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

Abstract

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.

Age, petrogenesis, and metamorphic modelling of high‐pressure garnet‐amphibolite from the Tethyan Himalayan Sequence of Bhagirathi Valley, Western Garhwal Himalaya

“…In the Bhagirathi Valley of Western Garhwal Himalaya (Figure 1b). The Jhala Normal Fault (JNF) is an extensional fault related to the STDZ (Sen et al, 2022 and references therein). At the structural top of the GHS, the north‐dipping fault, the JNF, forms the boundary between the GHS and low‐grade Tethyan sequence (THS) (Figure 1b).…”

“…At the structural top of the GHS, the north‐dipping fault, the JNF, forms the boundary between the GHS and low‐grade Tethyan sequence (THS) (Figure 1b). Further north, overlying the GHS, is the lowest part of the THS, namely the unfossiliferous Harsil Formation (Sen et al, 2022; Singh, 2018).…”

“…In the Bhagirathi Valley, the THS consists of Palaeozoic and Mesozoic strata metamorphosed to greenschist to epidote‐amphibolite to lower amphibolite facies, along with the occurrence of Palaeozoic two‐mica granites and Cenozoic leucogranites. The Palaeozoic two‐mica granite is referred to as the Bhaironghati Granite, which was emplaced in the THS following the assembly of Gondwana owing to extensional tectonics in a back‐arc or rift setting (Sen et al, 2022). The Cenozoic leucogranite is called Gangotri Leucogranite and is a product of syn‐Himalayan magmatism (Sen et al, 2022).…”

“…The Palaeozoic two‐mica granite is referred to as the Bhaironghati Granite, which was emplaced in the THS following the assembly of Gondwana owing to extensional tectonics in a back‐arc or rift setting (Sen et al, 2022). The Cenozoic leucogranite is called Gangotri Leucogranite and is a product of syn‐Himalayan magmatism (Sen et al, 2022). In the immediate hanging‐wall of JNF, garnet amphibolite appears interlayered within low‐grade chlorite‐biotite garnetiferous schist and occurs as veins, lenses, or dykes within the THS (Figure 2a).…”

“…The GHS is represented by garnet-bearing micaceous HP/HT metapelites and their mafic counterparts, meta-psammites, and calc-silicate rocks (Figure 1b) spanning in age across Neoproterozoic to Cenozoic (Catlos et al, 2020;Kawabata et al, 2021;Scaillet et al, 1995;Sen et al, 2019Sen et al, , 2022. It is bounded by two crustal-scale shear zones having opposite shear senses, namely the Main Central Thrust (MCT) in the south, which brought GHS above the Lesser Himalayan Crystalline Sequence (LHCS) (Heim & Gansser, 1939) and the South Tibetan Detachment Zone (STDZ) in the north, which separates the Tethyan Himalayan Sequence (THS) juxtaposed above the GHS (Burchfiel et al, 1992;Burg et al, 1984;Celerier et al, 2008;Searle et al, 2003) (Figure 1a).…”