Abstract

Geochemical data for a comprehensive suite of over 700 samples of massif anorthosite and associated rocks from the Adirondack Mountains, New York, exhibit a ubiquitous mixed tholeiitic and calc-alkaline signature indicating a complex petrogenesis. The origin and relationship of massif anorthosite to associated rocks has been a major petrological problem despite decades of study. The Adirondack Mountains is one of the best areas to study these rocks because it contains one of the world's most abundant occurrences of massif anorthosite and associated rocks forming bedrock over thousands of square kilometers. The suite of rocks analyzed includes both anorthosite suite rocks, consisting of anorthosites, leucogabbros, gabbros, oxide apatite gabbronorites (OAGNs), and oxide gabbronorites (OGNs) (defined in text), and mangerite suite rocks consisting of jotunites, monzodiorites, mangerites, and charnockites. Representative major-element compositions were determined largely by X-ray fluorescence (XRF) analysis for 352 massif anorthosites and associated rocks and a variety of trace elements were determined by XRF, instrumental neutron activation analysis (INAA), and inductively coupled plasma mass spectroscopy (ICP-MS) for 296 massif anorthosites and associated rocks. All rock types show a mixture of tholeiitic and calc-alkaline compositional characteristics with major elements exhibiting a strong iron enrichment tholeiitic trend and trace elements showing a depletion of Nb and Ta characteristic of calc-alkaline rocks. Prior to this study the geochemistry of these rocks in the Adirondacks has been only poorly characterized from scattered local studies.

The anorthosite suite of rocks exhibits two distinct compositional trends. Massif anorthosites, leucogabbros, gabbros, OAGNs, OGNs, along with jotunites, separate into two distinct compositional trends on P2O5-MgO and TiO2-MgO diagrams, whereas monzodiorites, mangerites, and charnockites have only one compositional trend. Two trends in anorthosites are caused by two varieties of anorthosite: one type with a characteristic mineralogy dominated by plagioclase plus pyroxene and another type dominated by plagioclase plus oxide minerals and apatite. Mafic enclaves at some localities near the margins of anorthosite masses contain gabbro, OGN, and OAGN in close spatial association, suggesting they represent crystallization from the same or similar parental magmas at different stages of evolution.

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