Bulletin of the Geological Society of Finland (Dec 1997)
Geology and Geochemistry of the Early Proterozoic Kortejärvi and Laivajoki Carbonatites, Central Fennoscandian Shield, Finland
Abstract
This paper provides for the first time extensive petrological, mineralogical and geochemical data on the early Proterozoic Kortejärvi and Laivajoki carbonatites, northern Finland, which form metamorphosed and highly strained bodies 2 and 4 km long within a Svecokarelian shear zone in central Fennoscandian Shield. They are not exposed, but have been penetrated by a couple of deep drill holes. In terms of modal mineralogy, both intrusions contain calcite carbonatite and dolomite-calcite carbonatite as their main rock types, but Kortejärvi also contains dolomite carbonatite and calcite-dolomite carbonatite, some glimmerite and olivine-magnetite rock and Laivajärvi tremolite-calcite carbonatite, tremolite-dolomite carbonatite, serpentine-talc-dolomite rock and glimmerite. The main country rock is an amphibolite which is not fenitized. No alkaline rocks have been detected in these intrusions. Calcite is most common mineral in both occurrences. Other carbonate minerals include dolomite with minor ankerite and occassional siderite. In addition to low-Ti phlogopite, tetraferriphlogopite is also encountered. Fresh olivine is rare, and its alteration products include titaniferous clinohumite. The amphiboles are mainly calcic amphiboles, including actinolite, tremolite and edenite. The only sodic-calcic amphibole is accessory richterite. Other essential minerals are Ti-poor magnetite with ilmenite exsolutions, fluorapatite (3.95-4.89 wt. % F), monazite, and allanite-(Ce). Geochemically, the Kortejärvi rocks are mostly magnesiocarbonatites, whereas those of Laivajärvi, due to their higher magnetite content, are ferrocarbonatites. Of the trace elements, Nb is much lower (8-30 ppm) in proper carbonatites than the average for carbonatites and U and Th (<0.9 ppm and<2.4 pm, respectively) lower than average. Sr is typical, but not high (1830-3480 ppm), and Ba is rather low (27-348 ppm). The REEs are hosted by allanite and monazite and their concentrations in the proper carbonatites are relatively low (REE(Y)tot 737-1452 ppm), but the LREE/HREE ratios are markedly high (La/Lu 244-1092). The mean δ13C values of dolomite and calcite for the Laivajoki samples are -4.3±0.1 ‰ +1sd, PDB) and -4.5±0.6 ‰, respectively, and for the Kortejärvi samples -3.9±0.2 ‰ and -4.2±0.2 ‰, respectively. The mean δ18O values of dolomite and calcite for the Laivajoki samples are 6.5±0.5 ‰ (±1sd, SMOW) and 7.0±0.8 ‰, respectively, and for the Kortejärvi intrusion 6.8±0.1 ‰ and 8.4±1.0 ‰, respectively. These isotopic characteristics are similar to the analytical results reported from other carbonatite complexes and are in the range expected for carbonate melts in equilibrium with mantle minerals. The Kortejärvi and Laivajärvi intrusions represent slightly metasomatized early - middle stage carbonatites which were metamorphosed in amphibolite facies and sheared during the Svecokarelian orogeny about 1880-1800 Ma ago.
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