Extensive Resetting of Feldspar Pb Isotopic Composition in Archean–Paleoproterozoic Granitic Rocks from the Kongling Terrane,South China:Implications for Tracing Thermal Evolutionary History

Feldspar Pb isotopes have been widely used to trace magmatic formation and evolution processes.However,it remains unclear whether post-magmatic thermal events can affect feldspar Pb isotopic ratios.Here,the in situ Pb isotopic composition of feldspar hosted in granitic rocks(thirteen Archean and one Paleoproterozoic)from the northern Kongling terrane,Yangtze Craton,South China,is analyzed.The samples reveal a substantial variation in their Pb isotopic composition,spanning the gap between the 1.9 Ga and present-day geochrons,which indicates extensive resetting by later tectonothermal events.This resetting was interpreted to have likely resulted from Paleoproterozoic and Neoproterozoic tectonothermal events related to the assembly and breakup of the Columbia and Rodinia supercontinents.These results suggest that Pb isotopes should be used cautiously when tracing magma sources and petrogenesis in magmatic rocks that have experienced post-magmatic reworking.However,the in situ Pb isotopic composition of feldspar in ancient granitoids may also potentially be used to reveal later tectonothermal events.The extensive resetting of the Pb isotopic composition in feldspar by regional thermal events may also provide new insights into our understanding of the Pb isotope paradox.

碳酸盐化榴辉岩对板内碱性玄武岩源区的贡献

碳在地球表层和内部圈层之间的循环控制了地球的环境气候变化。海底热液蚀变以及变质作用会使得碳酸盐沉淀到玄武质洋壳中,之后它们在俯冲带经历高压榴辉岩相变质作用会形成碳酸盐化的榴辉岩。俯冲过程可以将这些碳酸盐化的玄武质洋壳榴辉岩带入深部地幔,从而导致地幔在地球化学和岩石学上存在显著的不均一性,并深刻影响着包括部分熔融在内的地幔深部动力学过程。早期对碳酸盐化榴辉岩的实验研究多集中在俯冲带地温梯度下碳酸盐化洋壳榴辉岩的命运,近年来的实验开始研究地幔温压条件下碳酸盐化榴辉岩部分熔融对板内碱性玄武质岩浆源区的潜在影响。本文简要梳理了碳酸盐化榴辉岩在上地幔温压条件下的部分熔融实验,并结合一些天然样品的研究实例,说明碳酸盐化榴辉岩熔体不仅可以作为潜在的地幔交代介质,还可能是板内碱性玄武岩源区的重要组分,它们与橄榄岩发生反应或者与橄榄岩熔体发生混合可以解释板内碱性玄武岩的成分变化。