Xue W.-Q.,Wuhan University |
Li B.,Key Laboratory of Marine Mineral Resources |
Yan J.-X.,Wuhan University |
Ellwood B.B.,Louisiana State University |
And 3 more authors.
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2015
The Capitanian stage is a critical time interval when fierce eruption of the Emeishan large igneous province (LIP) occurred with onset of a double-episodic Paleozoic-Mesozoic bio-crisis. However, the time duration of the Capitanian stage is geochronologically poorly-constrained due to a lack of absolute age data with radioactive dating method constraining both the basal and the top boundary of the Capitanian stage. Cyclostratigraphy can provide a high-resolution astronomical time scale by tuning the cyclic stratigraphic records to astronomical solutions. In this study, a floating point time scale (FPTS) for the Capitanian stage was developed based on the result of time-series analysis of magnetic susceptibility (MS) and additional biostratigraphic data sets. In addition, we estimate the age and time duration of the Emeishan LIP eruption. 1456 unoriented samples (sampling interval ~0.1 m) for MS measurement and 45 samples for biostratigraphic study were collected from outcrops with stratigraphic interval of ~147.5 m. The bottom horizon of sampling range is located at ~31.2 m below the Wordian-Capitanian (W-C) boundary. We conducted continuous sampling with the top horizon at 2.6 m above the Capitanian-Wuchiapingian (C-W) boundary at Tieqiao section, Laibin in South China, as a supplementary reference section for the Guadalupian-Lopingian (Middle-Upper Permian) Global Boundary Stratotype Section and Point (GSSP). The stratigraphic succession consists of alternations between thin-bedded chert and micritic limestone recognized as typical slope-basin facies with carbonate cap on shallow-water platform. Representative samples were selected to be carried out thermomagnetic susceptibility measurements after analysis of low-field bulk susceptibility for all samples. Then spectral (time-series) analysis of the MS data was performed by means of Multi Taper Method (MTM) and Fourier Transform (FT) analysis. Samples for biostratigraphic study were processed using acetic acid digestion in order to obtain conodont fossils. Seven conodont zones are identified within the Capitanian stage, including Jinogondolella postserrata, J. shannoni, J. altudaensis, J. prexuanhanensis, J. xuanhanensis, J. granti, and Clarkina postbitteri hongshuiensis from the base to the top horizons of the Capitanian stage. The MS data sets are mostly negative (diamagnetic), suggesting that the Laibin area was isolated from terrigenous source during most period of the Capitanian stage. An abrupt increase in MS signal immediately below the Guadalupian-Lopingian (G-L) boundary is recorded. Although regression at the end of Capitanian might account for the increase, the coincidence of the increase with the onset of the Emeishan Large Igneous Province (LIP) eruption is intensively concerned for volcanic role in controlling the MS at the section. The results of MS and thermomagnetic susceptibility measurements indicate that the dominant magnetic constituents in this study are diamagnetic calcite and chert with subordinate paramagnetic and ferrimagnetic minerals producing the cyclicity observed in the data set. The variation of MS near the G-L boundary at Tieqiao section is essentially as same as previously published results for MS at Penglaitan Section (G-L boundary GSSP). Six Milankovitch bands are reported with eccentricity peaks E2 (~405 ka) at 0.08 cycles·m-1 and E1 (~100 ka) at 0.35 cycles·m-1, obliquity peaks, O2 (~44.1 ka) at 0.78 cycles·m-1 and O1 (~35.0 ka), and precessional peaks P2 (~20.95 ka) at 1.69 cycles·m-1, and P1 (~17.7 ka) at 1.94 cycles·m-1. Four peaks exceed 99% confidence levels. In terms of the results of magnetostratigraphic susceptibility (MSS) zones developed from smoothed data to conform to the 405 ka (E2) eccentricity band identified in the time-series data set, a high-resolution FPTS was developed for the entire Tieqiao succession reported here. This FPTS allows a temporal resolution of ~200 ka for the studying section because each MSS zone represents half of the E2 eccentricity cycle. The results indicate that the time duration of the Capitanian Stage is estimated as ~3.85 Ma (with +0~0.28 Ma error). The mean sediment accumulation rate (SAR) for the succession is ~2.91 cm·ka-1. Moreover, conodont zones within the Capitanian stage is estimated to spanning the shortest time interval of ~26.6 ka and the longest duration of ~2.3 Ma. In addition, the onset of the Emeishan LIP eruptions is estimated at ~262.67 Ma, 1.42 Ma earlier than the Guadalupian-Lopingian boundary. ©, 2015, Science Press. All right reserved. Source
Yang H.,Wuhan University |
Zhang H.,Wuhan University |
Luo B.,Wuhan University |
Zhang J.,Central South Bureau of China Metallurgical Geology Bureau |
And 3 more authors.
Lithos | Year: 2015
Voluminous Early Paleozoic intrusive rocks occur in the Qilian orogenic belt, northeastern margin of the Tibetan Plateau. Their petrogenesis can provide insights into the tectonic evolution of Qilian and its adjacent areas. This paper carries out an integrated study of U-Pb zircon dating, geochemical and Sr-Nd-Hf isotopic compositions for the Bamishan and Heishishan plutons in the eastern Qilian orogen. Both the Bamishan and Heishishan plutons consist of mafic, intermediate and felsic intrusive rocks. U-Pb zircon dating yielded the magma crystallization ages of 459-449. Ma for the Bamishan pluton and 447-438. Ma for the Heishishan pluton. Their geochemical and Sr-Nd-Hf isotopic data show that the parental magmas of the Bamishan monzogranites and quartz diorites were generated by slab (MORB and overlying sediments) melting. The Bamishan hornblende gabbros have extremely high Cr (1159-1163. ppm), Ni (220-353. ppm) and Mg# (73-74), indicating a cumulate origin. The Heishishan granodiorite-granite has adakitic geochemical signatures and they were produced by partial melting of the mafic lower crust. The Heishishan hornblende gabbro and quartz diorites were derived from the enriched lithospheric mantle metasomatized by subduction-related fluid/melt. In light of the available data, we propose that there was an Early Paleozoic ocean basin between the Central-South Qilian and Qaidam-West Qiling belts. The northward closure of this basin and subsequent continental collision and post-collisional processes resulted in the generation of the Early Paleozoic magmatism in the eastern Central Qilian. The Late Ordovician to Early Silurian (~. 460-440. Ma) intrusive rocks in the eastern Central Qilian formed in a post-collisional setting. A slab break-off model may account for the magma generation. © 2015 Elsevier B.V. Source
Li B.,Guangzhou Marine Geological Survey |
Li B.,Key Laboratory of Marine Mineral Resources |
Li B.,Wuhan University |
Xue W.,Wuhan University |
And 4 more authors.
Diqiu Kexue - Zhongguo Dizhi Daxue Xuebao/Earth Science - Journal of China University of Geosciences | Year: 2015
As a reliable proxy for paleoclimate and paleoenvironment, magnetic parameter could provide valuable data for the research of global environmental changes and climatic processes. A detailed study has been carried out on rock magnetism in order to reveal the change of climate and environment and its reasons across Guadalupian-Lopingian (G-L) boundary at Tieqiao section in Laibin area, Guangxi, China. The results show that the dominant magnetic minerals from Tieqiao section are paramagnetic minerals with a small amount of magnetite and hematite as well. There is a remarkable change in the properties of rock magnetism near the G-L boundary. Magnetic susceptibility increases first and then decreases. Magnetic carriers show the trend of being transformed from hard magnetic minerals (hematite) to soft magnetic mineral (magnetite) and then turning to hard magnetic minerals (hematite). All of these changes occur within 4 m-thick strata interval above and below the G-L boundary, and synchronize with the fluctuation of sea-level and ancient seawater temperature during late Middle Permian. The significant changes in magnetic parameters for Middle-Late Permian carbonates suggest that the migration and conversion of magnetic minerals between different spheres in earth system has shifted, which resulted from the climatic and environmental changes. High sea-level during Late Guadalupian and Early Lopingian resulted in the decrease in terrigenous supply in South China. Therefore, the magnetic carriers in Tieqiao section mainly are hematite through aeolian transportation during this period. However, pronounced regression during the Middle-Late Permian transition led to the expansion of exposed land area. Meanwhile, land plants experienced widespread extinction, which led to increased sediment source. The magnetic carriers in contemporaneous sediments of Tieqiao section are mainly fluvial magnetite. ©, 2015, China University of Geosciences. All right reserved. Source
Sun H.,Wuhan University |
Han J.,Qinhuangdao Team of Mineral hydrological Geology |
Shen Y.,Chinese Academy of Geological Sciences |
Liu L.,Guangdong Nonferrous Metals Geological Exploration Institution |
And 6 more authors.
Diqiu Kexue - Zhongguo Dizhi Daxue Xuebao/Earth Science - Journal of China University of Geosciences | Year: 2016
The Linglong and Jiaojia goldfields are the two most famous goldfields in the Northwest Jiaodong with voluminous publication focused on gold mineralization. Whereas it is still a barren land as far to pay attention to the post-mineralization exhumation. Utilizing the latest low-temperature thermochronological technique, this work carries out a tentative research on the exhumation degree in the two goldfields. The results show that the zircon (U-Th)/He ages in Linglong goldfield vary predominantly between 80 and 100 Ma, while they are of significant concentration between 90 and 105 Ma in Jiaojia goldfield. These ages yielded in this work are different from those of the previous obtained by other methods, they are not the geochronological ages in respond to magmatic-hydrothermal mineralization events, instead they must have recorded the thermochronological ages of the post-mineralization exhumation of the granitoid rocks. Based on the constrain of the zircon (U-Th)/He ages, it can be concluded that the degree of exhumation in this area is roughly in agreement with that from the statistics of the global orogenic gold deposits, about 60 m/Ma. The amount of post-mineralization exhumation in the Linglong goldfield is overall ca. 600-900 m more than that in the Jiaojia goldfield, so the prospecting potential at deep part is larger in the Jiaojia goldfield than that in the Linglong goldfield. © 2016, Editorial Department of Earth Science. All right reserved. Source
Bao Z.,CAS Guangzhou Institute of Geochemistry |
Li C.,Guangdong Nonferrous Metals Geological Exploration Institution |
Zhao Z.,CAS Guangzhou Institute of Geochemistry
Ore Geology Reviews | Year: 2016
The Dongping gold deposit is located at the northern margin of the North China Craton in Hebei Province, China; it is the largest alkaline pluton-related Au deposit in China. The ore deposit is hosted in the 400-386 Ma (LA-ICPMS and SHRIMP zircon U-Pb method) Shuiquangou syenite intrusion, which cuts Archean-aged metamorphic rocks. The ore bodies consist of a set of en echelon lenses and veins controlled by shear zones. The ores can be divided into auriferous quartz vein, stockwork, and disseminated ores. Two stages of gold mineralization can be distinguished, the first stage mineralization consists of high grade (> 10 g/t) grayish quartz veins enveloped by stockwork and disseminated ores, whereas the second stage mineralization occurs as low grade (< 10 g/t) milky white quartz veins. The ores are sulfide-poor and enriched in Te. Gold is mainly present as native gold, calaverite, electrum and petzite. Studies of fluid inclusions in quartz indicate that the ore-forming fluids have low salinity (0.7-8.1% NaCl equivalent) in the H2O-CO2-NaCl system and have homogenization temperatures varying from 310 to 350°C. The ore-forming fluids have δ18O values varying from -1.7 to +6.9‰ with an average of 2.9‰ and δD values from -108 to -66.5‰ with an average of -85‰, indicating a magmatic origin with some involvement of meteoric fluid. The ore-forming fluids have initial 87Sr/86Sr385Ma ratios of 0.705-0.706 and 3He/4He ratios of 0.3 to 5.2 Ra (Ra being the atmospheric ratio of 1.0), indicative of a mantle contribution of the fluids. The ores have bulk δ34S values varying from -1 to +2‰, similar to those of the host syenites (δ34S=1.8-3‰) and the Archean metamorphic rocks (δ34S=0.04 to +4.4‰). Lead isotope analyses of sulfides, stock and disseminated ores, quartz veins, fresh host syenites and the Archean metamorphic rocks have a linear correlation of 208Pb/206Pb and 207Pb/206Pb, indicating a genetic link between the gold mineralization and the syenites. Hydrothermal zircons from the high grade ores of the first stage can be distinguished from magmatic zircons based on internal textures on cathodoluminescence images and rare earth element patterns; these zircons have been dated at 389±1.0 to 385±5.7Ma, whereas those from the second stage low-grade auriferous quartz vein were dated at ~140Ma (LA-ICPMS and SIM U-Pb methods). We propose a model for the formation of the Dongping gold deposit involving a late-magmatic hydrothermal process related to a Devonian mantle sourced syenites with a Late Jurassic-Early Cretaceous hydrothermal overprint. © 2015 Elsevier B.V. Source