40Ar/ 39Ar geochronology of post-collisional volcanism in the middle Gangdese Belt, southern Tibet

Abstract

40Ar– 39Ar step-heating experiments on eleven mineral separates have been conducted on eight volcanic rocks and a granite-porphyry dike from the Yangying and Wuyu basins in the middle Gangdese Belt, southern Tibet. New radiometric ages for sanidine and biotite separates in four volcanic rock samples from Yangying ranges from 10.32 ± 0.07 to 11.40 ± 0.11 Ma, whereas plagioclase and biotite separates from a stratigraphic section of the Gaza Cun Formation in the Wuyu basin give ages from 12.57 ± 0.08 to 13.2 ± 0.2 Ma. A granite-porphyry which cuts the lower part of the Gaza Cun Formation gives an age of 11.09 ± 0.07 Ma, and a dacite from the margin of Wuyu basin gives an age of 15.48 ± 0.11 Ma. These age data, in conjunction with geochemical data, suggest that the mid-Miocene post-collisional volcanic rocks from these two basins have both similarities and difference in their petrogenesis. The high [Sm/Yb] N (>7) ratio and high Sr concentration (423–1065 ppm) in both suites are consistent with their parental melts derived from partial melting of eclogitized lower crust. However, the Yangying samples are more evolved than the Wuyu samples as manifested by lower MgO, Sr/Sr∗, Eu/Eu∗, and normative plagioclase. It is likely that the source material for the Yangying suite is likely more enriched in K 2O than that of the Wuyu suite, perhaps has a greater contribution from metasomatized lithospheric mantle. It is apparent that the post-collisional volcanism in southern Tibet occurred spatially scattered and temporally within a short period in each volcanic basin.