Kimberlites are often closely associated, both in time and space, with a wide variety of alkaline ultramafic rock types, yet the question of a genetic relationship between these rock types remains uncertain. One locality where these relationships can be studied within the same cluster is the Karelian craton in Finland. In this study we present the first petrographic, mineral and whole-rock geochemical results for the most recently discovered kimberlite cluster on this craton, which represents an example of the close spatial overlap of kimberlites with ultramafic lamprophyres. The Kuusamo cluster incorporates seven bodies [Kasma 45, Kasma 45 south, Kasma 47, Kalettomanpuro (KP), Kattaisenvaara (KV), Dike 15 and Lampi] distributed along a 60 km NE–SW corridor. Hypabyssal samples from KV, KP, Kasma 45 and Kasma 47 consist of altered olivine macrocrysts and microcrysts and phlogopite phenocrysts in a groundmass of perovskite, apatite, spinel, ilmenite, serpentine, and calcite. These petrographic features combined with mineral (e.g. Mg-rich ilmenite, Al–Ba-rich, Ti–Fe-poor mica) and whole-rock incompatible trace element compositions (La/Nb = 0·8 ± 0·1; Th/Nb = 0·07 ± 0·01; Nb/U = 66 ± 9) are consistent with these rocks being classified as archetypal kimberlites. These Kuusamo kimberlites are enriched in CaO and poor in MgO, which, combined with the absence of chromite and paucity of olivine macrocrysts and mantle-derived xenocrysts (including diamonds), suggests derivation from differentiated magmas after crystal fractionation. Samples from Lampi share similar petrographic features, but contain mica with compositions ranging from kimberlitic (Ba–Al-rich cores) to those more typical of orangeites–lamproites (increasing Si–Fe, decreasing Al–Ti–Ba), and have higher bulk-rock SiO₂ contents than the Kuusamo kimberlites. These features, combined with the occurrence of quartz and titanite in the groundmass, indicate derivation from a kimberlite magma that underwent considerable crustal contamination. This study shows that crustal contamination can modify kimberlites by introducing features typical of alkaline ultramafic rock types. Dike 15 represents a distinct carbonate-rich lithology dominated by phlogopite over olivine, with lesser amounts of titaniferous clinopyroxene and manganoan ilmenite. Phlogopite (Fe–Ti-rich) and spinel [high Fe²⁺/(Fe²⁺ + Mg)] compositions are also distinct from the other Kuusamo intrusions. The petrographic and geochemical features of Dike 15 are typical of ultramafic lamprophyres, specifically, aillikites. Rb–Sr dating of phlogopite in Dike 15 yields an age of 1178·8 ± 4·1 Ma (2σ), which is considerably older than the ∼750 Ma emplacement age of the Kuusamo kimberlites. This new age indicates significant temporal overlap with the Lentiira–Kuhmo–Kostomuksha olivine lamproites emplaced ∼100 km to the SE. It is suggested that asthenospheric aillikite magmas similar to Dike 15 evolved to compositions akin to the Karelian orangeites and olivine lamproites through interaction with and assimilation of MARID-like, enriched subcontinental lithospheric mantle. We conclude that the spatial coincidence of the Kuusamo kimberlites and Dike 15 is probably the result of exploitation of similar trans-lithospheric corridors.

中文翻译：

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芬兰库萨莫地区演化的金伯利岩和超镁铁质煌斑岩混合星团的成因

金伯利岩经常在时间和空间上与各种各样的碱性超镁铁质岩石类型密切相关，但是这些岩石类型之间的遗传关系问题仍然不确定。可以在同一集群中研究这些关系的一个地方是芬兰的卡累利阿克拉通。在这项研究中，我们展示了该克拉通上最新发现的金伯利岩团簇的第一个岩石学，矿物学和全岩石地球化学结果，这代表了金伯利岩与超镁铁质煌斑岩紧密空间重叠的一个例子。Kuusamo集群包含沿60公里NE-SW走廊分布的七个尸体[Kasma 45，Kasma 45南，Kasma 47，Kalettomanpuro（KP），Kattaisenvaara（KV），Dike 15和Lampi]。来自KV，KP，Kasma 45和Kasma 47由钙钛矿，磷灰石，尖晶石，钛铁矿，钛铁矿，蛇纹石和方解石组成的橄榄石大晶，微晶和金云母隐晶组成。这些岩石学特征与矿物（例如，富含镁的钛铁矿，富含Al-Ba的钛，贫Ti-Fe的云母）和全岩不相容的痕量元素组成（La / Nb = 0·8±0·1； Th / Nb = 0·07±0·01； Nb / U = 66±9）与这些被归类为原型金伯利岩的岩石一致。这些Kuusamo的金伯利岩富含CaO，而MgO贫乏，再加上没有铬铁矿和橄榄石大晶和地幔衍生的异晶（包括钻石）稀少，表明晶体分级分离后是由分化的岩浆衍生而来的。Lampi的样品具有相似的岩相特征，₂内容比Kuusamo金伯利岩。这些特征，再加上地层中石英和钛铁矿的出现，表明金伯利岩岩浆受到了地壳的严重污染。这项研究表明，地壳污染可以通过引入碱性超镁铁质岩石的典型特征来修饰金伯利岩。堤防15代表一种独特的富碳酸盐岩性，主要由金云母覆盖在橄榄石上，钛铁质次氯吡咯和锰锰铁钛矿较少。金云母（富铁钛）和尖晶石[高Fe ²⁺ /（Fe ²⁺ + Mg）]成分也不同于其他Kuusamo入侵。Dike 15的岩石学和地球化学特征是典型的超镁铁煌斑岩，特别是aillikites。在第15堤防中金云母的Rb–Sr测年产生的年龄为1178·8±4·1 Ma（2σ），比Kuusamo金伯利岩的〜750 Ma入土年龄大得多。这个新时代表明与距东南约100 km的Lentiira–Kuhmo–Kostomuksha橄榄石红岩在时间上有明显重叠。有人认为，类似于Dike 15的软流圈硅藻土岩浆通过与类似MARID的，富集的陆下岩石圈地幔相互作用和同化，演化成类似于卡累利阿橙岩和橄榄石红岩的成分。