Granitization of Paleoproterozoic high-pressure metagabbro-norites of the Belomorian Group in Gorelyi Island, Kandalaksha Bay area, Baltic Shield

Abstract

The process of granitization was studied in a small massif of high-pressure Paleoproterozoic (2.45–2.36 Ga) metagabbro-norites (coronites) of the Belomorian Group at contacts with Bt-Hbl-Kfs-Pl-Qtz gneiss-granites. The granitization process occurred at the culmination of Svecofennian metamorphism, at T = 660–700°C and P = 9.5–10.5 kbar. The process affected the marginal portion (15–20 m thick) of the massif and proceeded under the effect of silicic-alkaline H2O-Cl-CO2 fluids related to the gneiss-granites and was of infiltration nature. The metagabbroids became enriched in Na, K, Si, Cl, CO2, and H2O and depleted in Ca, Mg, Fe, and Cr, which were removed outside the reaction zone. The debasified metabasites underwent melting during the final stages of the process. The changes in the bulk-rock compositions became more intense closer to the contact with the gneiss-granite, as can be clearly seen in the variation diagrams. Before their transformations, the metagabbroids contained magmatic minerals: pigeonite, pigeonite-augite, high-Ti biotite, and labradorite-bytownite, which are partly replaced by metamorphic minerals (sometimes in the form of reaction coronas): garnet, clinopyroxene, orthopyroxene, magnesian hornblende, anthophyllite, biotite, and andesine. According to the changes in the phase relations and the composition of the rocks, the contact-reaction interval can be subdivided into the following four zones: (I) weakly amphibolized metagabbro-norite; (II) apogabbro Hbl-Pl±Scp ± Qtz plagioamphibolites with rims, and then with pseudomorphs, after magmatic and corona minerals; (III) K-feldspathized Hbl-Bt-Pl-Kfs ± Scp-Qtz apogabbro amphibolites; and (IV) gneiss-granites with skialiths of bleached Bt-Hbl-Pl-Kfs ± Scp-Qtz amphibolites. Zone I contains newly formed embryonic Hbl, Opx, and Ath reaction rims around magmatic and coronitic minerals. In zone II, Pig, Pig-Aug, and Grt are completely replaced by hornblende and anthophyllite, and Lbr is replaced by andesine and scapolite with a calcite admixture. In zone III, Hbl is further replaced by biotite; andesine is replaced by oligoclase, potassic feldspar, and quartz; and Ath and Opx are decomposed. Biotitization becomes more intense in zone IV, where Hbl and Bt are partly replaced by feldspars and quartz, which mark the maximum of the debasification and enrichment in alkalis of the metagabbro-norites. The paper presents descriptions of reaction textures and mineral reactions that mark the transitions between the zones and considers the stability conditions of Ath and Opx in zones I and II and the causes of the disappearance of these minerals from zones III and IV. Equilibria of the Pl-Scp pair are discussed. The variations in the compositions of minerals from zone I through zone IV are systematic and display clearly pronounced tendencies. The hornblende becomes progressively more ferrous (its iron mole fraction increases from 0.31 to 0.69), along with an increase in the concentrations of alkalis and the K/Na ratio. The biotite also becomes more ferrous (from 0.28 to 0.61) and depleted in Ti in comparison with magmatic biotite. The anorthite content in plagioclases decreases from 60–80 to 20–22%. The content of the meionite end member in the scapolite varies from 30 to 50% but depends not on the zone but on the CO2 and Cl concentrations in the fluid. The distribution of the Me and An end members between Scp and Pl is equilibrium. Calculations indicate that the fluid in the most strongly transformed rear zone IV was a highly concentrated carbon dioxide-chloride brine rich in alkalis and silica. The transformations of the gabbroids in zones I–III was of metasomatic character. Zone IV is marked by the onset of the fragmentation of the amphibolized and bleached metagabbronorites, their transformation into skialiths submerged into gneiss-granite, and their further debasification and enrichment in alkalis. This process ends with the progressively more intense melting and dissolution of the nebular remnants of the metabasites in the anatectic granite migma or magma. The aureole described in this publication makes it possible to trace all granitization stages in metagabbroids in compliance with the known model proposed by D.S. Korzhinskii.