Identification of Hercynian shoshonitic intrusive rocks in central Hainan Island and its geotectonic implications

摘要: An identification has been made of some shoshonitic intrusive rocks in central Hainan Island recently. These are K-rich (K2O=2.9%–5.1%, K2O/Na2O=0.95–2.12), distinctly enriched LILE and LREE, strongly depleted Nb, Ta, moderately Sr Ti, with (87Sr/86Sr)i = 0.70859–0.71425 eNd(t) (−2.77–−7.49). They were derived from an EM II-type mantle source. The enrichment process is related to metasomatism caused by a great amount fluid-melt released oceanic crust terrigenous sediments at depth (eclogite facies) during the subduction South China plate under Indochina-South Sea Carboniferous-Early Permian. A SHRIMP U-Pb zircon dating yields crystallization age 272±7 Ma for intrusions, which coeval peraluminous granites found Island. two kinds generally possess syn-intrusion ductile deformation structures. Thus they considered have generated early stage (syn-thrust phase) post-collisional event. primary magma was produced > 80 km decompression-dehydration melting previously lithospheric wedge, phlogopite-bearing garnet peridotite, turn break-off descendent slab upwelling hot asthenosphere. rising melts accompanied crustal contamination fractionation (AFC). Combining other data, it proposed that southwards amalgamation Indo-China-South took place ca. 287–278 Ma, part convergence Pangea supercontinent. suture zone probably located along line Song Ma-Beibu Gulf-north margin Yunkai Mountains-Wuyi Mountains.