Zircon SHRIMP U-Pb Ages, Geochemistry and Nd-Hf Isotopes of ∼1.0 Ga A-Type Felsic Rocks in the Southwestern Yangtze Block, South China: Petrogenesis and Tectonic Implications

Abstract

The petrogenesis and tectonic affinity of Late Mesoproterozoic igneous rocks in the Yangtze Block are important to understand its tectonic evolution within the context of the Rodinia supercontinent’s reconstruction. Here, we report the SHRIMP zircon U-Pb ages, geochemistry, and Nd-Hf isotopic data for dacite from the Zegu Formation of the Dengxiangying Group in southwestern Yangtze Block. The crystallization age of the Zegu dacite is 1 037 Ma. These rocks have relatively high Ga, Zr, Hf, and HREE contents, as well as high (10 000 × Ga)/Al (2.7–2.9) and FeOt/MgO (3.8–10.5) ratios, showing a geochemical affinity of A-type granitoids. Moreover, they are characterized by negative ε _Nd(t) (−3.6 to −7.5) and zircon ε _Hr(t) values (−0.3 to −14.4), indicating that they likely generated via partial melting of an ancient continental crust source. The tectonic discriminant diagrams showed that the Zegu dacites have high Y and Nb contents (plotted in the ‘within-plate’ magmatic rocks field). In conclusion, the geochemistry and isotopic data indicated that the 1 037 Ma felsic volcanic rock in the Dengxiangying Group generated during an intra-plate rift basin along a passive margin. Our work thus argues against a Grenvillian orogen’s existence in the Yangtze Block. In view of the subsequent Neoproterozoic subduction-related magmatism in the region, we suggest that the Yangtze Block might have been along the periphery of the Rodinia.

Keywords

A-type granotoids / zircon SHRIMP U-Pb dating / intra-plate magmatism / Late Mesoproterozoic / Southwestern Yangtze Block / geochemistry

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