32 Igneous Petrology (Posters)

Monday, 23 March 2026
1:30 PM - 5:30 PM
CCC, Ballroom C

Petrology and Paragenesis of a Peraluminous Pegmatite Dike in the Western Uplands of Connecticut

Paper number: 32-1
Program: Technical Sessions
Time: 1:30 PM – 1:30 PM

Authors

Abstract

A peraluminous pegmatite dike cross-cuts the foliation of the Waterbury gneiss in the vicinity of Hickory Street in Naugatuck, Ct.  The dike trends north/south and is steeply dipping, nearly vertically. It crops out on the surface for ~150m before being obscured by development and glacial till. The pegmatite is zoned, with a wall zone and quartz core, and scattered aplitic zones. The wall zone consists of muscovite and microcline, fine grained at the contact and coarsening towards the core. The core consists of euhedral andalusite, cordierite and almandine crystals surrounded by massive quartz. Andalusite and almandine favor the northern end of the exposure, and cordierite favors the southern end. Aplite zones consist of fine grained microcline and quartz. Other accessory minerals include scattered columbite, fluorapatite, periclase, pyrite, magnetite.

After emplacement, changing P/T conditions favored first the growth of a second generation of muscovite on the surface of the andalusite, the conversion of cordierite and almandine to chlorite minerals, and conversion of andalusite to kyanite. The growth of muscovite on the andalusite crystals is anisotropic, with alternating prism faces seeing thicker and thinner alteration to muscovite.

Tectonic deformation also occurred, as interpreted from deformed andalusite and muscovite crystals and strained quartz in thin section. Andalusite crystals are deformed in two ways: longer crystals exhibit bookshelf sliding or are are boudinaged, and prism cross sections have been squeezed from square cross sections to rhombic. Muscovite books in the wall zone have been bent.

As the dike was exhumed, weathering processes began occurring. The pyrite was oxidized to limonite and other iron oxides. Fluorapatite and periclase weathered out, leaving behind negative impressions in the andalusite crystals.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411259

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