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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Asia
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Himalayas
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Garhwal Himalayas (1)
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High Himalayan Crystallines (4)
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Indian Peninsula
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Bhutan (2)
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India
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Northeastern India
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Arunachal Pradesh India (11)
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Assam India (1)
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Rajasthan India
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Udaipur India (1)
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Uttarakhand India
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Garhwal Himalayas (1)
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Indian Shield (1)
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Nepal (1)
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Karakoram (1)
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commodities
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elements, isotopes
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Primary terms
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Asia
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Himalayas
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High Himalayan Crystallines (4)
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Kumaun Himalayas (1)
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Lesser Himalayas (6)
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-
Indian Peninsula
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Bhutan (2)
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India
-
Bhagirathi River (1)
-
Northeastern India
-
Arunachal Pradesh India (11)
-
Assam India (1)
-
-
Rajasthan India
-
Udaipur India (1)
-
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Uttarakhand India
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Garhwal Himalayas (1)
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Uttarkashi India (1)
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Indian Shield (1)
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Nepal (1)
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Karakoram (1)
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Main Central Thrust (4)
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Tibetan Plateau (1)
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carbon
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C-13/C-12 (1)
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magmas (1)
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metals
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rare earths (2)
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metamorphic rocks
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gneisses
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Bomdila Gneiss
Detection of a weak late-stage deformation event in granitic gneiss through anisotropy of magnetic susceptibility: implications for tectonic evolution of the Bomdila Gneiss in the Arunachal Lesser Himalaya, Northeast India
Structure, Stratigraphy and Magnetic Susceptibility of Bomdila Gneiss, Western Arunachal Himalaya, India
Abstract 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 Al IV = 2.50–2.90 apfu), biotite (FeO t /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 Al 2 O 3 /CaO + Na 2 O + K 2 O (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 207 Pb/ 206 Pb 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 ε Hf (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 207 Pb/ 206 Pb concordant ages (2436, 2136, 2013 Ma) of the inherited zircons.