Impact of slab tearing along the Yadong-Gulu rift on Miocene alkaline volcanism from the Lhasa terrane to the Himalayas, southern Tibet.

Slab tearing is widely reported in oceanic slabs; however, tearing in continental slabs is still not very well understood. Geophysical data have shown the existence of tearing of the Indian lithosphere underneath the Yadong-Gulu rift in southern Tibet. Along this rift, the Jiacun lamprophyres and the Yangying trachytes comprise the youngest alkaline volcanic rocks in the Himalayan-Tibetan orogen, and hence provide evidence for understanding the operation of continental slab tearing. Jiacun lamprophyres, with an age of 13 Ma as determined by Ar-Ar dating, are the only outcrop of alkaline volcanic rocks in the Himalayan block. Geochemical analysis indicates that they were derived from a peridotite source in the Indian lithospheric mantle near the spinel field. Yangying trachytes, dated at 8.81 ± 0.15 Ma by the U-Pb dating method, were derived from a pyroxenite melt in the Tibetan lithospheric mantle with a higher crustal influence. Both sites show high phlogopite and pyroxene temperatures, indicating a hot influx favoring the melting of these magmas, which is likely associated with the tearing of the Indian slab. Ages of this magmatism suggest that the activity along the rift lasted at least 4 m.y. and migrated from south to north. This shows that slab tearing can trigger over-thickened lithospheric melting in a collisional orogen. [ABSTRACT FROM AUTHOR]