Geochemistry of a Rapakivi Granite Suite in a Proterozoic Rift Trough in Beijing and Its Vicinity

Controlled by E-W-trending faults, a Proterozoic (1.4-1.8 Ga old) rapakivi granite suite was intruded inBeijing and the area to its east (within Hebei Province), forming three parallel belts of igneous rocks. Theisotopic, trace element and rare earth element geochemical data of a bimodal rock association made up ofanorthosite, gabbro and alkali basalt and olivine-bearing quartz-syenite, rapakivi granite and trachyte as wellas potassic A-type granites and anorogenic granites—— all suggest that there exists an incipient rift in thestudy area. Fractional crystallization of a mixed magma formed by the magma derived from the upper mantleand the magma derived by small degrees of fusion of the lower crust produced anorthosite cumulates. Thewater-deficient granitic magma was differentiated into a subalkaline series. When the fractional crystallizationwas incomplete, rhythmic eruptions took place.

Geochemistry of a Rapakivi Granite Suite in a Proterozoic Rift Trough in Beijing and Its Vicinity

Abstract Controlled by E-W-trending faults, a Proterozoic (1.4-1.8 Ga old) rapakivi granite suite was intruded in Beijing and the area to its east (within Hebei Province), forming three parallel belts of igneous rocks. The isotopic, trace element and rare earth element geochemical data of a bimodal rock association made up of anorthosite, gabbro and alkali basalt and olivine-bearing quartz-syenite, rapakivi granite and trachyte as well as potassic A-type granites and anorogenic granites— all suggest that there exists an incipient rift in the study area. Fractional crystallization of a mixed magma formed by the magma derived from the upper mantle and the magma derived by small degrees of fusion of the lower crust produced anorthosite cumulates. The water-deficient granitic magma was differentiated into a subalkaline series. When the fractional crystallization was incomplete, rhythmic eruptions took place.