HIGH-AL GABBROS IN THE LARAMIE ANORTHOSITE COMPLEX, WYOMING - IMPLICATIONS FOR THE COMPOSITION OF MELTS PARENTAL TO PROTEROZOIC ANORTHOSITE

High-Al gabbro represents one of the latest phases of magmatism in the 1.43 Ga Laramie anorthosite complex (LAG) in southeastern Wyoming. Th is lithology, which is mineralogically and geochemically the most prim itive in the LAG, forms dikes and small intrusions that cross cut monz onitic 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 Sr-87/Sr-86 (,, low as 0.7033) and highest initial epsilon(Nd) (up to + 2) in the LAG. Their Sr and N d isotopic characteristics indicate a mantle origin followed by crusta l assimilation during ascent. Intermediate plagioclase (An(50-60)) and mafic silicate (Fo(54-63)) compositions suggest that they are not pri mary mantle melts and that they differentiated prior to final emplacem ent. 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 gabbro s to represent mantle-derived melts produced by the differentiation of a basaltic magma in an upper mantle chamber. Continued evolution of t his magma eventually resulted in the formation of plagioclase-rich dia pirs which ascended to mid-crustal levels and formed the anorthositic rocks of the LAG. Because these gabbros intrude the anorthositic rocks , they do not represent directly the magma from which anorthosite crys tallized and instead are younger samples of magma formed by identical processes.