Lead isotope constraints on the mantle sources involved in the genesis of Mesozoic high-Ti tholeiite dykes (Urubici type) from the São Francisco Craton (Southern Espinhaço, Brazil)

ABSTRACT: The first results of Pb isotope compositions of the high-Ti Mesozoic dykes of the Southern Espinhaço are presented. The results do not show large variations and are significantly more radiogenic than the Pb isotope compositions of the high-Ti tholeiites from the Paraná Continental Flood Basalts. The data combined with published geochemical and Sr-Nd isotope results rule out crustal contamination processes in the genesis of the dykes, requiring magma generation in metasomatized subcontinental lithospheric mantle with the involvement of HIMU-type and carbonatite components. The magmas may have been also derived from a mantle source containing ~4 - 5% of pyroxenite and ~1% of carbonatite melts, agreeing with published Os isotope compositions of high-Ti rocks from the Paraná Continental Flood Basalts. These metasomatizing agents could be responsible for mantle source refertilization, as was also proposed in the literature to explain the characteristics of xenoliths of the Goiás Alkaline Province, which also occurs in the border of the São Francisco Craton. Additionally, to evaluate the risks of Pb contamination during sample preparation for analysis, several experimental tests were accomplished, which indicate the need of sawed surface removal and a careful washing of small-sized rock fragments before powdering, especially for rocks with [Pb]

摘要：本文首次报道了南埃斯皮尼亚苏（Southern Espinhaço）地区高钛中生代岩脉的铅同位素组成测试结果。测试结果未呈现显著波动，且其放射性成因铅占比显著高于巴拉那大陆溢流玄武岩（Paraná Continental Flood Basalts）中高钛拉斑玄武岩的铅同位素组成。结合已发表的地球化学与Sr-Nd同位素数据，本次研究结果排除了岩脉成因过程中发生地壳混染的可能性，表明岩浆起源于受交代作用改造的大陆下岩石圈地幔，且涉及HIMU型（HIMU-type）与碳酸岩（carbonatite）组分。该岩浆可能还源自含有约4%~5%辉石岩（pyroxenite）与约1%碳酸岩熔体的地幔源区，这与已发表的巴拉那大陆溢流玄武岩中高钛岩石的锇同位素组成特征相符。上述交代介质可能是导致地幔源区发生再肥沃化的原因，这一解释也被前人文献用于阐释戈亚斯碱性省（Goiás Alkaline Province）的捕虏体特征，该省同样位于圣弗朗西斯科克拉通（São Francisco Craton）的边界地带。此外，为评估样品制备与分析过程中铅污染的风险，本次研究开展了多组实验测试，结果表明在粉碎样品前需去除锯切表面，并仔细清洗小型岩屑，尤其针对含[Pb]的岩石。