Geochemical Variations of the Late Paleozoic Granitoids from the Baolidao Arc-Accrection Belt in Southeastern Segment of Central Asia Orogenic Belt: Implications for Tectonic Transition from Early Carboniferous to Early Permian.

Compositional changes in successively erupted felsic rocks can be used to infer physical changes in lower crustal conditions and to enhance the understanding of the tectonic regime. This study presents geochronological, geochemical and isotopic data for two I-type granitic plutons in the Sonid Left Banner of the Central Asian Orogenic Belt. Our new data, together with compiled I-type granitoid data, reveal the presence of magma compositional transition at ∼305 Ma in the Baolidao arc-accretion belt. The early stage granitoids (330–305 Ma) are medium-K calc-alkaline with higher Sr/Y ratios. The late stage granitoids (305–270 Ma) are high-K calc-alkaline with lower Sr/Y ratios. The two-stage granitoids have roughly similar predominately positive Sr−Nd−Hf isotope values, but with a decreasing trend from the early to late stages. Geochemical data indicate that the early stage granitoids were generated by dehydration melting of juvenile mafic crust at amphibole-dominated depths. In contrast, the late stage granitoids were produced by dehydration melting of a mixed lithology containing juvenile K-rich mafic lower crust and supracrustal materials at the plagioclase-stable crustal level. We propose that the compositional transition of these granitoids can be linked with different slab behaviors of the northward subducting Paleo-Asian oceanic plate, and also with the back-arc tectonic settings. [ABSTRACT FROM AUTHOR]