Utility of Rb–Sr geochronology in constraining Miocene and Cretaceous events in the eastern Karakoram, Ladakh, India

Multiple magmatic and deformational events, resulting from accretion of Gondwanan terranes to the northern margin of India and followed by post-accretion tectonics, have been recognized from the eastern Karakoram. The time resolution of these events is important for understanding the geodynamic evolution of the region. To constrain certain major events, conventional Rb–Sr geochronology was applied to the various phases of granitoids and their mineral separates, from mainly within the Pangong transpressional zone of the dextral Karakoram fault (KF), in order to show that this technique can date such young events.Calcalkaline gneissic granodiorite from within the transpression zone and calcalkaline tonalite enclaves within the Karakoram batholith (KB) at Skyangpoche, yield a whole rock date of 118±15 Ma, MSWD 1.5, Sri of 0.7045±0.0002 and whole rock-biotite age of 115±18 Ma with Sri of 0.7061±0.0002 and MSWD=1.5, respectively. Four samples of a mesocratic foliated granite within the Pangong migmatite, near the NE strand of the KF, yielded an errorchron corresponding to an age of 72±8 Ma, MSWD of 39 and Sri of 0.7041±0.0008. Phenocrysts of phlogopite from an ultrapotassic lamprophyre dyke within the Ladakh batholith, near Leh, yield an age of 24.3±2.2 Ma, with Sri of 0.7083±0.0001. A muscovite-whole rock isochron age of 13.3±0.6 Ma with Sri of 0.7249±0.0001, and MSWD=1.5 was measured from the Darbuk leucogranite, on the SW strand of the KF. A foliated granite, proximal to Muglib on the NE strand of the KF, yields a whole rock-biotite age of 12.4±0.7 Ma and Sri of 0.7056±0.0001.The Early Cretaceous ages of the granitoids, comprising early pre-Miocene plutonic components of the KB, are consistent with an U-Pb SHRIMP age of ∼106 Ma, and can be spatially correlated with the K2 gneiss (115–120 Ma) and the Hunza Plutonic Unit (106 Ma) of the KB in northern Pakistan. The Late Cretaceous age of foliated granite along the KF confirms that calcalkaline plutonism extended from Tirit, near the confluence of Nubra and Shyok rivers, to Rutog in western Tibet and was probably related to plutonism in the Ladakh magmatic arc. The earliest Miocene magmatism in SE Ladakh is represented by the ultrapotassic lamprophyre dyke at 24 Ma and spatially represents the westernmost extension of the voluminous ultrapotassic and potassic lavas in SW Tibet at this time (17–25 Ma). Following the intrusion of the main KB-forming leucogranite at 15 Ma (U-Pb SHRIMP crystallization age from zircon rims), the time of shearing on the SW strand of the KF is recorded by the age of muscovite from the Darbuk leucogranite at 13.3 Ma, comparable to that measured by Ar isotopic systematics (13.9 Ma) on mica aggregates from a granodiorite deformed by the KF in the Nubra valley. The biotite cooling age of the foliated granite within the Pangong migmatite is consistent with its cooling following high-grade migmatization, previously measured as having occurred between 17 and 14 Ma.