Geochemical characteristics and tectonic setting of the early Yanshanian South Zhuguangshan granite in the central segment of the Nanling Mountains

Abstract： The South Zhuguangshan pluton is a composite granite batholith formed in the Caledonian, Indosinian and early Yanshanian periods. This paper mainly discusses early Yanshanian granites, namely, the Middle Jurassic Sanjiangkou sequence. The sequence is composed of five rock-units, and the rock type is hornblende-biotite monzogranite-biotite monzogranite-two-mica monzogranite. The earlier units contain abundant mafic micro-enclaves, and the petrographic study of the enclaves indicates the crust-mantle mixing origin of the granitic magma. The average content of SiO2 increases from 68.95% in the early-formed granite units to 76.3% in the late-formed granite units. The rocks are alkali-rich. Their Na2O+K2O ranges from 7.11％ to 8.24%, with an average of 7.73% and K2O＞Na2O, and their ASI values range from 0.92 to 1.19, with an average of 1.05；so they belong to the magnesian-ferruginous, metaluminous-peraluminous, high-K calc-alkaline series. The rocks are of light REE-enriched type, with a moderate ΣREE content of 213μg/g and strong Eu depletion, and have negative anomalies of Ba, Nb, Sr, P, Ti, Eu and Ba and positive anomalies of Rb, U, Th, Nd, Zr, Sm and Y, which indicate an intracontinental tectonomagmatic environment. The rocks have ISr values of 0.7115 to 0.72466, εNd (t) values of －10.0 to －12.6 with an average of －10.97 and t2DM ages of 1.75 to 2.0 Ga with an average of 1.84 Ga. The average age is close to that (1.7 to 1.8 Ga) of Mesozoic granites in the Nanling Mountains, indicating that the rock-forming materials were mainly derived from the Mesoproterozoic crust. The tectonic setting discrimination diagrams based on multiple oxides and trace elements suggest that granites of the Sanjiangkou sequence formed in a post-orogenic extensional environment during the transition from compressional orogeny to non-orogeny. The authors consider that the early Yanshanian tectonic setting in the Nanling Mountains and even South China is post-orogenic rather than an intracontinental rift.