Origin of the Yuechengling caledonian granitic batholith, northeastern Guangxi: constraint from zircon U-Pb geochronology, geochemistry and Nd-Hf isotopes

The Yuechengling granitic batholith in the junction of Guangxi and Guangdong provinces contains Caledonian to Indonesian granitoids, and recently becomes a new target of geological survey and exploration for W-Sn mineralization in the Nanling Range. In this batholith, Caledonian granitoid mainly consists of (coarse to) medium grained and medium to fine grained monzogranite, with minor fine grained granodiorite. LA-MC-ICP-MS zircon U-Pb chronology, mineralogy, geochemistry and Nd-Hf isotopes of the Caledonian granitoids were carried out to constraint their temporal and spatial distribution, petrogenesis and tectonic setting. The results showed that the (coarse to) medium grained monzogranite and granodiorite are of I-type emplaced at 435~438 Ma, while the medium to fine grained monzogranite is S-type in origin emplace at 423~429 Ma, which is a little later than the former. I-type granitoid has abundant titanite and some magnetite as their typical accessory mineral, and has relatively low SiO2 (<70%), moderate to high CaO, FeO+MgO, and high TiO2 contents with low A/CNK (0.99~1.05), moderate (La/Yb)N ratios and moderate depletion of Ba, Sr, and Eu. By contrast, the S-type granite has low abundance of ilmenite and monazite and are characterized by high SiO2 (>70%) and alkaline, low CaO and FeO+MgO with high A/CNK(1.04~1.14), (La/Yb)N ratios and moderate to highly depleted Ba, Sr, and Eu. C/MF-A/MF diagram indicate that I-type granitoid derived from the metamorphic basic to intermediate igneous rocks. Those of the S-type granitoid has similar εNd(t) (-7.9~ -8.8) and tNd2DM (1.81~1.88 Ga) ratios as I type, but CaO/Na2O (0.28~0.64) and dispersed εHf(t) (-2.6~ -7.9) ratios suggest a source dominated of metagreywacks and metapelites with minor younger crust component. Intergrated with tectonic involvement of the Wuyi-Yunkai Orogen of South China, isothermal decompression of the middle to lower crust during stress transformation process of this orogen may be the key factor controlling the granitic intrusions of the the Yuechengling region. The decompression process perhaps was accomplished through relaxation of the deep fault nearby. © 2016, Science Press. All right reserved.