Correlation between magmatism of the Ladakh Batholith and plate convergence rates during the India–Eurasia collision

Evidence for episodic magmatism is found within large batholiths of ancient and modern convergent margin settings and is often attributed to regional geodynamic changes such as an increase in plate convergence rate, lithospheric extension and/or delamination, and intra-crustal/lithospheric shortening. The nature and timing of collision between India and Eurasia remain contentious as many models suggest that the “hard” collision occurred during the Early Paleogene (65 to 45 Ma) whereas other models suggest younger ages (~ 34 Ma) or a diachronous collision. New zircon LA-ICPMS U/Pb ages from rocks of the Ladakh Batholith range from 47.7 ± 0.7 Ma to 57.6 ± 0.7 Ma and correspond to two main episodes of magmatism. The two major episodes of magmatism are correlative to changes in the convergence rate of the Indian and Eurasian plates. The formation of adakitic rocks within the Ladakh Batholith at 49.2 ± 1.2 Ma is facilitated by partial melting of a garnet-bearing thickened lower crust. Therefore we suggest that crustal thickening was initiated by the “hard” collision between India and Eurasia. The convergence slowdown is concurrent with an increase in magmatism within the Ladakh Batholith. Thus the collision is constrained by the decreasing convergence rates at ~ 52 Ma and formation of the adakitic rocks at ~ 49 Ma.