CALCITE - AMPHIBOLE - CLINOPYROXENE ROCK FROM THE AFRIKANDA COMPLEX, KOLA PENINSULA, RUSSIA: MINERALOGY AND A POSSIBLE LINK TO CARBONATITES. III. SILICATE MINERALS

Abstract

Carbonate – amphibole – clinopyroxene rocks and carbonatites from the Afrikanda complex, in the Kola Peninsula, Russia, contain a number of oxysalt minerals, including major calcite (15–95 vol.%), subordinate hydroxylapatite, ancylite-(Ce), calcio ancylite-(Ce), and minor burbankite, khanneshite, nyerereite, shortite, bradleyite, strontianite, britholite-(Ce) and barite. T hree mineral parageneses differing in the mode of occurrence of calcite are distinguished: (1) calcite – magnesiohastingsite – diops ide rock, (2) segregations of perovskite and titanite, and (3) calcite carbonatite. Cathodoluminescence studies document a complex evolutionary history of primary Sr-enriched calcite (0.6–1.4 wt.% SrO) involving late-stage resorption and replacement by a low Sr variety (<0.5 wt.% SrO). The presence of nyerereite, shortite, bradleyite, burbankite and khanneshite as solid inclusions in the early-crystallized minerals (primarily oxides and hydroxylapatite) indicates initially high activities of Na in the system. The transition from nyerereite (inclusions in magnetite) to shortite (in perovskite) signifies evolution of the carbonatite system toward Ca-enriched compositions. Crystallization of ancylite-(Ce) and calcio-ancylite-(Ce) is related to low-temperature hydrothermal processes, whereas burbankite and khanneshite probably represent primary magmatic phases. Low-temperature (200–250 °C) hydrothermal alteration accompanied by isotope-exchange processes produced variations in the oxygen isotopic composition of the Afrikanda rocks ( 18 O in the range 9.3 to 12.1‰ SMOW). Subtle variations in the isotopic composition of carbon ( 13 C in the range –2.5 to –1.7‰ PDB) suggest interaction with a meteoric-hydrothermal fluid with a low CO 2:H2O ratio. The observed high 13 C values of the calcite are consistent with heterogeneity of the mantle beneath the Kola Craton; the heterogeneity probably was due to a subduction-related source of contamination.