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

Abstract

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 ~1990Ma and ~2450Ma from one sample and discordia intercept ages of ~3.5Ga, 2.5Ga, 1.86Ga and 233Ma from another in the Badu Group. Zircons in two samples from the Mayuan group yield intercept ages of 1859Ma and 249Ma in one sample and ~2.6Ga, 1.87Ga, 257Ma and a weighted mean 206Pb/238U age of 248Ma in the other. The ca. 1.86–1.87Ga and 230–250Ma 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.Evidence for Paleozoic metamorphic reworking that is considered to have affected the whole of Cathaysia block is not revealed in this study; in contrast our data clearly show obvious Mesozoic metamorphic reworking at ca. 230–250Ma. Zircon ɛHf(t) values range from −19 to +11 with a peak at −5.5 and show TDMC(Hf) ranging from 1.9 to 4.1Ga with a peak at ca. 2.7–3.0Ga suggesting that a major crustal growth took place during this time. This interpretation is consistent with the previously suggested crustal growth peaks of ~2.7Ga and ~2.9Ga. A synthesis of the reliable geochronological data gathered so far on Phanerozoic metamorphic reworking of the northeastern Cathaysia block reveals that the imprints of these tectonothermal events is differently distributed in the different zones. Rocks metamorphosed during the Paleozoic tectonothermal event dominantly occur in the western zone whereas those reworked by high-grade metamorphism during the Mesozoic tectonothermal event mainly outcrop in the eastern zone. Our study alerts the previous notion of a uniform distribution of the reworked rocks by high-grade metamorphism all across the northeastern Cathaysia block and provides new insights on the evolution of the South China Block.