狼山北部高Sr/Y花岗岩成因及其对阿拉善地块晚古生代构造演化的启示OA

Magmatic-tectonic events in theAlxa block since the Early Paleozoic are interpreted as a response to the southward subduction of the Paleo-Asian Ocean.However,the timing of the ocean's closure remains highly debated.Early Permian adakitic porphyritic granites,widely exposed in the Tugurige area of the northern Langshan,Alxa block,provide key insights into the tectonic setting of the Alxa block and the closure timing of the Paleo-Asian Ocean.LA-ICP-MS zircon U-Pb dating of the porphyritic granite yields a crystallization age of 286 Ma,coinciding with a Permian magmatic hiatus during in the Alxa block.Geochemical analysis reveals that the granite is aluminum-rich(Al2O3:15.26％～15.48％),alkaline-rich(Na2O+K2O:7.95％～8.25％),and has low MgO(0.73％～0.84％)and Mg#values(42.83～44.20).It is strongly depleted in heavy rare earth elements(Y:4.7 × 10-6～5.7 × 10-6;Yb:0.39 × 10-6～0.46 × 10-6)and exhibit high Sr(628× 10-6～743× 10-6)and high Sr/Y ratios(123.14～143.40),characteristic of adakitic rocks.Magmatic zircons have εHf(t)values ranging from-0.37 to+4.01,and whole-rock Nd isotopes show a slightly depleted signature(εNd(t):-4.08 to-3.76).Combined with the Late Paleozoic Hf and Nd isotope evolution characteristics of magmatic rocks and crustal thickness changes in the Alxa block,this study suggests that from the Early to Middle Permian,the block was in a tectonic setting dominated by prolonged subduction and slab rollback of the Paleo-Asian Ocean.The Early Permian adakitic intrusive rocks in Tugurige were likely generated in an extensional environment induced by slab rollback of the subducted Paleo-Asian Oceanic plate.This extensional environment triggered mantle-derived upwelling,leading to the partial melting of the thickened lower crust to form of adakitic magma.This magma subsequently experienced limited mixing with coeval mantle-derived mafic magma during its ascent.These findings suggest that the Paleo-Asian Ocean in the Alxa block had not yet closed prior to the Late Permian.