Detrital zircons in metasedimentary rocks of Mayuan and Mamianshan Group from Cathaysia Block in northwestern Fujian Province, South China: New constraints on their formation ages and paleogeographic implication

Abstract

Pre-Palaeozoic metasedimentary rocks are widespread in the Cathaysia Block but their accurate depositional ages are poorly constrained. Here we report a detailed study on U–Pb dating for zircons from the metasedimentary rocks in both Mamianshan and Mayuan groups from NW Fujian Province. The data show two models of age spectrum, i.e. multimodal and unimodal. Multimodal samples include detrital zircons from Daling and Longbeixi Formations of Mamianshan Group, Dajinshan Formation of Mayuan Group and Dikou Formation. Their age spectra are characterized by two prominent age groups of 1.3–0.9 Ga and 0.9–0.7 Ga, and two minor age groups of 0.7–0.5 Ga and 2.7–2.4 Ga. They all have the youngest age group of Ediacaran-Cambrian (ca. 0.61–0.53 Ga). Their age spectra resemble those of early Cambrian strata in Nanling and Yunkai regions of the Cathaysia Block, indicating that their depositional age is most likely Cambrian rather than previously proposed mid-Neoproterozoic or Paleoproterozoic. Therefore it is suggested that the originally named Mamianshan Group, Mayuan Group and Dikou Formation metamorphic strata are probably not anymore a traditional lithostratigraphic unit but a “complex” with rocks of multiple ages. The unimodal samples are all from Nanshan Formation of Mayuan Group, which show age spectra characterized by the dominating age cluster of mid-Neoproterozoic (ca. 0.88–0.74 Ga). The age spectra are very similar to those of the Cryogenian strata in Nanhua rift basin of the South China Block. Therefore, we suggest that the depositional age of those unimodal samples from Nanshan Formation is likely Cryogenian. In addition, detrital zircons from our Cryogenian samples are mostly euhedral-subhedral and they could be derived mainly from the South China Block itself. In contrast those Cambrian samples may have exotic East Gondwana derived detrital zircon compositions. Comparing age spectra of detrital zircons from different terranes, our studied Cambrian strata in the Cathaysia Block show a similarity to those of western and central North India-Himalaya. It is suggested that the Cathaysia Block should be more close to west and central India-Himalaya during Cambrian.