Constraints on the left lateral shearing and crustal melting of the Ailaoshan Massif, Yunnan Province, Southwest China

摘要

The Ailaoshan-Red River shear zone (ASRR) in SE Tibet accommodated about 20% convergence of the Cenozoic India-Asia continental collision which resulted in complicated thermal history during Paleocene to Miocene. The Ailaoshan massif, in the middle part of ASRR, is characterized by extensive mylonitization and migmatization, but the exact timing of the large scale left lateral shearing and whether the crustal partial melting occurred prior to or coeval with the left lateral shearing remain controversial. The detailed structural observation suggests the pre-kinematic nature of the Ailaoshan corundum with respect to the left lateral shearing. LA-ICP-MS U-Pb dating of metamorphic zircon and monazite inclusions in the pre-kinematic Ailaoshan corundum obtained a weighted Pb-206-U-238 ages of 28.0 +/- 0.5 Ma (MSWD = 3.3; N = 27) and 28.3 +/- 0.3 Ma (MSWD = 1.8; N = 18), respectively, which gave an upper limit on the initiation of the left lateral shearing in the Ailaoshan massif. Moreover, the presence of protogenetic siliceous melt inclusions in the pre-kinematic corundum indicates that the Cenozoic crustal anatexis of the Ailaoshan massif has occurred before the left lateral shearing. Along with available geochronological data, the crustal partial melting of the Ailaoshan massif was attributed to joint results, which at least include the Neoproterozoic convergence, the Triassic Indosinian Orogeny, and the Cenozoic evolution as a response to the India-Asia continental collision.