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, geochem- ical, 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 o f 302 ±9.2 Ma, 246.5±54.6 Ma, 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 monzo- granites （-235 Ma） are characterized by low A1203, but high Y and Yb with notably negative Eu anomalies. In contrast, the biotite granites （-229.5 Ma） show high A1203 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）.
Xin SHAJinrong WANGWanfeng CHENZheng LIUXinwei ZHAIJinlong MAShuhua WANG
