Archean Pon’goma-Navolok Granulite–Charnockite–Enderbite Complex, Northern Karelia: Geological Structure, Composition, and Parameters of Formation

Abstract

The paper presents original detailed data obtained by the authors on the Archean Pon’goma-Navolok granulite and charnockite massif in northern Karelia: a geological map of the massif and its surroundings, data on the petrography of the magmatic and metamorphic rocks, and the P–T parameters evaluated for major rock types by the techniques of multimineral thermomabometry and pseudosections. The Pon’goma-Navolok massif is determined to have formed as two intrusive phases at different crustal levels. The first intrusive phase corresponds to the massif of clinopyroxene–orthopyroxene charno-enderbites that crystallized at 8–11.2 kbar and 730–740°C. The second phase comprises dikes of orthopyroxene–biotite charnockites, which formed at 5.6–6.8 kbar and 830–850°C, and biotite granites, which crystallized at 6.8–7.0 kbar and 730–740°C. The dikes most likely correspond to different temperature and water-activity facies. The charnockites and granites were formed by processes of charnockitization and granitization of the charno-enderbites under the effect of saline aqueous solutions. The granulite-facies metamorphism of the metabasite blocks hosted in the charno-enderbite intrusion was of contact nature and was induced by the thermal effect of the charno-enderbites on the roof and wall rocks of the magma chamber. The high metamorphic temperatures of the metabasites (>900°C) and the absence of migmatization aureoles are explained by low water contents in the enderbites.