EPMA monazite U-Th-Pb chemical dating for monzogranites from Guangtoushan intrusion in western Qinling Mountains，Central China and its geological significance

he Guangtoushan granitoid pluton consists of early quartz diorites, tonalities and late intrusive monzogranites. Early lithological assemblages were deformed during the collision event between the South China Craton (SCC) and the Qinling-Dabie Microblock (QDM), and some rocks of them have been partly changed into mylonites, which indicates they were formed before the closure of Mianxian-Lüeyang paleo-ocean basin. However, later stage S-type monzogranites emplaced into early quartz diorites and tonalities and the ductile shear zones as one main mark of the closure of Mianxian-Lüeyang paleo-ocean basin, which reveals that the S-type monzogranites are correlated to the closure of the Mianxian-Lüeyang suture zone. Therefore, the geochronological data of Guangtoushan granitic pluton can be used to constrain the timing of collision between the SCC and the QDM. Two representative samples of monzogranites that formed at the later stage are dated using the electron microprobe monazite Th-U-Pb chemical dating method. The electron microprobe monazite Th-U-Pb data reveal three age groups and four age groups, respectively. Integrated distribution of the apparent ages on the BSE images, internal structures of analyzed monazite grains with isochron ages, the 209～196 Ma are interpreted as the main formation ages of the S-type monzogranites, which implies that the collisional event between the SCC and the QDM, namely closure of the Mianxian-Lüeyang paleo-ocean, occurred from Late Triassic to Early Jurassic. The older ages of 240±4 Ma, 318±5 Ma and 249±4 Ma could sign early geological events relating subduction of oceanic crust slab in the Mianxian-Lüeyang suture zone, which are ages of inherited monazite remnants. And the younger ages of 184±3Ma and 154±3 Ma reflect overprint events of deformation and fluid alteration during post-collision to late stages.