Accessory minerals with so-called granular texture have risen in importance as geochronological tools for U-Pb dating of meteorite impact events.Grain-scale recrystallization,typically triggered by a combina-tion of high-strain deformation and post-impact heating,can create a polycrystalline microstructure con-sisting of neoblasts that expel radiogenic Pb,which are thus ideal for isotopic dating.While granular domains in zircon and monazite from shocked rocks have been demonstrated to preserve impact ages,few U-Pb dating studies have been conducted on granular microstructures in titanite(CaTiSiO5).Here we report the occurrence of granular-textured titanite from～2020 Ma granite basement rock exposed in the rim of the 4-5 Ma Roter Kamm impact structure in Namibia.Orientation mapping reveals two microstructurally distinct titanite populations:one consisting of strained/deformed grains,and the other consisting of grains that comprise aggregates of strain-free neoblasts.In situ U-Pb geochronology on 37 grains shows that most grains from both titanite populations yield indistinguishable U-Pb dates of ca.1025 Ma,consistent with the observed microstructures forming during the Mesoproterozoic Namaqua Orogeny.Only four grains preserved older age domains,recording ca.1875 Ma Paleoproterozoic meta-morphism.Two significant observations emerge:(1)none of the analyzed titanite grains yield the 2020 Ma igneous crystallization age previously established from zircon in the same sample,and(2)no age-resetting was detected that could be attributed to the 4 to 5 Ma Roter Kamm impact event.Despite the similarity of the neoblastic microstructure to minerals from other sites with an established impact provenance,the granular texture and near-complete Pb-loss in titanite from Roter Kamm granite instead records a Paleo-to Mesoproterozoic polymetamorphic history,rather than Miocene age shock-related processes.These results highlight the critical importance of grain-scale context for interpretation of U-Pb data in granular titanite,and the potential for misinterpreting inherited(pre-impact)microstruc-tures as impact-related phenomenon in target rocks with a complex geological history.

Aaron J.Cavosie;Christopher J.Spencer;Noreen Evans;Kai Rankenburg;Robert J.Thomas;Paul H.Macey

Space Science and Technology Centre and The Institute for Geoscience Research,School of Earth and Planetary Science,Curtin University,Perth,WA 6102,Australia;Dept.of Geological Sciences and Geological Engineering,Queen's University,Ontario,Canada;John de Laeter Centre,Curtin University,Perth,WA 6102,Australia;Council for Geoscience,Bellville,Western Cape,South Africa

地学前缘（英文版）

[1]Aaron J.Cavosie;Christopher J.Spencer;Noreen Evans;Kai Rankenburg;Robert J.Thomas;Paul H.Macey-.Granular titanite from the Roter Kamm crater in Namibia:Product of regional metamorphism,not meteorite impact)[J].地学前缘（英文版）,2022(03):156-163

Roter,Kamm,Namibia,neoblasts,expel,radiogenic,microstructurally,Namaqua,Orogeny,resetting,neoblastic,polymetamorphic,misinterpreting

Granular,titanite,from,crater,Product,regional,metamorphism,not,meteorite,impact,Accessory,minerals,called,granular,have,risen,importance,geochronological,tools,Pb,dating,events,Grain,scale,recrystallization,typically,triggered,by,combina,deformation,post,heating,create,polycrystalline,that,which,are,thus,ideal,isotopic,While,domains,zircon,monazite,shocked,rocks,been,demonstrated,ages,few,studies,conducted,microstructures,CaTiSiO5,Here,we,report,occurrence,textured,Ma,granite,basement,exposed,rim,Orientation,mapping,reveals,two,distinct,populations,consisting,strained,deformed,grains,other,comprise,aggregates,free,In,situ,geochronology,shows,most,both,yield,indistinguishable,dates,consistent,observed,forming,during,Mesoproterozoic,Only,four,preserved,older,recording,Paleoproterozoic,Two,significant,observations,emerge,none,analyzed,igneous,previously,established,same,sample,was,detected,could,attributed,Despite,similarity,sites,provenance,near,complete,loss,instead,records,history,rather,than,Miocene,related,processes,These,results,highlight,critical,context,interpretation,data,potential,inherited,phenomenon,target,complex,geological

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