Geochronology and geochemistry of low-grade metamorphic rocks from the Erdaowa Group and its significance on the tectonic evolution of the Paleoproterozoic Khondalite Belt, North China Craton

Abstract

Khondalite Belt is located in the northwestern part of North China Craton, and is a typical continent-to-continent collisional orogen, along which the Yinshan Block collided with the Ordos Block at 1.95–1.85 Ga. However, this model was mainly relied on high-grade metamorphic records in the Khondalite Belt, whether the extensively exposed low-grade rocks were also involved in such collision remains poorly constrained. Erdaowa Group is an important lithological unit of the belt and consists of extensive low-grade metamorphic rock assemblages (i.e. greenschist to amphibolite facies), and provide crucial insights into understanding the issue. Based on petrological and geochemical characteristics, low-grade metamorphic rocks of Erdaowa Group show a large affinity to bimodal volcanic sequence characterized by basaltic and acidic endmembers. Meta-mafic rocks were derived from partial melting of lithospheric mantle with minor contamination of crustal materials, whereas meta-acidic rocks were generated from the middle-to-lower crust. Geochemical data of both rock assemblages show depletion of HFSE and enrichment of LILE, indicative of a subduction-related magmatic arc environment. U-Pb zircon dating results indicated that these rocks erupted at 2465 ± 42 Ma, and experienced metamorphism at 1940–1861 Ma. In combination with available petrological, geochemical and geochronological data, we infer that arc magmatism was developed at a back-arc basin area of a ~2.45 Ga active continental margin along southern Yinshan Block. Together with those high-grade metamorphic rocks, these low-grade rocks were also incorporated into the final collisional event at ~1.95–1.90 Ga.