Petrogenesis And Tectonic Setting Of The Middle Jurassic Granites In The Taerqi Region, Central Great Xing’an Range

This paper focuses on the Middle Jurassic granites from the Taerqi region in thecentral Great Xing’an Range. We present petrological, geochronological, geochemicaland Hf isotopic data for these granitic rocks, in order to determine their emplacementtiming, petrogenesis, and possible associated geodynamic setting. This data not onlyfurther consummate the geological information of the Taerqi region, but also provideevidence for the Middle Jurassic magmatic-tectonic evolution history in the GreatXing’an Range.In the study area, four granite samples were chosen for zircon U–Pb dating byLA-ICP-MS method. The zircons from these samples all display fine-scale oscillatorygrowth zoning and have high Th/U ratios, implying a magmatic origin. This indicatesthat the dated ages could represent the emplacement timing of the granites. The U-Pb agedata show that the weighted mean206Pb/238U ages of these samples are173±1.3Ma,169.4±1.6Ma,166.3±1.2Ma and166.7±0.8Ma, respectively, suggesting that the selectedgranites were emplaced in the Middle Jurassic.In addition, the Middle Jurassic granites in the Taerqi region are characterized byhigh Silicon and alkalis, and low Calcium and Magnesium, their A/CNK values aremainly concentrated in the range of0.9~1.1, indicating they are weakly peraluminoushigh-k calc-alkaline granites. The rare earth element patterns show that these granites are enriched in LREEs and depleted in HREEs with significantly negative Eu anomalies.Moreover, they are rich in LILEs (Rb, U and Th), and depleted in HFSEs (Nb, Ta and Ti),Sr and Ba. These above geochemical features all indicate that the Middle Jurassicgranites in the Taerqi region belong to I-type granites, which is also supported by linearvariation trend in the Harker diagrams. Furthermore, all the samples fall into the fields ofthe highly fractionated I-type and A-type granitoids in the discrimination diagrams.Nevertheless, the Zr contents of the Middle Jurassic granites in the study area are higherthan the lowest value of the A-type granitoids, and the average zircon saturationtemperature is lower than that of the A-type granitoids, suggesting that these MiddleJurassic granites all have features of highly fractionated I-type igneous rocks.The Middle Jurassic granites in the Taerqi region of the central Great Xing’anRange have similar Hf(t) values and two-stage model ages with those of the typicalgranitic rocks in the eastern Xing’an-Mongolian Orogenic Belt, indicating that thegranitic magmas could have been derived from the partial melting of juvenile crustalmaterial of Phanerozoic age. Meanwhile, taken all these petrological, geochemical, andgeochronological data into consideration together, we infer that the Middle Jurassicgranites in the Taerqi region were likely emplaced in an extensional environment duringthe evolution process of the Mongol-Okhotsk oceanic plate, resulting from the upwellingof asthenosphere due to thickened lithospheric delamination.