Mafic-ultramafic intrusions form a 32-km-long NE-SW-trending array within Paleoproterozoic arc-related calc-alkaline plutonic rocks of the Rio Piranhas terrane in the Borborema Province, Northeast Brazil. The mafic-ultramafic intrusions consist of wehrlite, clinopyroxenite, and troctolite. They outcrop as small elongated bodies hosted by orthogneiss and migmatites associated with minor amphibolite and supracrustal rocks. Primary magmatic features predominate in the core of the intrusions, whereas metamorphic minerals and textures characterize their outer zones. The composition of cumulus olivine from different intrusions ranges from Fo80.6 to Fo67.6 (forsterite), indicating moderately primitive parental magmas. The compositional range of cumulus plagioclase coexisting with cumulus olivine in troctolites is characterized by high anorthite content (An79.9–An89.0). This feature, together with interstitial magmatic amphibole in troctolites, is characteristic of gabbroic intrusions in subduction-related magmatic arcs. U-Pb zircon ages indicate that mafic-ultramafic intrusions crystallized at ca. 2190 ± 5 Ma. U-Pb zircon ages of host orthogneiss indicate that they crystallized from calc-alkaline felsic magmas at ca. 2220–2223 Ma, thus providing only ∼30 My for the reported events of felsic and mafic plutonism. U-Pb zircon ages of host rocks include older ages (2.47 and 2.69 Ga) indicating that older crustal components are associated with the 2.23 Ga calc-alkaline magmatism. The presence of older crustal components is consistent with arc-related features, indicated by geochemical data and variable εNd(t) values in the mafic-ultramafic intrusions. The tholeiitic and calc-alkaline magmatism is correlated with the ca. 2.15–2.25 Ga magmatic arc reported in previous studies of the Rio Piranhas terrane. The mafic-ultramafic magmatism is interpreted as having originated in a magmatic arc, possibly resulting from partial melting of a mantle wedge above the subduction zone in late stages of the orogeny and suggests a new window of opportunity for exploration of Ni-Cu–platinum group element deposits.

中文翻译：

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巴西东北部 Borborema 省北部的古元古代基性-超镁铁质岩浆作用：构造环境和沉积潜力

镁铁质-超镁铁质侵入体在巴西东北部 Borborema 省 Rio Piranhas 地体的古元古代弧相关钙碱性深成岩内形成 32 公里长的 NE-SW 趋势阵列。镁铁质-超镁铁质侵入体由纬长岩、斜长辉石和滑石组成。它们以细长的小体的形式露头，由正方麻岩和与次要角闪岩和上地壳岩石相关的混合岩组成。原生岩浆特征在侵入体的核心中占主导地位，而变质矿物和质地则表征了它们的外部区域。来自不同侵入体的积云橄榄石的成分范围从Fo80.6到Fo67.6（镁橄榄石），表明母体岩浆中等原始。凝灰岩中与橄榄石共存的积云斜长石的组成范围以高钙长石含量为特征（An79.9-An89.0）。这一特征与 troctolites 中的间隙岩浆角闪石一起，是与俯冲相关的岩浆弧中辉长岩侵入体的特征。U-Pb 锆石年龄表明，镁铁质-超镁铁质侵入体在约 2190±5毫安。寄主正方麻岩的 U-Pb 锆石年龄表明它们是在大约 10 年的钙碱性长英质岩浆中结晶的。2220-2223 Ma，因此对于所报告的长英质和镁铁质深成作用事件仅提供~30 My。主岩的 U-Pb 锆石年龄包括较旧的年龄（2.47 和 2.69 Ga），表明较旧的地壳成分与 2.23 Ga 钙碱性岩浆作用有关。较老地壳成分的存在与弧相关特征一致，由地球化学数据和镁铁质-超镁铁质侵入体中的可变εNd(t)值表明。拉斑岩和钙碱性岩浆作用与大约相关。2.15-2.25 Ga 岩浆弧在先前对 Rio Piranhas 地体的研究中报道。镁铁质-超镁铁质岩浆作用被解释为起源于岩浆弧，可能是造山运动后期俯冲带上方地幔楔的部分熔化造成的，并为镍-铜-铂族的勘探提供了一个新的机会窗口元素沉积。