The Karakoram fault zone is a dextral strike-slip fault bounded by the Pangong and Tangtse strands on its NE and SW flanks, respectively. In the Tangtse shear zone, the microstructures of mylonitic leucogranite exhibit superposition of high-temperature deformation followed by low-temperature deformation. The mylonites show fluid immiscibility, containing brine and carbonic inclusions. The occurrence of carbonic- and brine-rich inclusions in the oscillatory-zoned plagioclase indicates that they were trapped during the formation of the leucogranite. Eventually, these fluids recorded a near-isobaric drop in temperature down to <450 °C at the amphibolite-greenschist facies transition, when the zone of fluid mixing was established. The 40Ar-39Ar biotite ages indicate that the area cooled down to 400–350 °C over 10.34–9.48 Ma, and this period also coincides with a major phase of fluid infiltration and trapping of secondary reequilibrated carbonic and saline-aqueous inclusions. The 10.34–9.80 Ma period recorded a low-temperature deformation at greenschist conditions, when the involved fluid evolved following a near-isobaric path at ∼2 kbar. Subsequently, between 9.80 Ma and 9.48 Ma, the sudden drop in pressure (1.75–0.5 kbar) caused by mylonites produced reequilibrated fluid inclusion textures. These observations suggest that the Karakoram fault zone rocks show a single progressive deformation event with bimodal fluid evolution, in which the carbonic- and brine-rich inclusions were available prior to high-temperature deformation during the initiation of the Karakoram fault zone. The trapping of secondary inclusions between 10.34 Ma and 9.48 Ma with pressure decrease of ∼2–0.5 kbar yields an average uplift rate of 1 mm yr−1 for the Karakoram fault zone.
On account of their spatial relationship with crustal-scale shear zones, granitic rocks are often used as a marker for regional deformation events (Stöckhert et al., 1999; Vigneresse, 1999; Rosenberg, 2004). However, the utilization of granites or granitic mylonites to infer the exhumation history along a fault zone is far from straightforward. The lack of metamorphic index minerals makes pressure-temperature (P-T) estimation of deformation in granitic rocks a difficult task. For instance, S-type granites and leucogranites that are devoid of hornblende do not have a suitable barometer. To overcome these difficulties, a multiparametric approach is warranted that consists of microstructural studies, fluid inclusion analyses, and thermochronology. Deformation is preserved in granite by way of microstructures, which are related to different temperature conditions from crystallization to solid state to low-temperature brittle events (Vernon, 2000; Pawley and Collins, 2002). In the past, quartz and feldspar microstructures in granitic mylonites have been successfully used to infer the conditions of fabric formation and deformation of granitic rocks from midcrustal shear zones (Piazolo et al., 2002; Rosenberg and Stünitz, 2003; Ree et al., 2005; Pennacchioni et al., 2006; Ishii et al., 2007). The understanding of deformation conditions can be enhanced by fluid inclusion studies. Fluid inclusions, especially the reequilibrated ones, provide a fair estimate of P-T conditions of deformation when integrated with microstructures (Boullier, 1999; Lespinasse, 1999). The role of fluids in the evolution of fault zones has been investigated by various workers in order to reveal the depth of initiation of faults and to further constrain their tectonic evolution (O’Hara and Haak, 1992; Wawrzyniec et al., 1999). Once the conditions of progressive deformation of a fault zone are well established, the timing of tectonic activity can be constrained by thermochronology (40Ar-39Ar chronology). In this way, an evaluation of the role of fluids in the evolution of a fault zone in conjunction with 40Ar-39Ar chronology and microstructures can give valuable insights into the tectonics of a given area. In the present study, we investigated the mylonitized leucogranites of the Karakoram fault zone of the trans-Himalayan region using microstructural studies, fluid inclusion analyses, and thermochronology.
The dextral strike-slip Karakoram fault zone strikes NW-SE and extends from central Pamir to Gurla Mandata in southern Tibet. The fault cuts through rocks of the southern margin of Asia as well as in Karakoram and SW Tibet. The tectonic framework of the Karakoram fault zone is marked by two ∼500-m-wide shear zones marked by SW and NE splays, called the Tangtse and Pangong strands, respectively (Fig. 1). Along the Tangtse strand, mylonitized leucogranites are present, and their relationship with the Karakoram fault zone is debated. According to Phillips and Searle (2007), these mylonitized leucogranites exhibit evidence of noncoaxial deformation at greenschist to lower-amphibolite facies and are devoid of any submagmatic microstructures. Hence, they are not syntectonic with the activation of the Karakoram fault zone. There are arguments that the two strands of the present study area cut across all the leucogranites, and therefore these leucogranites were emplaced prior to activation of the Karakoram fault zone (Searle et al., 1998; Phillips et al., 2004; Searle and Phillips, 2009). On the other hand, Lacassin et al. (2004a) ascertained the presence of ca. 23 Ma synkinematic leucogranites in the Tibetan part of the Karakoram fault zone (Zhaxikang). These workers argued that the presence of a pervasive S-C fabric, combined with 40Ar-39Ar thermochronology, clearly indicates a high-temperature ductile shear regime at ca. 19 Ma (see also Lacassin et al., 2004b). Valli et al. (2008) provided U-Th-Pb data from the North Ayilari dextral shear zone, which is a part of the Karakoram fault zone, and envisaged a ca. 25–22 Ma magmatic event synchronous with dextral shearing. Weinberg et al. (2009) provided mesoscopic evidence from the Tangtse area that suggests shear-induced melt migration and ponding. According to them, leucogranitic magma accumulated due to shearing and anatexis within the Karakoram fault zone (see also Weinberg and Mark, 2008). Leech (2008, 2009) suggested that the penetrative Karakoram fault zone acted as a vertical conduit for the ascent of the leucogranitic melt. In view of these previous studies, and especially the mesoscopic observations of Weinberg and Mark (2008) and Weinberg et al. (2009), it can be stated that the leucogranites of the Karakoram fault zone are more likely to be syntectonic than pretectonic. In the following section, we provide some mesoscopic evidences that support this notion (Fig. 2).
The leucogranites of the Karakoram fault zone give a crystallization age of ca. 15 Ma (Phillips et al., 2004). These granites show evidence of dextral shearing related to the Karakoram fault zone. Dunlap et al. (1998) evaluated the postmetamorphic thermal history of the Pangong strand from 40Ar-39Ar chronology of muscovite and biotite of quartzofeldspathic migmatite and carbonate mylonites. He obtained a plateau age of 10.8 ± 0.1 Ma (muscovite) and 9.7 ± 0.1 Ma (biotite). However, indirect argon ages for the Tangtse strand (shear zone) obtained from the Ladakh granite suggest cooling through 350 °C at ca. 11.9 Ma (Dunlap et al., 1998). These granites were emplaced and then subsequently deformed and mylonitized due to repeated tectonic activity along the Tangtse strand and the Karakoram fault zone as a whole (Searle et al., 1998; Dunlap et al., 1998; Phillips et al., 2004; Sen et al., 2009). In the past, investigations were carried out on these leucogranites to decipher their age of crystallization, kinematics of deformation, and thermochronology (Searle et al., 1998; Dunlap et al., 1998; Phillips et al., 2004). Searle et al. (1998) carried out Ar-Ar thermochronology on muscovite from the deformed leucogranite of the Tangtse strand and determined a plateau age of 11.4 ± 0.1 Ma. Dunlap et al. (1998) obtained an age spectrum of 7.5–13 Ma on K-feldspar from the leucogranite of the same suite. However, there still remains a gap regarding the tectonic history of these leucogranites in terms of correlation among their fabric, fluid evolution, temperature, and timing of deformation. Since these leucogranites are directly linked to the deformation of the Karakoram fault zone, P-T and time (t) estimates of this suite alone will give the best idea about the tectonics and fluid evolution of the Tangtse strand and the Karakoram fault zone. In the present study, we used a combined approach including microstructures, fluid inclusions, and 40Ar-39Ar thermochronology to envisage the broader picture of the evolution of the Karakoram fault zone.
GEOLOGY OF THE STUDY AREA
The current plate motion reveals that the rate of maximum convergence between India and stable Asian or Tibetan crust is around 44 ± 5 mm/yr (Nakata, 1989) in the western Himalayan region. The postcollisional convergence of the Indian plate is believed to have been accommodated by the eastward extrusion of Tibet along the dextral strike-slip Karakoram fault zone (Molnar and Tapponnier, 1975; Searle, 1996). The 700-km-long Karakoram fault zone bounds the southwest boundary of the Tibetan Plateau, while in the north, it is bounded by the 1500-km-long Altyn Tagh fault (Figs. 1A and 1B). Armijo et al. (1989) suggested that the southern limit of extruding Tibetan crust occurs along a zone of right-lateral strike-slip faults in southern Tibet, which link the Karakoram fault zone with the Jiale fault. These two faults help in extruding Tibetan crust from depth to the surface (Molnar, 1984; Armijo et al., 1989). In the eastern part, the Karakoram fault zone is divided into two strands, namely, the Tangtse strand to the SW and the Pangong strand to the NE (Fig. 1C) (Rutter et al., 2007; Phillips, 2008). These two strands bind the Pangong transpressional zone. Phillips et al. (2004) reported two sets of leucogranitic dikes within this transpression zone, one of which is intensely mylonitized and the other of which is less deformed. The former has a mylonitic foliation concordant with the strike of the Karakoram fault zone, and the younger one crosscuts the regional fabric of the shear zone. U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) data of Phillips et al. (2004) indicate that the shear zone was initiated between 15.68 ± 0.52 Ma and 13.73 ± 0.28 Ma. These workers further calculated an offset of 40–150 km with an average slip of 2.7–10.2 mm/yr. According to them (see also Phillips and Searle, 2007), the low slip rate and limited offset indicate that large-scale uplift of Tibet through a fault such as the Karakoram fault zone is not possible.
The Karakoram fault zone is characterized by the presence of four main lithologies: (1) hornblende-biotite diorite, granodiorite, and granite, (2) the Baltoro granite, which is a sequence of monzogranite to two-mica granite, (3) the Karakoram metamorphic complex, consisting of garnet-staurolite schist, marbles, and amphibolites, and (4) the mylonitized leucogranite, which is the youngest of all the units. The transpressionally uplifted Pangong range consists of staurolite-grade metamorphic rocks, orthogneiss, and migmatite. According to Streule et al. (2009), this metamorphic event occurred at 108 ± 0.6 Ma, and it predates the collision and accretion of the Indian plate with Asia.
Along its NE margin with the Ladakh Batholith, the leucogranites of the Tangtse strand show intense mylonitization attributed to noncoaxial dextral shear. It is bounded by hornblende-biotite–bearing Ladakh granite to the southwest and Baltoro-type two-mica granite to the northeast (Fig. 1C). The hornblende-bearing Ladakh granite is deformed and mylonitized near the Tangtse strand and has developed tectonic foliation parallel to the strike of the Tangtse strand (Fig. 2A). Leucogranites have intruded these mylonites, and they also show concordant foliation with those of the Ladakh granite and the Tangtse strand (Fig. 2A). The Ladakh granite is further migmatized, and along its foliation, leucogranite has intruded. This leucogranite also contains a metamorphosed mafic enclave that initially was part of the Ladakh granite (Fig. 2B). There is also presence of leucosomes that are connected through boudin necks to the deformed Ladakh granite, indicating in situ partial melting during deformation (Fig. 2C). At some places, alternating layers of leucosomes and melanosomes are dragged into minor shear zones, and leucocratic melts have migrated through these melt channels (Fig. 2D). All these mesoscopic evidences indicate that formation and migration of leucocratic melts were synchronous with the deformation of the Ladakh granite, which was triggered by noncoaxial deformation along the Tangtse strand of the Karakoram fault zone.
In the Tangtse area, besides leucogranite, interlayered marbles, calc-silicates, granitic pegmatite, and boudins of amphibolitic gneisses are present. The pegmatites intruded the carbonates before or during the deformation in the Karakoram fault zone.
The microstructures of the mylonites were documented by conventional optical microscopy combined with backscattered electron (BSE) microscopy. Energy dispersive spectra (EDS) patterns were obtained using a Zeiss EVO 40EP scanning electron microscope at the Wadia Institute of Himalayan Geology, Dehradun, India, using an accelerating voltage of 20 kV and beam current of 3–6 nA. Element profiling of feldspars (GSA Data Repository Fig. DR11) was analyzed by EDS attachment using the QUANTAS software.
The mineral chemistry for biotite (Table 1) was measured with a CAMECA SX-100 electron microprobe at the Wadia Institute of Himalayan Geology with 15 kV acceleration voltage and 15 nA beam current. The acquisition time was 20 s for each element, and the beam diameter was 5 μm. Natural and synthetic oxides were used as standards for major elements.
The microthermometric observations and measurements of fluid inclusions were carried out on samples from the mylonitized leucogranite in order to constrain the P-T conditions of mylonitization and also to trace the exhumation history along the fault. The microthermometric analysis (Table 2) was done on a LINKAM heating-freezing stage calibrated to ±0.1 °C, −56.6 °C, 0 °C, and 374.4 °C. The precision of measurement was ±0.2 °C.
Laser Raman spectroscopy was chosen to confirm the presence of any carbon phase (? CH4-CO2) by using the Horiba Jobin Yvon Micro Raman Laser at the Wadia Institute of Himalayan Geology. Spectra were excited with an Ar ion laser at 514.54 nm through a 100× objective. The laser on the inclusion surface had a spot diameter of ∼2 μm and a power of 4 mW with dwell time of 5 s.
The 40Ar-39Ar chronology of the selected samples was conducted at the Isotope Laboratory of the Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, USA. The 40Ar-39Ar ages for each sample were measured using incremental heating experiments on multigrain samples. Methodology and procedural steps were adopted from Hodges et al. (1994) and Wobus et al. (2008).
The age gradients have been interpreted in terms of cooling rates, revealing the relationship between temperature and time. The plateau ages were plotted for help in interpretation. Inverse isochron plots were drawn for all the samples (GSA Data Repository Fig. DR2 [see footnote 1]).
The Tangtse mylonites are quartzofeldspathic with some amount of biotite, white mica, and chlorite. Plagioclase porphyroclasts (400–1000 μm in size) are included in a matrix of quartz, K-feldspar, plagioclase, and mica. The plagioclase porphyroclasts are subhedral in shape and usually show compositional and oscillatory zoning (Fig. 3A). Plagioclase and K-feldspar show evidence of grain boundary migration and recrystallization (Fig. 3B). Quartz grains show evidence of subgrain rotation (SGR) (Fig. 3C) and formation of stretched “ribbons” (Fig. 3D). Evidence of bulging recrystallization is also common in quartz. Recrystallized grains are present within the matrix and in the pressure shadow zones of the porphyroclasts, with the formation of “core and mantle” structure at places. The fabric present within the biotite indicates a variable amount of solid-state deformation and recrystallization. Kinking is very common, along with formation of “mica fish” (Fig. 3E). The biotite grains associated with recrystallized plagioclase grains show aggregates of slightly misoriented cleavage fragments. Some strongly deformed biotite aggregates appear to have been “shredded” into aggregates of cleavage fragments (Fig. 3F). The deformation and area reduction of the plagioclase porphyroclasts have taken place mainly by fracturing and replacement by K-feldspar (Fig. 3G). The BSE images show crystallization of K-feldspars along the rims, fractures, and pressure shadow zones of plagioclase porphyroclasts (Fig. 3H). The interstitial spaces between two porphyroclasts have also been replaced by K-feldspars.
Fluid Inclusion Study
Fluid activity plays a major role in controlling the development of fabrics of crustal rocks. Different rock types in crustal shear zones incorporate different sources of fluids and recycling volatiles (McCaig, 1988). This leads to a broad range of fluid composition, from aqueous liquid to gaseous phases, at all P-T conditions. Using the assigned deformation patterns discussed earlier, quartz and plagioclase of the Tangtse leucogranite were selected for fluid inclusion study. Primarily on the basis of compositional variation, the fluid inclusions were classified broadly into two groups: carbonic and saline-aqueous (NaCl-H2O). Texturally, the fluid inclusions show monophase (vapor [V]), biphase (liquid [L] + V), and triphase (solid [S] + L + V). The triphase NaCl daughter crystal coexisting with H2O-NaCl (no carbonic phase was observed by optical microscopy) inclusions and monophase CO2-rich inclusions are noted in a plagioclase grain from the Tangtse leucogranite (Figs. 4A, 4B, and 4C). They are invariably present in isolated form, are small in size (∼5 μm), and occupy the core part of the host mineral. These are recognized as primary-type fluid inclusions. In rare observation, the biphase NaCl-H2O fluid inclusions may act as a primary-type fluid (Fig. 4D). They are often present in the form of small isolated clusters, which lack any preferred orientation and are mainly restricted at or the around central part of the host mineral. The trail-bound reequilibrated CO2-rich fluid inclusions within recrystallized quartz are considered as primary trails with respect to the host mineral (Van Den Kerkhof and Hein, 2001). The fluid inclusions in these trails are minute and found to be associated with fractures along grain and subgrain boundaries in local domains. The physical appearances of carbonic trails within a single grain boundary suggest their primary or pseudosecondary nature. The NaCl-H2O–rich secondary inclusions are seen to be unevenly distributed in the matrix quartz (Figs. 4E, 4F, 4G, and 4H). They display both elongated and irregular-shaped inclusions containing two phases, liquid and gaseous, in varying ratios. Nevertheless, the rare monophase fluid inclusions are also observed within the mineral. The secondary NaCl-H2O–rich fluid inclusions occur in various sets of trails with different orientations (Fig. 4F). These trails are often found parallel to mineral foliation, in mineral microfractures, and they are either transgranular or clustered. In those clusters, various types of fluid inclusion texture show reequilibration. The reequilibration texture is a usual feature that is observed in primary as well as in secondary inclusions of NaCl-H2O, caused by a pronounced effect of deformation. Deformation features such as stretching, necking, elongated cavities, implosion-type textures, and annular-shaped morphology are commonly observed in Tangtse mylonites (Figs. 4E, 4F, 4G, and 4H). The CO2-H2O-NaCl fluids with two distinct phases are observed in very rare cases, which are only present in recrystallized parts of minerals. Due to scarcity and the extremely small size of these inclusions, only their petrographic observation has been incorporated in the present study.
Fluid inclusions microthermometry was carried out on doubly polished, ∼150-μm-thick rock wafers. During routine thermometry of fluid inclusions, the freezing and heating data were interpreted in terms of density, salinity, and the chemical composition of the fluid inclusions. More precisely, the study of Kooi et al. (1998) has been cited for the composition and density of carbonic-rich inclusions. The salinity of aqueous inclusions was calculated based on the program FLINCOR (Brown, 1989). A few confirmatory tests were also performed through Raman laser on minor phases like CO2. A summary of microthermometry values is shown in Table 2 and Figure 5, which also contains the abbreviations used in the text for phase transition temperature. The microthermometry run for the triphase NaCl-H2O–rich inclusions could not be carried out due to their extremely small sizes. The carbonic inclusions show large variation in Tim (temperature of initial melting) for CO2 of the primary type as compared to the reequilibrated one. The applied confirmatory test through Raman laser shows the presence of dense CO2 but does not show any other component, such as CH4, nor does it reveal any signature of another volatile component. The Tim CO2 values of −53.7 °C to −56.5 °C, and ThL (temperature of homogenization of liquid) CO2 values of −2 °C to −4.5 °C are shown for primary carbonic inclusions, whereas Tim CO2 values of −55 °C to −56.5 °C and ThL CO2 values of −3.2 °C to −4 °C were observed for reequilibrated carbonic inclusions and trails. The saline-aqueous fluid inclusions (H2O-salt) can be grouped into two types, which are based on mode of occurrences of fluid inclusions and their Te (eutectic temperature) and Tm (melting temperature) values. The eutectic Te values for isolated H2O-NaCl inclusions show narrow ranges, −20 °C to −25 °C, and final melting Tfm (temperature of final melting) of −2 °C to −6 °C and Th (temperature of homogenization) between 170 °C and 270 °C. The corresponding salinity ranges are estimated at 2.5–10 wt% NaCl equivalent, with density of 0.83–0.96 g/cm3. The secondary reequilibrated type inclusions occur in clusters and in transgranular trails. They show a wide range of Tfm values (−2 °C to −9 °C), Te (−20 °C to −26 °C), and Th (182 °C to 300 °C). The corresponding salinity ranges are estimated at 5–13 wt% NaCl equivalent, and density is in the range of 0.80–0.95 g/cm3. The clathrate melting temperature of the ternary system H2O-NaCl-CO2 is observed at around 8 °C.
The 40Ar-39Ar isotopic ages in K-rich minerals (mica and biotite) are used to infer the cooling history of rocks by deciphering their different closure temperatures (Hodges et al., 1994). This 40Ar-39Ar thermochronology analysis was applied to the biotite of the Tangtse mylonites to construct the time-temperature history of Karakoram fault zone. The integrated typical closure temperature for Ar in biotite is ∼300–450 °C (Dodson and McCellend-Brown, 1985; Verschure et al., 1980). The studied microstructure of biotite from the polydeformed Tangtse area, however, shows heterogeneous recrystallization and metamorphic overprinting. The electron probe data of such biotites display elemental zoning and a progressive increase of XMg from core to rim from 0.31 to 0.35, whereas XFe shows an opposite distribution from 0.64 to 0.68. The reverse pattern is observed in Ti composition at the core, which is 0.46, as compared to 0.15 at rim (Table 1). The deformed biotite shows kinking, a dislocation network, and compositional variations in response to changes in P-T and fluid composition during mylonitization at low-grade metamorphism (<450 °C). The isotopic and compositional heterogeneities in biotite are reflected in the 40Ar-39Ar step-heating spectra (Figs. 6A, 6B, and 6C) (Scaillet et al., 1990). Sample SSZ-04 from the Tangtse strand of the Karakoram fault zone shows flat plateau ages at 10.34 ± 0.06 Ma (SSZ-04, 70% 39Ar release) in the higher-temperature step, and SSZ-03 shows plateau ages at 9.80 ± 0.07 Ma (SSZ-03, 80% 39Ar release) in the lower-temperature step. On the other hand, sample SSZ-05 shows staircase-type 40Ar-39Ar age spectra at 9.48 ± 0.28 Ma (SSZ-05, 65% 39Ar release), which involves heterogeneity in temperature.
To understand the exhumation history in terms of pressure, temperature, and time, we focus on the mylonitized leucogranites of the Tangtse strand using the integrated approach with microstructures, fluid inclusion assemblages, and thermochronology data. In this section, we attempt to discuss our results and infer their tectonic significance by comparing and integrating our results with those of previous workers (Searle et al., 1998; Dunlap et al., 1998; Weinberg and Searle, 1998; Bhutani et al., 2003; Phillips et al., 2004; Weinberg et al., 2009).
The microstructures of the mylonites reveal superposition of solid-state deformation fabric over a primary magmatic fabric. Presence of oscillatory zoning in the plagioclase porphyroclasts indicates the mylonites are product of an igneous protolith (Fig. 3A). Quartz microstructures vary considerably, depending on the temperature and strain rate of solid-state deformation (Stipp et al., 2002; Piazolo et al., 2002). The presence of monocrystalline platten quartz in the recrystallized quartz-feldspar matrix indicates occurrence of a high-temperature solid-state deformation event. The presence of lobate grain boundaries in feldspar (Fig. 3B) also suggests the same (Rosenberg and Stünitz, 2003). Subgrain rotation in quartz indicates recrystallization at a temperature of 400–550 °C. On the other hand, the presence of stretched quartz ribbons and bulging recrystallization in these mylonites indicate deformation at a relatively low temperature (280–400 °C) (Stipp et al., 2002) (Fig. 3C). The replacement of plagioclase by K-feldspar and crystallization of K-feldspar along the fractures can also be attributed to deformation under greenschist-facies conditions (Fig. 3F) (Ree et al., 2005; Ishii et al., 2007). Presence of small fragments and misorientation along cleavage planes of biotite indicate brittle deformation (Vernon et al., 2004). From the microstructures of these mylonites, it is clear that there is a superposition of high-temperature solid-state deformation followed by low-temperature solid-state deformation over a preexisting magmatic fabric.
Phillips and Searle (2007) provided a detailed documentation of microstructures of the leucogranites of the Karakoram fault zone. The medium- to low-temperature deformation features we described here are very similar to their findings. Due to scarcity of magmatic and submagmatic textures, Phillips and Searle (2007) concluded that these leucogranites are post-tectonic with respect to shearing along the Karakoram fault zone. However, our study shows the presence of oscillatory-zoned plagioclase within a deformed and sheared matrix. This suggests that the magmatic fabric of these leucogranites is preserved. Even the plagioclase laths that define the tectonic foliation retain its magmatic texture in the form of oscillatory zoning in their core. The presence of zoned plagioclase crystals defining a fabric parallel to the Karakoram fault zone suggests that this leucogranite is synkinematic with the Karakoram fault zone (Paterson et al., 1998). Contrary to Phillips and Searle (2007), we suggest that magmatic and submagmatic microstructures within this leucogranite are scarce because this granite has undergone repeated tectonic activity along the Karakoram fault zone and its pristine igneous features have been obliterated and overprinted by solid-state deformation fabric of various temperature ranges, as also inferred by Weinberg et al. (2010).
The variation in Fe, Mg, and Ti contents of biotite from core to rim may also be attributed to this superposition of fabrics at different P-T conditions (Table 1). We further constrain and evaluate this superposed fabric with the help of fluid inclusion studies in the following section.
Stages of Fluid Entrapment
We investigated the fluid inclusions and their stages of entrapment through their physical properties, mode of occurrence, and chemical composition. Based on fluid inclusion petrography, we can infer that the fluid inclusions entrapped in the Karakoram fault zone rocks fall in two stages: (1) fluid inclusions restricted to the core of the plagioclase and (2) fluid inclusions in the matrix quartz. This reveals that the fluid inclusions entrapped in the core of the host mineral belong to leucogranite crystallization, whereas fluid inclusion trails along mineral grains and as transgranular inclusions were entrapped and reset by subsequent low-temperature deformation during the last stage of Karakoram fault zone exhumation.
The first stage of entrapment of carbonic and primary tri- and biphase saline-aqueous inclusions was restricted to the undeformed zone of the host mineral, i.e., the cores of the quartz and plagioclase, and they are mostly isolated and tiny and underwent partial reequilibration, which entrapped the inclusions at ∼4 kbar and ∼650 °C. These saline-aqueous and carbonic fluid inclusions show early (primary) entrapment during plagioclase crystallization. The two (carbonic and saline) fluids occur together and are inferred as immiscible at the prevailing P-T conditions. However, there are evidences of a primary saline-aqueous (tri- and biphase) system showing partial miscibility at ∼650 °C involving vapor and saline liquid phases (Sourirajan and Kennedy, 1962). In rare observation, another intermediate stage of fluid entrapment could also be identified in the form of reequilibrated trails of carbonic inclusions. On the other hand, a second stage of entrapment by secondary saline-aqueous fluid inclusions shows dominance of crustal fluid influxes in the system. Based on their mode of occurrence, the secondary saline-aqueous inclusions are further classified into cluster, transgranular, and crosscutting trails. These show a wider range of salinity and were likely entrapped at ∼2 kbar and <450 °C. In another observation, the mixed fluid (carbonic-saline-aqueous) usually occupies the transgranular or fracture zone of the mineral, which was possibly entrapped at ∼450 °C.
Fluid History of Karakoram Fault-Zone Rocks
The fluid evolution during deformation in the Karakoram fault zone can be constrained by a combination of geochronological data and mineral microstructures and comprehensive study of multiple fluid phases. Figure 7 shows typical compositional ranges of H2O, NaCl, and CO2 in widely varying P-T conditions. Fluid inclusion investigations from Tangtse mylonites show the significant role of the host minerals regarding fluid immiscibility. These immiscible fluids are preserved within plagioclase and quartz grains. The plagioclase grains preserve magmatic fabric in the form of oscillatory zoning (Fig. 3A), and this possibly formed during crystallization at a temperature of 600–680 °C (Fig. 7). The occurrence of brine and carbonic-rich inclusions (Figs. 4A, 4B, and 4C) implies that the fluid system was immiscible and fluids were trapped during plagioclase (leucogranite) crystallization at ∼650 °C and ∼4 kbar (Heinrich, 2007). The preserved brine and the carbonic-rich fluid system are assumed to be derived from calc-alkaline magma infiltrating fluids (Heinrich, 2007). Later on, these inclusions partially adjusted their densities during the mylonitization process. As a result of this, the estimated trapping condition for brine and carbonic inclusions occupies much lower P-T space, at ∼500 °C and 3 kbar (Fig. 7). Consequently, because of local scale inhomogeneity, the carbonic inclusion trails are observed in intracrystalline and fracture-filled zones, when their composition and density continually evolved (Bodnar, 2003). These processes apparently occurred when Tangtse mylonites underwent the brittle-ductile transition at amphibolites-facies metamorphism. This was the region of the first stage of fluid evolution, where the leucogranites cooled from 650 °C to 450 °C at 4–2 kbar.
This event can be correlated with the U-Pb geochronology data of Phillips et al. (2004), which indicate an event of crystallization and emplacement of these Tangtse leucogranites at ca. 15.7–13.7 Ma. By comparing our findings with those of Phillips et al. (2004), the magmatic fabric and high-temperature deformation fabric preserved in the plagioclase and platten quartz indicate a crystallization and ductile deformation event at ca. 17–13 Ma at a temperature of ≥500 °C.
During continuous mylonite deformation under amphibolite to greenschist metamorphic conditions (Maruyama et al., 1983), the biotites from the Tangtse shear zone show argon loss due to thermally activated volume diffusion, causing a decrease in apparent ages (Goodwin and Renne, 1991). Consequently, the data indicate that microstructures of biotite such as kink and dislocation networks are likely to record mylonite deformation under greenschist-facies conditions (∼450 °C). At this stage, the deformed mylonite became ductile due to excess inflow of syntectonic saline and carbonic fluids, and this established the isotopic and compositional heterogeneity reflected in the 40Ar-39Ar step-heating spectra (Scaillet et al., 1990). Kramar et al. (2003) demonstrated that argon diffusion in deformed micas is controlled by deformation-generated microstructures such as lattice-modifying structures and high-angle kink. In another observation, Dunlap (1997) inferred the time constraints in mylonite deformation through microstructural characterization to distinguish the inherited and neocrystallized mica.
In this paper, the 40Ar-39Ar system in biotite has been addressed to observe the effect of progressive mylonitization at low-temperature deformation conditions. This is consistent with the closure temperature of biotite (450–350 °C) (Dodson, 1973). This temperature range can be regarded as the stage of introduction of heterogeneous fluid flow in the system, and this is inferred as a fluid mixing stage. The 40Ar-39Ar biotite data from mylonitized leucogranites record young ages of 10.34–9.48 Ma (Figs. 6A, 6B, and 6C). During the last phase under greenschist-facies conditions (∼450 °C), the Tangtse shear zone became a prime candidate for fluid flow, either through deformation or by alteration of the mineral assemblages in the mylonitized leucogranites. Eventually, at ca. 10.34 Ma, the infiltration of secondary reequilibrated carbonic and saline fluid inclusions is inferred to have taken place. The compositional differences between these fluids and the associated metamorphic system indicate the presence of considerable amounts of salinity and CO2, which played a significant role in the fluid immiscibility in the Tangtse mylonites. This clearly shows that the two phases of fluid could coexist and were widespread during the last phase of Karakoram fault zone exhumation. Consequently, in the Tangtse mylonites, the saline-aqueous mixture became the most abundant secondary fluid in the system, and its entrapment conditions are estimated to be less than 400 °C and ∼2 kbar over 10.34–9.80 Ma (Figs. 6A, 6B, and 7), whereas presumed trapping conditions for reequilibrated saline and carbonic inclusions (Fig. 7) are supposedly plotted at >500 °C and ∼4 kbar. Bulging and “ribbon” microstructures in quartz indicate that their temperature of deformation falls in the 280–400 °C range (Stipp et al., 2002). The saline and carbonic inclusions within matrix quartz show an area of overlap for the secondary inclusions, and the temperature ranges for recrystallization and deformation of quartz define a P-T range of 300–400 °C at 2 kbar (Fig. 7). At this stage, the P-T path follows a near-isochoric path, indicating that the initial exhumation of the Karakoram fault zone was relatively slow over 10.34–9.80 Ma. Due to continuous influence of crustal fluids and hydration reactions, the Tangtse mylonites yielded a low-argon mica fluid partition coefficient (Kelley, 2002). This phenomenon is applied to explain the 9.8–9.48 Ma period when Tangtse mylonites suffered an isothermal drop in pressure at <375 °C, and this favors the production of reequilibrated fluid textures. Necking, fractures, stretching of cavities, satellite and annular inclusions (Figs. 4E, 4F, 4G, and 4H) are commonly observed. The majority of biphase (saline-aqueous) inclusions reflects necking and stretching (Figs. 4C and 4E), perhaps governed by reequilibration through high internal overpressure (2.8 kbar). The annular morphology shows that their formation was subjected to reequilibration through internal underpressure (Boullier et al., 1991). This type of texture is otherwise supported by a compressive loading P-T path at the internal underpressure of 0.8 kbar at 400–450 °C, as shown by Boullier et al. (1991) in natural quartz from the Higher Himalaya. The imprint of internal overpressure in the inclusions could be seen through satellite and decrepitation texture (Fig. 4). They were formed due to isothermal decompression (Boullier et al., 1991; Vityk and Bodnar, 1995). The isothermal decompression from 1.2 to 0.8 kbar (Fig. 7) is shown by Tangtse mylonites, and it is also reflected in irregular-shaped staircase-type 40Ar-39Ar spectra followed by initial flat-plateau spectra (Fig. 6). These indicate that a period of rapid exhumation prevailed over 9.8–9.48 Ma and below. This periodic exhumation (cooling) of mylonites in the Karakoram fault zone has also been observed by Dunlap et al. (1998). They envisaged an initial exhumation related to oblique thrusting at amphibolites-facies conditions at ca. 17–13 Ma and another event of oblique thrusting at greenschist-facies condition at ca. 8–7 Ma. They inferred that there was a prolonged interval between these two events dominated by strike-slip motion. However, our study shows that deformation at amphibolites-facies condition prevailed until ca. 10.34 Ma. Moreover, we found no prolonged hiatus between the two events of deformation, and the change from amphibolite to greenschist facies of deformation seems more like a transitional one. Secondly, Dunlap et al. (1998) obtained a deformation age of 9.7 ± 0.1 Ma on biotite from the quartzofeldspathic migmatite of the Pangong strand. By comparing this with our data, we can infer that both the Tangtse and Pangong strands experienced cooling through ∼350 °C at the same time (ca. 9.7–9.8 Ma), and there was no partitioning of deformation between these two strands until ca. 9.7 Ma. The study of Rutter et al. (2007) suggests active tectonics in the Pangong strand of the Karakoram fault zone until ca. 3 Ma. Hence, we can infer that the evidence of rapid exhumation showed by fluid inclusion morphology may have occurred and/or continued until a very late stage, and it postdated the youngest Ar-Ar date (9.48 Ma) obtained in this study.
The Karakoram fault zone is kinematically related along strike with the N-S Yaden-Gurla graben, which has accommodated E-W extension in southern Tibet (Searle, 1996). The rapid exhumation of the Ladakh Batholith during the Oligocene–Miocene period is also correlated with the uplift of the Tibetan Plateau and initiation of the Karakoram fault zone (Kirstein et al., 2009). Edwards and Harrison (1997) revealed that a part of south Tibetan crust was exhumed between 12.5 Ma and 8 Ma. During this interval, there was no evidence of rapid cooling or transpressive motion. The Yaden-Gurla graben is the largest graben in southern Tibet. Here, cooling took place at ca. 8 Ma (Harrison et al., 1995). The development of this structure in southern Tibet coincides with the transpression-related rapid isothermal exhumation of the Karakoram fault zone rocks that started ca. 10.4 Ma and continued to 6 Ma. In support of this view, the tectonothermal history, combined with fluid-evolution path, of the Karakoram fault zone indicates a single progressive deformation event with two stages of fluid evolution. During the first stage, the carbonic- and brine-rich inclusions were the dominant fluids available prior to high-temperature deformation during initiation of Karakoram fault zone. While during the second stage, the trapping of secondary reequilibrated carbonic and saline-aqueous inclusions took place between 10.34 Ma and 9.48 Ma. Consequently, the mica ages of Karakoram fault zone rocks reflect the time at which the source rock was exhumed from ∼8 to 10 km depth to the surface following an average geothermal gradient of 50 °C. The calculated average exhumation rate of Karakoram fault zone rocks is 1 mm yr−1. After 10.34 Ma, the rate of exhumation became comparatively slower. However, at later stages, near the surface, the rate of exhumation became much faster.
From the present study of the Tangtse mylonites of Karakoram fault zone, the following conclusions can be drawn:
(1) Microstructural features in relict plagioclase porphyroclasts show the presence of an igneous fabric in the core and dynamic recrystallization features in the rim. This suggests superposition of deformation microstructures over a primary magmatic texture.
(2) Preservation of carbonic and brine phase fluid inclusions within the core of the plagioclase (defined by igneous zoning) indicates that the fluids were derived from calc-alkaline magma.
(3) The fluid inclusions record two major fluid evolutionary stages. The first stage is represented by primary carbonic- and brine-rich inclusions trapped at ∼4 kbar and ∼650 °C, while the second stage is characterized by trapping of carbonic- and saline-aqueous–rich inclusions at ∼2 kbar and at less than 400 °C, which occurred between 10.34 Ma and 9.48 Ma or later.
(4) Excess inflow of crustal fluids within the mylonitic stage generated secondary textures, viz. decrepitation, implosion, and annular, which were formed between 9.80 Ma and 9.48 Ma or later.
(5) The 40Ar-39Ar biotite ages reveal that 10.34–9.48 Ma was the period when the area cooled from 400 °C to 350 °C, and this period is seen as a pronounced phase of fluid reequilibration and infiltration. This fluid inflow led to rapid exhumation of Karakoram fault zone rocks to the surface, averaging 1 mm yr−1.
We are grateful to the director of the Wadia Institute of Himalayan Geology, Dehradun, for his encouragement and support. We are thankful to K.V. Hodges and M. Pringle for providing facilities and assisting in 40Ar-39Ar chronological study in the laboratory at the Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. Ann Mary Boullier gave helpful suggestions on an earlier version of this manuscript. We thank two anonymous reviewers for their constructive reviews. John Pelletier is thanked for editorial handling.