Late Paleozoic magmatic flare-up in the Nuoergong-Langshan Belt, Alxa Block: Insights into tectonic evolution of the southern Paleo-Asian Ocean

Abstract

Magmatic flare-up events are widely recognized in continental arcs and have played a critical role in constructing arc crust. However, the geodynamic mechanism that triggers the Late-Paleozoic magmatic flare-ups in the Nuoergong-Langshan Belt (NLB) is still debated. We have identified a Late-Paleozoic magmatic flare-up in the Nuoergong-Langshan Belt (NLB), Alxa Block, based on new and published data. Zircon UPb dating shows that the diorites, intermediate dikes, and their host granites in the NLB crystallized at ca. 275–279 Ma, indicating contemporaneous emplacement of these rocks. Geochemical analyses, together with field observations, suggest that the intermediate dikes were probably formed through mixing of felsic and mafic magmas. The diorites have high MgO (3.2–4.0 wt%), Cr (70–80 ppm) contents, and enriched isotopic compositions and were probably derived from a lithospheric mantle source modified by subducted materials. The host granite of the intermediate dikes have similar isotopic compositions with the gabbro and diorite in the NLZ and were mainly generated by remelting of newly-formed mafic rocks and contaminated by ancient crustal components. Syntectonic folded veins and felsic veins intruding a fold core indicate a crustal shortening and thickening event in the early Carboniferous and early Permian in the NLZ. Our geochronological and geochemical data, coupled with published data, reveal that the magmatic flare-up events were simultaneous with crustal thickening in the southern CAOB, implying that flare-ups in continental arcs were probably related to crustal thickening. The temporal and spatial distribution of the late Paleozoic magmatic rocks in the northern Alxa Block reveals a retreating accretionary orogeny of the Paleo-Asian Ocean (PAO).