Himanshu K. Sachan

Discovery of coesite from Indus Suture Zone (ISZ), Ladakh, India: Evidence for deep subduction

Coesite, the high-pressure polymorph of quartz has been identified for the first time in the Tso-Morari Crystalline Complex, Ladakh (India) in the Himalayan belt. The preservation of coesite grains as inclusions in garnet within eclogite boudins indicates the existence of UHP metamorphism in this continental collision setting. The coesite was identified optically and its presence confirmed by its characteristic Raman bands. Both coesite and polycrys-talline quartz inclusions exhibit prominent radial fractures in their host garnet. The silica inclusions (monomineralic coesite, monomineralic quartz and bimineralic quartz+coesite) are associated with various textural features and well-developed chemical zonation within the garnet, which show the prograde nature of the UHP metamorphism. Preliminary P-T estimates suggest that the coesite growth took place at pressure > 28 kbar (> 90 km depth) and temperatures > 640°C. Significantly, the coesite inclusions are interpreted as having suffered decompression during exhumation without changing to quartz, most likely due to the rapid uplift. This finding also indicates deep subduction of the Indian plate beneath the Asian continent. The occurrence of coesite and subduction of the Indian plate was essentially governed by a low geothermal gradient which occurred prior to rapid exhumation. This was vital for generating the coherent picture of metamorphism and exposure of UHP rocks.

Carbonate-Bearing UHPM Rocks from the Tso-Morari Region, Ladakh, India: Petrological Implications

Evidence of ultrahigh-pressure metamorphism (UHPM) of subducted Indian continental crust in the form of carbonate-bearing coesite eclogite is preserved in the Tso-Morari Crystalline Complex (TMC) in eastern Ladakh, India. These eclogites, which occur as boudins in kyanite/sillimanite—grade rocks of the Puga Formation, contain essential mineral assemblages (garnet, clinopyroxeneomphacite, phengite, rutile, epidote-zoisite/clinozoisite and quartz), as well as coesite, talc, kyanite, magnesite, aragonite, dolomite, and Mg-calcite. Coesite, magnesite, and dolomite occur as inclusions in zoned garnet. The carbonate-bearing coesite eclogite underwent three stages of metamorphism—prograde, peak, and retrograde. The prograde assemblage is characterized by the presence of magnesite and a SiO2 polymorph, which is stable throughout the metamorphic process from the prograde to retrograde stage. At ultrahigh-pressure (27 kbar) and a temperature of 650°C, quartz transforms to coesite. Peak metamorphism was characterized by the development of coesite in garnet coexisting with high-Si phengite, clinopyroxene, magnesite, aragonite, dolomite, zoisite/clinozoisite, kyanite, and talc at a pressure of >39 kbar and temperature of >750°C. This is in good agreement with the estimated peak pressure and temperature judging from the composition of phengite, jadeite barometry, and garnet-clinopyroxene, garnet-phengite thermometry. Enstatite formed with talc and kyanite at a pressure of >31 kbar and temperature of 750°C. With a subsequent decrease in pressure, retrogression is constrained by the development of chlorite and chloritoid, which surround the garnet at a minimum pressure of 4-5 kbar and temperature of <500°C. Mineral assemblages in the carbonate-bearing coesite eclogite reveal that prograde metamorphism started with greenschist-facies conditions and reached the ultrahigh-pressure eclogite facies, passing through the intermediate blueschist facies. During UHP metamorphism, pressure abruptly doubled with a slight change of temperature, defining a geothermal gradient of 6–7°C/km. The UHP material was brought back to the surface along a path by rapid and almost isothermal exhumation.

The Malari Leucogranite, Garhwal Himalaya, Northern India: Chemistry, Age, and Tectonic Implications

In the Garhwal region, India, the Malari leucogranite cuts the South Tibetan detachment system, a large-scale normal fault system at the top of the High Himalaya. The leucogranite crosscuts ductile normal-sense shear fabrics and has experienced relatively little subsolidus brittle deformation or alteration. Its relatively evolved bulk chemical composition, high Rb/Sr ratio, and normative corundum indicate a (meta)sedimentary source, likely the underlying Greater Himalayan sequence. Zircon U-Pb ages, collected by laser-ablation inductively coupled plasma–mass spectrometry (ICP-MS) and corrected for initial U/Th disequilibrium, indicate emplacement at 19.0 ± 0.5 Ma. Thus, ductile normal shear on the South Tibetan detachment system must have ceased by 19 Ma. Studies elsewhere in the Himalaya suggest initiation of South Tibetan detachment system ductile movement not earlier than 24 Ma, and likely ca. 22 Ma. The short duration of extension (≤5 and likely ∼3 m.y.) and early cessation contrast with channel-flow models that predict long-duration ductile normal shear, and large displacements after ca. 20 Ma. Observations are instead better explained by critical taper models, in which internal weakening of the wedge, likely from partial melting, caused a brief interval of flattening and ductile extension in the rear of the wedge.

Fluids in coesite-bearing rocks of the Tso Morari Complex, NW Himalaya: evidence for entrapment during peak metamorphism and subsequent uplift

Fluid inclusions trapped in coesite-bearing rocks provide important information on the fluid phases present during ultrahigh-pressure metamorphism. The subduction-related coesite-bearing eclogites of the Tso Morari Complex, Himalaya, contain five major types of fluids identified by microthermometry and Raman spectroscopy. These are: (1) high-salinity brine, (2) N 2 , (3) CH 4 , (4) CO 2 and (5) low-salinity aqueous fluids. These fluids were trapped during both deep subduction and exhumation processes. The coesite-bearing rocks are inferred to have been buried to a depth of >120 km, where they experienced ultrahigh-pressure metamorphism. The fluid–rock interaction provides direct evidence for fluid derivation during a deep subduction process as demonstrated by silica–carbonate assemblages in eclogite. High salinity brine, N 2 and CH 4 inclusions are remnants of prograde and peak metamorphic fluids, whereas CO 2 and low-salinity aqueous fluids appear to have been trapped late, during uplift. The high-salinity brine was possibly derived from subducted ancient metasedimentary rocks, whereas the N 2 and CH 4 fluids were likely generated through chemical breakdown of NH 3 -bearing K minerals and graphite. Alternatively, CH 4 might have been formed by a mixed fluid that was released from calcareous sediments during subduction or supplied through subducted oceanic metabasic rocks. High density CO 2 is associated with matrix minerals formed during granulite-facies overprinting of the ultrahigh-pressure eclogite. During retrogression to amphibolite-facies conditions, low-salinity fluids were introduced from external sources, probably the enclosing gneisses. This source enhances salinity differences as compared to primary saline inclusions. The subducting Indian lithosphere produced brines prior to achieving maximal depths of >120 km, where fluids were instead dominated by gaseous phases. Subsequently, the Indian lithosphere released CO 2 -rich fluids during fast exhumation and was then infiltrated by the low-salinity aqueous fluids near the surface through external sources. Elemental modelling may improve quantitative understanding of the complexity of fluids and their reactions.

Fluid events and exhumation history of the main central thrust zone Garhwal Himalaya (India)

Mineral thermobarometric and fluid inclusion studies were carried out on the low to medium grade metamorphics which occur at different tectonic levels of the main central thrust (MCT) zone in Garhwal Himalaya (MCT-I in north, MCT-II in the centre and the southern most MCT-III). The augen gneiss and pelitic schists present in this zone show an increase in the grade of metamorphism from chlorite to kyanite towards the north. The pressure calculated from garnet‐biotite‐muscovite‐plagioclase phase equilibria, increases from 1.9 to 8 kbar and the temperature obtained through garnet‐biotite and chlorite geothermometer varies between 360 and 5628C. The mineral assemblages and these thermobarometric estimates reveal that the grade of metamorphism in the MCT zone increases from south to north. Considering the inclusion types, it is apparent that the carbonic fluid increases towards the MCT-I, which is related to increase in P‐T conditions whereas the aqueous phase is more pronounced near the MCT-III reflecting fluid assisted retrogression. This observation is also substantiated by the systematic increase in CO2 density from 0.75 g/cm 3 in CO2‐H2O inclusions near MCT-III to 1.01 g/cm 3 in pure CO2 inclusions near MCT-I. The pressure and temperature estimated using fluid isochores varies from 1.9 to 4.8 kbar and 360 to 5628C. When compared with mineral phase thermobarometry, these P‐T conditions suggest a post peak-metamorphic nature of the fluids. A smooth trend of mineral and fluid phase P‐T conditions is observed in the MCT zone. Based on combined mineral P‐T data and fluid isochores, a decompressional uplift path is suggested. The decrease in P‐T and the evidence of movement along well distributed shear fabric across the MCT zone indicate that exhumation from north to south occurred under decreasing P‐T conditions. q 2001 Elsevier Science Ltd. All rights reserved.

Fluid inclusions in charnockites from Kodaikanal massif (South India): P-T record and implications for crustal uplift history

SummaryThe Kodaikanal massif is part of a Precambrian terrane characterized by granulite facies rocks. It is dominated by the widespread occurrence of charnockites. The observed textural relationships in these rocks are consistent with the following main reactions:i.Biotite + Quartz = Orthopyroxene + Alkali feldspar ± Garnet + Vapourii.Garnet + Quartz = Orthopyroxene + Plagioclaseiii.Pyrope = Mg Tschermaks + Enstatite (in Opx) Garnet consuming reactions and the preservation of biotite-quartz-orthopyroxene-plagioclase symplectites are indicative of a decompression event. Progress of such reactions with decreasing pressure together with fluid inclusion data has implications for the construction of P-T vectors. Quartz from the charnockites contains the following fluid inclusions: (1) monophase high density CO2-rich (0.968/−1.014g/cm3) as the dominant fluid phase; (2) aqueous biphase CO2-H2O (0.888–0.915 g/cm3) and (3) late minor aqueous H2O inclusions with no visible CO2. CO2-isochores for the high density fluid inclusions yield a pressure limit of ca. 6.5 kbars, at granulite facies temperatures of ca. 800°C, which is in accordance with the estimation from mineralogical thermobarometry. The P-T path delineated from combined mineralogical and density data on carbonic inclusions is characteristically T-convex suggesting an isothermal decompression path and rapid uplift followed by cooling of a tectonically thickened crust.ZusammenfassungDas Kodaikanal Massiv ist ein Teil eines präkambrischen Granulitareals, das hauptsächlich aus Charnockiten aufgebaut wird. Die beobachteten, texturellen Beziehungen in diesen Gesteinen lassen sich mit den folgenden Mineralreaktionen in Übereinstimmung bringen:i.Biotit + Quarz = Orthopyroxen + Alkalifeldspat ± Granat + Vapourii.Granat + Quarz = Orthopyroxen + Plagioklasiii.Pyrop = Mg Tschermak Komponente in OPX + Enstatit Granatabbaureaktionen und das Auftreten von Symplektiten mit Biofit, Quarz, Orthopyroxen und Plagioklas, belegen ein Dekompressionsereignis. Das Ablaufen dieser Reaktionen mit fallendem Druck und die Daten der Untersuchungen an Flüssigkeitseinschlüssen sind für die Konstruktion des P-T Pfades bedeutsam. Quarz in Charnockiten führt folgende Flüssigkeitseinschlüsse: (1) Einphasige, sehr dichte, CO2-reiche Fluide (0.968–1.014g/cm3) dominieren. (2) Zweiphasige, wässrige, CO2-H2O (0.888–0.915 g/cm3) und (3) späte, CO2-freie, H2O-Einschlüsse treten untergeordnet auf. Aus den lsochoren der Einschlüsse hoher Dichte läßt sich ein Druck von 6.5 kbar bei granulitfaziellen Temperaturen von ca. 800 °C ableiten, was mit den geothermobarometrischen Ergebnissen übereinstimmt. Der aus den mineralogischen und Fl-Daten abzuleitendene P-T Pfad ist charakteristischerweise T-konvex ausgebildet und ist mit isothermischer Dekompression und rascher Hebung einer tektonisch vedickten Kruste erklärbar.

Exhumation history of a shear zone constrained by microstructural and fluid inclusion techniques: an example from the Satluj valley, NW Himalaya, India

Abstract The regional structures, rock microstructures and fluid inclusion trail patterns have been employed to determine the evolution of the Jakhri Thrust Zone (JTZ). The JTZ is a break back thrust cutting across the folded Lesser Himalayan Crystalline nappe and is best exposed in the Kulu-Rampur window zone of the NW Himalaya. The microstructures in the JTZ suggest SW directed ductile shearing and a progressively decreasing finite strain away from the thrust in the footwall. The quartz recrystallization, microstructures and presence of chlorite in the thrust zone indicate lower greenschist facies P–T conditions during deformation. The microstructures and fluid inclusion trails (secondary) show analogous patterns suggesting that the latter would have formed by the healing of microfractures during shearing in the footwall. The microthermic studies on these fluid inclusions suggest that the CO2–H2O inclusions have been emplaced and reequilibrated during peak deformation whereas the H2O–NaCl inclusions reequilibrated during footwall exhumation. The density and salinity of fluid inclusions were also reset during the same exhumation. The isochores of CO2–H2O and H2O–NaCl inclusions in the greenschist facies suggest an isothermal exhumation path from a depth of ∼15 to 17 km, assuming lithostatic pressure conditions. These results in the JTZ emphasize the utility of fluid inclusions in tectonic studies.

Exhumation history of the Karakoram fault zone mylonites: New constraints from microstructures, fluid inclusions, and 40Ar-39Ar analyses

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 40 Ar- 39 Ar 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.

Aluminous and alkali-deficient tourmaline from the Singhbhum Shear Zone, East Indian shield: Insight for polyphase boron infiltration during regional metamorphism

Abstract In the western part of the Singhbhum Shear Zone (SSZ), East Indian Shield, borosilicate-bearing veins of variable thickness (tens of micrometers to 1 m thick) are hosted in kyanite-quartzite and kyanite-mica schist. The veins have been classified into three types, which are, from oldest to youngest, generation I (tourmaline), II (dumortierite + tourmaline), and III (tourmaline) veins. Alkali- and Mg-rich tourmaline [XMg = Mg/(Mg + Fe) = 0.68 ± 0.09; X = Na, Ca, K, □ (vacancy) = 0.40 ± 0.12] is the sole borosilicate in generation I veins, which have been folded in response to regional deformation. Generation II veins were emplaced along shear bands (1 mm to 1 m thick) developed parallel to the axial planes of these folds. Long axes of fibrous dumortierite and prismatic tourmaline of generation II veins are oriented along the shear bands and have been bent around lenticular remnants of host kyanite-quartzite. Generation III veins have a dendritic pattern, crosscut generation II veins and show aggregates of fibrous to acicular tourmaline. Prismatic tourmaline in generation II veins is optically zoned with a green tourmaline core that is variably replaced and rimmed by blue tourmaline. Fibrous to acicular tourmaline in generation III veins is comprised up of blue tourmaline with compositions similar to the rim composition of prismatic tourmaline in generation II veins. Green and blue tourmaline is aluminous (Al total >7 apfu) and alkali-deficient (X = 0.71 ± 0.08). High YAl content, high X, low XMg (0.19 ± 0.10), and excess cation charge indicate tourmaline in generation II veins is rich in an “oxy-foitite” component. Foitite-rich tourmaline in generation III veins has tetrahedral Al and a slightly lower Mg-content and X than those of generation II veins. Optical zoning in prismatic tourmaline corresponds to an abrupt compositional change with paragenetically older green tourmaline having higher Al and XMg, but lower alkali content in the X-site than the blue tourmaline rim. The compositional variation in green and blue tourmaline can be explained by a combination of coupled substitutions represented by AlO[R(OH)]-1 and Al(NaR)-1, where R = (Fe2+ + Mg). Pseudosections in the system Na2O-K2O-Al2O3-SiO2-H2O constructed from bulk chemical compositions of the studied rocks and the P-T slopes of two isochors computed from brine-rich inclusions trapped in quartz grains indicate that borosilicate formation in generation II and III veins occurred within 4.1 ± 0.5 kbar and 377 ± 21 °C. The mineral assemblages and textures suggest that the borosilicate-bearing veins formed from infiltration-driven alteration of host kyanite-quartzite and kyanite-mica schist along structurally controlled conduits by more than one batch of chemically distinct boron-rich aqueous fluids.

Cooling history of subduction related granite from the Indus Suture Zone, Ladakh, India: evidence from fluid inclusions

The P-T history of the subduction related Ladakh granite in northwestern Himalaya has been recorded from silicate-melt inclusions and other high temperature solid inclusions in the rock forming quartz. The silicate-melt inclusions show initial melting from 650 °C–705 °C. These inclusions have compositions similar to calc-alkaline granitic magma which is supported by the trace and major element chemistry of the granite. The earliest fluids (75 equiv. wt.% NaCl) circulated at subsolidus conditions from 625 ° down to 425 °C, whereas invasion of dilute solution (<8 equiv. wt.% NaCl) in the granite took place from 300 °C down to 165 °C. On the basis of the above observations it is inferred that the cooling history of the Ladakh granite started at 705 °C and continued up to 650 °C in the range of 1.2–2.8 kbar. This, therefore suggest a shallow emplacement of granitic magma. The hydrothermal processes involved during the cooling reveals simultaneous drop in temperature and pressure along with the introduction of dilute solution (very low saline) through microfractures.