Crystallization Conditions of High-Magnesian Granitoids of the Novaya Melovatka Pluton, Voronezh Crystalline Massif

The petrogenesis of high-magnesian granitoids remains controversial. The crystallization conditions of such magmas have been poorly investigated. Mineralogical, chemical, and isotopic studies have been carried out to interpret the crystallization conditions of the Paleoproterozoic Novaya Melovatka pluton of high-magnesian granitites. The pluton consists of biotite–orthopyroxene quartz diorite and monzodiorite porphyrites (phase 1) and medium-grained biotite–amphibole quartz diorites, tonalites, and granodiorites with phase 1 mafic magmatic inclusions (phase 2). Calculations based on the compositions of minerals and rocks demonstrate that the parent magmas of the first phase were water-poor (less than 3 wt % dissolved H2O) with crystallization temperatures ranging between 902 and 720°C. The parent magmas of the second phase are water-rich (more than 6 wt % dissolved H2O) with crystallization temperatures between 820 and 716°C. Both phases crystallized at fO2 between NNO and NNO + 1. Geobarometry (aluminum-in-hornblende) indicates crystallization at upper crustal depths (i.e., 1.7–2.4 kbar). The maximum pressure estimates have been obtained for brown amphibole cores from phase 2 rocks and relict amphibole crystals of the mafic xenoliths (up to 7.8 kbar). The Rb–Sr ages of the mineral fractions and the whole rock compositions have been estimated: 2021 ± 18 Ma (phase 1) and 1994 ± 18 Ma (phase 2). The two parental melts (phases) of high-magnesium rocks of the Novaya Melovatka pluton crystallized at the same oxygen fugacity, but under different temperatures and water content at the upper crustal level. The brown amphibole were inherited from a mafic–ultramafic lower crustal source. The Rb–Sr isochrones are interpreted as the result of secondary recrystallization of pluton rocks due to reactivation of the lithosphere under remote stresses.