Petrology and mineral chemistry of shallow intrusive carbonatites : examples from the Kola Peninsula and Cape Verde

Analysis of hypabyssal carbonatite and associated silicate rocks from three locations was undertaken to establish the processes leading to their textural and chemical diversity. Carbonatite dykes from Vuorijarvi (Kola Peninsula) show good evidence of magmatic processes and were least affected by fenitization. Globular calcite and apatite inclusions in silicate phases indicate precipitation from a silicate melt; subsolidus recrystallisation of olivine to monticellite implies an early stage of carbonatite formation; complex substitution in perovskite formed Zr-garnet. Occurrences of xenocrysts suggest extensive wall-rock interaction. Primary tabular calcite was replaced by secondary mosaic calcite, and amphibole was formed by early fenitisation. Calcite and apatite host LREE; Zr-garnet and perovskite show strong compatibility for Zr, Nb and all REE. Hypabyssal carbonatites of Fogo, Cape Verde, contain glimmerite sheets which formed hydrothermal pathways in which accessory minerals crystallised. Shearing also formed pathways for oxidised low-temperature REE-rich hydrothermal fluids which caused veined and pervasive K- and Na-fenitization that modified the texture and chemistry of primary phases. Hf-bearing phlogopite is associated with Hf-poor titanite, zirconolite, zircon and pyrochlore; Nb-Th-U-rich pyrochlore formed via hydrothermal dissolution of Zr-rich precursors; post-magmatic changes progressively altered magnetite to titanomagnetite, ilmenite, garnet and titanite. Calcite and apatite are the main trace-element-bearing phases; post-magmatic garnet and titanite are enriched in REEs and U, Th and Nb. Carbonatites and syenites from Brava, Cape Verde, show extensive fenitisation. Sövites contain primary (Sr-calcite, Ca-strontianite, carbocernaite), metasomatic (parisite) and hydrothermal (burbankite, barite) phases. Apatite and calcite show several orders of REE enrichment. Clinopyroxene evolved from diopside towards aegirine enriched in Zr, Th, U and LREEs; amphibole is both primary and secondary; phlogopite mica evolved towards late-stage phengite and pennantite; magnetite progressively formed titanomagnetite, ilmenite, titanite, rutile and henrymeyerite. Lower temperature Mn-, Sr- and Zr-rich hydrothermal fluids exploited fractures, forming LREE-, Zr-rich hilairite, REE-törnebohmite; rhabdophane, lanthanite and LREE-rich ilmenite