Ages of Jurassic volcano-sedimentary strata in the Yanshan Fold-and-Thrust Belt and their implications for the coal-bearing strata of northern China
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Jurassic coal-bearing strata are widely distributed in the North China Craton (NCC) and other areas of northern China. These coal-bearing strata were previously considered to be Early–Middle Jurassic in age based on plant fossils, particularly the fossil assemblage of Coniopteris–Phoenicopsis. Since coal-bearing strata are interbedded with volcanic units in the basins of the Yanshan Fold-and-Thrust Belt (YFTB), northern NCC, isotopic dating of the volcanic units can therefore provide age constraints on the coal-bearing strata and the Coniopteris‒Phoenicopsis assemblage. In this paper, we performed a systematic geological survey and present the results of zircon U–Pb dating of the volcanic units and a pluton in typical basins of the YFTB. These data, combined with the results of previous studies, indicate that the ages of the Nandaling/Xinglonggou, Haifanggou, Jiulongshan, and Tiaojishan/Lanqi formations are 180–168, 169–161, 161–157, and 161–153 Ma, respectively. The ages of the interbedded coal-bearing Yaopo and Beipiao formations are constrained to be 169–161 and 177–169 Ma, respectively. Our results demonstrate that both the coal-bearing strata and the Coniopteris‒Phoenicopsis assemblage are Middle Jurassic in age, which is younger than that previously considered. This fossil assemblage plays a critical role in age constraints on the Jurassic coal-bearing strata. The refinement of its age permits a more precise dating of the coal-bearing strata, especially in northwestern China, where datable interbedded volcanic units are lacking. Stratigraphical framework for the Jurassic strata of Yanshan region is established.The fossil assemblage of Coniopteris‒Phoenicopsis occurred in Middle Jurassic.The Jurassic coal-bearing strata in northern China are mainly Middle Jurassic.Keywords:
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Io-ages obtained for glass-zircon pairs which were not consistent with expected ages were discussed on the basis of uranium and thorium distribution between zircon and glass. Uranium and thorium distribution between zircon and host rocks from dacites and granites were also discussed. Discordant Io-ages obtained for glass-zircon pairs with normal (Th/U)zircon/(Th/U)glass ratio are explained by early stage crysallization of the zircon in the magma. Discordant Io-ages with abnormal (Th/U)zircon/(Th/U)glass ratios suggest that the zircon was captured in the magma as xenoryst. The discrepancy between (Th/U)zircon/(Th/U)glass ratios (about 0.19) for dacites and (Th/U)zircon/(Th/U)whole rock ratios (about 0.12) for granites could be explained by crystallization of granitic zircon from liquid having chemical composition different from that of the whole rock of granite.
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The intrusive rocks in El Sela area can be arranged from the oldest to the youngest into: two-mica granite and postgranitic dikes which include microgranite, dolerite and bostonite dikes. Zircon is the most abundant accessory mineral. Zircon morphology and geochemical features are good indicators for evolution of rocks. The aim of the work is to determine the morphology, internal structure and chemical composition of zircon to identify the difference of zircon in various intrusive rocks. Results show that morphologically, zircon in the two-mica granite is euhedral coarse- grained with zonation. It is represented by crystals up to 125 µm and corresponds to S10 and P2. Zircon in post-granitic dikes exhibit irregular forms. Geochemically, zircon crystals have higher ZrO2 values in the core whereas HfO2 , UO2 , ThO2 increase at the peripheries of zoned crystals of the two-mica granite. Zircon of two-mica granite contains high HfO2 , UO2 , ThO2 and CaO contents but low Sc2 O3 content. HfO2 is not detected in zircon of microgranite. TiO2 in zircon of two-mica granite and bostonite dikes is under detection limits. REEs are not recorded in zircon of the studied intrusive rocks
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This paper presents a systematic study on morphology,geochemistry and geochronology of a zircon population from the Danzhu granodiorite located in the southwest region of the Zhejiang province.The analysis results show that there are two types of zircons from the Danzhu granodiorite:magmatic zircon and metamorphic zircon.The former is characterized by euhedral—subhedral and long-prismatic form,incolorness,transparence,presence of magmatic and mineral inclusions,homogeneous internal structure(occasionally with oscillatory zoning)on the section of grains revealed by backscattered electron(BSE)imaging,and analogue pattern of typological indices of zircon for those from the calco-alkaline granodiorite;the latter is characterized mainly by its occurrence as overgrowth on magmatic zircon,but it also occurs very occasionally as anhedral grains,which is yellow-brown,semi-transparent,and homogeneous on the BSE imaging.These two types of zircon differ also in trace element composition,i.e.,the magmatic zircon is significantly richer in the substituent elements like as U,Th and Y than the metamorphic zircon.The zircon U-Pb dating using LA-ICPMS shows that the magmatic zircon and metamorphic zircon were formed in two different periods:at 1875±33 Ma and at 209±12 Ma respectively,demonstrating the objective response for the Zhe-Min(Zhejiang—Fujian)movement and the Indosinian movement in the southwest region of the Zhejiang province.
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In this article we present a compilation of U-Pb zircon ages of the whole Xolapa terrane in coastal southern Mexico (dataset 1) as a curved line, obtained from plotting individual zircon grains versus its corresponding age. We identified five low-slope segments of the curved line, each one assigned to a high zircon-production (or preservation) event (HZE). Crystallization temperatures (CT) from Ti-in-zircon geothermometer data on Xolapa rocks were estimated separately from individual zircon grains (dataset 2), in order to compare CT ranges corresponding to each HZE identified. Datasets 1 and 2 are discussed for tectonic implications in the research article "The opening and closure of the Jurassic-Cretaceous Xolapa basin, southern Mexico" Peña-Alonso et al., 2017.
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