Several mafic-ultramafic layered intrusions were emplaced in the Fennoscandian Shield during wide-spread mantle-sourced magmatism at 2.5–2.4 Ga. The Näränkävaara intrusion (surface area 5 × 30 km 2 ), northeastern Finland, contains a 1.5–2 km thick basal dunite (not dated), and a 1.5 km thick layered series (2436 ± 5 Ma). A newly discovered marginal series between the basal dunite and the layered series indicates that the basal dunite is older, and highlights the need for further study on their relationship. Along its southern basement contact, the basal dunite contains a 200–300 m thick zone of olivine ortho- and mesocumulates, but the bulk of it is composed of olivine adcumulates and lesser olivine-orthopyroxene heteradcumulates. Based on whole-rock geochemistry, the basal dunite is divided into a low-Fe zone (average FeO t 10.2 wt% and Ni 2250 ppm) and a high-Fe zone (average FeO t 12.5 wt% and Ni 1700 ppm). Both zones have high MgO (32–47 wt%) and varying Cr (830–5160 ppm) and Al 2 O 3 /TiO 2 (16–26). Textural and geochemical layering is similar along the 30 km strike of the basal dunite. A LREE-enriched high-MgO basaltic parental magma composition (13–18 wt% MgO) is inferred for the basal dunite from olivine–melt mixing trends in orthocumulates. The dunite exhibits at least two geochemical reversals as well as abundant low-porosity adcumulates, poikilitic chromite, and bimodal olivine, suggesting formation in a high-volume open magmatic system. Significant similarity in major and trace element compositions with the Näränkävaara layered series and the Burakovsky intrusion and Vetreny belt extrusives in Russian Karelia suggests that the basal dunite belongs to the Fennoscandian 2.5–2.4 Ga mafic layered intrusions. An Archean komatiitic origin for the dunite body cannot be completely ruled out, however. Distinct Ni-depletion in olivine is found in the basal dunite from the low-Fe zone to the high-Fe zone (3200 versus 2200 ppm). This depletion does not correlate with Fo contents, which suggests that it is not related to olivine fractionation. The basal dunite may thus have potential for Ni-(Cu-Co-PGE) sulfide mineralization.

中文翻译：

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前寒武纪镁铁质-超镁铁质 Näränkävaara 侵入体的基底纯晶岩：岩石成因的考虑和对勘探的影响

在 2.5-2.4 Ga 的广泛地幔源岩浆活动期间，几个镁铁质-超镁铁质层状侵入体被安置在芬诺斯坎迪亚地盾中。 Näränkävaara 侵入体（表面积 5 × 30 km 2）位于芬兰东北部，包含一个 1.5-2 公里厚的基底纯晶（未注明日期）和 1.5 公里厚的层状系列（2436 ± 5 Ma）。新发现的基底纯粒岩与层状系列之间的边缘系列表明基底纯粒岩年龄较大，并强调需要进一步研究它们之间的关系。沿着其南部基底接触面，基底纯晶岩包含一个 200-300 m 厚的橄榄石正和中堆积带，但大部分由橄榄石堆积体和较小的橄榄石-邻辉石杂堆积体组成。根据全岩地球化学，基底纯晶分为低铁区（平均FeO t 10. 2 wt% 和 Ni 2250 ppm）和高铁区（平均 FeO t 12.5 wt% 和 Ni 1700 ppm）。两个区域都具有高 MgO (32-47 wt%) 和变化的 Cr (830-5160 ppm) 和 Al 2 O 3 / TiO 2 (16-26)。沿着基底纯晶岩 30 公里走向的构造和地球化学层状结构相似。从正堆积物中橄榄石-熔体混合趋势推断出基底纯晶岩具有富含轻稀土的高 MgO 玄武质母岩浆成分（13-18 wt% MgO）。纯晶岩表现出至少两次地球化学逆转以及丰富的低孔隙度堆积物、斑纹铬铁矿和双峰橄榄石，表明形成于大量开放岩浆系统中。主要和微量元素组成与俄罗斯卡累利阿的 Näränkävaara 层状系列和 Burakovsky 侵入体和 Vetreny 带喷出体的显着相似性表明基底纯纯岩属于 Fennoscandian 2.5-2.4 Ga 镁铁质层状侵入体。然而，不能完全排除纯晶体的太古代科马提岩起源。在从低铁区到高铁区（3200 与 2200 ppm）的基底纯晶中发现橄榄石中明显的 Ni 贫化。这种消耗与 Fo 含量无关，这表明它与橄榄石分馏无关。因此，基底纯石英可能具有 Ni-(Cu-Co-PGE) 硫化物矿化的潜力。在从低铁区到高铁区（3200 与 2200 ppm）的基底纯晶中发现橄榄石中明显的 Ni 贫化。这种消耗与 Fo 含量无关，这表明它与橄榄石分馏无关。因此，基底纯石英可能具有 Ni-(Cu-Co-PGE) 硫化物矿化的潜力。在从低铁区到高铁区（3200 与 2200 ppm）的基底纯晶中发现橄榄石中明显的 Ni 贫化。这种消耗与 Fo 含量无关，这表明它与橄榄石分馏无关。因此，基底纯晶岩可能具有 Ni-(Cu-Co-PGE) 硫化物矿化的潜力。