Identification and petrogenesis of a highly differentiated A-type granite formed in the late Mesozoic, central Jiangxi ProvinceFENG Shang-Jie1*,WANG Hong-Zuo2,1,CHEN Wei-Feng, ZHAO Kui-Dong3,CHEN Pei-Rong1,LING Hong-Fei1*
Abstract:Strong magmatic differentiation by fractional crystallization could significantly modify apparent geochemical features of A-type granites, resulting in ambiguous boundary between A-type and I/S-type granites. Thus to distinguish these types of highly differentiated granites is a significant challenge. The Dawushan granitic intrusive body in the central Jiangxi Province could be an exemplificative case to demonstrate the methods for discriminating a highly differentiated A-type granite. The Dawushan granite, composed of medium-grained biotite monzo-granite (phase I) and fine-grained muscovite-bearing biotite alkali feldspar granite (phase II), is located in center of the Cathaysia Block and the northern Nanling Range. Zircon U-Pb dating shows that the formation ages of phase-I and -II are156 Ma and 155~157 Ma, respectively. The Dawushan granite is characterized by low P2O5 (0.05~0.16%), high SiO2 (70.8~76.4 %) contents, high Rb/Sr (averaging 9.05) and Rb/Ba ratios (averaging 2.38), and weakly to strongly peraluminous feature. Plots in triangular diagram of Rb-Ba-Sr and bivariate diagram of Th/Nd-Th suggest the Dawushan granite experienced a highly differentiation process. Other characteristics, such as high Ga/Al ratio (2.89~3.48), zircon saturation temperature (839℃), and anhydrous feature indicated by interstitial biotite aggregation, suggest that it is an A-type granitic intrusion. Considering strong fractional crystallization of minerals enriched in zirconium, such as zircon and sphene, the Dawushan granite shows a linear trend distinctively different from I/S-type granite on the diagrams of 10000*Ga/Al vs. Zr+Nb+Ce+Y and Zr vs. SiO2. In addition, its Y/Nb ratios are greater than 1.2. Taken together, these characteristics suggest that the Dawushan granite belongs to a highly differentiated A2 granite rather than a differentiated I- or S-type granite. Therefore, on the basis of analyses of petrology, mineralogy and geochemistry, the distinctive evolving trends shown on the diagrams of 10000*Ga/Al vs. Zr+Nb+Ce+Y and Zr vs. SiO2 by I-&S-type and A-type granites can be effectively used for recovering and discriminating different primary rock types of highly fractionated granites. Isotopically, the Dawushan granite exhibits low εNd(t) (-9.23~-14.6) and εHf(t) values (-10.2~-6.50), and old two-stage Nd model ages of 1.7~2.0 Ga. Combined with the metamorphic rocks outcropped in this region, the Dawushan granite could be originated from a composite source consisting of orthometamorphic and parametamorphic rocks similar to the Zhoutan Group. In particular, such a source experienced an S-type melting extraction event during the Indosinian Period, leaving relatively refractory materials similar to diorite in composition. In the Yanshanian Period, the second melting event occurred in the residual source at higher temperatures with heat from the upwelling hot asthenospheric mantle under regional extension, forming the Dawushan A-type granite.