Petrogenesis of ~2.1 Ga mafic and granitic magmatism and tectonic implication of Jiaobei Terrane in North China Craton

Abstract

Paleoproterozoic magmatism in the Jiaobei Terrane has not been well studied, but it is important for understanding the tectonic evolution of Jiao-Liao-Ji belt in the east North China Craton. In this work, ~2.1 Ga mafic and granitic intrusions in the Jiaobei Terrane, including the Xiliu amphibolite, Dongbaishishan granite and Gujiacun granite, are studied in detail. The Xiliu amphibolite intruded the Archean basement and the Dongbaishishan granite intruded the Paleoproterozoic Jingshan Group. SHRIMP zircon UPb geochronology shows that both amphibolite and granite were emplaced simultaneously at ~2.1 Ga and were metamorphosed at ~1.87–1.84 Ga. The ~2.1 Ga Xiliu amphibolite samples show a tholeiitic basalt composition; they exhibit enriched light rare earth elements (REEs) and large ion lithophile elements (LILEs, e.g., U and K), but depleted high field strength elements (HFSEs, e.g., Nb, Ta, P, Zr, Hf and Ti). They experienced the fractional crystallization of clinopyroxene. Whole-rock Nd isotopes show that the ~2.1 Ga amphibolites were generated from partial melting of the Archean enriched mantle, and that the mantle source was affected by the addition of subduction-related fluids/melts. The ~2.1 Ga Dongbaishishan and Gujiacun granite samples in the Jiaobei Terrane show granodiorite to granite compositions with metaluminous and calc-alkaline characteristics. They show decreasing Fe2O3T, TiO2, Al2O3, CaO, Na2O and P2O5, but increasing K2O, with increasing SiO2 contents. This indicates that FeTi oxide, plagioclase and apatite were fractionated during magmatic evolution, which is confirmed by the negative Eu, Sr, P, and Ti anomalies. Compared with the 2.18 Ga A-type granites derived from partial melting of tonalites in the Jiaobei Terrane, major elemental geochemistry and zircon Hf isotopes showed that ~2.1 Ga granites were generated from partial melting of the low-K mafic lower crust possibly due to mantle-derived magmatic underplating.The ~2.1 Ga Jiaobei amphibolites show similar geochemical and isotopic characteristics to the contemporaneous mafic rocks in the north Liaoji and south Anhui areas of the Jiao-Liao-Ji belt. Together with the volcano-sedimentary series and A2- and I-type granitic gneiss in the belt, these ~2.1 Ga mafic rocks probably formed in a subduction-related back-arc extension environment. In addition, both episodes of the 2.18 Ga and 2.1 Ga granites in the Jiaobei Terrane show high-temperature magmatic characters, which can be attributed to mantle-derived magmatic heating. The younger episode (2.1 Ga) granites indicate that there was stronger mantle-derived magmatism in a back-arc extended zone.