The Petrology and Geochemistry of Mafic Igneous Rocks in the Anorthosite-Bearing Adirondack Highlands, New York

Igneous rocks of broadly basaltic composition are widely distributed in the anorthosite-bearing Adirondack Highlands of New York. They constitute a mafic series of rocks that occurs on the margins of the anorthosite series and up to 50 km away from the anorthosite. On an Sr reference diagram, the mafic series has an apparent initial 87Sr/86Sr value of 0.7036, which is consistent with 1100 Ma subcontinental mantle. The series was probably emplaced in the interval 1150–1100 Ma, during which time anorthosite and granitic magmas were also emplaced. The rocks were metamorphosed shortly thereafter (˜ 1088 Ma) under upper-amphibolite and granulite facies conditions. Individual bodies of mafic rocks range in thickness from small enclaves and layers ≫ 1 m thick to large lensoid masses several tens of meters thick. Despite deformation and metamorphic recrystallization, many of the rocks retain chiHed margins, cross-cutting relationships, and relict igneous textures. Selected samples of the mafic series have been analyzed for their major and trace element compositions. Metamorphism did not significantly alter the igneous geochemical relationships, and the rocks retain mantle-like values for Zr/Nb, K/Zr, K/Rb, Rb/Sr, and 87Sr/86Sr. The most primitive rocks of the series are silica-undersaturated gabbroic troctolites, and the more evolved rocks are basaltic in composition. The mafic series as a whole has high abundances of A12O3, FeO, the light rare earth elements (LREE), and other incompatible trace elements. Even the most geochemically primitive compositions have high FeO contents. The Fe enrichment and Si depletion that are shown by chemically evolved compositions are consistent with a Fenner trend of fractionation. Low levels of normative di indicate that high Fe is not a result of the extensive fractionation of cpx. The geochemical trends that are defined by the trace elements, including the REE, suggest the basaltic rocks may be differentiates of a parental magma of gabbroic troctolite composition. The main compositional trend of the mafic series can be simulated by ∼61% crystallization of olivine and plagioclase of a gabbroic troctolite, followed by 13% crystallization of olivine, plagioclase, clinopyroxene, and titanomagnetite at the final stages. The modelled ratio of olivine to plagioclase crystallization changes from 1–8: 1 to 0–64: 1. These non-cotectic ratios may reflect a delay in the crystallization of plagioclase relative to olivine, possibly as a resul