Mid-Paleozoic ridge subduction in the Central Beishan of the southern Altaids: evidence from geochemical, Sr-Nd and zircon U-Pb-Hf-O isotopic data of Gongpoquan volcanic rocks

摘要

The investigation of Mid-Paleozoic tectonics could constrain the relations of early and late Paleozoic subduction events in the southern Altaids and improve our understanding of the architecture of accretionary orogens. The Gongpoquan volcanic rocks are widespread in the Central Beishan and contain important information on the tectonics of the southern Altaids. Geochronological data constrain the eruptive time of the Gongpoquan volcanic rocks as early Silurian-early Devonian (434-411 Ma). The Gongpoquan andesites show similar geochemical characteristics to those of typical sanukitoids, including high MgO and compatible element concentrations, enrichments in large ion lithophile elements and light rare earth elements, and depleted isotopic compositions. The geochemical data show that the Gongpoquan andesites were generated through the partial melting of subducting sediments and subsequent melt-mantle peridotite interactions. The early Silurian basalts are Na-rich and exhibit high Nb/U and Nb/La ratios, an enrichment in high field strength elements and enriched isotopic compositions, making them akin to Nb-enriched basalts. Such Nb-enriched basalts should be derived from the melting of a mantle component similar to ocean island basalt in the mantle wedge. The early Devonian basalts are calc-alkaline, enriched in large ion lithophile elements and depleted in high field strength elements. They yield high Ba/Yb ratios and depleted isotopic compositions, indicating an origin via the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids. Our data prove that the Paleo-Asian Ocean may still have been subducted during the Mid-Paleozoic. During this process, a spreading ridge was subducted beneath the Central Beishan, resulting in the opening of a slab window, which generated the observed 431-411 Ma sanukitoids and Nb-enriched basalts.