Petrogenesis of late Permian–middle Triassic volcanic rocks in the Linxi area, southeastern Inner Mongolia, China: implication for late-stage tectonic evolution of the Central Asian Orogenic Belt

The late Paleozoic–early Mesozoic magmatic rocks along the Solonker suture zone in southeastern Inner Mongolia provide significant insights into the late-stage tectonic evolution of the Central Asian Orogenic Belt (CAOB). Here, we report zircon U–Pb ages, whole-rock geochemistry, and Sr–Nd isotopic compositions for volcanic rocks from the Linxi area, south of the Solonker suture zone. The late Permian (256 Ma) Xingfuzhilu volcanic suites (XFVS) are dominated by trachyandesite, andesite and trachydacite with minor basaltic andesite, dacite and rhyolite; whereas, the middle Triassic (237 Ma) Heishantou volcanic suites (HSVS) consist of basaltic trachyandesite, trachyandesite and small amounts of trachydacite. Two groups of the XFVS and HSVS have been identified based on their geochemical and Sr–Nd isotope features, respectively. Group 1 rocks of the XFVS show calc-alkaline affinity and have relatively low SiO 2 and variable MgO and total alkalis contents. Group 2 rocks have high SiO 2 and low MgO contents. Both of them show variable Cr and Ni contents, LREE and LILE enrichments, Nb–Ta–Ti depletion, low initial 87 Sr/ 86 Sr values, and high ɛ Nd ( t ) (+ 5.2 to + 6.7) values. These features suggest that the parental magma of the XFVS was likely derived from a metasomatized lithospheric mantle. The petrogenesis of Group 1 rocks were controlled by the process of fractional crystallization, whereas Group 2 rocks were dominated by both fractional crystallization and crustal contamination. In comparison, Group1 rocks of the HSVS are mafic in composition and have high MgO contents and depleted isotopic compositions ( ɛ Nd ( t ) = + 4.0 to + 4.2), and were probably derived from a metasomatized lithospheric mantle. Group 2 rocks show high Sr/Y and La/Yb ratios, and low Y and Yb concentrations, with variable ɛ Nd ( t ) values (+ 1.8 to + 5.6), and were likely generated by partial melting of delaminated lower crust. Generation of the late Permian XFVS were probably in an extensional setting associated with the break-off of previously subducted Paleo-Asian oceanic slab along the Solonker suture zone. In contrast, the middle Triassic HSVS were formed in the post-orogenic setting, as a magmatic response to the destruction of the CAOB.