Nyainqentanglha Shan: A window into the tectonic, thermal, and geochemical evolution of the Lhasa block, southern Tibet

Abstract

In the Nyainqentanglha (NQTL) massif, southern Tibet, a late Cenozoic, SE dipping, normal fault exhumed an oblique section of crust in its footwall. U-Th-Pb dating of zircon and monazite from footwall exposures reveals a collage of felsic intrusions including Cretaceous-early Tertiary and Miocene granitoids. Ages of the latter span >10 m.y., suggesting semicontinuous or episodic Miocene magmatism. Geochemical and isotopic analyses show a Gangdese arc affinity, indicating significant mantle heat and mass transfer in their formation and semicontinuous calc-alkaline magmatism throughout the Cenozoic Indo-Asian collision. The undeformed nature of the footwall Cretaceous and Miocene granitoids suggests that Mesozoic-Cenozoic Lhasa block deformation was "thin-skinned," being concentrated in supracrustal assemblages. This, coupled with the lack of migmatites exposed in the NQTL, implies the exposed crust was not a partial melt zone nor involved in large-scale channel flow. Some 40Ar/39Ar thermochronologic studies of footwall K-feldspars reveal that samples collected within several kilometers below the normal fault cooled prior to emplacement of young leucogranites, indicating little perturbation of the background thermal structure since ∼15 Ma. This plus high melting temperatures and the lack of penetrative granitoid deformation requires that the melts formed at lower crustal levels and were emplaced rapidly to the midcrust. Seismic reflection results showing high "bright spot" anomalies in the midcrust along the NQTL rift may have imaged the youngest magmatic episode or its associated hydrothermal system.