Charnockitization and Enderbitization of Mafic Granulites in the Porya Bay Area, Lapland Granulite Belt, Southern Kola Peninsula: I. Petrology and Geothermobarometry

摘要

Charnockitization of mafic Opx–Cpx–Grt–Hbl–Bt–Pl ± Qtz granulites and Hbl–Opx–Bt horn- blendites was studied in the southeastern part of the Lapland Granulite Belt. The evolutionary trends of the whole-rock compositions and mineral assemblages indicate that the rocks were affected by Na–K–Si– H_2O–CO_2–Cl brines, which came from outside, alkalinized and debasified the granulites, introduced Na, K, and Si into them, and depleted them in Mg, Fe, and Ca prior to the onset of charnockite melting; the latter began in the granulites only in their most extensively debasified domains. In the course of alkaline metasom- atism, pyroxene were replaced by secondary hornblende and biotite with high Ti concentrations, analogous to those in the unaltered granulites. This suggests that the pre-charnockite amphibolization and biotitization were induced not by a temperature decrease but by the effect of Na- and K-bearing fluid during the metamor- phic culmination. The metasomatically altered granulites, which were transformed into leucocratic disinte- grated amphibolite skialiths, were gradually resorbed and dissolved in the charnockite melt, whose bulk com- position corresponded to low-alkaline granites and tonalites. Hence, no contamination took place, and the excess Mg, Fe, and Ca amounts with respect to the eutectic composition were removed from the reaction zone. Variation diagrams indicate that the whole-rock composition of the granulites is gradually shifted toward the composition of charnockitoids. In certain instances, however, melanocratic Hbl–Grt–Opx–Cpx–Pl rims were formed along the granulite–charnockite interface, with the bulk composition of these fringes richer in Mg, Fe, and Ca than that of the ambient granulites. The reason for this was the sporadic redeposition of Mg, Fe, and Ca, which were mobilized from during charnbockitization and redeposited immediately in the reaction zone. In addition, rocks around the charnockite veins bear autonomous melanocratic Grt–Opx–Cpx- Hbl ± Mag ± Ilm ± Scp ± Pl ± Qtz veins whose mineral assemblages and bulk composition are close to those of the melanocratic rims around charnockitoids. The veins were formed via the transportation of Mg, Fe, and Ca for long distances outside the charnockitization zones. TWQ thermobarometric calculations indicate that the pre-charnockite alkaline metasomatism and debasification (amphibolization, biotitization, and feldspa- thization), anatectic formation of charnockite migma or magma, and the development of the melanocratic veins took place at the peak of the high-pressure granulite metamorphism at the same P–T parameters: approximately 800°C and 9–9.5 kbar. The calculated composition of the charnockitizing fluids suggests that they were homogeneous brines with X_(H_2O) = 0.45, X_(CO_2)= 0.10, X_(NaCl) = 0.30, and X_(KCl) = 0.15.