P-T paths of cooling and metamorphism under conditions of low-grade amphibolite facies in the xenoliths of granulites in the Siberian craton

Abstract

The paper presents the results of detailed petrological studies of mafic granulites from the Yubileinaya and Novinka kimberlite pipes, where retrograde metamorphic P-T paths were reconstructed for the first time using mineral geothermobarometry. These P-T paths demonstrate subisobaric cooling of the rocks from the P-T conditions of lower granulite facies to lower amphibolite facies in the middle crust depth. It was found that compositions of garnet, clinopyroxene, and orthopyroxene depend on the contacting mineral, reflecting the different temperatures closure temperatures of the exchange mineral reactions. The higher temperatures are determined using a two-pyroxene geothermometer, while lower ones are determined using garnet-clinopyroxene and garnet-orthopyroxene geothermometers. Using phase equilibria modeling we obtained thermodynamic conditions corresponding to the lower amphibolite facies: 540 °C, 0.76 GPa, lgfO2 = QFM + 1.7 (Yubileinaya pipe); 530 °C, 0.72 GPa, lgfO2 = QFM + 2.2 (Novinka pipe). The stability of the granulite paragenesis garnet+clinopyroxene+orthopyroxene+plagioclase under such P-T conditions is poorly known phenomenon that mirror a deficit of aqueous fluid during the crystallization of gabbro melts and their subsequent cooling in deep areas of cratons.