Initial opening of the southern Paleo-Asian Ocean: evidence from Neoproterozoic magmatism in the Beishan area, southern central Asia orogenic belt, NW China

Abstract

The Beishan area (BA), located along the southern margin of the Central Asian orogenic belt, represents a key region for studying the opening of the southern Paleo-Asian Ocean (PAO) during the Neoproterozoic Era. Current understanding of BA’s Neoproterozoic tectonic evolution remains limited because of insufficient constraints. This study presents petrological, whole-rock geochemical, Sr–Nd isotopic, zircon U–Pb dating, and Hf isotopic analyses of gabbro (765 Ma), dacitic tuff (756 Ma), syenogranite (764 Ma), and monzogranite (770 Ma) from the BA. The gabbros show high MgO (8.06–8.31 wt%) and low K₂O (0.23–1.09 wt%) contents, depletion of high-field-strength elements (Nb, Ta, and Ti), (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7051–0.7057, and positive ε_Nd(t) values (+ 2.55 to + 3.19). These geochemical signatures suggest derivation from a depleted mantle metasomatized by subducted slab-derived melts, consistent with formation in a back-arc rift setting. The dacitic tuffs display negative ε_Nd(t) values (− 16.38 to − 1.29) and Zr-Hf enrichment, indicating an origin from ancient crustal melting. The syenogranites and monzogranites are characterized by high 10,000 Ga/Al and (Zr+Nb+Ce+Y) values, classifying them as A1-type granites. They yield negative ε_Nd(t) values (− 6.66 to − 2.29) and variable ε_Hf(t) values (− 3.7 to + 4.8), indicating partial melting of Paleoproterozoic crust material. Based on these findings and previous studies, we propose that the initial opening of the southern PAO in the BA occurred no earlier than 770 Ma, likely driven by combined subducted slab rollback and mantle plume upwelling.