INVESTIGATING THE METAMORPHIC HISTORY OF THE PASSADUMKEAG RIVER PLUTON AUREOLE

The Passadumkeag River Pluton is a major component of the Bottle Lake igneous complex in east-central Maine. A contact aureole surrounds the complex, with a narrow inner zone of sillimanite succeeded outward by broad cordierite+andalusite and biotite zones in pelites. The aureole is poorly exposed at Passadumkeag Mountain but test borings for a proposed wind farm offer a unique opportunity to explore the thermal history of the aureole. A sequence of prograde-retrograde-prograde textures suggests that the pluton delivered two pulses of heat to the Cambro-Ordovician host rocks (Bastas-Hernandez and Ludman, 2014).

This project examines the rocks from the aureole more closely - in particular garnet growth patterns and chemistry of associated mineral assemblages - in order to establish a detailed thermal history and understand processes associated with heat transfer and metamorphic crystallization. Petrographic and SEM analyses were used to identify textures within and relationships between phases, and EDX studies helped to estimate bulk and mineral chemistry and determine the degree of homogeneity or nature of zoning in the metamorphic minerals.

Samples are heterogeneous on the millimeter scale, with different mineral assemblages and textures in adjacent layers. Multiple generations of polymineralic veins cross-cut primary layering and a number of unique accessory minerals occur throughout including Fe and Ti oxides, Fe and As sulfides, and several phosphates. Some samples contain large sector-zoned cordierite porphyroblasts, replaced to varying degrees by aggregates of muscovite flakes; others exhibit garnet crystals with a range of size and morphologic characteristics, including radial “wagon wheel” growth patterns.

Garnets are typically high-Mn, low-Mg and display zonation, including high Ca concentrations at the core or in annular rings, and a depletion of Mn toward the rim. Hydrothermal activity evidenced by Na-rich veins is associated with more well-defined garnet crystals. Further investigations will include electron probe analyses and petrologic modeling for specific mineral assemblages with the ultimate goal of determining P-T and fluid activity conditions associated with the thermal pulses and an estimate of their duration.