下扬子青阳-九华山复式岩体成因:锆石年代学和Hf-O同位素制约

Yanshanian magmatic rocks are widely cropped out in the southern Anhui Province. The Qingyang-Jiuhuashan complex plutons is one of the most famous complex one which consist of dominant granodiorite, monzogranite (Qingyang pluton) and K-feldspar granite (Jiuhuashan pluton). For them, LA-ICPMS zircon U-Pb dating gives results of 138.3±1.4, 132.8±1.5, and 128.4±1.4 Ma, respectively. Yanshanian magmatism in the southern Anhui Province can be subdivided into three stages: the early stage of 136-153 Ma, the middle stage of 136-129 Ma, and the latest stage of 129-122 Ma. Granodiorite and monzogranite of the Qingyang pluton have SiO2 contents ranging from 66% to 68%, relatively lower MgO contents (0.6%-2.2%), total alkali contents ranging from 7.5% to 8.6%, K2O/Na2O ratios between 1.1-1.3, and lower A/CNK ratios (0.9-1.1), a geochemical signatures which belong to the high potassium calc-alkaline series and metaluminous I-type granite. K-feldspar granite of the Jiuhuashan pluton is charactered by high SiO2 (>75 %), low CaO (0.51%-0.86%), A/CNK ratios ranging from 0.9 to 1.2, and K2O/Na2O ratios ranging from 1.1 to 1.3, also showing the high potassium calc-alkaline series affinities, but being A-type granite. The εHf(t) values of zircons in granodiorite and monzogranite from the Qingyang pluton range from -8.01 to -6.71 and from -11.40 to -9.43, with the zircon δ18O values are between 7.16‰ to 8.04‰ and 7.26‰ to 8.06‰. However, zircons in granite from the Jiuhuashan pluton have relatively higher εHf(t) values of -7.62 to -4.78, and similar δ18O values of 7.03‰ to 8.84‰. Integrated studies of the major elements and zircon Hf-O isotopes indicate that the Qingyang granodiorite and monzogranite were formed by melt mixing which separately derived from partial melting of the lithospheric mantle which metasomatized by the fluid/melts from the subducted slab, and the late Archaean-early Proterozoic crust which were induced to partial melting by the underplating basaltic magma, under a tectonic setting of the Pacific plate low-angle subduction. Associated with increase of the subducted angle and roll-back of the subducted slab, the tectonic setting in the southern Anhui Province transformed into back arc extension. K-feldspar granites of the Jiuhuashan pluton were formed by anatexis of the Mesoproterozoic basement under a condition of relatively low pressure and high temperature.