苏鲁造山带北缘五莲地区五莲群变质地层特征及形成时代——锆石MCICPMS UPb测年的证据

五莲群下部为海眼口组,一段以斜长角闪岩为主,二段以云母变粒岩、片岩为主;上部为坤山组,主要为白云石大理岩、方解石大理岩,下部夹石英岩、云母片岩。五莲群下部地层海眼口组电气石石英岩与黑云变粒岩最年轻的碎屑锆石MC-ICPMS U-Pb年龄数据为1685～1727Ma,这限定了五莲群原岩沉积时代属中元古代。而侵入海眼口组黑云变粒岩的花岗岩脉岩浆锆石206Pb/238 U表面年龄加权平均值为(777.5±1.8)Ma。表明花岗岩脉所代表的热事件与扬子北缘Rodinia超大陆裂解事件有关;说明南华纪时期,五莲群位于扬子地台的北缘,而不是华北地台的东南缘。所以将五莲群再从粉子山群中重新划出,恢复为原五莲群及海眼口组、坤山组。

Characteristics and Formation Era of Metamorphic Strata of Wulian Groupin Wulian Area in the North Margin of Sulu Orogenic Bel

The lower part of Wulian group is Haiyankou formation. Its first section is mainly composed of plagioclase amphibolite, and the second section is mainly composed of mica granulite and schist. Its upper part is Kunshan group, which is mainly composed of dolomite marble and calcite marble, while its lower part is accompanied by folder quartzite and mica schist. MCICPMS UPb age data of tourmaline quartzite in Haiyankou formation in lower part of Wulian group zircons and the youngest detrital zircon of biotite granulite is 1685~1727Ma, which limited the sedimentary stage of Wulian group as the Mesoproterozoic period. Weighted mean of zircon 206 Pb / of 238U surface age of veins and magma which intruded into biotite granulite of Haiyankou formation is (777.5±1.8) Ma. It is showed that thermal events which has been represented by granite veins have close relation with Rodinia supercontinent breakup events in the North margin of Yangtze plate. That is to say, during Nanhuaan period, Wulian group is located in the northern margin of the Yangtze platform, rather than the southeast edge of the North China platform. Thus, Wulian group should be marked out from Fenzishan gourp, and restored into the original Wulian group, Haiyankou group and Kunshan group.