Three types of Triassic granitoids in Changning‐Menglian suture zone: Petrological, geochemical, and geochronological constraints for source characteristics and petrogenesis

The Triassic granitoids are widely exposed along the Changning‐Menglian suture zone (CMSZ) in SW China, which are the products of Palaeo‐Tethyan closure and are characterized by highly variable geochemical compositions. However, details of the implied source heterogeneity and magma genesis have not been well constrained. In this study, we present zircon U–Pb isotopes and trace elements, whole‐rock major and trace element compositions and Sr–Nd isotopes of Triassic granitoids from the giant Yunling and Lincang plutons developed in the CMSZ. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating indicates that they were emplaced at ca. 227–216 Ma. Geochemically, these Triassic granitoids can be classified into three types. The Yunling low‐silica granodiorites (Type 1) are characterized by the presence of amphibole‐bearing mineral assemblages, high TiO2 and FeOT + MgO, belonging to I‐type granite and calc‐alkaline series. The Lincang low‐silica granodiorites (Type 2) are metaluminous to peraluminous with the occurrence of amphibole, consistent with transitional I‐S‐type granites. Compared to Type 1 and 2, the Lincang high‐silica (SiO2 > 70%) alkali feldspar granites and syenogranites (Type 3) display lower CaO, TiO2, P2O5, MgO + FeOT, Eu, Sr, and Zr contents, belonging to fractionated‐type granites. Three types of granitoids collectively show enriched Rb, Th, K, and Pb, depleted Ba, Sr, Ti, and Eu, negative whole‐rock εNd(t) (−11.0 to −10.7) and old TDM2 ages of 1,886–1,855 Ma, indicating the affinity of middle‐upper crustal derivation. New zircon U–Pb dating yields an age range of 2,391–322 Ma for inherited zircons with two clusters at ca. 460 and 956 Ma, suggesting that their crustal source might be the basement of the Simao Block rather than the basement of the Proterozoic Lancang Group. Diverse geochemical compositions indicate that the parental magma of the Triassic granitoids in the CMSZ originated from the partial melting of a heterogeneous source, and experienced magma mixing, assimilation with Lancang Group meta‐sandstone and fractional crystallization. Considering the exhumation of the high‐pressure metamorphic rocks in the CMSZ at Triassic, the upwelling of the asthenosphere after the Palaeo‐Tethyan closure increased the geothermal gradient of the lithosphere and triggered extensive crust melting in a post‐collisional setting.

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