Barun K. Mukherjee

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.

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.

Tectonics of the Himalaya: an introduction

SOUMYAJIT MUKHERJEE1*, RODOLFO CAROSI2, PETER VAN DER BEEK3, BARUN KUMAR MUKHERJEE4 & DELORES M. ROBINSON5 Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, Maharashtra, India Dipartimento di Scienze della Terra, v. Valperga Caluso, 35 10125 Torino, Italy Universite Grenoble Alpes, Institut des Sciences de la Terre, 38041 Grenoble, France Petrology and Geochemistry Group, Wadia Institute of Himalayan Geology, 33 GMS Road, Dehra Dun 248001, India Department of Geological Sciences, University of Alabama, 201 7th Ave, Room 2003 Bevill Building, Tuscaloosa, AL 35487, USA

Geosciences of the Himalaya–Karakoram–Tibet orogen

The Himalaya–Karakoram–Tibet orogen, a product of India–Eurasia collision *55 Ma back, continues to attract global attention for its many geoscientific uniqueness. This orogen is the most exciting ‘natural laboratory’ to study continental dynamics and its evolution. During past few decades, phenomenal progresses have been made in uplift and extrusion models, dynamic metamorphism, magmatism, climate–tectonics interaction, etc. This thematic volume on the geosciences of the Himalaya–Karakoram– Tibet consists of eighteen papers and two Geosites. In the first paper, Kirby and Harkins correlated variation in slip rate along the Kunlun fault with topography of the Anyemaqen Shan Mountain at the eastern Tibetan plateau. Their work indicates that fault terminations might be associated with crustal thickening of the plateau and demonstrate that the upper crustal deformation of the Tibetan plateau to be pervasive and dispersed throughout the crustal blocks rather than localized along narrow fault zones. Robinson and Pearson compiled an orogen-wide correlation of footwall and hanging wall units of the Ramgarh and Munsiari thrust sheets. They considered the Ramgarh–Munsiari thrust sheet as a single ‘major orogenscale fault’ system. They hypothesized that the extensional shear within the South Tibetan Detachment was triggered possibly by ‘slip transfer’ from the Main Central Thrust into the Ramgarh–Munsiari Thrust during the Miocene. Thakur reviewed the tectonics of the Siwalik range of the Himalaya. He concluded in-sequence deformation and critical taper mechanism acted in this terrain. Presuming the crustal channel flow model to be correct, Mukherjee estimated a viscosity of 10–10 Pa s, and a Prandtl number of 10–10 for the Greater Himalayan Crystallines. Moharana et al., described the Munsiari Thrust from Kumaun Himalaya to consist of a core and a damage zone and described their structural geology in detail. Mukherjee described a ‘basal detachment’ of extensional ductile shear from the base of the Greater Himalayan Crystallines at Bhagirathi section of the Indian Himalaya and explained it in terms of a shifting crustal channel flow. Ubiquitous backthrusts within this terrain were another new finding and were explained by southward subduction of Eurasian plate below the Indian plate. Based on balanced crosssection studies, Khanal and Robinson estimated slip along major Himalayan faults from the Budhi-Gandhaki river section of central Nepal. Sen and Collins inferred dextral transpression and changing angle of convergence of the formation of the Ladakh magmatic arc. They also reported Late Eocene S-type granite magmatism in its central part. Mathew et al. discuss the tectonothermal evolution of the Arunachal Himalaya. They obtained isothermal decompression for the Greater Himalayan Sequence and isobaric cooling from the Main Central Thrust region. Jayangondaperumal et al. reevaluated co-seismic slip of the Himalayan Frontal Thrust. The estimated shortening around the Himalayan Frontal Thrust would lead a bigger earthquake in the western part of Indian Himalaya. Shah presented a geomorphologic study from Kashmir Basin in India, identified faults from remote sensing studies, and predicted a future earthquake of Mw 7.6. Srivastava et al. S. Mukherjee (&) Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India e-mail: soumyajitm@gmail.com

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.

Tectonics of the Himalaya

The Himalayan mountain belt, which developed during the India–Asia collision starting about 55 Ma ago, is a dramatically active orogen and it is regarded as the classic collisional orogen. It is characterized by an impressively continuous 2500 km of tectonic units, thrusts and normal faults, as well as large volumes of high-grade metamorphic rocks and granites exposed at the surface. This constitutes an invaluable field laboratory, where amazing crustal sections can be observed directly in very deep gorges. It is possible to unravel the tectonic and metamorphic evolution of litho-units, to observe the mechanisms of exhumation of deep-seated rocks and the propagation of the deformation. Himalayan tectonics has been the target of many studies from numerous international researchers over the years. In the last 15 years there has been an explosion of data and theories from both geological and geophysical perspectives. This book presents the results of integrated multidisciplinary studies, including geology, petrology, magmatism, geochemistry, geochronology and geophysics, of the structures and processes affecting the continental lithosphere. These processes and their spatial and temporal evolution have major consequences on the geometry and kinematics of the India–Eurasia collision zone.

Bimodal stable isotope signatures of Zildat Ophiolitic Melange, Indus Suture Zone, Himalaya: implications for emplacement of an ophiolitic melange in a convergent setup

Zildat Ophiolitic Mélange (ZOM) of the Indus Suture Zone, Himalaya, represents tectonic blocks of the fragmented oceanic metasediments and ophiolite remnants. The ZOM is sandwiched between the Zildat fault adjacent to a gneissic dome known as Tso Morari Crystalline (TMC) and thin sliver of an ophiolite called as the Nidar Ophiolitic Complex. The ZOM contain chaotic low-density lithologies of metamorphosed oceanic sediments and hydrated mantle rocks, in which carbonates are present as mega-clasts ranging from 100 meters to few centimeters in size. In this work, calcite microstructures, fluid inclusion petrography and stable isotope analyses of carbonates were carried out to envisage the emplacement history of the ZOM. Calcite microstructure varies with decreasing temperature and increasing intensity of deformation. Intense shearing is seen at the marginal part of the mélange near Zildat fault. These observations are consistent with the mélange as a tectonically dismembered block, formed at a plate boundary in convergent setup. The δ18O and δ13C isotope values of carbonates show bimodal nature from deeper (interior) to the shallower (marginal, near the Zildat fault) part of the mélange. Carbonate blocks from deeper part of the mélange reflect marine isotopic signature with limited fluid–rock interaction, which later on provide a mixing zone of oceanic metasediments and/or hydrated ultramafic rocks. Carbonates at shallower depths of the mélange show dominance of syn-deformation hydrous fluids, and this has later been modified by metamorphism of the adjacent TMC gneisses. Above observations reveal that the mélange was emplaced over the subducting Indian plate and later on synchronously deformed with the TMC gneissic dome.

In situ peridotitic diamond in Indus ophiolite sourced from hydrocarbon fluids in the mantle transition zone

In recent years ophiolitic diamonds have been reported mostly from podiform chromitites. However, the mechanism of such diamond formation remains unknown. We report in situ diamond, graphite pseudomorphs after diamond crystals, and hydrocarbon (C-H) and hydrogen (H 2 ) fluid inclusions in ultrahigh-pressure (UHP) peridotitic minerals of the Nidar ophiolite, Indus suture zone. Diamond occurs as octahedral inclusion along with nitrogen (N 2 ) in orthoenstatite. Methane (CH 4 ) also occurs with UHP clinoenstatite (>8 GPa) in orthoenstatite. The graphite pseudomorphs after diamond crystals and primary hydrocarbon (C-H), and hydrogen (H 2 ) fluids are included in olivine. Oriented hematite (α-Fe 2 O 3 ) exsolutions are also present in the olivines, indicating a precursory β-Mg 2 SiO 4 phase of the host olivines. This assemblage of diamond, graphite, C-H and H 2 has not previously been reported from any ophiolitic peridotite. The hydrocarbon fluids in UHP clinoenstatites and retrogressed β-Mg 2 SiO 4 strongly suggest their source from the mantle transition zone or base of the upper mantle. We conclude that the peridotitic diamonds precipitated from C-H fluids during mantle upwelling beneath the Neo-Tethys Ocean spreading center.

Peridotitic minerals of the Nidar Ophiolite in the NW Himalaya: sourced from the depth of the mantle transition zone and above

Abstract The Nidar Ophiolite Complex (NOC) consists of a c. 10 km thick ophiolite suite in the NW Himalaya, India. The c. 7 km thick lower ultramafic part of the ophiolite body terminates against the Tso Morari Crystallines, which represent the leading edge of the Indian continental margin. Mineral inclusions from the peridotites in the lower ultramafic part of the NOC were studied, including C2/c clinoenstatite, disordered coesite and high-pressure Mg2SiO4 (probably β-Mg2SiO4). These minerals, found in two lherzolite bodies from the ophiolites mantle section, were characterized by laser Raman spectroscopy and electron micro-probe analysis. Textural evidence supporting decompression from an ultra-high-pressure condition was also observed, such as Cr spinel exsolution needles in olivine crystals. The systematic mineral phase transitions of coesite→quartz, high-pressure clinoenstatite→orthoenstatite and β-Mg2SiO4→Cr spinel exsolution needles in olivine suggest that the mantle section of the Nidar Ophiolite evolved from the deep mantle beneath a palaeo-spreading centre. The phase stabilities of these high-pressure minerals require derivation from the depth of the mantle transition zone (410–660 km). A transport mechanism for these minerals is suggested via dunite channels along a mantle adiabat in the focused convective flow below the spreading centre. This mechanism brought these deep mantle phases into the ultramafic part of the NOC. These observations suggest that some part of the mantle section of the NOC in the NW Himalaya originated in a mid-ocean ridge setting. Supplementary material: Representative mineral chemical compositions of the lherzolites (1M1 and 1NU27) and the host channelized dunites are available at http://www.geolsoc.org.uk/SUP18836.