Geochemical characteristics and tectonic setting of the Early Paleozoic greenschist in the western segment of the southern Altyn Tagh marginal tectonic belt

Abstract：There outcrop many fresh greenschist tectonic blocks in the western segment of the southern Altyn Tagh marginal tectonic belt. Intensive petrological and geochemical studies show that: the protoliths of the greenschist are basic volcanic rocks (tholeiites), and in addition three types of basalt are also recognized, namely, oceanic island (within plate) basalt (OIB), mid-oceanic ridge basalt (E-MORB) and island arc basalt (IAT). The E-MORB have similar geochemical characteristics, with low Ti (TiO2=0.93%-1.75%, average 1.34%) and P (P2O5=0.09%-0.15%, average 0.13%) and the high field strength elements Zr=(79-146)×10-6, Hf=(2.6-3.5)×10-6 and Nb=(6.6-15.3)×10-6, being LREE-depleted or slightly enriched. IAT have lower Ti (TiO2=0.4%) and P (P2O5=0.08%) than those of MORB and the high field strength elements Zr=56×10-6, Hf=1.6×10-6 and Nb=5.9×10-6, being LREE-slightly enriched. OIB have high Ti (TiO2 = 1.96%-3.08%, average 2.49%) and P (P2O5=0.23%-0.34%, average 0.26%), with the high field strength elements Zr=(204-237)×10-6, Hf=(4.2-7.2)×10-6 and Nb=(14.6-22.1)×10-6, being LREE-enriched. The presence of the OIB, E-MORB and IAT assemblage suggests the existence of an ocean basin in the southern Altyn Tagh marginal tectonic belt during the Early Paleozoic.