MID CRUSTAL PLUTON CONSTRUCTION IN THE EASTERN ADIRONDACK HIGHLANDS, SOUTHERN GRENVILLE PROVINCE: A FIELD ANALOGUE FOR THE TRANSPORT AND COALESCENCE OF COMPOSITE GRANITE BATHOLITHS

The Lyon Mountain granite gneiss (LMG) is a late leucogranite rimming the granulite-facies Adirondack Highlands. The LMG was emplaced after granulite facies metamorphism. 1:24,000 and 1:12,000 scale geologic mapping within the Eagle Lake quadrangle, NY provides detailed structural information pertaining to the emplacement mechanisms of the LMG. Layering within the LMG is defined by variations in both grain size and the mode of mafic minerals. However, little evidence of a shape-preferred orientation is present, and petrographic analysis shows limited evidence of subsolidus deformation. Therefore, the layering within the LMG is interpreted as magmatic in origin. Truncations of layering in the LMG by subsequent LMG layers as well as foliation-parallel sills of LMG within adjacent host rocks suggest that the LMG was emplaced as lit-par-lit sill-like injections along pre-existing folia.

Three localities illustrate the various states of the LMG plutons (from smallest to largest) in the area: 1) east of Penfield Pond, 2) Skiff Mountain, and 3) the historic mining district of Hammondville. East of Penfield Pond, LMG cores open folds that plunge moderately to the east defined by rocks of the Grenville Supergroup. At Skiff Mountain, the LMG pluton was emplaced into a broad zone of E-W trending granulite facies paragneisses, and it forms km-scale sills in these host-rocks. Two km-scale amphibolite xenoliths are oriented at high angles to host rock tectonites to the east of the Skiff Mountain pluton, and mimic the geometry of folds present east of Penfield. The Hammondville pluton preserves numerous km-scale xenoliths and screens of amphibolite, paragneiss, and charnockitic gneiss. These pre-LMG rocks maintain surprising continuity such that they can still be used to identify pre-pluton isoclinal folding present in the granulite-grade host rocks as well as overprinting open, large amplitude folds. At all three localities magmatic layering in LMG is parallel to the older foliation present in the surrounding host rocks and xenoliths. We interpret these data to suggest that the LMG was emplaced during the development of large-amplitude folds. This folding provides a mechanism to transport, accommodate, and ultimately coalesce batches of granite, with minimal rotation of km-scale xenoliths.