Himalayan Geology, Vol. 42 (2), 2021, pp. 213-246, Printed in India
Tectono-magmatic evolution of granitoids in the Himalaya and Trans-Himalaya
SANTOSH KUMAR*, SHAILENDRA PUNDIR
Department of Geology, Centre of Advanced Study, Kumaun University, Nainital 263001, India
*Email (Corresponding author): skyadavan@yahoo.com
Abstract: The Himalaya represents an active young mountain belt formed during Tertiary collisional orogeny, and is mainly composed of Indian shield and cover rocks. The granitoids can be traced all along the Himalayan orogen and Trans-Himalayan belts. They are commonly represented by pre-collision (subduction-related) and syn- to post-collisional granitoids, granite gneisses, leucogranites, and pegmatites. This contribution presents a review on the nature, origin and tectono-magmatic evolution of pre-Himalayan to Himalayan and Trans-Himalayan granitoids and associated mafic rocks using published records. Pre-Himalayan granitoids (ca. 1900-1750 Ma, 900-800 Ma, 550-475 Ma) display calc-alkaline metaluminous (I-type) to peraluminous (S-type) character, which are derived from ancient crustal sources or mixed sources (mantle and crust) in the syn- to post-collisional environments. They have contributed to crustal growth to the northern part of the Indian lithosphere during Columbia, Rodinia and Gondwana supercontinent cycles, which are now tectonically juxtaposed predominantly in the Lesser Himalayan Sequence (LHS) and partly in the Greater Himalayan domain. A few metamorphic records of early Paleozoic thermal events over the Precambrian crystallines can be recognized but they are obliterated due to superimposed Tertiary orogeny. Two episodes of Tertiary (ca. 30-37 Ma, ca. 21-22 Ma) metamorphism, related to crustal shortening and thickening of the Indian plate margin followed by crustal melting and emplacement of leucogranites. The Himalayan leucogranites (ca. 9-25 Ma) are syn- to late kinematic, two-mica granites, and mainly occur in the Higher Himalayan anatectic zone bounded between Main Crystalline Thrust (MCT) in the south and South Tibetan Detachment (STD) in the north, formed by water-fluxed partial melting of metapelitic source, in a wide range of P=4-8 kbar and T=600-850°C. However, high-temperature (~850°C) Himalayan leucogranites probably generated due to asthenosphere upwelling along the convergent plate boundary during post-collision and volcanic arc tectonic environments.
Trans-Himalayan magmatic arc in the Ladakh and Karakoram regions represented by Andean-type, calc-alkaline, metaluminous to peraluminous diorites, granitoids, mafic to hybrid microgranular enclaves, syn- to post-magmatic dykes, and volcanics. They originated episodically during ca. 160-50 Ma in the context of northward subduction of Neo-Tethyan oceanic slab below the southern margin of the Asian continent. Detrital zircon ages from post-collisional Indus Group reveals that magmatism within the Ladakh arc continued until ca. 41 Ma, even after the collision of Indian-Asian continents at ca. 55±10 Ma. They bear signatures of mixing and mingling of crust- and mantle-derived magmas, as well as recycling of Proterozoic-Paleozoic crustal components in their genesis. In the eastern Karakoram and Ladakh, the arc rocks and associated heterogeneous (mixed) source rocks have undergone syn- to post-collisional deformational and melting events, mainly along Karakoram Fault (KF) zone, which produced leucogranite-pegmatite melts forming either dyke network or some at places small stock-like bodies.
Keywords: Himalaya, Trans-Himalaya, Granitoids, Petrology, Geochronology, Geodynamics
