THE ORIGIN OF DUNITE AND HARZBUGITE ENCLAVES IN LAYERED CUMULATES OF THE NINTYONE MILE CREEK CANYON PERIDOTITE, GRAND CANYON, USA

A 50 meter thick unit of Proterozoic cumulate layered wehrlite, lherzolite, and olivine websterite exposed at river mile 91 in the Ninetyone Mile Creek Canyon, a side canyon to the Grand Canyon, Arizona, USA, hosts numerous spheroidal to ellipsoidal dunite and olivine-rich harzburgite and wehrlite inclusions (enclaves) from 1 to 20 cm in diameter. The occurrence is unusual in that the enclaves appear to have been included in the cumulate layers as bodies of cohesive crystal mush. The ellipsoidal and tear-drop shapes of many of the enclaves suggests that they behaved ductilely during emplacement. Diffusion profiles suggest emplacement while both the enclaves and the surrounding cumulate peridotite were still at a high enough temperatures for rapid diffusion. Textural and geochemical characteristics suggest that the dunite enclaves and their cumulate peridotite host are co-genetic, and that the dunite is an early differentiate of the same melt that produced the lherzolite host. The dunite enclaves are suggested to be remnants of an early differentiated cumulate layer that was disrupted, perhaps by an infusion of new magma into the chamber, and ultimately deposited and covered by more evolved cumulate peridotite. A harzburgite enclave does not appear to be related by fractionation to either the surrounding cumulate peridotite or the dunite enclaves. The unusual major and minor element composition of the harzburgite enclave is also not a product of reaction with a percolating melt. Geochemical evidence suggests that the harzburgite enclave formed as a residue of partial melting of a less primitive peridotite. The harzburgite enclave(s) were probably dislodged from their original location by eruptive events, tectonic events, or convective flow and were carried into the pluton and deposited, like the dunite enclaves, as cohesive enclaves during the accumulation of more evolved cumulate layers.