Petrology, geochemistry and zircon U–Pb–Lu–Hf isotopes of Paleoproterozoic granite gneiss from Bomdila in the western Arunachal Himalaya, NE India
Published:September 25, 2019
Manjari Pathak, Santosh Kumar, 2019. "Petrology, geochemistry and zircon U–Pb–Lu–Hf isotopes of Paleoproterozoic granite gneiss from Bomdila in the western Arunachal Himalaya, NE India", Crustal Architecture and Evolution of the Himalaya–Karakoram–Tibet Orogen, Rajesh Sharma, Igor M. Villa, Santosh Kumar
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Felsic magmatic bodies are exposed widely in the Bomdila region of the western Arunachal Himalaya, NE India. The litho-units of this region are primarily composed of two-mica (muscovite–biotite (ms–bt)) granite gneiss, referred to herein as the Bomdila granite gneiss (BGGn), and the metasediments of the Bomdila Group forming an integral part of the NE Lesser Himalayan thrust sheet. Phase petrology, whole-rock elemental geochemistry and zircon U–Pb–Lu–Hf isotopes of the BGGn have been investigated in order to decipher the origin and timing of felsic magmatism and its implications for understanding the pre-Himalayan tectonic environment. Modally, the BGGn can be classified into monzogranite, syenogranite and quartz-rich granitoids. The composition of muscovite (Ti = 0.03–0.07, average Na = 0.07 and AlIV = 2.50–2.90 apfu), biotite (FeOt/MgO = 3.1–4.6, 2Al⇌3Fe and Mg⇌Fe substitutions, and the presence of siderophyllite) and tourmaline (Fe/Fe + Mg = 0.56–0.96, Ca < 0.17 apfu) implies their primary magmatic nature crystallized typically in a peraluminous (S-type) felsic parental melt. This is further supported by the presence of ms–bt, whole-rock molar Al2O3/CaO + Na2O + K2O (A/CNK = 1.03–1.64) and normative corundum. Whole-rock multi-cationic parameters indicate a syn-collisional tectonic environment. However, the content of Rb (average 300 ppm) and high-field strength elements (HFSEs) in the BGGn indicates syn- to post-collisional tectonic settings. The BGGn parental melt was most likely to have been generated by dehydration melting of metasedimentary sources at middle–upper crustal depths. Geochemical modelling constrains the evolution of the parental melt of the BGGn by a moderate degree of fractional differentiation (F = 0.45) involving a biotite–plagioclase–K-feldspar–muscovite–titanite–apatite (bt–pl–Kfs–ms–ttn–ap) assemblage. Laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS)-analysed zircons from the BGGn yield a weighted mean 207Pb/206Pb age of 1752 ± 23 Ma as the crystallization age for the zircons in the BGGn melt, which falls well within the period of magmatism formed during the Columbia supercontinent accretionary orogen. The observed negative εHf(t) values (−1.67 to −7.99) and three-stage Hf-model ages (2818, 2586–2424 and 2393–2250 Ma) of zircons, strongly point to the involvement of ancient continental crust and source heterogeneity (Neoarchean–Paleoproterozoic continental crust) in the generation of the BGGn melt. The reworked ancient crustal components would have once been part of the northern Indian lithosphere, as indicated by the observed 207Pb/206Pb concordant ages (2436, 2136, 2013 Ma) of the inherited zircons.
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Crustal Architecture and Evolution of the Himalaya–Karakoram–Tibet Orogen
CONTAINS OPEN ACCESS
This volume comprises 17 contributions that address the architecture and geodynamic evolution of the Himalaya–Karakoram–Tibet (HKT) system, covering wide aspects, from the active seismicity of the present day to the remnants of the Proterozoic orogen. The articles investigate the HKT system at different scales, blending field research with laboratory studies. The role of various lithospheric components and their inheritance in the geodynamic and magmatic evolution of the HKT system through time, and their links to global geological events, are studied in the field. The laboratory research focuses on the (sub-)micrometre scale, detailing micro-structural geology, crystal chemistry, geochronology, and the study of circulating fluids, their preservation (trapped in fluid inclusions) and their evolution, distribution, migration and interaction with the solid host. An orogen over 2000 km long can be understood only if the processes at the nanometre and micrometre scales are taken into account. The contributions in this volume successfully combine these scales to enhance our understanding of the HKT system.