Zircon U–Pb and Hf isotopic constraints on petrogenesis of the Cretaceous–Tertiary granites in eastern Karakoram and Ladakh, India

Abstract

Multiple subduction events occurred preceding the collision of Greater India with the Karakoram terrane that consumed the Neotethyan Ocean between Early Cretaceous and Eocene time. Zircon U–Pb dating and Hf isotope analyses of granitic rocks collected from the Karakoram terrane and Ladakh batholith have been conducted to constrain these geological processes. Undeformed Ladakh batholith granodiorite yielded zircon U–Pb ages of 68 and 64 Ma with ε Hf( t) values of + 6.4 and + 10.3, and three diorites near Leh and Darbuk, gave zircon U–Pb ages of 50.4–51.0 Ma and ε Hf( t) values of + 7.4 to + 8.9, indicating that the Paleocene–Eocene was the most important period of granitic magmatism that derived from a juvenile source. Three tonalite–granodiorite enclaves at Skyangpoche within the Karakoram batholith have zircon U–Pb ages of 101–103 Ma with ε Hf( t) values of − 2.3 to − 3.7, indicating generation of granitic rocks by partial melting of the Karakoram basement. Three samples of the Miocene Karakoram batholith, from around Darbuk, yielded ages of 16.6–19.1 Ma with ε Hf( t) values of − 8.5 and − 10.5, that indicate an input of ancient crust from the Indian plate. Granodiorite within the Karakoram metasedimentary sequence (zircon U–Pb age ∼ 74 Ma, ε Hf( t) of + 5.4 from the Tangtse gorge) and granite around Muglib (zircon U–Pb age of 63 and 56 Ma, ε Hf( t) of + 6.8 and + 6.0) indicate similar geochronological and isotopic characteristics to those of the Ladakh batholith. Thus the time resolved Hf isotopic compositions of magmatic zircons from the granites in the Karakoram and Ladakh batholith show a large variation that indicate their derivation from both ancient and juvenile continental crusts, in active continental margin since at least the Mid Cretaceous.