Phlogopite-rich and phlogopite-poor kimberlite intrusions within the Du Toitspan kimberlite pipe, South Africa: Petrogenetic relationships and localised source heterogeneity
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Phlogopite
Country rock
Xenolith
Phlogopite
Lapilli
Diatreme
Country rock
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ABSTRACT Assimilation of country rock xenoliths by the host kimberlite can result in the development of concentric reaction zones within the xenoliths and a reaction halo in the surrounding contaminated kimberlite. Petrographic and geochemical changes across the reaction zones in the xenoliths and the host kimberlite were studied using samples with different kimberlite textures and contrasting xenolith abundances from the Renard 65 kimberlite pipe. The pipe, infilled by Kimberley-type pyroclastic (KPK) and hypabyssal kimberlite (HK) and kimberlite with transitional textures, was emplaced into granitoid and gneisses of the Superior Craton. Using samples of zoned, medium-sized xenoliths of both types, mineralogical and geochemical data were collected across xenolith-to-kimberlite profiles and contrasted with those of fresh unreacted country rock and hypabyssal kimberlite. The original mineralogy of the unreacted xenoliths (potassium feldspar-plagioclase-quartz-biotite in granitoid and plagioclase-quartz-biotite-orthopyroxene in gneiss) is replaced by prehnite, pectolite, and diopside. In the kimberlite halo, olivine is completely serpentinized and diopside and late phlogopite crystallized in the groundmass. The xenoliths show the progressive degrees of reaction, textural modification, and mineral replacement in the sequence of kimberlite units KPK — transitional KPK — transitional HK. The higher degree of reaction observed in the HK-hosted xenoliths as compared to the KPK-hosted xenoliths in this study and elsewhere may partly relate to higher temperatures in xenoliths included in an HK melt. The correlation between the degree of reaction and the kimberlite textures suggests that the reactions are specific to and occur within each emplaced batch of magma and cannot result from external post-emplacement processes that should obliterate the textural differences across the kimberlite units. Xenolith assimilation may have started in the melt, as suggested by the textures in the xenoliths and the surrounding halos and proceeded in the subsolidus. Elevated CaO at the kimberlite-xenolith contact appears to be an important factor in producing the concentric mineralogical zoning in assimilated xenoliths.
Xenolith
Phlogopite
Metasomatism
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Diatreme
Xenolith
Phlogopite
Breccia
Lapilli
Phenocryst
Baddeleyite
Country rock
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Breccia
Phlogopite
Diatreme
Phenocryst
Peridotite
Ultramafic rock
Amphibole
Caldera
Country rock
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Abstract 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.
Phlogopite
Ultramafic rock
Phenocryst
Petrogenesis
Ilmenite
Chromite
Country rock
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Dike
Phlogopite
Ultramafic rock
Carbonatite
Pyroxene
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Abstract Previous studies of the Prairie Creek occurrence have identified three main rock types namely: “volcanic breccias”, “tuffs and fine-grained breccias” and “hypabyssal kimberlite or peridotite”. Our investigation confirms the presence of three distinct rock groups which include both magmatic and crater-facies types. The so-called “volcanic breccias” and “tuffs” are both considered to be predominantly of pyroclastic origin. Many features of these rocks are atypical of kimberlite and indicate a complex intrusion history. The magmatic rocks contain two generations of relatively abundant olivine in a fine-grained matrix composed of phlogopite, clinopyroxene, amphibole, perovskite, spinel, serpentine and glass. Although some petrographic features of these rocks are similar to those of kimberlites, the form of the euhedral olivine, presence of abundant glass and occurrence of potassic richterite are uncharacteristic of kimberlite but typical of lamproitic rocks. Both the groundmass phlogopite and the bulk rock have compositions intermediate between known lamproites and kimberlites. The data presented here shows that the Prairie Creek intrusion is not a kimberlite. Although in many respects Prairie Creek appears to be transitional between kimberlite and lamproite, it is considered that these rocks form an extension of the lamproite field.
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Inclusions within garnet and diopside megacrysts from kimberlite in eastern Kentucky contain Ti-rich phlogopite, serpentine, and calcite. The garnet inclusions also contain spinel, aluminous clinopyroxene, and chlorite. It is inferred that the inclusions in garnet were once the same as those in diopside, but spinel, aluminous clinopyroxene, and aluminous orthopyroxene (now serpentine + chlorite) formed at the expense of host garnet and included olivine. These polymineralic inclusions may represent included liquids of the same composition as the liquid from which the megacrysts crystallized, which may be a phlogopite-bearing kimberlite, unlike the kimberlite host rocks in Kentucky.
Phlogopite
Diopside
Ultramafic rock
Country rock
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Phlogopite
Xenolith
Lapilli
Diatreme
Country rock
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Diatreme
Phenocryst
Xenolith
Breccia
Magma chamber
Country rock
Melt inclusions
Phlogopite
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