THE ORIGIN OF GARNET OF THE STRONGLY PERALUMINOUS GARNET TONALITES OF THE FLAGSTAFF LAKE IGNEOUS COMPLEX, MAINE

The garnet tonalite of the Flagstaff Lake Igneous Complex of western Maine contains 40 - 70% garnet. These strongly peraluminous rocks have increasing amounts of normative corundum with increasing maficity, indicating that they do not represent liquid compositions; garnet accumulation accounts for their bulk-rock A/CNK compositions. The purpose of this study is to identify the origin of garnet in these rocks and to elucidate the process of garnet accumulation. We evaluated the textures of the garnet crystals and obtained ẟ¹⁸O analyses of garnet and zircon to distinguish garnet phenocrysts from xenocrystic, restitic, and peritectic garnet. The garnet tonalite consists of the Loon Lake phase and Quarry phases A and B. Loon Lake garnets are anhedral to subhedral, inclusion rich, and average 2 cm in diameter. They have an average ẟ¹⁸O value of 10.20 ±0.35‰. In contrast, the majority zircon analyses have ẟ¹⁸O values between 11.45-12.00‰, showing that the garnets are not in equilibrium with the melt. These inclusion-rich garnets are either xenocrysts or restite. Quarry phase A has garnets that are euhedral and inclusion-poor. They average ẟ¹⁸O of 10.50 ±0.35‰ and show rimward decreases of ~ 0.70‰. Zircons in this phase have ẟ¹⁸O values between 10.05-11.80‰, overlapping the range of the garnets, indicating the garnets are phenocrysts. Quarry phase B also has euhedral garnets that are inclusion-poor. The cores have ẟ¹⁸O compositions that average 9.90 ±0.35‰ with a gradual rimward increase of ~ 0.60‰. In contrast, rims and mantles of zircons in this phase have ẟ¹⁸O values between 11.25-12.50‰, showing that the cores of these garnets are not in equilibrium with the melt and are either xenocrystic or peritectic cores with magmatic overgrowths. Quarry phase A garnets are zoned to lower ẟ¹⁸O values with rim compositions similar to the core compositions of Quarry phase B garnet. In contrast, Quarry phase B garnets are zoned to higher ẟ¹⁸O values, similar to the core compositions of Quarry phase B garnets. Each phase therefore records the magmatic interaction of the other Quarry phase magma in the late stages of evolution as the two magmas mixed and hybridized. Both these phases have an abundance of garnet phenocrysts, indicating that they are cumulates and the whole-rock compositions are reflective of magma chamber accumulation processes.