Browsing by Author "Koushik Sen"

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Age and geochemistry of the paleoproterozoic bhatwari gneiss of garhwal lesser himalaya, nw india: Implications for the pre-himalayan magmatic history of the lesser himalayan basement rocks
(Geological Society of London, 2019) Aranya Sen; Koushik Sen; Hari B. Srivastava; Saurabh Singhal; Purbajyoti Phukon
The Bhatwari Gneiss of Bhagirathi Valley in the Garhwal Himalaya is a Paleoproterozoic crystalline rock from the Inner Lesser Himalayan Sequence. On the basis of field and petrographic analyses, we have classified the Bhatwari Gneiss into two parts: the Lower Bhatwari Gneiss (LBG) and the Upper Bhatwari Gneiss (UBG). The geochemical signatures of these rocks suggest a monzonitic protolith for the LBG and a granitic protolith for the UBG. The UBG has a calc-alkaline S-type granitoid protolith, whereas the LBG has an alkaline I-type granitoid protolith; the UBG is more fractionated. The trace element concentrations suggest a volcanic arc setting for the LBG and a within-plate setting for the UBG. The U–Pb geochronology of one sample from the LBG gives an upper intercept age of 1988 ± 12 Ma (n = 10, MSWD = 2.5). One sample from the UBG gives an upper intercept age of 1895 ± 22 Ma (n = 15, MSWD = 0.82), whereas another sample does not give any upper intercept age, but indicates magmatism from c. 1940 to 1840 Ma. Based on these ages, we infer that the Bhatwari Gneiss has evolved due to arc magmatism and related back-arc rifting over a time period of c. 100 Ma during the Proterozoic. This arc magmatism is related to the formation of the Columbia supercontinent. © 2019 The Author(s). Published by The Geological Society of London. All rights reserved.
Characterizing anatexis in the Greater Himalayan Sequence (Kumaun, NW India) in terms of pressure, temperature, time and deformation
(Elsevier B.V., 2019) Purbajyoti Phukon; Koushik Sen; Praveen Chandra Singh; Aranya Sen; Hari B. Srivastava; Saurabh Singhal
We applied field observations combined with P-T pseudosection modelling, zircon U-Pb geochronology and bulk rock geochemistry along the Kali River Valley, Kumaun Himalaya to understand conditions of peak metamorphism and partial melting of the Greater Himalayan Sequence (GHS) along with spatiotemporal relationship between anatexis and fault activation. The southern tectonic boundary of GHS or the Main Central Thrust (MCT) is marked on the basis of structural, metamorphic and chronological evidences. Outcrop-scale observations suggest generation of partial melt at the base of the MCT. This partial melt migrated to higher structural levels and finally emplaced as tourmaline bearing leucogranite in the northern tectonic boundary of the GHS, which is marked by the South Tibetan Detachment Zone (STDZ). P-T pseudosection modelling shows that GHS have experienced muscovite dehydration melting at 9.2–9.8 kbar and 720°–725 °C, 8.4–8.7 kbar and 700°-710 °C, 7.8–8.4 kbar and 700–720 °C respectively at its lower, lower-middle and middle structural levels. Zircon U-Pb geochronology suggests that the GHS underwent suprasolidus peak metamorphism and post-peak anatexis during a time span of ~26–22 Ma at the base of the MCT and ~32–27 Ma at the middle structural level. The MCT is at least ~22 Ma old, being synkinematic to the partial melting event that took place at its base. Diachronous and brief episodes of partial melting and absence of sillimanite zone at the base of the GHS help us envisage a ‘critical taper wedge’ scenario, where partial melt weakened the overlying Himalayan wedge and triggered gravity collapse that formed the STDZ. © 2019 Elsevier B.V.
Exhumation mechanisms of the Himalayan metamorphic core in the Bhagirathi Valley, NW India: Insights from an integrated structural and metamorphic analysis
(Elsevier Ltd, 2024) Aranya Sen; Purbajyoti Phukon; Koushik Sen; Subham Bose; Hari B. Srivastava
The Himalayan metamorphic core in the Bhagirathi valley (NW India) consists of low-grade rocks belonging to the Lesser Himalayan Sequence to the south, and of high-grade rocks of the Greater Himalayan Sequence (GHS), to the north. The contact between these two units is a 2–3 km thick high-strain zone known as the Main Central Thrust zone (MCTz). Structural studies in the GHS identify the superposition of foliation (S2) related to tight isoclinal fold (F2) on a pre-existing foliation (S1). S2 was further overprinted by D3 crenulations, and a set of conjugate fractures (D4) overprinted these ductile fabrics. Deformation in the GHS was characterized by top-to-the-SW shearing, which, in its upper structural level, was overprinted by top-to-the-NE extensional shear. Metamorphic analysis shows a moderate T/high P inverted metamorphic sequence with pre-to syn-kinematic, inclusion-rich garnet in the structurally lower part of the GHS and high T-moderate P prograde metamorphism with post-tectonic inclusion-free garnet and evidence of partial melting, in its upper structural level. We envisage that the uppermost part of the GHS and the lower part of the GHS, including the MCTz, are two distinct tectonic slices, separated by a tectono-metamorphic discontinuity. © 2023 Elsevier Ltd
Tectonothermal evolution of the Lohit Valley, Eastern Himalaya: New low-temperature thermochronological constraints
(John Wiley and Sons Ltd, 2022) Kunal Mukherjee; Vikas Adlakha; Sayandeep Banerjee; Koushik Sen
The present contribution provides the first low-temperature thermochronological record from the Lohit Valley, Eastern Himalaya, presenting eight ZHe and two AHe cooling ages across the major tectonic boundaries. The ZHe cooling ages range from 6.94 ± 1.17 Ma to 12.51 ± 2.84 Ma, whereas AHe ages vary between 1.73 ± 0.15 Ma and 3.56 ± 0.42 Ma. The ZHe cooling ages suggest that the Mishmi Crystallines exposed at the southwestern mountain front are the slowest exhuming domain since ~12 Ma. The ZHe ages are youngest in the Demwe Thrust (DT) zone, and the contact between the frontal low-grade metamorphic rocks of Mishmi Crystalline and high-grade gneissic rocks of the Mayodia Group. The rapid exhumation in the DT zone as obtained from the ZHe cooling ages suggests an out-of-sequence thrusting at ~7 Ma. The QTQt thermal history modelling of the co-genetic pairs of ZHe and AHe cooling ages of the northeasternmost Lohit Plutonic Complex suggests that the exhumation rates in this region were as high as ~3.7 mm/year during the Pliocene-Quaternary. These high exhumation rates are in good correlation with the local topographic relief, hill slopes, and channel steepness, which suggests the establishment of the present-day topography of the Lohit valley region at the latest by Pliocene-Quaternary. Variation in exhumation rates does not correlate with the present-day precipitation pattern. Tectonics appears to be the prime driver for exhumation rates of the Lohit valley region of the easternmost Himalaya. © 2022 John Wiley & Sons Ltd.
Tracing Late-Stage Fluid Sources and Vein Formation within Ophiolitic Mélanges from the Indus Suture Zone, Ladakh Himalaya
(University of Chicago Press, 2021) Aditya Kharya; Himanshu K. Sachan; Christopher J. Spencer; Koushik Sen; Divya Prakash; Shashi Ranjan Rai; Vikash Kumar
Quartz-calcite veins in the Zildat ophiolitic mélange (ZOM) and Shergol ophiolitic mélange (SOM) of the Indus Suture Zone preserve a diversity of fluid activity in the late stages of ophiolitic mélange formation. This article presents fluid-inclusion and isotope geochemistry of these veins to understand their source and evolution in terms of pressure and temperature. The microstructures of quartz and calcite veins indicate deformation temperatures between 2007 and 4007C. The d13 C and d18 O values of calcite veins from the ZOM and SOM are within the mixing hyperbolas of marine and primitive-mantle fields in the mixing model. The Sr and Pb isotopic values of calcite veins from the ZOM suggest a mid-ocean ridge basalt (MORB) fluid source of vein formation that was radiogenically enriched by metasomatism in a suprasubduction zone. For the SOM, fluids may have originated from the enriched-mantle (EM) and the depleted-MORB-mantle rocks. It is inferred that the carbonic fluids were derived from ultramafic lithologies and oceanic crust that formed the ophiolitic mélange rocks, which also host these veins. These source rocks have EM and MORB geochemical signatures that are also obtained in the quartz-calcite veins, as characterized by their C-O-Sr-Pb isotopic ratios. The magmatic saline fluid is inferred to have formed in the early stages of vein formation and to have been subsequently diluted, as exemplified by the presence of low-saline secondary aqueous inclusions. The microthermometry fluid pressure-temperature estimation of veins from the studied sections suggests that the maximum depth of emplacement of veining fluid was about 24.5 MPa (corresponding to ∼2.5 km) at 3367C. The vein-forming fluids (calcareous and siliceous) were introduced into the fractures that developed in the host as a result of deformation. © 2021 The University of Chicago. All rights reserved.
U-Pb geochronology and geochemistry from the Kumaun Himalaya, NW India, reveal Paleoproterozoic arc magmatism related to formation of the Columbia supercontinent
(Geological Society of America, 2018) Purbajyoti Phukon; Koushik Sen; Hari B. Srivastava; Saurabh Singhal; Aranya Sen
Columbia was a Proterozoic supercontinent that formed ~2.0 b.y. ago by amalgamation of almost all the present-day continental blocks. One major controversy regarding the formation of this supercontinent is the position and tectonic setting of the north Indian continental margin during this amalgamation. One school of thought suggests that this margin remained passive throughout the continental accretion process, and another school contradicts that by suggesting subduction and an active continental arc setting. The Paleoproterozoic basement rocks of the Himalaya consist of granitic gneisses from the Lesser Himalayan Sequence that belonged to the north Indian continental margin during the formation of Columbia. In this work, we present bulk-rock geochemistry and U-Pb geochronology of zircon from augen gneisses of the Lesser Himalayan Munsiari Formation and granite gneisses of the Chiplakot Crystalline Belt of the Kumaun Himalaya, from Kali River Valley, India. Our objective was to evaluate the tectonic setting of these rocks to infer the nature of the Proterozoic north Indian continental margin. Bulk-rock geochemistry of four samples from the Munsiari augen gneiss and six samples from the granite gneisses from Chiplakot Crystalline Belt shows calc-alkaline and shoshonitic composition, respectively. Depletion in Nb, Sr, P, and Ti point toward a magmatic arc origin for both units. U-Pb chronology of zircon was carried out on two samples from the Munsiari augen gneiss and three samples from the Chiplakot Crystalline Belt. All of these samples yielded ages ranging from ca. 1970 Ma to ca. 1860 Ma, with crystallization ages in the Musiari augen gneiss varying from ca. 1970 to ca. 1950 Ma and ages in the Chiplakot Crystalline Belt at ca. 1920 Ma. Based on these results, it is inferred that both the Munsiari augen gneiss and the Chiplakot Crystalline Belt belong to the Inner Lesser Himalayan Sequence, and the magmatism took place within a span of ~100 m.y. related to active subduction along the Proterozoic north Indian continental margin. It is envisaged that the slightly older and highly fractionated calc-alkaline Munsiari augen gneiss reflects mixing of mantle and crustal melt along with assimilation and fractional crystallization. On the other hand, the Chiplakot Crystalline Belt is less fractionated and may have intruded during a later part of continental arc magmatism into a thinner crust, associated with crustal extension probably driven by slab break-off and/or slab rollback. Our study indicates that the north Indian continental margin was indeed an active subduction zone during the formation of Columbia. © 2018 The Authors.
Understanding pre-and syn-orogenic tectonic evolution in western Himalaya through age and petrogenesis of Palaeozoic and Cenozoic granites from upper structural levels of Bhagirathi Valley, NW India
(Cambridge University Press, 2022) Aranya Sen; Koushik Sen; Amitava Chatterjee; Shubham Choudhary; Alosree Dey
The Himalaya is characterized by the presence of both pre-Himalayan Palaeozoic and syn-Himalayan Cenozoic granitic bodies, which can help unravel the pre- to syn-collisional geodynamics of this orogen. In the Bhagirathi Valley of Western Himalaya, such granites and the Tethyan Himalayan Sequence (THS) hosting them are bound to the south by the top-to-the-N extensional Jhala Normal Fault (JNF) and low-grade metapelite of the THS to its north. The THS is intruded by a set of leucocratic dykes concordant to the JNF. Zircon U-Pb laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) geochronology of the THS and one leucocratic dyke reveals that the two rocks have a strikingly similar age distribution, with a common and most prominent age peak at ∼1000 Ma. To the north of the THS lies Bhaironghati Granite, a Palaeozoic two-mica granite, which shows a crystallization age of 512.28 ± 1.58 Ma. Our geochemical analysis indicates that it is a product of pre-Himalayan Palaeozoic magmatism owing to extensional tectonics in a back-arc or rift setting following the assembly of Gondwana (500-530 Ma). The Cenozoic Gangotri Leucogranite lies to the north of Bhaironghati Granite, and U-Pb dating of zircon from this leucogranite gives a crystallization age of 21.73 ± 0.11 Ma. Our geochemical studies suggest that the Gangotri Leucogranite is a product of muscovite-dehydration melting of the lower crust owing to flexural bending in relation to steepening of the subducted Indian plate. The leucocratic dykes are highly refracted parts of the Gangotri Leucogranite that migrated and emplaced along extensional fault zones related to the JNF and scavenged zircon from the host THS during crystallization. © 2021 The Author(s). Published by Cambridge University Press.
Variation in mechanism of dynamic recrystallization and differential stress across the Chiplakot Crystalline Belt, Kali River Valley, Kumaun Himalaya: Implications for exhumation of basement rocks in a 'critical taper wedge' setting
(Wadia Institute of Himalayan Geology, 2019) Purbajyoti Phukon; Aranya Sen; Koushik Sen; Hari Bahadur Srivastava
In this study, we carried out mesoscopic and magnetic fabric analysis along with differential flow stress based on quartz piezometer and fractal dimension (D) of quartz grain boundaries from area–perimeter method to understand variation in fabric and deformation intensity across the Chiplakot Crystalline Belt (CCB) from the Kumaun Lesser Himalaya of Kali River Valley, to understand strain variation in relation syn-Himalayan deformation and the mechanism of exhumation of the CCB. The CCB is separated from the low grade metasedimentary rocks that include siliciclastics and carbonates of the Lesser Himalaya by binding thrust zones. The northern and southern contacts of the CCB are demarcated by North Chiplakot Thrust (NCT) and South Chiplakot Thrust (SCT) and the core part of the CCB is also demarcated by a thrust called the Central Chiplakot Thrust (CCT). Both mesoscopic and magnetic fabrics are concordant and follows the NW dipping attitude of all the major thrust zones, including the Munsiari Thrust and the Main Central Thrust of this area. High temperature dynamic recrystallization texture such as Grain Boundary Migration (GBM) is more prominent in and around the CCT and progressively this texture is replaced by Sub Grain Rotation (SGR) towards the north nearing the NCT and Grain Boundary Bulging (BLG) and Sub Grain Rotation (SGR) at SCT. Differential flow stress (σ) decreases exponentially from the CCT to the SCT with increasing grain sizes. Similarly, towards the north from the CCT, value of σ gradually decreases with sudden increment within the NCT zone. We infer that the intensity of brittle-ductile shearing is more prominent along the CCT compared to the NCT. On the basis of our findings, we envisaged a 'critical taper wedge' scenario where internal deformation and forward propagation of thrusts are the key mechanisms of late stage exhumation. We opine that the CCT is an out of sequence thrust formed due to internal deformation and has played a pivotal role in the exhumation of the Chiplakot Crystalline Belt. © 2019, Wadia Institute of Himalayan Geology. All rights reserved.
Zircon U–Pb geochronology, mineral and whole-rock geochemistry of the Khardung volcanics, Ladakh Himalaya, India: Implications for Late Cretaceous to Palaeogene continental arc magmatism
(John Wiley and Sons Ltd, 2020) Nongmaithem Lakhan; Athokpam Krishnakanta Singh; Birendra Pratap Singh; Koushik Sen; Mutum Rajanikanta Singh; Shoraisam Khogenkumar; Saurabh Singhal; Govind Oinam
In this study, we present new mineral and whole-rock geochemical data with zircon U–Pb ages of the Khardung volcanics (KV) from the western Himalaya and discuss their tectono-magmatic evolution. These volcanics are sandwiched between the Ladakh batholith and Karakoram batholith and classified as intermediate volcanics (basaltic andesite-andesite) and felsic volcanics (dacite-rhyolite). The intermediate volcanics are marked by low SiO2 (52.80–61.31 wt.%), enriched LREEs, and depleted HFSEs (Nb, Ti, Zr), whereas more evolved felsic volcanics exhibit quartz, K-feldspar, and plagioclase as dominant mineral phases and felsic compositions are characterized by high SiO2 (64.52–79.19 wt.%) content with pronounced negative Eu anomalies, enriched LREEs, and depleted HREEs and HFSEs (Nb, Ti). New zircon U–Pb ages of intermediate volcanics (andesite) yield 69.71 Ma, whereas felsic volcanics (rhyolites) range between 62.49 and 66.55 Ma, indicating that the Khardung magmatism overlaps with the last phase of the Ladakh batholith magmatism. Geochemical characteristics indicate that the KV were generated from a same parental magma source through fractional crystallization along with crustal assimilation from an older crust, and they show genetic affinity with the adjacent Ladakh batholith. Therefore, the KV and Ladakh batholith could be considered as a product of the mature stage arc magmatism generated during subduction of the Neo-Tethyan oceanic crust prior to the main collision between the Indian and Eurasian continents. © 2019 John Wiley & Sons, Ltd.