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Mafic magmatism in the Adirondacks, New York
Abstract
Late Proterozoic mafic magmatism was widespread in the Adirondack region of New York State, and was characterized by a distinctive Al-Fe geochemical signature in association with anorthosite. Basaltic rocks from the 20,000 km$\sp2$ Highlands define a path of geochemical evolution that resembles the trend shown by calculated liquids of the Kiglapait Layered Intrusion. Similar Al-Fe magmas occur in other anorthosite-bearing regions, and are distinct from most common magma types and from typical high-Al or high-Fe basalts. The Highlands signature has a moderately alkaline character which is critically undersaturated in silica, Al- and Fe-enriched, and shows extreme Fe enrichment on fractionation. The rocks are enriched in the incompatible trace elements, particularly K and Rb, and the light rare-earth elements. The range in observed rock compositions is consistent with fractionation of olivine and plagioclase, and late-stage titanomagnetite and clinopyroxene, from a gabbro-troctolite parent magma. The Lowlands signature shows silica enrichment and low iron, yet trace-element contents resemble Highlands rocks. They may represent similar magmas which have fractionated under more oxidizing conditions. Widespread emplacement of geochemically kindred mafic magmas in the Adirondack region was coeval with anorthosite in an anorogenic tectonic setting. It is postulated that a deep-seated mantle plume or channelized magma swell contributed an Fe-rich astehnosphere component to gabbro-troctolite parent magmas, which fractionated mafic minerals at lower crustal levels and became sufficiently hyperfeldspathic to generate anorthosite on decompression. Samples of these magmas that were periodically emplaced into higher crustal levels are now represented by the Highlands suite. Identification of Al-Fe magmas and their association with anorthosites may help to resolve questions of Proterozoic magmatism.
Subject Area
Geology
Recommended Citation
Olson, Karin Elizabeth, "Mafic magmatism in the Adirondacks, New York" (1990). Doctoral Dissertations Available from Proquest. AAI9100527.
https://scholarworks.umass.edu/dissertations/AAI9100527