Fenites of the Miaskite–Carbonatite Complex in the Vishnevye Mountains, Southern Urals, Russia: Origin of the Metasomatic Zoning and Thermodynamic Simulations of the Processes

Mineral zoning in fenites around miaskite intrusions of the Vishnevye Mountains complex can be interpreted as a magmatic-replacement zonal metasomatic aureole (in D.S. Korzhinskii’s understanding): the metasomatic transformations of the fenitized gneisses under the effect of deep alkaline fluid eventually resulted in the derivation of nepheline syenite eutectic melt. Based on the P–T–fO2 parameters calculated from the composition of minerals coexisting in the successive zones, isobaric–isothermal fO2–aSiO2 and µNa2O–µAl2O3 sections were constructed with the Perplex program package to model how the fenites interacted with H2O–CO2 fluid (in the Na–K–Al–Si–Ca–Ti–Fe–Mg–O–H–C system). The results indicate that the fluid–rock interaction mechanisms are different in the outer (fenite) and inner (migmatite) parts of the zonal aureole. Its outer portion was dominated by desilication of rocks, which led, first, to quartz disappearance from these rocks and then to an increase in the Al# of the coexisting minerals (biotite and clinopyroxene). In the inner part of the aureole, fenite transformations into biotite–feldspathic metasomatic rocks and nepheline migmatite were triggered by an increase in the Na and Al activities in the system alkaline H2O–CO2 fluid–rock. As a consequence, the metasomatites were progressively enriched in Al2O3 and alkalis, and these transformations led to the development of biotite in equilibrium with K–Na feldspar and calcite at the sacrifice of pyroxene. The further introduction of alkalis led to the melting of the biotite–feldspathic metasomatites and the origin of nepheline migmatites. The simulated model sequence of metasomatic zones that developed when the gneiss was fenitized and geochemical features of the successive zones (differences in the LILE and REE concentrations in the rocks and minerals of the fenitization aureole and the Sm–Nd isotope systematics of the rocks of the alkaline complex) indicate that the source of the fluid responsible for the origin of zonal fenite–miaskite complexes may have been carbonatite, a derivative of mantle magmas, whereas the miaskites were produced by metasomatic transformations of gneisses and subsequent melting under the effect of fluid derived from carbonatite magmas.