New Evidence For The Formation Age Of The Luliang Group

The Luliang Group is an important part of the Luliang 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. 5Ga for meta-gabbro intruding the Yuanjiacun Formation of the Luliang 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)ph ages of 2530 +/- 10Ma (SHRIMP) and 2518 +/- 7Ma (LA-ICPMS). Hf isotope analysis on the zircons yeilded epsilon(Hf) (t) values of 4. 26 similar to 9. 69 and t(DM) of 2448 similar to 2646Ma with a mean value being 2588 Ma. Therefore, the gabbro was formed at 2520 similar to 2530Ma and was derived from depleted mantle. Combined with the age distribution of detrital zircons from the Yuanjiacun Formation, it is suggested that the Luliang 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 Luliang Group is considered to form in a tectonic setting of the Neoarchean back-arc basin. The Yuanjiacun BIF ( banded iron formation) of the Luliang 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 Luliang Group is similar to that of Wutai Group in Wutai Mountain area, and Wutai complex and Luliang complex are closely connected through Yunzhongshan area, indicating that they may belong to the same Archean block. The Luliang 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.