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.

我们报道了产自巴西南里贝拉造山带（Southern Ribeira Belt）沃图韦拉瓦群（Votuverava Group）佩劳组（Perau Formation）与贝塔拉组（Betara Formation）沉积变质岩系（metasedimentary units）中呈岩床（sills）状夹层产出的基性变质岩（metabasic rocks）的全新岩石学（petrographic）、全岩地球化学（whole-rock geochemistry）及高灵敏度高分辨率离子探针（Sensitive High-Resolution Ion Microprobe, SHRIMP）锆石U-Pb年代学（zircon U-Pb geochronological）数据。上述两组地层覆于约1.8 Ga的古元古代（Paleoproterozoic）变质花岗质岩（metagranitoids）之上，该套岩石属于帕拉帕内马克拉通（Paranapanema Craton）的基底残块。通过岩石学与全岩地球化学分析，可将样品划分为从原始到演化的四个类别：i）堆积型变辉绿岩（cumulatic metadolerites，G1组）、ii）各向同性变辉绿岩（isotropic metadolerites，G2组）、iii）正角闪岩（ortho-amphibolites，G3组）以及iv）铁质正角闪岩（ferroan ortho-amphibolites，G4组）。这些岩石均具有拉斑玄武质亚碱性成分（tholeiitic subalkali basaltic composition），稀土元素（Rare Earth Elements, REE）配分模式近乎平坦，地球化学特征介于正常洋中脊玄武岩（Normal Mid-Ocean Ridge Basalt, N-MORB）与富集型洋中脊玄武岩（Enriched Mid-Ocean Ridge Basalt, E-MORB）之间。低氧逸度（oxygen fugacity, fO₂）条件下的分离结晶作用（fractional crystallization）是控制岩浆演化的主要成岩过程，这一结论可通过Ti/V、Cr/Y比值及拉斑系列趋势（tholeiitic trends）得到验证。地球化学特征显示存在地壳混染（Crustal contamination）信号：包括铌负异常（negative Nb anomaly）、Th/Nb-Ti/V代用指标（proxies）趋势、大离子亲石元素（Large Ion Lithophile Elements, LILE）富集，以及年龄约为2.2 Ga的捕掳晶锆石（assimilated xenocryst zircons）。稀土元素与微量元素（trace element）系统学特征表明，基性岩浆起源于尖晶石相（spinel facies）域内的软流圈地幔（asthenospheric mantle）源区约10%~20%的部分熔融（partial melting）。TiO₂/Yb-Nb/Yb、Nb/Yb-Zr/Yb比值代用指标，以及沃图韦拉瓦群前人文献中报道的贫型洋中脊玄武岩（Depleted Mid-Ocean Ridge Basalt, P-MORB）基性变质岩，均指示该岩浆受到地幔柱（plume）的影响。本次研究对一件G3组正角闪岩样品开展SHRIMP锆石U-Pb定年，获得年龄为1448 ± 11 Ma。我们提出佩劳组与贝塔拉组内的变辉绿岩形成于陆内构造环境（intracontinental tectonic setting），该伸展事件与帕拉帕内马克拉通的裂谷作用（riftting）相关。此外，本次研究的基性变质岩的成因可能与卡里姆纪（Calymmian times，约1.5~1.45 Ga）的区域性伸展事件有关，该事件在刚果克拉通（Congo Craton）、圣弗朗西斯科克拉通（São Francisco Craton）、西伯利亚克拉通（Siberian Craton）、劳伦克拉通（Laurentian Craton）及中国克拉通（China Cratons）等多个克拉通内均引发了基性岩浆作用，这与哥伦比亚超大陆（Columbia Supercontinent）的裂解过程存在关联。