TEXTURAL EVIDENCE IN THE CONTACT AUREOLE OF THE PASSADUMKEAG RIVER PLUTON, EAST-CENTRAL MAINE, FOR MULTIPLE MAGMA INJECTION AND DISEQUILIBRIUM MINERAL GROWTH

The Passadumkeag River pluton (PRP) is the western part of the post-Acadian (380Ma) Bottle Lake igneous complex (BLC) that intruded shallow crustal early Paleozoic strata in east-central Maine (Ayuso, 1984). It created a broad contact aureole that is developed prominently in Cambro-Ordovician strata of the Miramichi terrane on Passadumkeag Mountain at the southwest corner of the body.

Test borings for a wind farm along the crest of Passadumkeag Mountain provided samples of fresh hornfels from which 14 thin sections were prepared for petrographic analysis. The cores and detailed geologic mapping require that the pluton-host rock contact be shifted two hundred meters north of the position shown in the Maine bedrock geologic map (Osberg et al, 1985). Assemblages in carbonaceous and non-carbonaceous pelite are appropriate for the shallow emplacement depth: qtz-plag-musc-bio±cordierite±garnet±pyrite±graphite.

Textural evidence supports two principal conclusions:

1. PRP emplacement involved multiple magmatic injections: The initial injection generated cordierite and garnet porphyroblasts, followed by an episode of cooling and retrograde metamorphism in which cordierite was nearly completely replaced by a fine-grained aggregate of muscovite and chlorite. A subsequent injection caused a second prograde event evidenced by coarse muscovite flakes growing in the matrix and cutting across the fine-grained, retrograde aggregate. This is in agreement with Ayuso’s model for pluton emplacement.
2. Disequilibrium garnet growth: Garnet porphyroblasts display unusual zonation in which cores of radiating lath-shaped garnet are surrounded by clear rims. These textures appear to be a low-P analogue of amphibolite facies fabrics in Connecticut reported by Wilbur and Ague (2006). They proposed that nucleation occurred after garnet stability conditions were overstepped, causing extremely rapid growth of the core laths. Normal growth rates later produced the clear rims.