Geochronology and Geochemistry of the Kuwei Mafic Intrusion, Southern Margin of the Altai Mountains, Northern Xinjiang, Northwest China: Evidence for Distant Effects of the Indo‐Eurasia Collision

Abstract

The Kuwei mafic intrusion, consisting of hornblende gabbro, gabbro, gabbro norite, and olivine norite, lies in the southern Altai Mountains, northern Xinjiang. A combined field, geochronological, and geochemical study of the Kuwei intrusion is reported here. This study provides the first reliable SHRIMP U‐Pb zircon dating results for the intrusion, and these yielded an age of $$47\pm 1$$ Ma, which is the first documented report of Eocene magmatism in the region. The chondrite‐normalized rare earth element patterns for the Eocene intrusions are flat, and most of the incompatible elements are comparably depleted. Thus, geochemical data suggest that the Kuwei mafic intrusion was produced by partial melting of asthenospheric mantle that was slightly contaminated by lithospheric material. We interpret the 47‐Ma magmatism to result from asthenospheric mantle upwelling following the progressive India‐Eurasian collision. Although the Kuwei intrusion is laterally beyond the limit of Eocene deformation normally attributed to the India‐Asia collision, the timing of magmatism in the intrusion suggests that lateral extension may have initially affected a wider region than the area later thickened by convergence in the Tibetan Plateau. The Kuwei intrusion and other plutons likely related to it may have been emplaced into dilational jogs in fault systems activated by the India‐Asia collision. The emplacement depth is estimated to be ∼6 km, based on geobarometric determinations. Erosion was imperceptible before 25 Ma but has worn away an average of 0.024 cm of uplift every year since 25 Ma. The 6 km of exhumation since the late Oligocene is also attributed to far‐field effects of the India‐Asia collision.