Zhou Zuyi

The Southward Extension of Cathaysia Block: Evidence from Zircon U-Pb Dates of Borehole Volcanics in the Northern South China Sea

: Five Paleogene volcanics sampled from the northern South China Sea were analyzed via LA-ICP-MS zircon U-Pb dating, including basalt and andesite from Borehole SCSV1 and volcanic agglomerate from Borehole SCSV2, respectively. A total of 162 zircon U-Pb dates for them cover an age range from Neoarchean to Eocene, in which the pre-Paleocene data dominate. The Paleogene dates of 62.5±2.2 Ma and 42.1±2.9 Ma are associated with two igneous episodes prior to opening of South China Sea basin. Those pre-Paleocene zircons are inherited zircons mostly with magmatogenic oscillatory zones, and have REE features of crustal zircon. Zircon U-Pb dates of 2518–2481 Ma, 1933–1724 Ma, and 1094–1040 Ma from the SCSV1 volcanics, and 2810–2718 Ma, 2458–2421 Ma, and 1850–993.4 Ma from the SCSV2 volcanics reveal part of Precambrian evolution of the northern South China Sea, well comparable with age records dated from the Cathaysia block. The data of 927.0±6.9 Ma and 781±38 Ma dated from the SCSV2 coincide with amalgamation between Yangtze and Cathaysia blocks and breakup of the Rodinia, respectively. The age records of Caledonian orogeny from the Cathaysia block are widely found from our volcanic samples with concordant mean ages of 432.0±5.8 Ma from the SCSV1 and of 437±15 Ma from the SCSV2. The part of the northern South China Sea resembling the Cathaysia underwent Indosinian and Yanshannian tectonothermal events. Their age signatures from the SCSV1 cover 266.5±3.5 Ma, 241.1±6.0 Ma, 184.0±4.2 Ma, 160.9±4.2 Ma and 102.8±2.6 Ma, and from the SCSV2 are 244±15 Ma, 158.1±3.5 Ma, 141±13 Ma and 96.3±2.1 Ma. Our pre-Paleogene U-Pb age spectra of zircons from the borehole volcanics indicate that the northern South China Sea underwent an evolution from formation of Precambrian basement, Caledonian orogeny, and Indosinian orogeny to Yanshannian magmatism. This process can be well comparable with the tectonic evolution of South China, largely supporting the areas of the northern South China Sea as part of southward extension of the Cathaysia.

Modelling of low-temperature exhumation rate in Dabie Mountain based on (U-Th)/He and fission-track thermochronological data

While the high-temperature exhumation process in the Dabie Mountain has been well documented, the low-temperature exhumation of this area since Cretaceous, especially since Late Cretaceous, is relatively less studied. Low-temperature thermochronology provides one of the important approaches to solve this problem. Based on the data of fission track and (U-Th)/He analysis of apaptites and zircons from the granitoid and metamorphic rocks in the Dabie Mountain, this paper applies Mancktelow’s and Braun’s methods to estimating the exhumation rates and to drawing the regional differential exhumation pattern since Cretaceous, especially since Late Cretaceous by taking into consideration factors such as heat transport, heat advection, topography and heat production, which could influence geothermal field in the shallow crust. Since Cretaceous, the exhumation rate (0.08–0.10 km/Ma) in the region around Tiantangzhai and in the south of Tanlu fault zone is larger than the rate (0.04–0.07 km/Ma) in other areas of the Dabie Mountain. The regional differential exhumation pattern might be related to the push-up effect caused by differential strike-slip movement along NNE-trending faults.

Constraints on the early cooling rates of Mt. Dabie from diffusion modeling of chemical zoning of garnet

Diffusion modeling of zoning profiles in eclogite garnets from three different tectonic units of Mt. Dabie, UHPM unit, HPM unit and northern Dabie, was used to estimate the relative time span and cooling rates of these rocks. Modeling result for the Huangzhen eclogite garnet shows that the maximal time span for the diffusion-adjustment process is about 22 Ma since the peak-temperature metamorphism, which is the maximum time span from amphibolite facies metamorphism to greenschist facies metamorphism. The Bixiling eclogites had subjected to a cooling process at a rate of ∼10°C/Ma from 750°C to 560°C during 20 Ma. The second cooling stage of the Raobazhai eclogite following granulite-facies metamorphism is an initial fast cooling process at a rate of about 25°C/Ma and then slowed down gradually. All these belong to a coherent Dabie collision orogen with differences in subduction depth and exhumation/uplifting path.