Late Paleozoic Post-Collisional Multi-Stage Magmatism in Baiyinwula Area of CentralInnerMong,China

The Baiyinwula area is located in the west segment of the Xing’an-Mongolia orogenic belt, the southeast margin of the Central Asian orogenic belt. Although voluminous Paleozoic magmas have been reported in this area, their activity sequences and tectonic settings are still in debate. According to the field relationships, the sequences of magmas in the study area are set up in this paper: breccia-bearing rhyolitic ignimbrite + grey phenocryst-poor rhyolite (wall rock)→ green-grey phenocryst-rich rhyolite→ brick-red rhyolite porphyry→alkaline granite. LA-ICP-MS zircon U-Pb dating reveals that the breccia-bearing rhyolitic ignimbrite contains two group ages of 312.2±2.8 Ma (n=5，MSWD=0.098) and 293±2 Ma (n=10，MSWD=0.95). The green-grey phenocryst-rich rhyolite and brick-red rhyolite porphyry were emplaced at 296±2 Ma (n=21, MSWD=1.4) and 294±2 Ma (n=18, MSWD=0.2), respectively, slightly older than the age of alkaline granite (290~285 Ma). As a result, the magmatism exposed regionally can be recognized with three episodes of ca. 311 Ma, 297 Ma, and 291 Ma respectively. Geochemically, the grey phenocryst-poor rhyolite and green-grey phenocryst-rich rhyolite are characterized by potassic calc-alkaline series.In contrast, the brick-red rhyolite porphyry and alkaline granite are sodic alkaline-peralkaline series. Those volcanic-intrusive rocks exhibit an evolution trend from calc-alkaline to alkaline-peralkaline, which are highly consistent with that of the magmatism in the northern magmatic belt of the Erenhot-Hegenshan area, and those of the post-collisional magmatism in the typical orogenic belts. Accordingly, it is proposed that the Late Carboniferous-Early Permian magmatic rocks in the northern magmatic belt of the Erenhot-Hegenshan area were formed in a post-collisional tectonic setting.