海南岛变碎屑沉积岩锆石SHRIMP U-Pb年龄及地质意义

U-Pb SHRIMP-Dating of Zircon Domains from Metaclastic Sedimentary Rocks in Hainan Island, South China, and Its Geological Significance

CL成像制约下的锆石SHRIMPU—Pb定年结果表明，海南岛北西部中新元古代石碌群的源区岩可能主要为同区的古中元古代抱板群和中元古代花岗质岩，推测其沉积上限约960Ma、下限约1300Ma；而岛中东部早古生代变沉积岩物源较为复杂，可能与新太古宙-古元古代或更老的洋壳基底、同期的基性火成岩及岛北两部的抱板群和中元古代花岗质岩有关，推测其沉积上限约470Ma、下限约514Ma或稍早。锆石SHRIMP年龄谱及Nd模式年龄，虽揭示海南岛东南部与北西部可能具不同的地壳增长历史，但两者约在1000~1200Ma同时经历格林威尔造山事件，并组成Rodinia超大陆一部分。由于晚元古代-寒武纪洋内弧残余在岛中东部可能出现，并于440～509Ma普遍经历一次变质事件，而岛北西则出现中元古代花岗质岩，因而我们认为Rodinia超大陆聚合前，海南岛北西部最可能是劳伧大陆南缘西延部分，而岛东南部更可能位于澳大利亚大陆东部边缘。研究结果还暗示，中新元古代石碌群可能产于陆-陆碰撞有关的前陆盆地，而早古生代火山-碎屑沉积岩可能产于洋内弧相关的环境，因而晚前寒武纪以来海南岛大地构造环境具有由被动大陆边缘经裂谷向活动大陆边缘演化的特点。

The diopside-tremolite rocks occur as interlayer with iron ore bodies at the top of the late MesoNeoproterozoic 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 continentcontinent 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.