Abstract:

Zircon U-Pb ages of four granitoids in Xinkailing region, Northwestern Xiao Hinggan Mountain, are reported in this paper. They are (187.7±1.4) Ma for the Datoushan quartz diorite, (170.7±1.3) Ma for the Dapingshan biotite monzogranites, (128.0±1.1) Ma for the East Fudiyingzi biotite monzogranite porphyry and (120.6±0.6) Ma for biotite granite porphyry dyke, respectively. Combined with previous zircon age datas, the Mesozoic granitic magmatism in this area can be divided into two stages, Early-Middle Jurassic (188-164 Ma) and Early Cretaceous (128-106 Ma), which are comparable with NE China and Far East Russia. From Jurassic to Cretaceous, the granitoids show clear evolution characteristics, from metaluminousweakly peraluminous, high-K calc-alkaline (or over with calc-alkaline type) I-type granitoids to weakly peraluminous, high-K calc-alkaline to shoshonitic highly differentiated I-type granitoids, Sr/Y ratios become significantly lower and zircon ε_Hf (t) values increased slightly. These suggest that the early high Sr/Y granitoids probably melted from the lower crust of compressional and thicken crust transferred into the late highly fractionated I-type granitoids formed in a relative extensional and thinning setting and evolved by mantle-derived materials. Evolution characteristics of granitoid magmatisms, combined with the regional tectonic evolution on the periods, indcates that the crust in the region experienced a transition from compaction and thickening setting to a relative extension and thinning setting during ca. 160 Ma.

Key words: granite, zircon U-Pb age, Hf isotope, magmatic evolution, Xinkailing region