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    U-Pb dating of zircon from the bed parallel anatectic graniteic intrusion in the Baban group in Hainan Island and the tectonic implication [查看] DINGShijiang、HUJianmin、SONGBiao、CHENMulun、XIEShengzhou、FANYuan
    Abstract The petrological and geochemical features of the bed parallel granitic intrusion in the Pre-Cambrian Baoban group in Hainan Island attest to the anatexis origin of the granites.U-Pb dating analyses of zircons from the anatectic granite and the biotite two-feldspar gneiss in the Baoban group, using SHRIMP II in the Beijing Ion-probe Center, acquire 206Pb/238U ages of 368±3.5 Ma (of granite, 95% confidence level, MSDW=1.23) and 362.9±6.1 Ma (of gneiss, 95% confidence level, MSDW= 2.04) respectively. The two late Devonian ages indicate consistently a tectonic- thermal event experienced in Hainan Island, and are the first discovered record of the Devonian geological process in the district. This event possibly resulted from the deep thermal-dynamic process when the Gondwana continent began to break up in the Devonian period.
    Identification of Hercynian shoshonitic intrusive rocks in central Hainan Island and its geotectonic implications [查看] XIECaifu、ZHUJinchu、DINGShijiang、ZHANG
    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 moder-ately depleted in Sr and Ti, with (87Sr/86Sr)i = 0.70859-0.71425 andεNd(t)=(− 2.77―− 7.49). They were derived from an EMⅡ-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 terri-genous sediments at great depth (eclogite facies) during the subduction of the South China plate under the Indochina-South China Sea plate in the Carbon-iferous–Early Permian. A SHRIMP U-Pb zircon dating yields a crystallization age of 272±7 Ma for the sho-shonitic 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 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 as-thenosphere. The rising of melts was accompanied by crustal contamination and crystallization fractiona-tion (AFC). Combining with other related data, it is proposed that the southwards subduction and amal-gamation 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 proc-ess of the Pangea supercontinent. The suture zone was probably located along the line of Song Ma-Beibu Gulf-north margin of the Yunkai Moun-tains-Wuyi Mountains.
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