High-Al gabbros in the Laramie Anorthosite Complex, Wyoming: implications for the composition of melts parental to Proterozoic anorthosite

Abstract

High-Al gabbro represents one of the latest phases of magmatism in the 1.43 Ga Laramie anorthosite complex (LAC) in southeastern Wyoming. This lithology, which is mineralogically and geochemically the most primitive in the LAC, forms dikes and small intrusions that cross cut monzonitic and anorthositic rocks. High-Al gabbro is characterized by high Al2O3 (15–19 wt%), REE patterns with positive europium anomalies (Eu/Eu*=1.2–3.8), and the lowest initial 87Sr/86Sr (as low as 0.7033) and highest initial ɛNd (up to +2) in the LAC. Their Sr and Nd isotopic characteristics indicate a mantle origin followed by crustal assimilation during ascent. Intermediate plagioclase (An50–60) and mafic silicate (Fo54–63) compositions suggest that they are not primary mantle melts and that they differentiated prior to final emplacement. High-Al gabbros of the LAC are similar compositionally to gabbros from several other Proterozoic anorthosite complexes, including rocks from the Harp Lake complex and the Hettasch intrusion in Labrador and the Adirondack Mountains of New York. These gabbros are considered to be parental to their associated anorthositic rocks, a theory that is supported by recent experimental work. We interpret LAC high-Al gabbros to represent mantle-derived melts produced by the differentiation of a basaltic magma in an upper mantle chamber. Continued evolution of this magma eventually resulted in the formation of plagioclase-rich diapirs which ascended to mid-crustal levels and formed the anorthositic rocks of the LAC. Because these gabbros intrude the anorthositic rocks, they do not represent directly the magma from which anorthosite crystallized and instead are younger samples of magma formed by identical processes.