Zircon U-Pb Dating, Geochemical Characteristics of the Fangjidao Granitic Rocks in the Southeasternmost Cathaysia Block and Their Tectonic Implications

Abstract

Early Paleozoic granites are widely distributed in the Fangjidao area, the western Guangdong province, i.e., the southeastern Cathaysia Block. They experienced intensive shearing zone deformation and metamorphism, and now occur as mylonized granitic gneiss. The later undeformed monzogranitic veins intruded and truncated the major foliation of the granitic gneiss. However, the crystallization age, petrogenesis and deformation characteristics of these granites are poorly constrained, which hinder our understanding of the tectonic evolution of the region and its relation to the Cathaysia Block. LA-ICP-MS zircon U-Pb dating results show that the granitic gneisses contain abundant inherited zircon of ca. 1183-700 Ma, and have crystallization age of 436-432 Ma. In contrast, the undeformed monzogranitic veins have a younger crystallization age of ~427 Ma. This can well bracket the timing of regional anatexis and regional deformation in 436-427 Ma. The granitic gneisses and monzogranite samples are characterized by a peraluminous signature with A/CNK values varying from 1.10 to 1.24. The rocks are enriched in LREE, with LREE/HREE ratios ranging from 2.07 to 3.98, and (La/Yb)N from 5.93 to 22.27 (mean value is 10.36). All the samples exhibit distinct negative Eu anomalies. Zircon crystals from the granitic rocks have negative εHf(t) values mainly ranging from -22.8 to -4.3, with two stage model ages of 2820-1688 Ma. New data of this study reveal that the Early Paleozoic granitic rocks in the Fangjidao area were generated by partial melting of the Paleoproterozoic crustal components. Combined with the previously published data, we infer that the granitic rocks in the Fangjidao area have a similar origin with the coeval granites in the adjacent areas (e.g. the Yunkai area). They were most likely derived from partial melting of the crustal materials during the extension stage of the intra-plate orogeny.