Zircon U-Th-Pb-Hf isotopes of the basement rocks in northeastern Cathaysia block, South China: Implications for Phanerozoic multiple metamorphic reworking of a Paleoproterozoic terrane

摘要

The Cathaysia block records a complex tectonic history, the understanding of which is central to the debate on the evolution of the South China Block and its position in various supercontinent assemblies. Here we investigate two key formations from this block, the Mayuan Group in the northern Fujian Province, northeastern Cathaysia block and its equivalent the Badu Group in southwestern Zhejiang Province. Previous studies traced the Paleoproterozoic records from the Badu Group whereas a Neoproterozoic age was proposed for the Mayuan Group. The rocks sampled in this study from both groups show similar mineral assemblages of garnet + sillimanite + biotite + plagioclase + quartz +/- K-feldspar +/- muscovite +/- graphite as well as high contents of SiO2 and Al2O3, typical of amphibolite- to granulite-facies metapelitic rocks. Zircon U-Pb data yield two discordia intercept ages of similar to 1990 Ma and similar to 2450 Ma from one sample and discordia intercept ages of similar to 3.5 Ga, 2.5 Ga, 1.86 Ga and 233 Ma from another in the Badu Group. Zircons in two samples from the Mayuan group yield intercept ages of 1859 Ma and 249 Ma in one sample and similar to 2.6 Ga, 1.87 Ga, 257 Ma and a weighted mean Pb-206/U-238 age of 248 Ma in the other. The ca. 1.86-1.87 Ga and 230-250 Ma ages are interpreted to represent the time of metamorphic reworking because zircon grains of these ages tend to have low Th/U ratios, flat HREE patterns and unzoned internal texture as revealed by cathodoluminescene (CL) images. These results confirm that the Badu Group is a Paleoproterozoic lithostratigraphic unit and also suggest that at least part, if not all, of the Mayuan Group is Paleoproterozoic.