Paper No. 5-5
Presentation Time: 2:55 PM
PERITECTIC, XENOCRYSTIC/RESTITIC, AND PHENOCRYSTIC GARNET ACCUMULATION AND THE ORIGIN OF STRONGLY PERALUMIOUS GRANITIC ROCKS: THE FLAGSTAFF LAKE IGNEOUS COMPLEX, MAINE
The Flagstaff Lake Igneous Complex of Maine contains a metaluminous main phase that ranges from diorite to granite, and three phases of strongly peraluminous, garnet-bearing rocks (Loon Lake Phase and Quarry Phases A & B) with silica contents that range between 43 to 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 spiral inclusion patterns, suggestive of porphyroblasts, either as entrained restite or entrained xenocrysts. 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. Other grains have resorbed cores that are inclusion-poor that are mantled by inclusion-free overgrowths. The cores have low concentrations of compatible elements and high concentrations of incompatible elements, whereas the mantles have the opposite trace element abundances. These cores are resorbed phenocrysts that were mingled in a more primitive, garnet crystallizing magma. Also present are inclusion-rich garnets with random inclusion distributions. These are interpreted as peritectic garnet, entrained from the source or from digested wall rocks. They are mantled by inclusion-free garnet with the same composition as the phenocrysts and appear to be phenocrystic overgrowths. Thus, the original anatectic melt was mildly peraluminous with the current bulk-rock compositions of the garnet tonalites dominated by the accumulation of variable proportions of phenocrystic garnet, the entrainment of peritictic or restitic garnet, and the enrichment of xenocrystic garnet, any of which generates increasing peraluminocity with increasing maficity. Enclaves present in the peraluminous rocks are similar to the metaluminous rocks of the main phase of the complex, suggesting that the coeval basaltic magmas provided the heat for anataxis to generate the peralumious magmas.