Petrogenesis, geochronology, and tectonic setting of the Calymmian 1.45–1.50 Ga basic magmatism in the Southern Ribeira Belt, Brazil: evidence of intracontinental rifting and Columbia break-up

We present new petrographic, whole-rock geochemistry, and SHRIMP U – Pb zircon geochronological data of metabasic rocks interlayered as sills in the metasedimentary units of the Perau and Betara formations, Votuverava Group, Southern Ribeira Belt, Brazil. These formations overlie ~ 1.8 Ga Paleoproterozoic metagranitoids that represent basement inliers from the Paranapanema Craton. Petrography and whole-rock geochemistry allow the recognition of four groups, ranging from primitive to evolved compositions: i) cumulatic metadolerites (G1), ii) isotropic metadolerites (G2), iii) ortho-amphibolites (G3); and iv) ferroan ortho-amphibolites (G4). They exhibit tholeiitic subalkali basaltic composition, near-flat REE patterns, and signatures between N- and E-MORB. Fractional crystallization under low fO2 conditions is the main petrogenetic process controlling the magmatic evolution, as observed by Ti/V and Cr/Y ratio and tholeiitic trends. Crustal contamination signatures are observed through negative Nb anomaly, Th/Nb-Ti/V proxies trends, LILE enrichment, and, also, assimilated xenocryst zircons aged ~ 2.2 Ga. REE and trace element systematics indicate that the basic magmas were generated by ~ 20–10% partial melting of a model asthenospheric mantle source within spinel facies. TiO2/Yb-Nb/Yb and Nb/Yb-Zr/Yb ratio proxies and P-MORB metabasic rocks, previously described in the literature within the Votuverava Group, suggest a plume-influenced melt. A new SHRIMP U – Pb zircon age of 1448 ± 11 Ma was determined for an ortho-amphibolite sample (G3 group). We propose an intracontinental tectonic setting for the genesis of the metadolerites whitin the Perau and Betara formations, suggesting that this extensional event is associated with the Paranapanema Craton rifting. Furthermore, the genesis of the studied metabasic rocks could be associated with a regional extensional event during Calymmian times (~1.5–1.45 Ga), which generated basic magmatism in several other cratons, such as the Congo, São Francisco, Siberian, Laurentian, and China cratons, linked to the break-up of the Columbia Supercontinent.

本文报道了巴西南部里贝拉造山带Votuverava群Perau组与Betara组变沉积岩单元中以岩床形式夹层产出的变基性岩的最新岩石学、全岩地球化学及SHRIMP U-Pb锆石定年（SHRIMP）数据。该两套地层不整合覆盖于约18亿年的古元古代（Paleoproterozoic）变花岗岩类之上，该变花岗岩类属于Paranapanema克拉通的基底残块。通过岩石学与全岩地球化学分析，可识别出四组从原始到演化的不同成分组合：①堆积型变辉绿岩（cumulatic metadolerites，G1组）、②各向同性变辉绿岩（isotropic metadolerites，G2组）、③正角闪岩（ortho-amphibolites，G3组）以及④铁质正角闪岩（ferroan ortho-amphibolites，G4组）。这些岩石均具有拉斑质亚碱性玄武岩成分，稀土元素（REE，Rare Earth Elements）配分模式近于平坦，地球化学特征介于正常洋中脊玄武岩（N-MORB，Normal Mid-Ocean Ridge Basalt）与富集型洋中脊玄武岩（E-MORB，Enriched Mid-Ocean Ridge Basalt）之间。低氧逸度（fO2，oxygen fugacity）条件下的分离结晶作用是控制其岩浆演化的核心成岩过程，这一结论可通过Ti/V、Cr/Y比值及拉斑系列演化趋势得到佐证。地壳混染的地球化学特征可通过铌负异常、Th/Nb-Ti/V代用指标趋势、大离子亲石元素（LILE，Large Ion Lithophile Elements）富集，以及年龄约22亿年的同化捕获捕虏锆石得以识别。稀土元素与微量元素体系特征表明，基性岩浆由尖晶石相域内的软流圈地幔源区经历约10%~20%的部分熔融形成。TiO₂/Yb-Nb/Yb、Nb/Yb-Zr/Yb比值代用指标，以及此前文献中报道的Votuverava群内P型洋中脊玄武岩（P-MORB）型变基性岩，均指示该岩浆熔体受地幔柱作用影响。本次研究对一件G3组正角闪岩样品开展SHRIMP U-Pb锆石定年，获得了1448±11 Ma的新年龄数据。我们认为Perau组与Betara组内变辉绿岩的形成具有陆内构造背景，指示该伸展事件与Paranapanema克拉通的裂谷作用相关。此外，本次研究的变基性岩的成因，可与卡罗米安期（Calymmian，约15亿~14.5亿年）的区域性伸展事件相关联——该时期在刚果、圣弗朗西斯科、西伯利亚、劳伦、华夏等多个克拉通均形成了基性岩浆作用，这与哥伦比亚超大陆（Columbia Supercontinent）的裂解过程密切相关。