Zircon U-Pb ages and geochemical characteristics of the quartz monzonite diorite rocks from Hanmiao area in the southern segment of the Da Hinggan Mountains

In order to determine the geochronology, petrogenesis and tectonic background of the quartz monzonite diorite rocks in Hanmiao area, the authors conducted the research on petrology by means of LA-MC-ICP-MS zircon U-Pb geochronology and geochemistry. The results show that the zircons from the quartz monzonite diorite rocks are magmatic zircons, and the zircon ²⁰⁶Pb/²³⁸U dating yielded 134±1Ma(n=22, MSWD=0.42), suggesting late Early Cretaceous. The values of SiO₂, K₂O and Na₂O/K₂O are 63.44%~68.96%, 2.04%~3.84% and 0.47~1.17 respectively. The aluminum saturation indexes (A/CNK) range from 2.12 to 2.55. The rocks thus belong to calc-alkaline-high-K calc-alkaline series. The rocks are enriched in LILE (Rb, K) and incompatible elements(Th, U), strongly depleted in HFSE such as Nb, Ta, P and Ti. Combined with the study of regional tectonics the authors hold that the quartz monzonite diorite rocks might have been affected by subducted slab fluids. It was closely related to back-arc extension tectonics and lithospheric thinning after the subduction of Paleo-Pacific plate towards Eurasia plate.