ZIRCON U-PB AGES OF MAGMATIC ROCKS FROM MEISU FORMATION IN GERZE AREA IN TIBET AND ITS GEOLOGICAL SIGNIFICANCE

The Meisu Formation exposed in Gerze basin of southern Qiangtang, middle Tibet, is composed of red sandstones and conglomerates with volcanic rock interlayers. At present, its formation age is still lack of reliable geochronologic constraints as other researchers only acquired Paleocene-Eocene K-Ar ages(34.0~69.1 Ma) from type section and other places of Meisu Formation. Laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) method is used to determinate zircon U-Pb isotope from rhyolite, quartz trachyandesite, and granite secondary volcanic rock of sedimentary strata of Meisu Formation in Gerze basin. The results indicate that the average ²⁰⁶Pb/²³⁸U age of magma zircons of 3 volcanic samples and 1 intrusive rock sample are respectively 38.3±0.5 Ma, 39.0±0.3Ma, 37.4±0.4 Ma and 38.8±0.6 Ma, which defines the formation age of Meisu Formation of this area is late Eocene. The forming age of Meisu Formation of Gerze area provides chronological evidence of reactivation of Bangonghu-Nujiang Suture to regulate internal rigid block of Tibet Plateau "escape" to the East after India-Eurasia continental collision.