Petrogenesis and implications of the Dupangling compound granite in southern Hunan Province, China: Constraints from mineralogical chemistry, zircon U-Pb age, geochemistry and Nd-Hf isotope

Abstract

In this paper, zircon SHRIMP U-Pb geochronology, petrology, mineral chemistry, petrogeochemistry, Sm-Nd and Lu-Hf isotopes were studied for the eastern part of the Dupangling pluton, which is located at the western section of Nanling at the junction of Hunan and Guangxi provinces. The zircon SHRIMP U-Pb dating results show that the age of coarse-to-medium-grained porphyritic biotite monzogranite is 215.6±2.1 Ma, medium-grained porphyritic biotite monzogranite 220.5±1.8 Ma, medium-grained rapakivi biotite monzogranite 222.8±1.5 Ma. Combined with the age of fine-grained muscovite monzogranite as 209.7±3.1 Ma obtained in the previous research, it is suggested that the emplacement time ranges from 222.8 Ma to 209.7 Ma, and the pluton is derived from the Indosinian magmatic activity rather than the Yanshanian as previously thought. The mineral-chemical characteristics of rapakivi K-feldspar and biotite phenocryst indicate that the magma temperature, pressure and composition have gone through fluctuation during the formation process, and biotite phenocryst was formed by multiple melting under the underplating by basaltic magma in this area. The Dupangling biotite monzogranites have higher contents of SiO2 and K2O+Na2O, A/CNK value ranges between 1.02 and 1.39 and Rittman index (δ) between 0.93 and 2.18, belonging to the peraluminous calc-alkaline. Moreover, these monzogranites are enriched in REE, Rb, Th and U with higher content of HFSE (Nb, Y and Ga), depleted in Ba, Sr and Eu with higher ratios of TFeO/MgO and Ga/Al, showing the characterisitics of A-type granite. The isotope εNd(t) value ranges from -8.74 to -8.13, and the T2DM value from 1.71 to 1.66 Ga; the zircon isotope εHf(t) value ranges from -14.1 to -1.4, and the T2DM value from 2.14 to 1.34 Ga. It reveals that the Dupangling biotite monzogranites mainly originated from the partial melting of ancient crustal materials and were mixed with a certain degree of depleted mantle materials. The metamorphic peak period of the Indosinian movement is from 258 Ma to 243 Ma. South China was in an extensional tectonic setting after 233 Ma and mantle-derived basalt magma was in a wide range of underplating, which caused the crustal material remelting to form the Dupangling Indosinian aluminous A-type granite (rapakiwi granite) under an extensional background.

Keywords

• dupangling pluton
• rapakiwi granite
• shrimp u-pb age
• mineral chemistry
• a-type granite
• extensional setting