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首页> 外文期刊>Contributions to Mineralogy and Petrology >U-Pb geochronology of apatite and zircon from the Brent impact structure, Canada: a Late Ordovician Sandbian-Katian boundary event associated with L-Chondrite parent body disruption
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U-Pb geochronology of apatite and zircon from the Brent impact structure, Canada: a Late Ordovician Sandbian-Katian boundary event associated with L-Chondrite parent body disruption

机译：磷灰石和锆石的U-PB地理学从加拿大的Brent Impact结构：奥陶涅迪扬妇 - 堪前亚岛边界事件与L-Chondrite父母身体中断相关

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In situ LA-ICP-MS U-Pb geochronology has been performed on apatite and zircon within thermally recrystallized clast-laden and clast-poor impact melt rocks from the Brent impact structure. A total of 377 laser analyses on 120 impact melt-grown (n = 9) and impact-recrystallized zircon grains (n = 111) were obtained, from which a concordia age of 452.8 +/- 2.7 Ma (MSWD 0.57,n = 11), and a weighted average mean(206)Pb/U-238 age of 453.2 +/- 2.9 Ma (MSWD 0.60) (n = 11) are calculated. A total of 300 laser analyses from 100 relict apatite grains were obtained, with an unanchored regression through all data yielding a lower intercept age of 453.2 +/- 6.0 Ma (MSWD 5.8,n = 300), that overlaps within error of zircon.Pb-207/Pb-206 ratios obtained from feldspar clasts within clast-laden impact melt retain the same initial Pb composition as the target rocks from which they are derived, while feldspars that crystallized from impact melt have(207)Pb/Pb-206 ratios indicative of isotopic re-equilibration between basement lithologies of two different ages. A similar variability in(207)Pb/Pb-206 is recorded by apatite. This provides evidence for the involvement of Neoproterozoic Lake Nipissing alkaline suite, as well as Mesoproterozoic Grenville gneisses in the production of impact melt at Brent. Recrystallized apatite grains exhibit enrichments in light rare earth elements (LREEs) along neoblast grain boundaries, indicative of trace element substitution and phase precipitation during impact-induced recrystallization. An age of 452.8 +/- 2.7 Ma from zircon and 453.2 +/- 6.0 Ma from apatite places the impact event in the Late Ordovician, at or near the Sandbian-Katian boundary, confirming Brent's involvement in the Middle to Late Ordovician crater cluster event-a period of enhanced impactor flux to Earth related to the L-Chondrite parent body disruption.
机译：在原位La-ICP-MS U-PB地形学上已经在磷灰石和锆石内进行了热再结晶的碎屑和填充型从灌木局部冲击结构的裂隙性熔体岩石中进行的。获得120次冲击生长（n = 9）和冲击再结晶锆石（n = 111）的377激光分析，协和452.8 +/- 2.7 mA（mswd 0.57，n = 11 ），并计算加权平均值（206）Pb / U-238岁453.2 +/- 2.9 mA（mswd 0.60）（n = 11）。获得100个relictirt磷灰石晶粒的300个激光分析，通过所有数据的所有数据都有未经支承的回归，其截止时代的较低年龄为453.2 +/- 6.0 mA（mswd 5.8，n = 300），其在zircon.pb的错误内重叠-207 / Pb-206从加氏撞击熔体内的长石泥浆获得的比率保持与衍生的靶岩体相同的初始Pb组合物，而从冲击熔体结晶的长石具有（207）Pb / Pb-206比率表示两种不同年龄的基底岩性的同位素重新平衡。 （207）PB / PB-206的类似可变性由磷灰石记录。这为Neoproteroiczoic Lake Nipissing碱性套件的参与提供了证据，以及Mesoproterozoice Grenville片状在德伦特的冲击熔体中的产生。重结晶的磷灰石晶粒沿新细胞晶粒边界表现出轻质稀土元素（LEREE）的富集，指示冲击诱导的重结晶期间的痕量元素取代和相沉淀。来自锆石的452.8 +/- 2.7 ma，距离磷灰石的453.2 +/- 6.0 ma占据了奥陶诺维安末期，在Sandbian-Katian边界或附近的影响事件，确认Brent在中间到末orovician火山口集群活动的参与 - 与L-Chondrite父母身体破坏有关的地球增强的影响。

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