Anorthositic sills of the southern Adirondack Mountains of New York state

Abstract

The Speculator Sheet and the Wells-Tenantville Sill are intruded into the Rooster Hill/Little Moose Mountain Formations in the southern Adirondacks. The Wells Sill consists of a medium-grained gabbro forming the top and basal layers, and an inner core of anorthositic gabbro rimmed by a fine-grained chill margin called the Tenantville facies. The Speculator Sheet consists of gabbroic anorthosite and has a mafic lower margin due to gravitational settling of mafic phases.

Plagioclase and pyroxene compositions suggest that the Wells gabbro is distinct from the rest of the sills and is related to the Oregon Dome. Whole rock data show that the gabbroic anorthosite and the rocks of the Thirteenth Lake Dome have a differentiation trend of minor iron enrichment, while the Wells gabbro, Oregon Dome and Snowy Mountain Dome have a trend of more extreme iron enrichment. Field evidence indicates that the Wells gabbro was intruded first as a mafic differentiate of the Oregon Dome/Snowy Mountain Dome magma, and was later intruded by the anorthositic gabbro. This later intrusion was concurrent with the emplacement of the Speculator Sheet and the Thirteenth Lake Dome. These data suggest that the contacts of anorthosites against country rock in the southern Adirondacks are igneous and not the product of sedimentation on an anorthositic basement.

Textural and analytical evidence and graphical analysis suggest that the differing metamorphic mineral assemblages are controlled by variations in water fugacity. The development of orthopyroxene-plagio-clase coronas on some of the garnets is dependent on the iron contents of the garnets.