhanced textural weakening during the development of greenschist facies mylonites and its relevance for other ductile fault systems is discussed. GEOLOGICAL SETTING AND BACKGROUND The Tres Arboles fault zone marks the western margin of migmatitic rocks of the Sierras de Co rdoba in central Argentina (Fig. 1). This east dipping fault zone is exposed discontinuously for 250 km and contains amphibolite facies mylonitic and ultramylonitic rocks that reach a thickness of km in southern sections. The fault accommodated the westward thrusting of upper amphibolite to granulite facies gneisses and migmatites over biotite + muscovite quartzites and quartzofeldspathic rocks (Whitmeyer & Simpson, 2003). Discrete greenschist-facies shear bands overprint the ultramylonite along localized, metres-wide zones and contain abundant east-dipping mineral elongation lineations and east-over-west kinematic indicators (Simpson et al., 2003; Whitmeyer & Simpson, 2003). Samples were collected from the interior of the southern portion of this fault zone (Fig. 1) with the goal of understanding the deformational and metamorphic processes responsible for producing such a wide fault zone and which facilitated subsequent greenschist facies local reactivation. The Tres Arboles fault zone is interpreted as the Early Palaeozoic suture between the Sierras de Co rdoba and the Sierra de San Luis (Whitmeyer & Simpson, 2003, 2004). These two terranes are components of the Eastern Sierras Pampeanas and contain evidence of east-directed subduction along the western Fig. 1. Simplified geological map of the Sierras de Co rdoba and north-eastern portion of the Sierra de San Luis, Argentina. Sample location within Tres Arboles fault zone is indicated by arrow. Map modified from Whitmeyer & Simpson (2003). margin of Gondwana that initiated in the Early Cambrian (Rapela et al., 1998; Simpson et al., 2003). Previous work on the Tres Arboles fault zone suggests that the most significant phase of convergence occurred after the regional intrusion of Middle Ordovician plutons (Whitmeyer & Simpson, 2004), and ceased by the Late Devonian (Dorais et al., 1997) when undeformed plutons intruded the mylonite zone (Fig. 1). RESULTS The Tres Arboles fault zone is dominated by ultramylonitic schists containing biotite + ioclase ± quartz in a poorly foliated aggregate of lm grains. This schist contains ovoid clasts of coarse grained garnet and ioclase, and fragments of quartzofeldspathic and pelitic rocks (Figs 2 & 3) relict of a precursor rock. Well-defined mylonitic shear bands defined primarily by muscovite ± chlorite with

3 REACTION SOFTENING IN REACTIVATED MYLONITES 413 abundant, elongated quartz ribbons overprint the ultramylonite matrix. Understanding the relationship among successive generations of fabric-forming minerals within the Tres Arboles fault zone is important for determining the metamorphic and microstructural evolution of these rocks. Therefore, the petrology and textures of each of the major minerals is discussed below. Plagioclase Plagioclase in the most abundant mineral in these mylonitic rocks. In the ultramylonite matrix it occurs as lm grains, in a poorly aligned aggregate with biotite. Grains are typically sub-equant and blocky, with aspect ratios of 1:1 to 1:2 (Fig. 3d). A more conspicuous occurrence of ioclase is as porphyroclasts or rock fragments. Here lm grains are rounded to elliptical with width-to-length aspect ratios between 1:1 and 1:3 (Fig. 2a). Some ioclase porphyroclasts occur as elliptical theta and sigma grains (Passchier & Simpson, 1986), the latter with quartz tails that typically contain muscovite + quartz ± chlorite (Fig. 3b,c). Other grains with similar shapes and sizes are composed of a patchwork intergrowth of ioclase and quartz (Fig. 3b). Margins of ioclase porphyroclasts are commonly intergrown with lm biotite (Fig. 3c). Finally, ioclase occurs as the major component of well-defined mantles that isolate garnet porphyroclasts from the ultramylonite matrix (Fig. 2d,e). The compositions of ioclase vary systematically with mode of occurrence. Anorthite content of matrix ioclase is rather uniform at An30 32 (Table 1). This is in contrast to the porphyroclasts that tend to be zoned, with cores slightly more calcic ( An 35%) than rims (32 33%; Fig. 4). This normal zoning pattern is reversed in the few relict feldspar that are now composed of a patchwork of ioclase + quartz (Fig. 3b). In these grains cores are relatively Na-rich (c. An30) and rims are slightly more calcic (c. An33; Table 1). Anorthite content of ioclase that armours garnet porphyroclasts decreases from the interior garnet margin (c. An40) to the outer matrix boundary (c. An30). Thus ioclase compositions on the margins of zoned grains all approach the matrix composition of c. An33 (Fig. 4). Biotite Biotite is the major mineral in all of the rocks of the Tres Arboles fault zone. In the ultramylonitic schists that dominate the zone, it occurs as weakly oriented lm grains associated with ioclase and locally quartz. It is much coarser-grained in included porphyroclasts and rock fragments. Here grains up to lm occur as inclusions within garnet porphyroclasts (Fig. 2d), or associated with garnet + sillimanite + quartz (Fig. 2c). Coarse biotite inclusions in garnet have aspect ratios of 1:2 to 1:4 and exhibit rounded margins and well-defined crystal cleavage planes. Biotite also occurs floating in a ioclase host in a box-work texture that forms rims around garnet porphyroclasts. Here randomly oriented lm flakes with aspect ratios of >1:10 together with ioclase strongly embay garnet porphyroclasts, isolating them from the surrounding ultramylonite matrix. Finally, biotite also occurs as lm long grains in well-oriented and overprinting mylonitic shear bands. These grains may be straight or curved with aspect ratios from 1:2 to 1:5, intergrown with muscovite, and locally chlorite. Chemical compositions of biotite reveal a clear difference between coarse-grained biotite inclusions in garnet, and finer-grained rim and matrix biotite. Biotite inclusions in garnet define a distinctly low Ti and high Al VI field relative to fine-grained matrix biotite and medium-grained biotite in garnet porphyroclast rims (Fig. 5a). Biotite inclusions in garnet also tend to be richer in Mg than rim and matrix biotite (Fig. 5b). Little chemical variation is apparent between matrix biotite and box-work biotite. Garnet Garnet within the Tres Arboles fault zone occurs as 1 5 mm grains as rock fragments or porphyroclasts that range from elliptical to elongated and often preserve an internal gneissic foliation defined by aligned sillimanite and biotite grains (Fig. 2b,c). Coarse, lm, biotite ± sillimanite inclusions are common in garnet cores, with rare quartz and ioclase inclusions near the garnet margins (Fig. 2d). Garnet is mantled by a lm rim of ioclase + randomly oriented biotite flakes, which frequently fill embayments into the garnet margin (Fig. 2d,e). In some occurrences, these ioclase + biotite mantles have almost completely replaced garnet porphyroclasts, leaving only a small core of original garnet (Fig. 2f). Garnet is almandinerich (Table 2) and shows no systematic zoning. Garnet is not present in the fine-grained ultramylonite matrix or in overprinting shear band fabrics. Sillimanite Sillimanite has several modes of occurrence. It is most abundant in rock fragments where coarse-grained sillimanite (up to 20 lm in cross-section) is present within and adjacent to garnet grains (Fig. 2c). Finergrained (20 60 lm long) sillimanite needles also occur as aligned inclusions within garnet porphyroclasts. These record an earlier fabric that is not preserved in the enclosing ultramylonite matrix (Fig. 2c). Sillimanite is also locally present within the ultramylonitic matrix as fibrolite bundles in some porphyroclast pressure shadows, and as individual coarser porphyroclasts preserved within lm muscovite grains (Fig. 3a).

4 414 S. J. WHITMEYER & R. P. WINTSCH (a) fsp (b) gar (c) bio gar (d) gar gar (e) (f) gar D3 D3

5 REACTION SOFTENING IN REACTIVATED MYLONITES 415 Quartz Quartz occurs as rare inclusions in garnet and ioclase porphyroclasts and as a significant component of the overprinting shear band fabric. Quartz inclusions in garnet grains are rounded to slightly elongated, lm in length and occur near the garnet margins or rarely within ioclase rims around garnet (Fig. 2d). Some ioclase porphyroclasts contain lm quartz grains along their margins (Fig. 3c). Less common patchwork ioclase + quartz porphyroclasts contain abundant quartz inclusions and exhibit quartz tails in pressure shadow regions (Fig. 3b). Small, lm quartz grains occur within the ultramylonitic matrix, but constitute a small percentage of the mode of the ultramylonites. Abundant, elongated quartz ribbons wrap around garnet porphyroblasts, and are commonly parallel to the shear band fabric (Fig. 3e). Quartz also occurs in pressure shadows and forms tails on ioclase porphyroclasts (Figs 3b & 6). Quartz grains within such tails exhibit sutured boundaries and undulatory extinction (Fig. 3f). Muscovite Muscovite (and chlorite) are retrograde minerals whose occurrence is restricted to overprinting shear bands, or to sites that can be related to shear band generation. In well foliated domains fine-to-mediumgrained ( lm) muscovite defines mylonitic shear bands, where it commonly interfingers with biotite and quartz ribbons (Figs 2e & 3e). Less well-oriented grains occur in replacement settings. This is most conspicuous where lm muscovite grains embay porphyroclasts of sillimanite (Fig. 3a,b). Locally, very fine-grained muscovite appears to have replaced biotite and ioclase in ultramylonite (upper right and centre, Fig. 3d), or nucleated along internal fractures in ioclase porphyroclasts (Fig. 3c) and in extension sites between biotite clasts (Fig. 3d). The compositions of muscovite are sensitive to associated mineral assemblages. Coarse muscovite replacing sillimanite is richer in Al and Na and poorer in Mg than matrix muscovite (Table 2). Chlorite Chlorite is another retrograde mineral restricted to overprinting shear bands. Chlorite typically interleaves with quartz ribbons and well-aligned muscovite-rich bands to define shear band folia. In the latter chlorite composes c % of shear band, with minor quartz or biotite. Chlorite also occurs as discrete aggregates of helicitic lm grains in pressure shadows (Fig. 2e) and as pseudomorphic replacements of biotite. Small, lm chlorite grains are common as partially to completely altered biotite grains in narrow shear bands around porphyroclasts (Fig. 6). The resulting biotite chlorite intergrowth is too finegrained to be resolved on the microprobe, but the percentage of chlorite alteration can be monitored with the K cations of the aggregate. In altered regions, K cations of the aggregate range between 1.5 (mostly biotite) and 0.0 (pure chlorite) and reveal the extent and distribution of biotite-to-chlorite alteration. Alteration is most extensive near porphyroclast margins in regions of higher strains, whereas little to no alteration occurs in porphyroclast pressure shadows (Fig. 6). DISCUSSION Protolith of the ultramylonites Rock fragments and porphyroclasts of ioclase, garnet and sillimanite in Tres Arboles ultramylonites and mylonites show that these rocks were derived from a high-grade pelitic gneiss. In fact the gneisses immediately east of, and in the hangingwall of, the Tres Arboles fault zone (Fig. 1) contain migmatitic mineral assemblages and structures indistinguishable from those in rock fragments within the fault zone. These hangingwall rocks consist of well-foliated, upper amphibolite facies quartz + ioclase + biotite + garnet gneisses and K-feldspar + sillimanite migmatites, which record temperatures and pressures of C and 7 8 kbar (Otamendi et al., 1999). The identical mineralogy, fabrics (here termed D1), and appropriate structural setting make these hangingwall rocks compelling candidates as the protoliths of the Tres Arboles fault zone ultramylonites (Whitmeyer & Simpson, 2003). Modification and digestion of included fragments of hangingwall rocks In spite of the apparent preservation of clasts of D1 rocks, these inclusions in the ultramylonitic rocks have undergone some changes. Temperatures obtained from Fig. 2. Transmitted polarized light and back scattered electron images of fault rocks viewed perpendicular to foliation and parallel to lineation. (a) Photomicrograph of ioclase porphyroclast, showing bent crystallization twins cross-cut by dagger-shaped deformation twins, surrounded by a (D3) fabric of quartz + muscovite + biotite. Crossed polarized light. (b) Photomicrograph of garnet porphyroclast with ioclase + biotite rim, in fine-grained ultramylonite (D2) matrix. Plane light. (c) Photomicrograph of a gneissic rock fragment dominated by garnet surrounded by and including a quartz biotite sillimanite schist. Crossed polarized light. (d) Back scattered electron image of a garnet porphyroclast with a coarse biotite inclusion and a mantle composed of an intergrowth of ioclase + randomly oriented biotite. (e) Back scattered electron image of a garnet porphyroclast with ioclase + biotite rim, surrounded by a mylonitic matrix (D3) of biotite + muscovite + quartz. D3 shear bands indicated; note the chlorite + quartz in pressure shadow of the garnet porphyroclast. (f) Back scattered electron image of an intergrowth of ioclase + randomly oriented biotite around a relict garnet core. Scale bars: (a) 100 lm; (b), (d), (e), (f) 200 lm; (c) 60 lm.

6 416 S. J. WHITMEYER & R. P. WINTSCH (a) bio (b) sill bio sill (c) D3 (d) bio bio musc bio (e) (f)

7 REACTION SOFTENING IN REACTIVATED MYLONITES 417 Table 1. Representative chemical analyses of ioclase. Cations based on 8 oxygen. Weight % Core Porphyroclast Margin Patchwork core Garnet rim next to gar Matrix SiO Al2O FeO MnO CaO Na 2 O K2O Total Cations based on 8 oxygen Si Al IV Fe Mn Ca Na K Total (a) VI Al (b) Matrix biotite Biotite in garnet rim Biotite inclusions in garnet Ti (cation) 2.0 Siderophyllite Eastonite IV Al 0.0 Annite Phlogopite 0 Mg# 100 VI Al Mg/(Mg+Fe 2+ ) Matrix biotite Biotite in garnet rim Biotite inclusions in garnet Fig. 4. Plot of Al cations across representative ioclase porphyroclast and patchwork ioclase, showing chemical zonation from core region to the margins. Note: Anorthite content is equivalent to Al (cations) )1. Fig. 5. (a) Al VI v. Ti plot of matrix biotite, biotite needles on garnet rims, and coarser-grained biotite inclusions in garnet porphyroblasts. Note the low-ti field defined by biotite inclusions. (b) Al VI v. Mg/(Mg + Fe 2+ ) plot of matrix biotite, biotite needles on garnet rims, and coarser-grained biotite inclusions in garnet porphyroblasts. Inset shows analysed biotite samples (hatched area) plot on a chart of the end-member biotite components. coarse-grained sillimanite and biotite in garnet porphyroclasts from the Tres Arboles fault zone (Fig. 2c) suggest re-equilibration of the garnet biotite exchange thermometer at lower temperatures between 540 and 590 C (Whitmeyer & Simpson, 2003). The occurrence of sillimanite needles with biotite + ioclase + quartz in the pressure shadows of some garnet porphyroclasts (Whitmeyer & Simpson, 2003) suggests that at least some early deformation occurred at these middle amphibolite facies metamorphic conditions. Plagioclase porphyroclasts are chemically zoned with the cores more calcic (up to An40) than the Fig. 3. Transmitted polarized light and back scattered electron images of fault rocks viewed perpendicular to foliation and parallel to lineation. (a) Back scattered electron image of sillimanite porphyroclast partially replaced by muscovite, in a matrix of D3 muscovite + biotite + quartz. (b) Back scattered electron image of D2 matrix hosting ioclase and sillimanite porphyroclasts and a patchwork grain of ioclase + quartz with quartz beards developed in pressure shadows. D2 matrix is cut and partially replaced by muscovite + quartz, and overprinted by muscovite + biotite as D3 shear bands. (c) Back scattered electron image of ioclase porphyroclast in a D2 biotite + ioclase + quartz matrix. This matrix is locally cut by muscovite-dominated D3 shear bands (dashed line), and very fine-grained muscovite locally replaces ioclase in a fracture in the porphyroclast. (d) Back scattered electron image of a D2 aggregate of c. 20lm grains of biotite and ioclase, set in a 2 5 lm matrix of muscovite (of D3 generation). D3 muscovite also fills the gap in the biotite grain. (e) Photomicrograph of quartz ribbons around ioclase porphyroclasts and parallel to D3 shear band foliation. D3 matrix consists of fine-grained muscovite + chlorite + ioclase ± biotite. Plane light. (f) Photomicrograph of quartz in pressure shadow region, showing sutured grain boundaries and deformation bands. Scale bars for (a) 100 lm; (b), (c) 200 lm; (d) 20 lm; (e) 300 lm; (f) 50 lm.

8 418 S. J. WHITMEYER & R. P. WINTSCH Table 2. Representative chemical analyses of phyllosilicates: biotite, chlorite and muscovite. Biotite and muscovite cations based on 22 oxygen, and chlorite cations based on 28 oxygen. Weight % Biotite Muscovite Matrix Inclusion chlorite In matrix In ioclase Coarse muscovite mantling sillimanite SiO Al 2 O TiO FeO MnO MgO CaO Na2O K 2 O F NA H 2 O Total Total-H2O Cations based on 22 oxygen (biotite, musc), 28 oxygen (chlorite) Si Al IV Al VI Ti Fe Mn Mg Ca Na K F 0 0 NA Total margins (Fig. 4). This composition is similar to that within granulite gneisses east of the fault zone (Otamendi et al., 1999). However, the normal zoning that converges to c. An33 suggests exchange with the ioclases in the ultramylonite matrix. Replacement of D1 clasts Clasts of rock fragments and of single crystals of garnet and ioclase are common in the ultramylonite. However, the textures marking the margins of these grains suggest that they were not in chemical equilibrium with the P T-fluid environment that produced the ultramylonitic host (here termed D2). Garnet Most garnet porphyroclasts are rimmed and isolated from the D2 matrix by a ioclase + biotite rim. Deep embayments of biotite that invade the garnet mantles are strong evidence for the replacement of garnet by ioclase + biotite. Further support is gained from the variable width of the ioclase biotite rims, which in the extreme can leave only a very small garnet in the centre of the texture (Fig. 2f). This suggests that ioclase and biotite grew at the expense of garnet by the reaction: garnet þ quartz ¼ biotite þ ioclase ð1þ Using representative analyses for the garnet and associated rim and D2 matrix biotite and ioclase (Table 3), one balanced reaction that could explain the texture is: 31ðMg,Fe,MnÞ 2:9 Ca 0:1 Al 2:0 Si 3:0 O 12 þ 11SiO 2 þ 2H 2 O þ 2TiO 2 þ 18K þ þ 12Na þ þ 4:9Ca þþ þ 36H þ ¼ 20K 0:9 ðmg,feþ 2:6 Ti 0:1 Al 1:7 Si 2:6 O 10 ðohþ 2 þ 20Na 0:6 Ca 0:4 Al 1:4 Si 2:6 O 8 þ 37:9ðMg,Fe,MnÞ þþ ð2þ While the reaction is not unique, it does show that SiO 2 is a reactant, consistent with the absence of quartz from the biotite ioclase intergrowth. It also shows that the reaction consumes both H 2 O and H +, suggesting that the reaction required the introduction of aqueous fluids into the fault zone. bio K cations µm Fig. 6. Composite backscatter electron image of ioclase and patchwork porphyroclasts, with a region of random D2 matrix above the ioclase porphyroclast just right of centre. D3 shear bands are visible in the top left and bottom right, and are indicated by dashed white lines; sinistral movement direction is indicated by white arrows. Biotite matrix grains are preferentially altered to chlorite in high-strain regions (indicated by yellow arrows); K cations chart indicates compositional range between predominantly biotite grains (red) and predominantly chlorite grains (green).

9 REACTION SOFTENING IN REACTIVATED MYLONITES 419 Table 3. Representative chemical analyses for principal minerals in the D2 reaction: garnet ¼ biotite + ioclase. Weight % Garnet Biotite Plagioclase SiO Al2O TiO NA FeO MnO MgO NA CaO Na 2 O NA K 2 O NA H2O NA 3.86 NA Total Total-H2O Cations based on 24 oxygen (garnet), 22 oxygen (biotite), 8 oxygen (ioclase) Si Al VI Al VI Ti NA Fe Mn Mg NA Ca Na NA K NA Total Garnet cations based on 24 oxygen, biotite cations based on 22 oxygen and ioclase cations based on 8 oxygen. Orthoclase The ioclase + quartz patchwork feldspar (Fig. 3b) may also have been modified, but more completely. Similar patchwork grains are not found in hangingwall rocks, but cm-size orthoclase grains are common in both the hangingwall (Otamendi & Rabbia, 1996) and in less deformed, marginal regions of the fault zone (Whitmeyer & Simpson, 2003). It is thus possible that the patchwork ioclase + quartz pattern resulted from the consumption of original K- feldspar porphyroclasts by the reaction: K-feldspar þ Na þ þ Ca þþ ¼ ioclase þ quartz þ K þ ð3þ Using a representative analysis for patchwork ioclase from the core region (Table 1), the balanced reaction is: 13KAlSi 3 O 8 þ 7Na þ þ 3Ca þþ ¼ 10Na 0:7 Ca 0:3 Al 1:3 Si 2:7 O 8 þ 12SiO 2 þ 13K þ ð4þ In support of the operation of this reaction, volumetric proportions of ioclase and quartz as reaction products calculated using 1 atmosphere molar volumes of the solids is 100:27, qualitatively similar to that found in patchwork grains (e.g. Fig. 3b). Orthoclase is not preserved in cores as they are in the garnet replacements, and so it is likely that this reaction has gone to completion, consuming all inherited orthoclase. best preserved within strain shadows adjacent to ioclase and garnet porphyroclasts (e.g. Figs 2 & 3; the top-right region of Fig. 6), where D2 textures are protected from later strain and reactions related to overprinting shear bands. The mechanisms that led to the formation of these fine-grained and poorly aligned matrix fabrics are not well understood at present, but may have resulted from grain boundary sliding at significant strain rates (Whitmeyer & Simpson, 2003). The D2 reaction above (Eq. 2) shows that both quartz and H 2 O are consumed, which is supported by the almost complete absence of quartz within garnet mantles and D2 matrix fabrics. The consumption of H 2 Oand alkalis indicates that the system was open, at least to these components, and is consistent with the reactions occurring during active deformation, where faults and shear zones often facilitate fluid migration (Sibson, 1977; Scholz, 1990; Wintsch et al., 1995). The fact that these reactions are arrested indicates that the activities of the reactants became too low to completely consume garnet. Thus, the low activity of SiO 2 and possibly H 2 O may indicate the local cessation of D2 deformation. The abundance of biotite in the D2 matrix and around garnet rims would normally be interpreted as representative of a mechanically weak fabric (e.g. Boullier & Gueguen, 1975; Schmid, 1982). However, ioclase within the ultramylonitic matrix and within rims around garnet grains appears to pin the elongate biotite needles in variable orientations (Fig. 2d,f) to create a poorly to non-aligned fabric, which would not easily facilitate grain boundary sliding (Schmid, 1982; Shea & Kronenberg, 1993). The absence of quartz within D2 fabrics would also result in a texturally strong fabric at deformation temperatures below the onset of feldspar ductility (at least 450 C; De Paor & Simpson, 1993; Rosenberg & Stu nitz, 2003). The development of a strong, somewhat interlocked biotite + ioclase matrix devoid of quartz may have facilitated the widening of the Tres Arboles zone, as surrounding wellfoliated and favourably oriented gneisses were likely mechanically weaker than reaction-hardened, poorly foliated shear zone fabrics. This suggestion is supported by the presence of metres-thick, discrete ductile deformation zones that lie east of, but in close proximity to, the eastern margin of the Tres Arboles fault zone proper (Simpson et al., 2003). The implication is that deformation became less continuous, and therefore less intense, in marginal regions where it was infiltrating the host gneisses. Although appropriate equilibrium compositions during D2 deformation are not available for geothermometry, the presence of fibrolite needles in porphyroclast pressure shadows (Whitmeyer & Simpson, 2003) suggests that deformation at least initiated at lower to middle amphibolite facies. Development of D2 ultramylonite Most of the matrix surrounding D1 clasts consists of poorly aligned biotite and ioclase. This texture is Overprinting D3 shear band fabrics and reaction processes The above D2 ultramylonitic schists that include clasts of D1 gneiss have further been overprinted by a

10 420 S. J. WHITMEYER & R. P. WINTSCH greenschist facies mylonitic fabric (D3). These are defined primarily by strongly oriented chlorite and muscovite in discrete mylonite shear bands. Quartz ribbons are aligned with the long axes of phyllosilicate minerals and contribute to defining the fabric (Figs 2e & 3e). Growth of muscovite and chlorite in a preferred alignment within mylonitic shear bands probably occurred through several reactions in locally open fluid systems. Muscovite was likely produced by three locally independent reactions, involving the consumption of sillimanite, ioclase and biotite. Muscovite Crystallization of muscovite and chlorite in a preferred alignment within mylonitic shear bands probably occurred via several locally metasomatic reactions. Muscovite was produced by three separate reactions, involving the consumption of sillimanite, biotite and ioclase. Individual coarse grains of sillimanite within the mylonitic matrix are always mantled by muscovite (Fig. 3a), suggesting the reaction: sillimanite + SiO 2 ¼ muscovite (e.g. Carmichael, 1969). Using the Ômuscovite mantling sillimaniteõ analysis from Table 2, the balanced reaction is: 59Al 2 SiO 5 þ 59SiO 2 þ 67H 2 O þ 36K þ þ 2Na þ þ 2Ca þþ þ 6ðMg,FeÞ þþ ¼ 20K 1:8 Na 0:1 Ca 0:1 ðmg,feþ 0:3 Al 5:9 Si 5:9 O 20 ðohþ 4 þ 54H þ ð5þ This reaction can be rewritten with theoretical mineral compositions with no loss of applicability as: 3Al 2 SiO 5 þ 3SiO 2 þ 3H 2 O þ 2K þ ¼ 2KAl 3 Si 3 O 10 ðohþ 2 þ 2H þ ð6þ This reaction requires the local introduction of K +, SiO 2, and H 2 O, and results in a 30% solid volume increase. Fine-grained muscovite in D3 folia cuts poorly oriented D2 biotite (Fig. 3a,b), or grows in cracks between biotite grains (Fig. 3d). These truncating structures suggest the replacement of biotite by muscovite. The reaction balanced on Al and using theoretical mineral compositions is: 3KðMg,FeÞ 3 AlSi 3 O 10 ðohþ 2 þ 20H þ ¼ KAl 3 Si 3 O 10 ðohþ 2 þ 6SiO 2 þ 12H 2 O þ 2K þ þ 9ðMg,FeÞ þþ ð7þ This metasomatic reaction requires the local introduction of H +, and produces H 2 O, SiO 2 and other aqueous ions. It also results in a 39% mineral volume decrease. The reaction produces muscovite and quartz in nearly equal volumes, explaining the high proportion of quartz in some patchy intergrowths (Fig. 3a) and in shear bands (Fig. 3e). The simplified compositions used above ignore small amounts of Ti in biotite (Table 2). However, rutile occurs locally in D3 shear bands, suggesting that rutile is a minor byproduct of this reaction. The low solubility of TiO 2 probably restricted its transport such that it was precipitated locally as isolated needles during D3 deformation. Muscovite is also produced at the expense of ioclase, both within mylonitic shear bands and within rare ioclase porphyroclasts where muscovite nucleates along fractures (Fig. 3c). The reaction balanced on Al using theoretical muscovite and ioclase of An40 composition is: 15Na 0:6 Ca 0:4 Al 1:4 Si 2:6 O 8 þ 7K þ þ 14H þ ¼ 7KAl 3 Si 3 O 10 ðohþ 2 þ 18SiO 2 þ 9Na þ þ 6Ca þþ ð8þ This reaction consumes K + and H+ and produces muscovite and quartz, with a 9% volume decrease. Chlorite Chlorite occurs with muscovite as lm aggregates (Fig. 2e), and as partial alteration products of biotite grains (Fig. 6). In both of these cases, chlorite crystallizes at the expense of biotite, particularly in locations of high strain. The balanced reaction using theoretical biotite and chlorite formulae is: 2KðMg,FeÞ 3 AlSi 3 O 10 ðohþ 2 þ 4H þ ¼ðMg,FeÞ 5 Al 2 Si 3 O 10 ðohþ 8 þ 3SiO 2 þ 2K þ þðmg,feþ þþ ð9þ The reaction requires the introduction of H + (but not H 2 O) and produces quartz at 25 volume % of chlorite. This is in qualitative agreement with proportions of quartz and chlorite in some intergrowths (e.g. Fig. 2e). The total reaction produces an c. 30% volume loss showing a strong theoretical dependence on local high stress. This dependence is in agreement with the degree of reaction progress of biotite replacement shown in Fig. 6. Here the replacement is most complete along the margins of the ioclase grain facing the shortening direction (yellow arrows). Quartz SiO 2 is a product of most D3 reactions (Eqs 7 9), however quartz grains are not always present next to muscovite or chlorite in D3 shear bands (e.g. Fig. 3a). The high solubility of quartz probably means that some of the SiO 2 was free to migrate and accumulate into porphyroclast pressure shadows (Fig. 3b) and as largely monomineralic ribbons (e.g. Figs 2e & 3e). This is supported by the local absence of quartz from high strain regions (Fig. 6), which suggests that SiO 2 reac-

11 REACTION SOFTENING IN REACTIVATED MYLONITES 421 3H 2 O + 2K + 18H + 3 sillimanite = 2 muscovite 2H + 3 biotite = muscovite Fig. 7. Diagram after Carmichael (1969), showing the four principal D3 reactions. Local open system behaviour imports H 2 O and H + ions and enables the aqueous transfer of SiO 2, alkalis and other cations. Excess SiO 2 precipitates as quartz ribbons and beards. Note that most aqueous cations and minerals in solution are components within a larger chemical system, which is mostly closed on a scale of centimetres. 14H + 5K + 9Na + 6Ca ++ 3SiO2 15 ioclase = 7 muscovite quartz ribbon 15SiO 2 6SiO2 2K + 3SiO 2 2 biotite = chlorite 9(Mg,Fe) ++ 9H 2 O 4H + ++ (Mg,Fe) tion products migrated during deformation. The abundance of quartz in D3 shear bands suggests that far more quartz was produced by D3 reactions (Eqs 7 9) than was consumed in reaction (Eq. 6). Significance of D3 reactions The D3 reactions discussed above (Eqs 5 9) depend on chemical systems that are locally open to the transfer of SiO 2,H 2 O and aqueous cations. However, many of the ionic reactants external to the local replacement site can be provided by products from other D3 reactions at the centimetre scale (Fig. 7). Centimetre-scale diffusive transfer of aqueous ions is well known in metamorphic and reaction-enhanced deformation processes (e.g. Carmichael, 1969; Wintsch & Yi, 2002; Wintsch et al., 2005). Most of these reactions involve a local volume decrease, particularly if SiO 2 does not precipitate locally as quartz. The volume decrease would have produced an increase in porosity and permeability, which in turn would have facilitated the infiltration of fluids. Therefore, a local positive feedback loop may have existed that enhanced the open system behaviour required by these syntectonic reactions. D3 reactions are typically concentrated in regions of high strain and therefore were likely driven by local compressive stresses. This is supported in the samples by the predominance of reaction products, such as muscovite and chlorite, along porphyroclast margins in regions of apparent maximum shortening (Fig. 6). Quartz is produced by three of four D3 reactions for which there is strong textural evidence, but is not abundant in local high strain sites. This can be explained by an open system distribution of aqueous ions and fluids that would facilitate the accumulation of quartz ± chlorite in micro-extensional sites and pressure shadows, including some porphyroclast tails (Figs 3b & 6). Thus, the net reaction of sillimanite + biotite + ioclase ¼ muscovite + chlorite + quartz ± rutile involved the net introduction of H 2 O and H +, and the removal of Fe 2+,Mg 2+,K +,Na + and Ca 2+ (Fig. 7) and could not have operated in a completely closed system. Several ions, such as (Fe,Mg) ++ are not balanced in Fig. 7, but could easily be components in a chlorite replaces ioclase reaction. This reaction probably did occur, but there are no textures sufficiently compelling to propose it directly. Other D3 deformation processes Deformation-induced chemical reactions were significant in forming the D3 shear band fabric, however other deformation processes were likely operating simultaneously. Evidence for dislocation creep and grain boundary migration is seen in quartz ribbons and tails, in the form of deformation bands and sutured grain boundaries (Tullis, 1977; Hirth & Tullis, 1992; Fig. 3f). Biotite apparently is not produced by D3 reactions; however aligned biotite grains are common within D3 shear bands. This suggests that dissolution-precipitation (pressure solution) of biotite (e.g. Mares & Kronenberg, 1993; Shea & Kronenberg, 1993) was active during D3 deformation, although the similarity in grain size between D2 and D3 biotite suggests that some mechanical rotation of existing D2 biotite grains was likely. Approximate temperature during D3 deformation Chemical reactions active during D3 deformation produced both muscovite and chlorite at the expense of biotite and ioclase. The breakdown and consumption of biotite and growth of chlorite suggests that temperatures were below the biotite stability field

12 422 S. J. WHITMEYER & R. P. WINTSCH (less than c. 400 C; Spear & Cheney, 1989). However, some biotite may have dissolved and reprecipitated within D3 shear bands, which would indicate temperatures at or above 400 C. Quartz microstructures within D3 ribbons and tails are consistent with Regime I recrystallization (Hirth & Tullis, 1992) at temperatures of c. 250 ± 50 C (De Paor & Simpson, 1993). Stipp et al. (1998) suggested that quartz deforms by bulging recrystallization (roughly equivalent to Regime I recrystallization) up to temperatures of c. 400 C. Therefore, D3 deformation is estimated to have occurred at greenschist facies conditions at temperatures of C. These temperatures are consistent with typical estimates for faulting at or near the brittle ductile transition in the crust (e.g. Scholz, 1990; Imber et al., 2001; Gueydan et al., 2004). Initiation of D3 deformation The above sections argue for reaction- and textureenhanced, greenschist facies deformation that was driven by local compressive stresses and largely selfsustained through feedback loops of interchanging SiO 2,H 2 O and free ions in solution. We suggest that most of the reactants necessary for these chemical reactions are available as products from other simultaneously occurring reactions as part of a system that is open on the scale of tens of millimetres, but largely closed on a scale of several centimetres. However, the processes that caused D3 deformation to initiate in zones that were at least partially reaction-hardened during earlier D2 deformation are not readily apparent. Imber et al. (2001) suggested that the formation of greenschist facies phyllonites (mica-rich mylonites) is often preceded by brittle microseismicity. In their model initially brittle faulting at the grain scale can produce microfractures that weaken the rock and facilitate the infiltration of fluids. Subsequent work by Gueydan et al. (2003) suggested that feldspar-to-white mica reactions which occur near the brittle ductile transition nucleate along pre-existing fractures in feldspar grains. Evidence for such a feldspar-to-mica reaction in these rocks is apparent in Fig. 3c, and is described by reaction (Eq. 7), discussed above. This reaction requires the input of H 2 O and ions in solution and appears to have nucleated along an internal fracture in a feldspar porphyroclast. Therefore, although not conclusive, our data are consistent with the suggestion that greenschist facies ductile deformation processes may initiate with fracturing and microseismicity at the grain scale, and that such a process may have provided local pathways for early fluid migration, reaction and thus strain localization. However, we contend that initially brittle processes were superceded by ductile reaction-enhanced processes that incorporated a reaction feedback loop that was largely closed at the several centimetre scale, except for the introduction of H + and H 2 O. We suspect that much of the evidence for early microseismicity was quickly obliterated by subsequent ductile deformation. SUMMARY AND CONCEPTUAL MODEL FOR REACTION-ENHANCED FABRIC DEVELOPMENT IN THE TRES ARBOLES FAULT ZONE The Tres Arboles shear zone is defined by a wide zone of biotite + ioclase ultramylonite. Relic D1 protolith fabrics from hangingwall gneisses are preserved within rock fragments and include a sillimanite + biotite foliation (likely D1-related). Incorporation of these fragments into a poorly aligned, fine-grained biotite + ioclase ultramylonitic matrix (D2) was followed by reactions of the clasts with their new matrix. This included the development of ioclase + biotite rims around garnet grains, and the modification of feldspar porphyroclasts. Strain rates must have been quite high, and the intrusion of H 2 O was limited, such that reactions that could have produced weak phyllosilicates did not go to completion. The development of poorly oriented D2 fabrics was a textural and possibly a reaction-hardening process that probably caused the migration of active deformation zones into well-foliated (and thus weaker) gneissic rocks of the hangingwall, thus widening the zone of ultramylonite. The D3 reactivation of the Tres Arboles fault zone may have initiated with local fracturing and microseismicity. However, if this occurred, it was quickly overprinted by ductile deformation processes that enabled slip along discrete mylonitic shear bands at greenschistfacies conditions. Local, metasomatic reactions produced muscovite and chlorite that principally formed in regions of high strain, and also produced quartz that accumulated in ribbons and tails. Thus the distribution of reaction products was strongly influenced by local compressive stresses that especially led to the diffusive mass transfer of SiO 2 into quartz ribbons and porphyroclast pressure shadows. Preferential alignment of muscovite, biotite and chlorite within shear bands presumably facilitated grain boundary sliding, and the alignment of biotite within shear bands may have been enhanced by dissolution precipitation processes. Quartz ribbons and tails also exhibit evidence of dislocation creep and grain boundary migration. Thus, simultaneous chemical and mechanical processes acted to produce softening of the Tres Arboles rocks and enabled at least local reactivation of the fault zone along a network of discrete shear bands under greenschist facies metamorphic conditions. IMPLICATIONS OF THIS WORK FOR DUCTILE DEFORMATION PROCESSES The classic Sibson Scholtz crustal model for fault zones (Sibson, 1977; Sibson 1986; Scholz, 1990) depicts a ductile deformation zone of increasing thickness below the brittle ductile transition. However, this

13 REACTION SOFTENING IN REACTIVATED MYLONITES 423 model does not fully address the mechanisms and processes that facilitate such an increase in fault zone width. More recent work has re-interpreted the probable depth of the brittle ductile transition (Imber et al., 2001) and addressed the driving mechanisms for fault zone reactivation (Imber et al., 2001; Holdsworth, 2004). In this contribution we have focused on syntectonic reaction processes and provided a conceptual model that may at least partially explain fault zone widening at mid-crustal depths and the formation of discrete overprinting shear bands near the brittle ductile transition. Given the abundant documented evidence of fault zone reactivation in many locations around the world (e.g. Holdsworth et al., 2001) we suggest that reaction-facilitated deformation processes, such as those described here, are significant factors in the formation of most mylonitic fault zones. The ideas expressed in this contribution are testable in a variety of exhumed fault zone settings, and it is suggested that continued investigation into syntectonic reaction processes will provide an important addition to the more extensive historical focus on mechanical deformation processes. ACKNOWLEDGEMENTS The authors thank C. Simpson for helpful discussions and introduction to the Tres Arboles fault zone. Microprobe analyses were conducted on a Cameca SX50 electron microprobe at Indiana University with assistance from C. Li. This manuscript benefited from reviews by G. Solar, R. Holdsworth and an anonymous reviewer. The research was partially supported by NSF grants EAR to C. Simpson and EAR to R. P. Wintsch. REFERENCES Boullier, M. T. & Gueguen, Y., SP-mylonites: origin of some mylonites by superplastic flow. Contributions to Mineralogy and Petrology, 50, Carmichael, D. M., On the mechanism of prograde metamorphic reactions in quartz bearing pelitic rocks. 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