Peritectic and phenocrystic garnet accumulation and the origin of strongly peraluminous granitic rocks: The Flagstaff Lake Igneous Complex, Maine

Abstract

The Flagstaff Lake Igneous Complex of Maine contains a metaluminous phase that ranges from diorite to granite, and three phases of strongly peraluminous, garnet-bearing rocks with silica contents that range between 43 and 59 wt%. These peraluminous rocks have increasing amounts of normative corundum with increasing maficity, indicating that they do not represent liquid compositions, rather garnet accumulation accounts for their bulk-rock A/CNK compositions. Garnet in these rocks displays a variety of textures, inclusion patterns, and trace element compositions. The Loon Lake phase is characterized by garnet with abundant, randomly distributed apatite inclusions. Some grains have spiral inclusion patterns that are peritictic grains formed during partial melting of source rocks while they were undergoing differential stress. In contrast, the Quarry Phases A and B contain inclusion-free garnet with euhedral grain boundaries. These grains have trace element profiles indicative of crystal fractionation processes and are interpreted to be phenocrysts. Some of these phenocrysts have inclusion poor, resorbed cores that are mantled by inclusion-free overgrowths. The cores have lower concentrations of compatible trace elements and higher concentrations of incompatible elements than the mantles. These cores are resorbed phenocrysts from an evolved magma that were mingled in a more primitive, garnet crystallizing magma. Also present in Quarry Phase B rocks are a minor population of inclusion-rich garnets with random inclusion distributions. These are also interpreted as peritectic garnet, entrained from the source, or from dehydration melting of country rock biotite. They are mantled by inclusion-free garnet with the same composition as the phenocrysts and are phenocrystic overgrowths. Thus, the original Flagstaff Lake anatectic melts were mildly peraluminous with the current bulk-rock compositions of the garnet tonalites dominated by the accumulation of variable portions of phenocrystic garnet or the entrainment of peritictic garnet, both of which generated the increasing peraluminosity with increasing maficity of the complex. Magmatic, mafic enclaves present in the peraluminous rocks are similar to the metaluminous, main phase rocks of the complex, suggesting that the coeval basaltic magmas provided the heat for anataxis to generate the peraluminous magmas.