GEOCHEMICAL EVIDENCE FOR PULSED MAGMA AND FLUID INTERACTION IN THE MILE 91 PERIDOTITE, GRAND CANYON, USA: IMPLICATIONS FOR INTERPRETATION OF PROTEROZOIC TECTONIC SETTING

Small Proterozoic ultramafic bodies, tectonically bounded by pelitic schist, occur in the Upper Granite Gorge of the Grand Canyon, Arizona at river miles 83, 91, 96, 98, 108, and 112. These bodies are spatially associated with the Mojave/Yavapai suture zone. The Mile 91 Peridotite is a 50-meter-thick unit of cumulate layered lherzolite that hosts numerous ellipsoidal dunite inclusions that range from 5-8 cm long and show systematic variations in mineral composition. Microtextural analysis, quantitative electron microprobe analyses, and x-ray mapping of minerals in these inclusions and in the surrounding lherzolite were used to evaluate the significance of the inclusions in terms of the magmatic evolution of this body and its implications for interpretation of the tectonic setting into which the ultramafic body was emplaced. Several reaction relationships that have been documented on the basis of petrographic analysis can be explained with a series of balanced chemical reactions. Inclusions of metasomatic minerals such as hornblende and phlogopite in olivine in the lherzolite indicate that significant amounts fluids reacted with the mile 91 peridotite relatively early in its evolution. These fluids may have been recycled hydrothermal fluids in a spreading center, or they may have originated from dewatering of a subducting slab in an arc setting. Opx and cpx inclusions in olivine in the lherzolite indicate reactions that involve the dissolution of pyroxene to form olivine and a silica-enriched liquid (basaltic magma) in an ascending melt The reaction of diopside with olivine to form olivine and a SiO2-rich melt increases the Fo concentration of olivine and may explain the systematic variations in olivine composition in these rocks. An origin for the dunite inclusions as dismembered dunite dikes that formed as opx-undersaturated melt ascended to interact with the lower ultramafic cumulate reaches of a gabbroic magma chamber is consistent with the mineral textures and compositions preserved in the Mile 91 peridotite. The ability to distinguish ophiolites from arc cumulates in the absence of original field context by understanding the geochemical evolution of a single body of rock provides a powerful tool for understanding the accretionary history of structurally complex areas such as Southwestern North America.