A Devonian arc–back-arc basin system in the southern Chinese Altai: Constraints from geochemical and Sr-Nd-Pb isotopic data for meta-basaltic rocks

Abstract

The early to middle Paleozoic accretionary history of the Chinese Altai orogen remains controversial, especially along its southern margin facing the Paleo-Asian Ocean. This study conducts a systematic investigation on the Devonian meta-volcanic sequences from the Altay Formation in the southern Chinese Altai to constrain their petrogenesis and tectonic implications. LA-ICP-MS zircon UPb dating for rocks from the bottom and top layers of the studied volcanic sequences indicates their eruption age at ca. 380 Ma. The basaltic rocks are low-K tholeiitic with high TiO2 (average = 1.98 wt%), and have rare-earth element (REE) patterns similar to the mid-ocean ridge basalt (MORB), in spite of a large variation in light REE (LREE). They also show enrichment in large ion lithophile elements (LILE; e.g. Ba, U, and Th) and depletion in high field strength elements (HFSE; e.g. Nb and Ta), which are hallmarks of supra-subduction zone basaltic magma. Thus, the elemental data indicate that these rocks are compositionally intermediate between MORB and island-arc tholeiite (IAT).The basaltic samples have positive εNd(t) values (+3.1 to +10.3) and elevated initial 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotopic ratios. Whole-rock elemental and isotopic data suggest that the rocks were likely derived from a dominant N-MORB type depleted asthenosphere mantle, which was variably metasomatized by or interacted with the relatively enriched sub-arc mantle. In combination with the field relationship and regional geology, our study suggests that the meta-basaltic rocks from the middle Altay Formation formed in a mature back-arc spreading environment. We propose an evolving tectonic scenario from a magmatic arc to arc rifting, and finally back-arc opening in the southern Chinese Altai, which corresponds to an extensional regime and resultant asthenosphere upwelling event in this region during Devonian time, probably in response to a trench retreat.