Formation age and tectonic environment of the Gantaohe Group,North China Craton:Geology,geochemistry,SHRIMP zircon geochronology and Hf-Nd isotopic systematics

The Gantaohe Group is an important early Precambrian unit in the Trans-North China Orogen,North China Craton,and is mainly composed of greenschist-facies metabasalt,meta-sandstone and dolomitic marble.We report whole-rock geochemical compositions and SHRIMP zircon ages as well as LA-ICP-MS Hf-in-zircon isotopeic analyses for metabasalts from the Gantaohe Group.SHRIMP dating yielded a weighted mean 207Pb/206Pb age of 2087±16 Ma(MSWD=1.3) for magmatic zircons,but there are also abundant ca.2.5 Ga inherited zircon xenocrysts.The magmatic zircons shows a large Hf(t) variation in Hf(t) from 7.17 to +0.45,suggesting an isotopically highly heterogeneous source for the metabasalt.Chemically all samples show no distinct Zr or Hf anomalies,and some samples show no Nd or Ta anomalies in a primitive mantle-normalized trace element variation diagram,and their whole-rock Nd(t) values range from 4.0 to 0.8.We suggest that the basalt is formed by partial melting of a depleted mantle source,followed by significant crustal contamination.Field observations,the presence of abundant inherited zircon,as well as isotope and trace elements geochemistry support formation of the Gantaohe Group on top of a continental basement.These data and the regional geology lead us to conclude that the Trans-North China Orogen constituted an intracontinental rift during the Paleoproterozoic that was connected to the Eastern Block since the end of the Archean.

The Gantaohe Group is an important early Precambrian unit in the Trans-North China Orogen, North China Craton, and is mainly composed of greenschist-facies metabasalt, meta-sandstone and dolomitic marble.