Migmatization and intrusion of “S‐type” granites in the trans‐Himalayan Ladakh Magmatic Arc of north India and their bearing on Indo‐Eurasian collisional tectonics.

The Ladakh Magmatic Arc (LMA) of trans‐Himalaya was formed due to subduction of the Tethyan Ocean beneath the Eurasian continental plate. This was followed by the Indo‐Eurasian continental collision that gave rise to the Himalayan orogen. In this work, we present results from U–Pb geochronology of zircon of 2 samples from a previously unknown migmatite body within the LMA and one two‐mica granite body that lies adjacent to this migmatite and intrudes the LMA. One sample of mesosome from this migmatite gives a crystallization age of 66.9 ± 1.5 Ma (n = 11 zircon) along with a younger population of zircon (n = 24) giving an age of 62.1 ± 1.3 Ma. This sample also shows the presence of a younger cluster of zircon (n = 7) giving a concordant age of 50.0 ± 2.9 Ma. The other sample of one leucosome contains a cluster of inherited zircon (n = 5) giving a concordant age of 72.86 ± 0.83 Ma. However, the majority of the population (n = 10) gives a concordant crystallization age of 55.31 ± 1.5 Ma. Limited Hf isotopic analysis of the leucosome shows positive εHft values ranging from 7 to 15, comparable to that of the LMA. High and positive εHft values and absence of any Palaeozoic xenocrystic zircon help us infer that the Indian continental crust was not involved in the partial melting and leucosome generation process. Therefore, this event of migmatization predates Indo‐Eurasian collision. By comparing Th/U ratio versus age for both the mesosome and leucosome and their respective age spectra, we infer that this part of the LMA underwent pronounced partial melting between 55 and 60 Ma and generated leucosome till ~50 Ma. The two‐mica granite contains muscovite, indicating its crustal origin. It gives 2 closely spaced age spectra of 38.1 ± 1.1 Ma (n = 7) and 34.31 ± 0.48 Ma (n = 4). This two‐mica granite also contains xenocrysts of Palaeozoic ages. By integrating our studies with earlier works, we infer that this migmatization of the LMA indicates its collision with the Indian plate, prior to Indo‐Eurasian continental accretion, at 55–50 Ma. The two‐mica granite contains xenocrystic material from both the LMA and the Indian continental crust and indicates S‐type granite magmatism related to India–Eurasia continental collision at 35–40 Ma. [ABSTRACT FROM AUTHOR]