Mineral chemistry, trace elements and Sr-Nd-Hf isotope geochemistry and petrogenesis of Cailing and Furong granites and mafic enclaves from the Qitianling batholith in the Shi-Hang zone, South China
文摘
The Shi-Hang zone is an important NE trending Mesozoic magmatic belt composed of granites with relative high ¦ÅNd(t) values and young TDM model ages in South China. However, the petrogenesis and the tectonic environment for the Shi-Hang zone magmatic rocks remain controversial. We report here mineral chemistry, geochemical and Sr-Nd-Hf isotopic data for the Cailing and Furong granites and mafic microgranular enclaves (MMEs) from the Qitianling granite batholith in southern Hunan province, South China. The Qitianling granite batholith is a multi-staged composite pluton with three phases (Cailing, Furong, and Huangtangling) according to their ages and petrography. The Cailing (163-160 Ma) and Furong (157-153 Ma) phases are mainly composed of porphyritic amphibole-biotite monzogranite, and they share similar geochemical and isotopic characteristics. Both of them show similar SiO2 contents from 66.50 to 70.28 % , and metaluminous A/CNK values of 0.80 to 0.98. The granites are characterized by high contents of large ion lithosphile elements (LILE) such as Rb, Th, U, Pb; high field strength elements (HFSE) such as Nb, Ta, Zr, Hf; and Zr + Nb + Ce + Y contents > 350 ppm, and high 10,000 ?#xA0;Ga/Al ratios > 2.6. Chondrite-normalized REE patterns show relative enrichment of light rare earth elements (LREEs) and significant negative Eu anomalies. Mineralogical and geochemical features suggest that the Cailing and Furong granites are A-type, which can be further classified as A2 subtype. They have relatively lower (87Sr/86Sr)i ratios (0.7091-0.7132), higher ¦ÅNd(t) values (?#xA0;5.5 to ?#xA0;7.6) and younger Nd isotopic model ages (1.48-1.56 Ga) than those common S-type granites in South China. Zircon ¦ÅHf(t) values vary from ?#xA0;8.1 to ?#xA0;3.7. The MMEs in the Cailing phase show similar trace element and Sr-Nd isotopic characteristics with the host granites. But zircons from the MMEs show different ¦ÅHf(t) values (?#xA0;6.4-+ 2.6) with those from the host granites (?#xA0;8.1 to ?#xA0;3.7). This indicates that the MMEs and host granites were crystallized from different sources of magmas, providing direct evidence for mafic-felsic magma mixing processes. The isotope data indicate that the Cailing and Furong granites from the Qitianling batholith were derived from a hybrid magma consisting of about 80 % felsic magma derived from old crust and about 20 % mantle-derived mafic magma. The strong magma mixing at about 160-155 Ma caused by intra-arc rifting or back arc extension related to subduction of the Paleo-Pacific plate, is favored to explain the petrogenesis of the Cailing and Furong granites, as well as the Shi-Hang zone.
