Geochemistry of Massif Anorthosite from the Adirondack Mountains, New York

Representative major and trace element compositions have been determined by XRF and INA analysis for 89 anorthosite suite samples selected from over 300 samples collected from several anorthosite plutons in the NE Adirondack Mountains. The anorthosite suite consists of anorthosites, leucogabbros, sodic leucogabbros, gabbros, sodic gabbros, and a variety of mafic iron-titanium-phosphorous-rich OAGNs. In addition 34 plagioclase, augite, hypersthene, magnetite, garnet, and hastingsite mineral compositions were determined by INA analysis of minerals separates and microprobe analyses of plagioclase, potash feldspar, augite, hypersthene, garnet, magnetite, and hastingsite in polished sections. Thin section study defined textural relationships and the mineralogy of these rocks. Whole rock compositional variations are explained entirely by mineralogical variations and mineral compositions; no evidence was found for an uncrystallized trapped liquid component in these rocks. Anorthosite to gabbro represents decreasing cumulus plagioclase and increasing mafic minerals whereas gabbro to OAGN represents increasing cumulus oxide minerals and apatite. Most gabbros contain essentially no cumulus minerals and represent candidates for parental magmas with tholeiitic affinities on an AFM diagram. The scatter on compositional diagrams relates to variable mineral compositions resulting from crystallization of multiple parental magmas at varying stages of magmatic evolution. Variations in the magmatic evolution of parental magmas and rocks crystallized from them are best revealed in hyerbolic plots of incompatible elements with compatible elements. Spider diagrams of anorthosite suite rocks exhibit negative Nb and Ta anomalies and irregular alkali element concentrations similar to alkali basalts indicating a calc-alkaline component. Existing neodymium and oxygen isotope data and the ubiquitous calc-alkaline component suggests that mantle derived tholeiitic parental magmas have interacted with lower crust or subcrustal mantle prior to emplacement. We have not considered the contentious issue of a possible genetic relationship between anorthosite suite rocks and spatially associated alkali-rich silicic rocks.