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

Abstract

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