Early Proterozoic Polymigmatites in a Zone of Granulite Metamorphism: P–T–X Conditions, Age, and Duration of Migmatization (Northern Ladoga Area, Russia)

Abstract

Abstract —Polymigmatites in the Early Proterozoic metamorphic complex of the Northern Ladoga area trace the evolution of thermodynamic conditions during anatexis. The P–T conditions inferred for anatectic leucosome correspond to the onset of partial melting under granulite facies conditions of 5.5–6.2 kbar and 720–810 °C. After the peak of metamorphism, pressure and temperature show a coupled decrease to 4 kbar and ~550 °C, respectively. The latest granitic veins were intruded into metamorphic rocks during an event of brittle deformation. The P–T trend correlates with changes in migmatitic mineral assemblages and in chemistry of minerals. Newly formed leucocratic material changed from plagiogranitic to granitic composition when melting involved plagiogneiss but melt derived from Al-rich metapelite remained granitic. The analyzed leucosomes and granitic veins originated by multistage melting, under P–T conditions changing from granulite to amphibolite facies, between 1875 and 1865 Ma. Judging by their ages, the leucosome and granitic vein bodies from the Lakhdenpokhia and Priozersky zones of the area differed in the total duration of crystallization. Therefore, a single thermal event involving different lithologies can produce intrusions of different ages.