Petrogenesis of the 2090 Ma Zanhuang ring and sill complexes in North China: A bimodal magmatism related to intra-continental process

Abstract

It is widely debated as to whether the ∼2100Ma bimodal magmatic events in the Eastern North China craton represent intra-continental rifting or marginal processes. The genetic relationship between the two components of bimodal suites is the key solution to this issue; and this is investigated on a group of newly identified ring complexes, as well as the apparently associated sills in Zanhuang (Southern Taihang Mts.). These ring complexes comprise varied components: pyroxenite, gabbro, anorthosite, K-rich A-type granite and minor Na-rich A-type granite. U-Pb analyses on baddeleyite and zircon grains liberated from a gabbro sample both yield a ∼2090Ma age, which represents crystallization timing and is contemporaneous with A-type granites of the ring complexes, as well as the sills in the Gantaohe Group. The mafic rocks of the ring complexes and sills show similar chemical features: they were characterized by enriched light rare earth elements compared to middle and heavy ones (La/Ybn=1.4−2.8), enriched large lithosphile elements but depleted high field strength elements, and enriched Sr-Nd isotopes (εNdt (t=2090Ma)=−1.9–0.6, Nd TDM=∼2.7Ga). They were most likely originated from the same magma source resided in the sub-continental lithospheric mantle, which formed in the Late Archean. The compositional variation of the mafic sills may be resulted from the fractionation of olivine and plagioclase. The K-rich A-type granite, as well as the dacitic sills, could be originated from this same source with firstly olivine+plagioclase-dominated fractionation in the lower magma chamber to more felsic compositions, and then with significant removal of clinopyroxene±plagioclase in the upper magma chamber, which at the same time produced pyroxenite-gabbro and minor anorthosite cumulates in the ring complexes. Na-rich A-type granite was likely formed locally under relatively high-water fugacity with fractionation of K-feldspar+plagioclase±clinopyroxene from a parental magma similar to the K-rich A-type granite. This bimodal Zanhuang ring and sill complexes, and other comparable bimodal suites widely distributed in the Eastern North China craton, were likely intra-continental, representing a craton-scale extension at ∼2100Ma.