P-T Paths of Cooling and Metamorphism under Conditions of Low-Grade Amphibolite Facies in the Xenoliths of Granulites in the Siberian Craton

The results of detailed petrological studies of mafic granulites from the Yubileinaya (Alakit-Markha field) and Novinka (Upper-Muna field) kimberlite pipes, where retrograde metamorphic P-T paths were reconstructed for the first time using mineral geothermobarometry, are provided. These P-T paths demonstrate subisobaric transition of the rocks from the P-T conditions of low-grade granulite facies to low-grade amphibolite facies at the depth of the middle crust. It was found that the compositions of garnet, clinopyroxene, and orthopyroxene depend on the adjacent mineral, reflecting the different closure temperatures of the exchange mineral reactions. The higher temperatures are determined using a two-pyroxene geothermometer, while the lower ones are found by application of garnet-clinopyroxene and garnet-orthopyroxene geothermometers. Phase equilibria modeling showed the thermodynamic conditions of the low-grade amphibolite facies: 540 degrees C, 0.76 GPa, lgf(O2) = QFM + 1.7 (Yubileinaya pipe); 530 degrees C, 0.72 GPa, lgf(O2) = QFM + 2.2 (Novinka pipe). The stability of the granulite paragenesis of garnet + clinopyroxene + orthopyroxene + plagioclase under such P-T conditions is a poorly known phenomenon reflecting a deficit of aqueous fluid during the crystallization of gabbroids and their subsequent cooling in the deep segments of cratons.

Automated summary

This study reconstructs the retrograde metamorphic P-T paths of mafic granulites from the Yubileinaya and Novinka kimberlite pipes using mineral geothermobarometry. The results show the subisobaric transition of the rocks from low-grade granulite facies to low-grade amphibolite facies at the middle crust depth. The compositions of garnet, clinopyroxene, and orthopyroxene are found to depend on the adjacent mineral. The thermodynamic conditions of the low-grade amphibolite facies are determined using phase equilibria modeling.