Early migmatites in the prograde metamorphism zone of gneisses in the northern domain of the Ladoga Region: U-Pb evidence based on monazite

Abstract

The North Ladoga region is the southeastern fragment of the Svecofennian Belt. The metamorphic rocks of this region were traditionally regarded as constituents of a zonal metamorphosed complex composed largely of the Kalevian (late Paleoproterozoic) turbidite formation [1]. Later data [2] provided evidence for tectonic juxtaposition of the high-temperature core of the complex with less metamorphosed rocks. These relationships made it possible to distinguish the northern (ND) and southern (SD) domains [3]. The relatively low-grade metamorphic rocks of the ND are related to the Sveco-Karelian pericratonic zone; the high-grade metamorphic rocks of the SD, to the Svecofennides that are devoid of the Archean granitic basement (Fig. 1). The geochronological data obtained over recent years furnished convincing evidence that ultrametamorphism of rocks in the SD occurred within a rather narrow time interval (1880‐1870 Ma ago) [4]. The results of geochronological study of migmatites in rocks of the Sveco-Karelian pericratonic zone (ND) are reported in the present communication for the first time. The ND structure is controlled by mantled gneiss domes with the Archean granite‐gneiss basement in their cores. The ND is a domain of zonal metamorphism with southward increasing grade from greenschist to amphibolite facies and poorly developed intrusive magmatism (Fig. 1). The thermobarometric estimates of metamorphic grade in the ND and metamorphic mineral assemblages confirm a decrease in metamorphic grade from south to north. The parameters of peak metamorphism in the amphibolite-facies zone are T = 650‐730 ° C and P = 3.8‐5.0 kbar [5]. The ND rocks also bear indications of retrograde transformations of the andalusite‐muscovite subfacies, which are most intense along the narrow submeridional tectonic zones. The SD is characterized by the predominance of granulite-facies metamorphic rocks. Hence, the SD is a high-temperature granulitic block with numerous gabbro, enderbite, tonalite, and granite intrusions. This block is considered a high-temperature core of the zonal metamorphic complex. The SD is thrust over the ND along the low-angle Meier fault zone [3].