Zircon SHRIMP U-Pb dating for olivine gabbro at Wangmuguan in the Beihuaiyang zone and its geological significance

摘要

Zircon SHRIMP U-Pb dating was performed for olivine gabbro at Wangmuguan in the Beihuaiyang zone of the Dabie orogen and its country rock (garnet-bearing epldote-mica-quartz schist). The results show that the gabbro was crystallized at 635 +- 5 Ma, in the late Neoproterozoic rather than in the late Paleozoic as previously suggested; its country rocks formed at 464 +-7 Ma, younger than the enclosed gabbro. The U-Pb age for the gabbro is in good agreement with ages for tuff interbedded with sediments from the Doushantuo Formation in the South China Block and late-Neoproterozoic basic dyke swarms distributed on a large scale over areas of Suizhou to Zaoyang of Hubei Province in the northern margin of the South China Block. This suggests a large-scale magmatic activity occurred at the late Neoproterozoic in the South China Block, so that the gabbro at Wangmuguan in the western segment of the Beihuaiyang zone is geotectonic affinity to the northern margin of the South China Block. Since the olivine gabbro occurs within the schist of Ordovician protolith with tectonic contact between them but forming in different tectonic settings, it is concluded that the late-Neoproterozoic gabbro was detached from the Precambrian basement of the South China Block during the Triassic subduction of the South China Block, and tectonically thrusted over the metamorphosed rocks in the southern margin of the North China Block.