吕梁群时代归属新证据

New Evidence for the formation age of the Lüliang Group

吕梁群是吕梁变质杂岩的重要组成部分。吕梁群的形成时代和构造背景长期存在争议,因其赋存有袁家村式铁矿和特殊的大地构造位置而倍受前寒武纪地质研究者关注。本次工作在宁家湾附近发现了侵入吕梁群袁家村组的变质辉长岩岩体,从中获得的锆石具有辉长岩中结晶锆石的典型特征,锆石U-Pb测定获得了2530±10Ma(SHRIMP)和2518±7Ma(LAICPMS)的年龄,代表辉长岩形成时代。锆石Hf同位素测试显示εHf(t)为4.26~9.69,tDM为2448~2646Ma,平均值为2588Ma,与锆石U-Pb年龄相近,表明辉长质岩浆来自亏损地幔源区。结合袁家村组的碎屑锆石年龄信息,可以厘定吕梁群的形成时代为新太古代晚期,而非以往许多人认为的古元古代。这与华北克拉通BIF几乎均形成于新太古代的认识一致。结合前人研究成果,吕梁群应形成于新太古代弧后盆地构造背景,故袁家村组含铁建造不属于典型的苏必利尔湖型铁矿,应是苏必利尔湖型与阿尔戈马型的过渡类型。吕梁群的形成时代与五台山区的五台群的形成时代相近,五台杂岩与吕梁杂岩通过云中山地区紧密相连,两者可能是同一太古宙陆块的组成部分。古元古代期间吕梁杂岩与五台杂岩的地质演化历史有所不同,吕梁山区古元古代的岩浆作用比五台山区更为强烈,既表现在岩浆活动的期次上也体现在规模上。

The Lüliang Group is an important part of the Lüliang metamorphic complex,due to its Yuanjiacun-type BIF and special tectonic location,but with formation age and tectonic setting having been controversial for a long time.This paper reports for the first the age of 2.5 Ga for meta-gabbro intruding the Yuanjiacun Formation of the Lüliang Group.Zircons from two meta-gabbro samples near Ningjiawan village show the typical characteristics of magmatic zircons from gabbroic magma and have weighted mean 207 Pb/206 Pb ages of 2530±10 Ma(SHRIMP)and 2518±7 Ma(LA-ICPMS).Hf isotope analysis on the zircons yeildedεHf(t)values of 4.26~9.69 and tDM of 2448~2646 Ma with a mean value being 2588 Ma.Therefore,the gabbro was formed at 2520~2530 Ma and was derived from depleted mantle.Combined with the age distribution of detrital zircons from the Yuanjiacun Formation,it is suggested that the Lüliang Group was formed at the end of the Neoarchean,rather than the Paleoproterozoic,as thought before.Based on previous studies and the zircon dating data in this work,the Lüliang Group is considered to form in a tectonic setting of the Neoarchean back-arc basin.The Yuanjiacun BIF(banded iron formation)of the Lüliang Group does not belong to the typical Lake Superior type,but a transitional type between Lake Superior and Algoma type.Most BIFs around the North China Craton formed in the Neoarchean.The formation age of Lüliang Group is similar to that of Wutai Group in Wutai Mountain area,and Wutai complex and Lüliang complex are closely connected through Yunzhongshan area,indicating that they may belong to the same Archean block.The Lüliang complex is somewhat different in Early Precambrian evolution from the Wutai complex with the Paleoproterozoic magmatism in the former being stronger,as indicated by being more frequency in periods and larger in scale.