Mineralisation associated with the fractionated Cretaceous Baoshan Monzogranite: Tectonic implications for South China

摘要

Jurassic and Late Cretaceous granites are spatially and temporally associated with mineralisation in the Paleozoic Dayaoshan Terrane in South China (also known as South China Block). The porphyritic Baoshan Monzogranite of Late Cretaceous is an example that is petrographically studied in this contribution. Sensitive High-Resolution Ion-Microprobe (SHRIMP) zircon U-Pb ages, in-situ zircon O-Hf isotopic analyses, and whole rock geochemistry are here used to better constrain the genesis of the monzogranite, which is porphyritic, and located in the Baoshan Cu mining area. SHRIMP zircon dating yields a weighted mean Pb-206/U-238 age of 89 +/- 1 Ma, interpreted to be the crystallisation age of the porphyritic monzogranite. Its geochemical data indicates it is derived from partial melting of the lower to crust, followed by fractionation, and emplacement in secondary faults related to the major Bobai-Cenxi Fault. The monzogranite has a Paleo- to Mesoproterozoic source in the crust, which was metasomatised during Neoproterozoic subduction. The rotation of Izanagi Plate's subduction from NW to NE resulted into the reactivation of NW and NE-trending thrust faults as transpressional or extensional ones. It was during this period that Late Cretaceous intrusions such as the Baoshan porphyritic monzogranite were emplaced in the terrane near the NW-trending faults and other intrusions at the edges of basins such as Yangchun, Luoding and Bobai basins near NE-trending faults.