Precambrian protoliths and Phanerozoic overprinting on the Wuyishan terrain (South China): New evidence from a combination of LA-ICPMS zircon and EMP monazite geochronology

Wuyishan is one of the most important terrains exposing the Precambrian basement of the Cathaysia Block, South China. Conventionally, Paleoproterozoic and Neoproterozoic sequences have been proposed in this terrain, however, the thermal histories affecting each unit within these sequences remain unclear. We report here spot analysis of 448 LA-ICPMS U-Pb zircon and >1600 EMP Th-U-Pb monazite ages for twenty samples, including schist, gneiss, migmatite and amphibolite, from different units within the basement. U-Pb zircon dating yield some concordant Neoproterozoic and/or Early Paleozoic age clusters with a wide spread of Neoarchean to Mesoproterozoic inherited ages (except amphibolite), indicating the formation of the Mayuan/Chencai core in the Early Neoproterozoic (887-827 Ma) and peripheral units in two stages of the Late Neoproterozoic (746-731 Ma and 681-660 Ma). The in-situ EMP monazite analyses give consistent Early Paleozoic ages (440-426 Ma) from inclusions in various host minerals for all but two samples from same locality that are Triassic in age (239-237 Ma). A large population of Early Paleozoic interstitial monazites shows breakdown texture and ages of secondary monazites range from 376 Ma to 358 Ma, conforming with the U-Pb zircon age of the amphibolite (370 ± 5 Ma). A few samples exhibit relict ages of ∼540 Ma from monazite cores. We conclude that the Wuyishan terrain is dominated by Early and Late Neoproterozoic basement sequences related to collisional and continental rifting orogenies, respectively, while the Badu Group in Zhejiang as a Paleoproterozoic protolith. The Neoproterozoic basement incorporating some Early Cambrian detritus, has thoroughly metamorphosed during the Silurian and further affected by the regional fluid-aided metasomatism coeval with the basic magma, now represented by amphibolites, around ∼370 Ma. The EMP monazite dating results for the Wuyishan terrain are quite different from the Yunkai massif where Triassic ages predominate, although the Wuyi-Yunkai metamorphic belt is often referred to as the same orogen in the Cathaysia Block.