Deciphering garnet genesis in meta-igneous rocks: An example from the Jiao-Liao-Ji Belt, North China Craton

Abstract

Garnet is common in metamorphic rocks and can be used to efficiently constrain pressure–temperature conditions experienced during orogenesis; however, it may also crystallize as a primary phase in many types of magma, which can complicate deciphering the petrogenesis of meta-igneous rocks. In this study, geochronological and geochemical analyses of garnet and zircon were integrated to constrain garnet genesis in plagioclase-absent mafic granulite from the Jiao-Liao-Ji Belt, North China Craton. The rare earth element patterns show an equilibrium relationship between garnet and metamorphic zircon, indicating that garnet has a metamorphic origin. U–Pb dating shows that garnet has a whole-grain age of c. 1.83 Ga, with no difference between core and rim domains, which is slightly younger than metamorphic ages recorded by zircon (c. 1.87 Ga). The age difference between garnet and zircon is likely a result of differences in their closure temperature for the U–Pb isotope system. The mineral assemblage garnet + clinopyroxene + K-feldspar + titanite shows textural equilibration and was used to determine peak pressure–temperature conditions of 0.8–1.2 GPa at 830–880 °C. Given the age of high-pressure metamorphism (c. 1.87 Ga) calculated here being younger than c. 1.95–1.90 Ga proposed by previous studies for the west Liaodong peninsula, we suggest that c. 1.95–1.87 Ga represents the timing of progressive collision between the Longgang Block and the Nangrim Block, with the western part having collided slightly earlier than the eastern part.