The petrogenesis of the fenghuangshan pluton at the southern margin of the cathaysia block

Abstract

Researching on the Mesozoic magmatism is critical to understanding the tectonic evolution of the Cathaysia Block, and therefore has become a hot topic in the past decade. Numerous petrological and geochemical studies were implemented in the eastern and middle parts of the block, but few studies focused the southernmost area in western Guandong Province. Therefore, the geochronological, geochemical, and Hf isotopic data from the Fenghuangshan granitoids, which are located at the southern margin of the Cathaysia Block, were used to constrain the crystallization age, petrogenesis and tectonic environment. The Fenghuangshan granitoids are dominated by biotite-bearing monzogranites and biotite-bearing syenogranites. LA-ICP-MS zircon U-Pb data show that the former is (156±2) Ma and the latter is (158±3) Ma, respectively. All of the samples contain high silicon (SiO2=72.66%-76.63%), high alkali (Na2O+K2O =7.89 %-9.43%), and low magnesium (MgO=0.05%-0.54%) concentrations. They show weakly peraluminous characteristics (A/CNK=1.02-1.04). The samples are enriched in Rb, Th, K and depleted in Ba, P, Sr, Ti, with a negative correlation between SiO2 and P2O5. Mineralogical and geochemical features suggest that the biotite-bearing syenogranite is I-type granite. By contrast, the biotite-bearing monzogranite falls into the area of highly fractionated I-type granite, with high differentiation index (DI=93.91-95.74), high Rb/Sr (=99.12-243.75) and Rb/Ba (=67.45-203.80) ratio, low consolidation index (SI=0.48-0.66) as well as significant negative Eu anomalies (δEu=0.01-0.02). The samples of monzogranite and syenogranite have relatively homogeneous Hf isotopic compositions, with εHf(t) values and two-stage model ages (TDM2) of -9.4 to-5.4 and 1 545-1 804 Ma, respectively. The Hf isotopic features indicated that the monzogranites and syenogranites might share a same magma source, which was sourced from reworking of the Paleoproterozoic-Mesoproterozoic crust in the Jurassic. Combined with the previous studies, the Fenghuangshan pluton was suggested to be formed by underplating of the mantle-derived magma in an extensional tectonic setting, which was related to the roll-back or breakup of the subducting oceanic lithosphere.