The lopolithic Pados-Tundra layered complex, the largest member of the Serpentinite belt–Tulppio belt (SB–TB) megastructure in the Fennoscandian Shield, is characterized by (1) highly magnesian compositions of comagmatic dunite–harzburgite–orthopyroxenite, with primitive levels of high-field-strength elements; (2) maximum values of Mg# in olivine (Ol, 93.3) and chromian spinel (Chr, 57.0) in the Dunite block (DB), which exceed those in Ol (91.7) and Chr (42.5) in the sills at Chapesvara, and (3) the presence of major contact-style chromite–IPGE-enriched zones hosted by the DB. A single batch of primitive, Al-undepleted komatiitic magma crystallized normally as dunite close to the outer contact, then toward the center. A similar magma gave rise to Chapesvara and other suites of the SB–TB megastructure. Crystallization proceeded from the early Ol + Chr cumulates to the later Ol–Opx and Opx cumulates with accessory Chr in the Orthopyroxenite zone. The accumulation of Chr resulted from efficient cooling along boundaries of the Dunite block. The inferred front of crystallization advanced along a path traced by vectors of Ol and Chr compositions. Grains and aggregates of Chr were mainly deposited early after the massive crystallization of olivine. Chromium, Al, Zn and H₂O, all incompatible in Ol, accumulated to produce podiform segregations or veins of chromitites. This occurred episodically along the moving front of crystallization. Crystallization occurred rapidly owing to heat loss at the contact and to a shallow level of emplacement. The Chr layers are not continuous but rather heterogeneously distributed pods or veins of Chr–Ol–clinochlore segregations. Isolated portions of melt enriched in H₂O and ore constituents accumulated during crystallization of Ol. Levels of fO₂ in the melt and, consequently, the content of ferric iron in Chr, increased progressively, as in other intrusions of the SB–TB megastructure. The komatiitic magma vesiculated intensely, which led to a progressive loss of H₂ and buildup in fO₂. In turn, this led to the appearance of anomalous Chr–Ilm parageneses. Diffuse rims of Chr grains, abundant in the DB, contain elevated levels of Fe³⁺ and enrichments in Ni and Mn. In contrast, Zn is preferentially partitioned into the core, leading to a decoupling of Zn from Mn, also known at Chapesvara. The sulfide species display a pronounced Ni-(Co) enrichment in assemblages of cobaltiferous pentlandite, millerite (and heazlewoodite at Khanlauta), deposited at ≤630 °C. The oxidizing conditions have promoted the formation of sulfoselenide phases of Ru in the chromitites. The attainment of high degrees of oxidation during crystallization of a primitive parental komatiitic magma accounts for the key characteristics of Pados-Tundra and related suites of the SB–TB megastructure.

中文翻译：

---

俄罗斯科拉半岛蛇纹岩带帕多斯-通德拉超镁铁质复合物的结晶PGE-铬铁矿矿化区

伦诺斯帕多斯-通德拉层状复合体，是芬诺斯堪的纳斯盾构蛇纹岩带-图尔皮奥带（SB-TB）巨型结构的最大成员，其特征是：（1）岩浆辉长岩-辉石-正辉石的高镁质组成，原始水平为高场强元素；（2）Dunite区块（DB）中橄榄石（Ol，93.3）和铬尖晶石（Chr，57.0）中Mg＃的最大值超过了Chapesvara窗台中的Ol（91.7）和Chr（42.5）中的Mg＃值， （3）由数据库托管的主要接触式铬铁矿-IPGE富集区的存在。一批原始的，未脱铝的科玛替岩岩浆通常结晶成榴辉岩，靠近外部接触，然后朝向中心。类似的岩浆引起了Chapesvara和SB-TB巨型结构的其他套件。结晶从早期的Ol + Chr累积到后期的Ol-Opx，而在邻辉石区，Opx伴随着辅助的Chr累积。Chr的积累是由于沿Dunite块边界有效冷却所致。推断的结晶前沿沿着由Ol和Chr组成的载体追踪的路径前进。Chr的颗粒和聚集体主要在橄榄石大量结晶后早期沉积。铬，铝，锌和氢 Chr的颗粒和聚集体主要在橄榄石大量结晶后早期沉积。铬，铝，锌和氢 Chr的颗粒和聚集体主要在橄榄石大量结晶后早期沉积。铬，铝，锌和氢₂ O（在O1中均不相容）累积，产生豆状偏析或铬铁矿脉。这是沿着结晶运动前沿发生的。由于接触处的热损失和较浅的位置，结晶迅速发生。Chr层不是连续的，而是Chr-0.clinochlore分离的豆荚或叶脉的分布不均。在Ol的结晶过程中，富集了H ₂ O和矿石成分的熔体的孤立部分积聚。熔体中的f O ₂含量以及随之而来的Chr中三价铁的含量逐渐增加，与SB-TB大结构的其他侵入一样。火山口岩浆强烈泡化，导致H ₂逐渐损失并在f O _2中积累。反过来，这导致出现异常的Chr–Ilm同种生物。在DB中丰富的Chr晶粒的扩散边缘含有较高水平的Fe ³⁺和富集的镍和锰。相比之下，Zn优先分配到芯中，导致Zn与Mn解耦，这在Chapesvara中也是众所周知的。硫化物在沉积于≤630°C的钴方型五方岩，Millerite（以及Khanlauta的heazlewoodite）的组合中表现出明显的Ni-（Co）富集。氧化条件促进了亚铬铁矿中Ru的亚磺酰亚胺相的形成。原始亲代岩浆岩浆结晶过程中获得的高氧化度是Pados-Tundra及其相关的SB-TB大结构套件的关键特征。