Paleogene magmatic flex and flux in the Ladakh Arc, NW Himalaya: Chronostratigraphy of the Khardung Formation

Abstract The Ladakh Arc was formed by subduction of Neotethyan oceanic lithosphere along the southern Eurasian margin before magmatic activity ceased upon the continental collision between India and Eurasia. The onset of arc magmatism is critical in terms of understanding when this subduction began. Termination of arc activity is associated with continental collision; thus, the youngest arc magmatism provides a maximum age for this collision. Intra-arc volcaniclastic sequences provide a stratigraphic record of arc magmatism, but also information on input from older basement rocks. We provide detailed U Pb detrital zircon chronostratigraphy for the Khardung Formation, which represents the volcanic equivalent of the Ladakh Batholith. The lower section of the Khardung Formation is mafic to andesitic and has a maximum depositional age of c. 60 Ma and the oldest inherited zircon age population from this lowermost section is c. 69 Ma. A switch to felsic volcanism occurred soon after and continued until at least c. 52 Ma. The youngest portion of the sequence has been removed by erosion or faulting along the Miocene dextral Karakoram Fault. However, detrital zircon data from the post-collisional Indus Group indicate that magmatism within the Ladakh Arc continued until c. 41 Ma. A compilation of U Pb zircon data for the Ladakh Batholith paired with our new data from the volcaniclastic sequence, reveal an inverse relationship between alternating pulses of volcanic and plutonic activity within the arc. We suggest, this magmatic pattern may relate to alternating episodes of extension (volcanism) and compression (plutonism) imposed on the arc.