关键词: A型花岗岩, 锆石U-Pb年龄, 地球化学特征, Hf同位素, 吉林中部靠山地区

Abstract:  The Early Jurassic medium-grained syenogranite in Kaoshan area of central Jilin Province is exposed in the southern part of Zhangguangcai range, and emplaced in contact with the Middle Triassic granitoids and Devonian-Carboniferous intrusive rocks. Its main rock-forming minerals are composed of K-feldspar, quartz, plagioclase and biotite. In order to explore the diagenetic age, geochemical characteristics and geodynamic background of the granites in Kaoshan area, we carried out petrographic, chronological, geochemical and Hf isotope studies on the Early Jurassic non-orogenic A-type medium-grained syenogranite exposed in the study area. Zircon U-Pb geochronology and geochemical studies show that: The concordant age of the medium-grained syenogranite in the study area is (193.2±0.9) Ma, and the diagenetic time should be attributed to the Early Jurassic. It has a high SiO2 (75.49%-75.82%) and total alkali (8.28%-9.17%) mass fraction, together with high A/CNK (1.08-1.13), LREE (20.89×10-6-46.38×10-6) and FeO/MgO (3.29-5.83) values. It is also enriched in large ionic lithophile elements (LILE, such as Rb, K and Pb) and high-field strength elements (HFSE, such as Th, U, Hf, Nb and Zr). It has low MgO (0.06%-0.14%), TiO2 (0.05%-0.12%), P2O5 (0.02%-0.04%), Sr (24.20×10-6-62.59×10-6), HREE (5.14×10-6-11.80×10-6) mass fraction and medium δEu (0.41-0.56) value, which is similar to the lithogeochemical characteristics of non-orogenic A-type granite.  Hf isotopic analysis results of zircons show that the εHf (t) values range from 9.7 to 15.7, and the two-stage Hf-isotope model ages (TDM2) range from 612 to 225 Ma. Combined with the regional research data, we think the diagenetic parent magma of the medium-grained syenogranite in the Kaoshan area was probably derived from the juvenile lower crust in an extensional environment. The tectonic environment of asthenosphere mantle upwelling and back-arc extension caused by the subduction of the early paleo-Pacific plate, may be an important geodynamic factor for the Early Jurassic non-orogenic A-type granitic magmatism in the study area.

Key words:  , A-type granite, zircon U-Pb age, geochemistry characteristics, Hf isotope, Kaoshan area of central Jilin ,