Deconvolving episodic age spectra from zircons of the Ladakh Batholith, northwest Indian Himalaya

Abstract

We present U/Pb SHRIMP dates of zircons from 7 key samples of the Ladakh Batholith to reconcile the many, often conflicting ages that have been proposed according to various geochronological techniques. Various crosscutting magmatic phases were dated to determine the age of granitoid emplacement. Our results indicate that granitoid emplacement occurred between 66Ma and 46Ma. The final phase of magmatism is characterized by the emplacement of crosscutting fine-grained granite (leucogranite), and pegmatite near the village of Chumatang. These dykes are defined as I-type granitoids according to their mineralogy. This indicates that I-type magmatism in the Ladakh Batholith continued until at least ~46Ma. This implies either that India had not accreted to Eurasia by 46Ma, or that one of the widely used geological criteria to define the timing of India-Asia collision is incorrect. In addition, SHRIMP analyses of metamorphic zircon rims yield ages between 20 and 10Ma. These ages may relate to the exhumation of the batholith or to metamorphic activity after exhumation. Evidence of metamorphic zircon growth between 20 and 10Ma may provide an explanation for many of the similar ages obtained from geochronometers that are affected or reset by temperatures lower than those expected for zircon (e.g. K/Ar and40Ar/39Ar dating of mica).