MESOZOIC MAGMATISM, METAMORPHISM AND EXHUMATION RECORDED BY THE AMDO BASEMENT, CENTRAL TIBET

The Middle Jurassic – Early Cretaceous Bangong suture zone marks the boundary between the two terranes, Qiangtang and Lhasa, which make up southern Tibet. The apparent absence of a Jurassic magmatic arc, Jurassic high-grade metamorphic rocks and major mid-Mesozoic deformation has contributed to the notion that subduction of the Bangong ocean and the resulting Lhasa-Qiangtang collision were relatively insignificant events in the tectonic development of central Tibet. The Amdo basement, an exposure of Precambrian gneisses and intruding granitoids along the Bangong suture, has been thought to have experienced amphibolite grade metamorphism only in the Cambrian. We present here new mapping, thermobarometry, thermochronology and geochronology of the Amdo basement which reveals a high-grade Middle Jurassic metamorphic event accompanied by extensive magmatism. U-Pb analyses of zircons from intruding granitoids show ~170-180 Ma crystallizations ages and suggest that the Amdo basement could represent an exhumed portion of the Bangong arc. In addition, our mapping and thermochronology reveal two periods of exhumation, one in the Middle Jurassic and one in the Early Cretaceous. 40Ar/39Ar ages of horneblende and mica, as well as a U-Pb analysis on titanite, show a rapid exhumation from lower-crustal levels to the mid-crust from about 185 Ma to 165 Ma. 40Ar/39Ar diffusion modeling ages on potassium-feldspar reveal a second period of relatively rapid exhumation ~130 Ma to upper-crustal levels. Shortening continued into the mid-Cretaceous as shown by a thrust fault involving the gneisses and associated metasedimentary rocks cutting a ~117 Ma granitoid. These new observations imply major crustal thickening and shortening associated with the Lhasa-Qiangtang collision. We interpret the prolonged exhumation, lack of observable arc and apparent lack of mid-Jurassic thrust faulting as a result of significant underthrusting of the Lhasa terrane beneath the Qiangtang. This crustal thickening, combined with the lack of post-Cretaceous erosion indicated by the 40Ar/39Ar potassium-feldspar data, indicate that central Tibet was relatively thick prior to the Indo-Asian collision.