Abstracts Earth and Environmental Sciences

Petrology and geochemistry of a metamorphosed anorthosite-gabbro intrusive suite near Dresden, NY

by Ethan T. Lake

Massive type anorthosite suites are complex large scale igneous bodies consisting primarily of plagioclase rich anorthosites and some residual mafic Fe-Ti bearing gabbros. These bodies formed during the early to mid Proterozoic, some 2500 to 1000 million years ago and are relatively common throughout the Adirondacks. This study involves the mapping and sampling of a small anorthosite suite near Dresden, New York. The site was mapped during a week of field days and 30 samples were collected for petrographic analysis, whole rock geochemical analysis using an ICP-AES, and rare earth element analysis using an ICP-MS. Much of the field area contains significant high grade metamorphic overprinting from the late stages of the Grenville Orogeny (around 1080 Ma). Field mapping revealed the presence of a gradient of plagioclase bearing rocks ranging from mafic ferro-gabbro to more plagioclase rich gabbroic anorthosite. Petrographic analyses confirmed the subdivision of the field area into three lithologies based on percent plagioclase composition as well as identified younger mafic intrusions into the anorthosite suite. Rare earth element analysis indicated that the suite likely differentiated from a single plagioclase rich magma source. Both the rare earth element analysis and whole rock geochemistry linked the anorthosite suite at Dresden to the broader Adirondack Proterozoic anorthosite event and indicated that the Dresden suite likely comprised a small, mafic off-shoot of that regional event.