Petrogenesis of newly identified mantle-plume-related ca. 3.3 Ga tholeiitic-komatiitic rocks in the Anshan area, North China Craton

Abstract

The geodynamic regime that governed the crustal evolution of Earth during the early Archean remains intensely debated. The North China Craton (NCC) preserves a geological history spanning 3.8–2.5 Ga, particularly in the Anshan area, making it an ideal natural laboratory for probing the formation and evolution of the NCC and its associated geodynamic regime. Here, we document new geochronology, geochemistry, and zircon Hf-O isotope data from newly discovered Paleoarchean meta-mafic rocks in the Anshan area. Based on their whole-rock geochemistry, these unit can be divided into tholeiitic and komatiitic series. Of which, the komatiitic rocks are characteristic by high MgO contents (up to 20.6 wt%). Zircon U-Pb dating shows that all meta-mafic rocks formed contemporaneously, with crystallization ages of ca. 3.3 Ga. Most of the studied meta-mafic rocks display radiogenic zircon Hf isotopes, with TDM1 ages ranging from 3592 to 3346 Ma and εHf(t) values from −1.94 to + 3.49. They also exhibit mantle-like zircon δ18O values (+4.92 ‰ to + 5.98 ‰). These zircon Hf-O isotope data indicate that the protoliths of the ca. 3.3 Ga meta-mafic rocks originated from the partial melting of a depleted mantle without significant crustal contamination. Furthermore, the parental magmas of the komatiitic rocks and some of tholeiitic rocks were generated from 20 % and 5–10 % partial melting of spinel-lherzolite, respectively, whereas other tholeiitic rocks were the result of a low degree (1–5 %) of partial melting of spinel-garnet lherzolite. The ca. 3.3 Ga tholeiitic-komatiitic rocks were most likely formed in an intraplate rifting setting, associated with a mantle plume.