江南造山带存在新元古代(～850Ma)俯冲作用--来自皖南SSZ型蛇绿岩锆石SHRIMP U-Pb年龄证据

Neoproterozoic(～850 Ma) subduction in the Jiangnan orogen:evidence from the SHRIMP U-Pb dating of the SSZ-type ophiolite in southern Anhui Province

皖南伏川蛇绿岩是江南造山带上出露最完整的蛇绿岩，主要由地幔橄榄岩、堆晶岩、枕状熔岩和少量硅质岩组成。地幔橄榄岩主要是方辉橄榄岩，具有U型的稀土元素配分模式，副矿物尖晶石的Cr^#较高（54-60），堆晶岩组合为纯橄岩-易剥橄榄岩-辉长岩（DPG型），这些特征指示伏川蛇绿岩形成于俯冲带之上的构造环境，属SSZ型蛇绿岩。堆晶岩异剥橄榄岩的锆石SHRIMP U-Pb年龄为827±9Ma（n=12，MSWD=1．4），侵入到方辉橄榄岩中的辉长岩岩脉的锆石SHRIMP U-Pb年龄为848±12Ma（n=7，MSWD=1．9），说明扬子板块和华夏板块至少在848Ma之前就开始拼合，它们之间的古大洋在新元古代（～850Ma）之前就已经俯冲削减。

The Fuchuan ophiolite is preserved well in the Jiangnan orogen of South China. In Fuchuan area, ophiolite is composed of the largest mantle peridotite massif in the orogen, emulated rocks, pillow lavas and some ribbon cherts. The mantle peridotite massif includes major massive harzburgite with high-Cr^# value （54 to 60） spinels and U-shaped REE patterns. The cumulated rocks are composed of dunite, wehrlite and gabbro, thus being of the DPG type. All these features are consistent with those of the Izu-Bonin-Mariana peridotites, indicating that the Fuchuan ophiolite was formed in the supra-subduction zone （SSZ）. The SHRIMP U-Pb ages of the co-magmatic zircon domains from gabbro that intruded the harzburgite and the wehrlite are 848±12 Ma （ n = 7, MSWD= 1.9） and 827±9 Ma （ n = 12, MSWD = 1.4）, respectively. The SSZ-type ophiolite and its ages suggest that the subduction of the ancient ocean between the Yangtze terrain and the cathaysian （called ＂Jiangnan Ocean＂） terrain started before Neoproterozoic. It is inferred that the tectonic transform of the Jiangnan Ocean from spreading to subduction began before Neoproterozoic in southern Anhui Province, and the Yangtze terrain and the Cathaysian terrain started to collide before 848 Ma.