The effect of source compositions on the geochemical diversity of continental granites

1. The effect of source compositions on the geochemical diversity of continental granites Peng Gao, Zi-Fu Zhao, Yong-Fei Zheng School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China (Email: gaopeng05@mail.ustc.edu.cn) 3. Discussion Previous studies on granite classification were primarily based on either lithochemistry or radiogenic Sr-Nd isotopes. However, high aluminium saturation indices are always associated with high O isotope ratios. This allows an integrated interpretation of the all geochemical data. Therefore, the three Triassic granites belong to the S-type. However, it is probable that the three granite intrusions were generated from different sources based on their different variation trends of major-trace elements (Figs. 2-4) and their slightly different isotope compositions (Fig. 5). The Luxi intrusion has MgO+FeOT contents higher than melts derived from sedimentary rocks, much more uniform Sr-Nd isotope compositions (Figs. 2 and 5), and regular major-trace element variations. It is inferred that the magma of this intrusion contained some source-derived solid materials but then was homogenized because the only mafic mineral is biotite. The Xiazhuang intrusion is nearly pure melt generated at lower temperatures compared with the Luxi one. 2. Results The three intrusions show different major-trace element variations in Hacker plots (Figs. 2-4). The Luxi intrusion is biotite granodiorite, while the Xiazhuang and Fucheng ones are mostly composed of two-mica granites. So all three intrusions are peraluminous (Fig. 3a). Meanwhile, all samples have high zircon d18O values of >9.0 ‰ (Fig. 5d). 1. Introduction Geochemical diversity is one of the most important topics in the study of granite petrogenesis. Reasons for such diversity can be revealed by a combined study of petrology, mineralogy and geochemistry. Granites derived from meta-sedimentary rocks, referred as S-type, are characterized by peraluminous and high zircon d18O values. They are viewed as low temperature magmas relative to I-type and A-type granites. Source heterogeneity are often reflected by S-type granites. A-type granites which were defined from geochemical features are usually thought to derive from basaltic magmas by fractional crystallization or metaigneous rocks by partial melting. However, there are little examples of A-type granite having a metasedimentary source (e.g., Huang et al., 2011). Fig. 1. (a) Simplified map showing distribution of Triassic granites in South China. Modified after Mao et al. (2013) and Wang et al. (2013); Schematic geological maps of (b) the Guidong complex and (c) the Fucheng-Hongshan complex. Modified after Zhou (2007). In this study, we carried out a combined study of whole-rock major-trace elements and Sr-Nd isotopes, zircon U-Pb ages and O isotopes, as well as biotite geochemistry for three Triassic granite intrusions from the Nanling Range in South China (Fig. 1). The results provide insights into the effects of source compositions on geochemical variations of granites. Fig. 6. Plots of fluorine (a) and A/CNK (b) with Mg# for biotites from the studied Triassic granites as well as literature data of biotite. SP A-type represents the strongly peraluminous A-type granites (Dahlquist et al., 2013); MP-WP A-type represents the metaluminous to weakly peraluminous A-type granites (Dahlquist et al., 2010); I- and S-type represents the calc-alkaline I- and S-type granites (Dahlquist et al., 2010). Figure 2. Hacker plots of major elements for the studied Triassic granites with MgO+FeOT as x-axis. Also shown is A-type melt from Skjerlie and Johnston (1993). The Fucheng intrusion is similar to A-type melt, with high (Na2O+K2O)/CaO, TiO2/MgO, Ga/Al and Zr+Nb+Ce+Y, but low CaO and Mg# (Figs. 2-4). Biotites of this intrusion are high in F, FeOT/MgO, and A/CNK (Fig. 6), resembling strongly peraluminous A-type granite. 4. Conclusions (1) The three intrusions belong to S-type granites, but the Fucheng one also resembles A-type granite generated from a fluorine-rich and residual sedimentary source at high temperatures; (2) The Luxi intrusion represents magmas that entrained source-derived solid materials, but were homogenized subsequently. The Xiazhuang intrusion represents low temperature pure melts; (3) Source compositions and melting temperatures control the geochemical diversity of these granites. Fig. 3. Hacker plots of major elements ratios with MgO+FeOT as x-axis for Triassic granites. A-type melt in this figure is same as Fig. 2. Fig. 5. Plots of (a) initial Sr-Nd isotopic compositions, (b) 87Rb/86Sr with initial Sr isotopic compositions, initial Nd (c) and zircon oxygen (d) isotopic compostions with MgO+FeOT for the studied Triassic granites. Fig. 4. Hacker plots of trace elements and their ratios with MgO+FeOT as x-axis for the studied Triassic granites. A-type melt in this figure is same as Fig. 2.