INTRAPLATE TECTONICS IN ASIA - A PRECISE AGE FOR LARGE-SCALE MIOCENE MOVEMENT ALONG THE AILAO-SHAN-RED-RIVER SHEAR ZONE, CHINA

Tertiary left-lateral movement along the 1000 km long Ailao Shan-Red River shear zone appears to have played an important role in absorbing post-collisional northward penetration of India into Asia. Crustal strike-slip shear along this zone caused the formation of a gneiss belt, metamorphosed to amphibolite grade, including anatectic melting. Both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the Ailao Shan-Red River belt. 17 U-Pb isotope analyses were performed on small size-fractions of zircon, monazite and xenotime, extracted from two different leucogranitic layers. The two samples are located about 50 km apart in the central segment of Ailao Shan, in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation. All mineral U-Pb analyses had to be corrected for excess or deficit amounts of radiogenic 206Pb, originating from initial 230Th disequilibrium in the 238U decay series. In monazite, such disequilibrium 206Pb reaches 20% of total radiogenic 206Pb. The corrected U-Pb ages of monazite and xenotime lie between 22.1 and 23.9 Ma, whereas zircon yields significantly discordant U-Pb ages between 30.5 and 33.9 Ma pointing to Precambrian material in the magma source region. Inherited components could also be detected in monazite. The set of U-Pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early Miocene age of 23.0 ± 0.2 Ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the Ailao Shan-Red River shear zone. © 1990.