U-Pb SHRIMP-DATING OF ZIRCON DOMAINS FROM METACLASTIC SEDIMENTARY ROCKS IN HAINAN ISLAND, SOUTH CHINA, AND ITS GEOLOGICAL SIGNIFICANCE

Abstract

The diopside-tremolite rocks occur as interlayer with iron ore bodies at the top of the late Meso-Neoproterozoic Shilu Group in Shilu iron ore mine, the northwest Hainan Island. The Early Paleozoic biotite schists are present probably as interlayer with the Chenxing metabasites in Tunchang area, the East-central Hainan Island. These metaclastics were dated by SHRIMP U-Pb analysis assisted by CL imaging of zircon, respectively. Zircons in the diopside-tremolite rocks of the Shilu Group yielded 207Pb/206Pb ages ranging from 1890 Ma to 900 Ma, with a primary cluster between 900 and 1330 Ma. In contrast, zircons in the biotite schists gave an older 207Pb/206Pb age of spanning from 2500 Ma to 980 Ma, as well as a younger, weighted concordant U-Pb mean age of 470 ± 16 Ma (MSWD=0.12), respectively. Because the Shilu sequences were deformed and metamorphosed from assembly of Rodinia, deposition probably occurred at ca. 960-1300 Ma, soon after crystallization of the youngest population of zircons and before or during the onset of Granville deformation. The. source rocks for the Group most likely are composed of the Paleo-Mesoproterozoic Baoban Group and the Mesoproterozoic granitoids in the same area. However, the provenances for the Early Paleozoic metaclastics in Tunchang area were rather complex and probably include about 2.5 Ga or older oceanic crust, Paleo-Mesoproterozoic Baoban Group, Mesoproterozoic granitic intrusive and about 514 Ma or earlier basic igneous rocks. Because the unit has a similar metamorphism to the Tasmanian fold belt adjacent to east Australia continent of East Gondwana, deposition probably occurred between about ca. 470 Ma and about 514 Ma or earlier, soon after crystallization of the youngest population of zircons and before or during the onset of Ross-Delamerian Orogeny (Boger and Miller, 2004). We infer that the southeast Island not only has an older basement than, but also have a less consistent crust growth history with the northwest Island. However, both of them probably had constituted a part of Rodinia supercontinent due to ca. about 1000 - 1200 Ma Grenville orogenic event, being earlier that that for South China (ca. about 800 - 1000 Ma). Thus the northwest Island more likely was attached to westerly extension of the southern Laurentia whereas the southeast Island to eastern Australia before assembly of Rodina. A foreland fold basin related to continent-continent collision is suggested for the deposition of the Shilu Group, whereas an intra-oceanic arc environment for the early Paleozoic volcanic-clastic sediments, respectively. The change of tectonic conditions at the late Precambrian to early Paleozoic transition probably reflected a change from a passive continental margin via rifting to an active continental margin.