The formation of large neoblasts in shocked zircon and their utility in dating impacts
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Abstract:
Uranium-lead (U-Pb) geochronology of individual shocked zircon grains has unique potential for dating bolide impact events. Neoblasts in granular-textured zircon have been recognized as the shock-related feature most effective at recording the impact age. Here we report the discovery of large neoblasts (5–100 µm in dimension) in shocked zircon at the Sudbury impact structure, Canada—the first report of in situ coarsely granular zircon from a terrestrial impact site other than the Vredefort structure, South Africa. The neoblast-bearing sample was taken from a heterogeneous, lithic clast–rich igneous unit associated with the roof rocks of the impact melt sheet, making this the first time a crater has been dated using neoblastic zircon from the upper part of its stratigraphy. Previous in situ discoveries of coarsely granular zircon at Vredefort were all in impact-generated mafic melt emplaced beneath the impact melt sheet. Electron backscatter diffraction analysis of the impact-aged neoblasts indicates that the high-pressure conditions inferred in the formation of many small neoblasts were not necessarily involved in the formation of these large ones. Their large size, internal zonation, and occurrence in a slowly cooling environment collectively suggest that large neoblasts at Sudbury formed by relatively protracted, post-impact growth in shocked zircon incorporated into impact-related melt. Based on insight from large neoblast growth in terrestrial settings, we suggest that the ca. 4.33 Ga neoblasts recently reported in lunar zircon may imply a major basin-forming event on the Moon at that time. New knowledge of the cratering environments in which large neoblasts form also raises the prospect of possibly linking ex situ granular zircon in lunar breccias with specific impact structures—and thus better calibrating the lunar cratering record with radiometric ages.Keywords:
Metamictization
Impact structure
Large igneous province
Geochronology
Ultramafic rock
Large igneous province
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Abstract The Emeishan large igneous province (ELIP) was a major geological event that took place around the Middle–Late Permian boundary. The major episode of the ELIP is generally considered to have occurred at 260–259 Ma, although an increasing number of geological data have suggested that multiple episodes of magmatic activity might have occurred. Here we report a study of a Lower Permian dolomite whose formation was influenced by the ELIP in the Sichuan Basin of southwestern China. Results show two new pulsed activities at 273.2 ± 9.7 Ma to 269.1 ± 12.0 Ma (ELIP-I) and 263.2 ± 10.3 Ma to 258.7 ± 8.1 Ma (ELIP-II). The two stages of the ELIP magmatism and thermal activity controlled the regional tectonic, sedimentary, and diagenetic evolution and exerted a profound influence on the oil and gas reservoirs, source rocks, and lithofacies paleogeography. This study is the first to interpret the timing of the ELIP from analysis of hydrothermal dolomite and provides new clues and data for improving our understanding of the dynamic formation mechanism and Earth system of the ELIP.
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Large volumes of mafic igneous rocks are commonly emplaced during Large Igneous Province (LIP) eruptions, and these mafic rocks are often contemporaneous with periods of environmental disturbances, such as global ocean anoxia, and as a result, mass extinctions. The Paraná-Etendeka LIP is no exception, and has been previously correlated with, and interpreted to be the cause of the Valanginian oceanic anoxic event (OAE), a small global environmental disturbance. Here, we present new U-Pb ID-TIMS baddeleyite ages from high-Ti-Sr mafic intrusive rocks from the Paraná LIP, in Brazil. While these data are potentially complicated by the presence of Pb-loss and inheritance, it is possible to interpret geologically meaningful ages from them. The first high-precision age is reported for the type-locality of the Florianópolis Dyke Swarm, in North Santa Catarina Island, which yields an age of 132.53 ± 0.40/0.40/0.42 Ma. We also report an age of 132.07 ± 0.27/0.30/0.33 Ma for a dolerite sill, which intrudes organic-rich sedimentary rocks of the Paraná Basin. The emplacement of high-Ti-Sr magmas at ca. 132 Ma suggests that there was up to 2 Myr of intrusive magmatism in the southern part of the Paraná LIP. Further investigation on the mafic intrusive magmatism from the Paraná LIP through robust high-precision geochronology is required to elucidate the proposed linkage more clearly with environmental changes during the Early Cretaceous.
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Large igneous province
Sill
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