Late Paleozoic-Early Mesozoic tectonic evolution of the Paleo-Asian Ocean: geochronological and geochemical evidence from granitoids in the northern margin of Alxa,Western China

摘要

The Paleo-Asian Ocean (Southern Mongolian Ocean) ophiolitic belts and massive granitoids are exposed in the Alxa block,in response to oceanic subduction processes.In this work,we report petrographic,geochemical,and zircon U-Pb age data of some granitoid intrusions from the northern Alxa.Zircon U-Pb dating for the quartz diorite,tonalite,monzogranite,and biotite granite yielded weighted mean 206pb/238U ages of 302±9.2 Ma,246.5±4.6Ma,235±4.4 Ma,and 229.5±5.6 Ma,respectively.The quartz diorites (～302 Ma) exhibit geochemical similarities to adakites,likely derived from partial melting of the initially subducted Chaganchulu back-arc oceanic slab.The tonalites (～246.5 Ma) display geochemical affinities of I-type granites.They were probably derived by fractional crystallization of the modified lithospheric mantle-derived basaltic magmas in a volcanic arc setting.The monzogranites (～235 Ma) are characterized by low Al2O3,but high Y and Yb with notably negative Eu anomalies.In contrast,the biotite granites (～229.5 Ma) show high Al2O3 but low Y and Yb with steep HREE patterns and the absence of negative Eu anomalies.Elemental data suggested that the biotite granites were likely derived from a thickened lower crust,but the monzogranites originated from a thin crust.Our data suggested that the initial subduction of the Chaganchulu oceanic slab towards the Alxa block occurred at ～ 302 Ma.This subduction process continued to the Early Triassic (～246 Ma) and the basin was finally closed before the Middle Triassic (～235 Ma).Subsequently,the break-off of the subducted slab triggered asthenosphere upwelling (240-230 Ma).