Hybrid genesis of Jurassic fayalite-bearing felsic subvolcanic rocks in South China: Inspired by petrography, geochronology, and Sr-Nd-O-Hf isotopes

Fayalite-bearing felsic (FBF) magmatic rocks are a special type of granitic rocks with controversial origins. A suite of fayalite- and ferrosilite-bearing subvolcanic rocks, namely, the Xishan FBF rocks in South China, is investigated in this study. The Xishan FBF rocks have high SiO2 contents of 69-70 wt.%, high K2O/Na2O ratios of 1.71-1.95, and high FeOt/(FeOt + MgO) ratios of 0.88-0.89. Fayalite (Fo = 73-9.6) and ferrosilite (Fs = 74.1-76.5) minerals are found in the Xishan FBF rocks. According to the Unmix function of Isoplot, the zircon U-Pb ages and initial Hf isotope compositions are categorized into two groups with ages of 156.6 Ma and 151.5 Ma and epsilon(Hf)(t) values of -7.1 and -5.2, respectively. The minerals show delta O-18 values of 8.8-9.8 parts per thousand for zircon, 6.0-8.1 parts per thousand for fayalite, and 7.0-8.2 parts per thousand for ferrosilite. The oxygen isotope fractionations between ferrosilite and fayalite (Delta(Opx-OI)) vary from -0.8 parts per thousand to +1.5 parts per thousand, which indicates disequilibrium crystallization. Whole-rock analyses show high initial Sr-87/Sr-86 ratios of 0.7169 to 0.7180 and negative epsilon(Nd)(t) values of -7.3 to -6.8; zircon analyses show epsilon(Hf)(t) values of -9.1 to -3.8 and delta O-18 values of 8.8-9.8 parts per thousand. So whole rock and zircon isotopes indicate a crustal signature. Based on these mineralogical and geochemical data, the Xishan FBF rocks were attributed to A-type granites and derived from the mixing of two batches of crustal magmas, which were all derived from the partial melting of ancient igneous protolith under the conditions of high temperature (683-893 degrees C), moderate water (3-5 wt.%), and low oxygen fugacity (1g fO(2) = -1.21). Such rigorous physical conditions may be common for the FBF igneous rocks all over the world, which may be the primary factors controlling occurrence of the FBF rocks in limited volume and quantity.(C) 2016 Elsevier B.V. All rights reserved.