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

• Published in: Chinese science bulletin Vol. 51; no. 20; pp. 2507 - 2519
• Main Authors: Xie, Caifu, Zhu, Jinchu, Ding, Shijiang, Zhang, Yeming, Fu, Tai’an, Li, Zhihong
• Format: Journal Article
• Language: English
• Published: State Key Laboratory for Mineral Deposits Research and Department of Earth Sciences, Nanjing University, Nanjing 210093, China 01.10.2006; Yichang Institute of Geology and Mineral Resources, Yichang 443003,China%State Key Laboratory for Mineral Deposits Research and Department of Earth Sciences, Nanjing University, Nanjing 210093, China%Hainan Bureau of Geology and Mineral Resources, Haikou 570226,China%Yichang Institute of Geology and Mineral Resources, Yichang 443003,China
• Subjects: U-Pb定年; 侵入岩; 海南岛; 海西期; 碰撞事件; SHRIMP U-Pb dating; Hercynian; shoshonitic intrusive rocks; post-collisional event; enriched mantle; Hainan Island
• ISSN: 1001-6538; 1861-9541
• DOI: 10.1007/s11434-006-2122-0

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 CNd（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 decompressiondehydration melting of previously enriched litho- spheric 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 IndoChina-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.