An oxygen isotopic study of garnet and zircon of the Flagstaff Lake Igneous Complex: Identification of peritectic and phenocrystic garnet with implications for the petrogenesis of strongly peraluminous granites

The Flagstaff Lake Igneous Complex of western Maine contains three phases of garnet-bearing, strongly peraluminous rocks, the Loon Lake, Quarry Phase A, and Quarry Phase B. The Loon Lake phase contains garnet with 818O values that average 10 permil, matching a minor subset of 818O values of accompanying zircon. Most zircons in this phase have higher 818O values, ranging between 11.3 and 12.1 permil, suggesting contamination after the garnet and the minor set of zircons crystallized. The subhedral grain morphologies, the abundant apatite inclusions of the grains, and the similar isotopic compositions of this zircon subset, indicate that the garnet is peritectic. Garnets ceased crystallization before the contamination event, attaining the subhedral morphologies. In contrast, the euhedral shapes and inclusion-free nature of garnet in Quarry Phase A, combined with the identical 818O values of both garnet and zircon, indicate that these garnets are phenocrysts. Similar inclusionfree, euhedral garnet is present in Quarry Phase B, but the garnets have lower 818O values (9.3 to 10.5 permil) compared to zircon (>11 permil). These garnets are also phenocrysts, having the same textural characteristics of those in Quarry Phase A, but they crystallized after assimilation of a lower 818O component, producing the mismatch of oxygen isotopic compositions. Both Quarry Phases owe their strongly peraluminous, whole-rock compositions to phenocryst accumulation whereas the Loon Lake phase is peraluminious because of peritectic garnet entrainment.

Automated summary

The Flagstaff Lake Igneous Complex in western Maine contains three phases of garnet-bearing, strongly peraluminous rocks: Loon Lake, Quarry Phase A, and Quarry Phase B. The garnet in the Loon Lake phase has δ818O values of 10 permil, matching a minor subset of accompanying zircon. Most zircons in this phase have higher δ818O values, suggesting contamination after garnet crystallization. The garnets in Quarry Phase A and Phase B have different isotopic compositions, indicating different crystallization processes.