He Y.-Y.,China University of Geosciences |
He Y.-Y.,Key Laboratory for Paleontology and Coevolution of Life and Environment |
Niu Z.-J.,Key Laboratory for Paleontology and Coevolution of Life and Environment |
Yang W.-Q.,Key Laboratory for Paleontology and Coevolution of Life and Environment
Geology in China | Year: 2016
Ordovician strata in central-southern Hunan consist of "finer clastic-chert-bearing series-coarser clastic rock", and their thickness varies regularly. Spatially, the thickness contours are distributed northeastwards nd meanwhile, the thickest tends to move southeastwards. In the study area, chert samples of Yanxi Formation in Lingkou section show high Si02 values (89.08 %-94.32%) and Al/(A1+Fe+Mn) ratios (0.52-0.79), and are characterized by slight enrichment of LREE and no obvious cerium and europium anomalies. Samples of equivalent formation in Daqiao section shows high Si02 values (91.74 %-95.14%), and the A1/(A1+ Fe+Mn) ratios range from 0.34 to 0.56, with characteristics of LREE enrichment, intermittently negative cerium anomaly and no europium anomaly, and low Y/Ho ratios (20.65 ±1.63). Geochemical characteristics and discrimination diagrams of middle-upper Ordovician cherts in central-southern Hunan indicate that these rocks were bio-genetic and formed in an open continental margin. A correlation with equivalent formation in adjacent areas shows that these cherts have similar origins and sedimentary environments, which indicates that they formed in the same basin. Combined with the analysis of the iso-thickness diagram, it could be suggested that the basin in central-southern Hunan experienced the change from passive margin to foreland basin during Ordovician, and the cherts probably resulted from this transition. In addition, there is no obvious hydrothermal activity, suggesting the geodynamics that caused the uplift of Cathaysian block might have been located to the further southern or southeastern extension range of the cherts series in central-southern Hunan.
Tian Y.,Wuhan Center |
Tian Y.,Key Laboratory for Paleontology and Coevolution of Life and Environment |
Xie G.,Wuhan Center |
Xie G.,Key Laboratory for Paleontology and Coevolution of Life and Environment |
And 8 more authors.
Diqiu Kexue - Zhongguo Dizhi Daxue Xuebao/Earth Science - Journal of China University of Geosciences | Year: 2015
For revealing provenance and tectonic settings of Triassic Xujiahe Formation in Qiyueshan area, Southwest Hubei, detrital compositions, major, trace and rare earth elements, and zircon isotope ages of clastic rocks from Zhoujiawan and Yanjiagou sections were tested and analyzed. All sandstone samples are rich in quartz (Q), poor in lithic fragments (L) and feldspar (F), with an average of 76.15%, 8.90% and 3.45% respectively, Q/(Q+F+L) being averaged at 0.86, featuring with heavy mineral assemblage of zircon-brookite-magnetite-sphene-tourmaline, which suggests an acidic or low-grade metamorphic source. Dickinson discrimination diagrams show provenance mainly from recycled orogen. The sandstones are characterized by low (TFe2O3+MgO)* (1.3%, 1.5%, average for Zhoujiawan and Yanjiagou sections respectively), low TiO2 * (0.37%, 0.39%), low Al2O3/SiO2 (0.10, 0.12) and high K2O/Na2O(4.89, 40.01), which are most similar to the characteristics of passive continental margin. The index of compositional variation (ICV: 0.32~0.79, average: 0.56) for mudstones implies the source materials are mainly recycled sediments. The REE patterns, contents and ratios of specific trace elements indicate passive and active continental margin settings during Late Triassic. Detrital zircons can be divided into rounded and euhedral categories. As recycled zircons mostly, rounded zircons (>500 Ma, n=133) are characterized by “core-edge” or “core-mantle-edge” structure. Three prominent age peaks, i. e. 2 480 Ma, 1 880 Ma and 832 Ma are obtained, suggesting fingerprint of Yangtze craton. Having two prominent age peaks 435 Ma and 217 Ma, cylindrical euhedral zircons (<500 Ma, n=42) with typical oscillatory zoning possess the characteristics of Xuefeng and Qinling orogens. Combined with provenance and tectonic discrimination diagrams, these characteristics all above suggest that Xujiahe Formation deposited in passive (main) and active continental margin, and its provenances were from Xuefeng orogenic belt (main source) and Qinling orogen. © 2015, Editorial Department of Earth Science-Journal of China University of Geosciences. All right reserved.