| The origin of monzonitic intrusions that are associated with Proterozoic massif-type anorthosite complexes is controversial. A detailed oxygen isotope study of the Sybille intrusion, a monzonitic intrusion of the Laramie Anorthosite Complex (Wyoming), indicates that it was likely derived from a basaltic magma of mantle-origin with a metasedimentary component (∼20%) incorporated early in its magmatic history. This is broadly consistent with radiogenic isotope and other petrologic findings of previous studies on the Sybille intrusion.; The oxygen isotope compositions of plagioclase, pyroxene and zircon from the Sybille monzosyenite, the dominant rock type in the Sybille intrusion, have been analyzed in order to assess the magmatic oxygen isotope compositions. We interpret the higher plagioclase delta18O values (average 8.69 +/- 0.30‰, n = 19) to be magmatic, lower plagioclase delta 18O values (average 7.51 +/- 0.44‰, n = 22) to be the result of variable deuteric alteration, and pyroxene delta18O values (average 6.34 +/- 0.38‰, n = 19) to be the result of closed-system diffusional exchange during cooling. Low magnetic zircons, which have been shown to retain magmatic oxygen isotope values despite high grade metamorphism and extensive subsolidus hydrothermal alteration, have delta18O values (7.40 +/- 0.24‰, n = 11) which are consistent with our interpretation of the plagioclase and pyroxene results.; Oxygen isotope data from all three minerals indicate that the magmatic oxygen isotope composition of the Sybille intrusion is enriched in 18O relative to the compositions of average mantle-derived magmas. Furthermore, this enrichment is approximately twice the oxygen isotope enrichment that would result from closed-system fractionation, rendering a closed-system, comagmatic petrogenic model between the Sybille intrusion and the mantle-derived anorthositic lithologies of the Laramie Anorthosite Complex improbable. |
