XENOLITHS FROM THE WESTERN GRAND CANYON: A GLIMPSE OF THE MODERN AND PROTEROZOIC LOWER CRUST

Quaternary basalt flows in the western Grand Canyon contain a variety of inclusions (xenoliths and autoliths) of crustal and (possible) mantle origin. This study investigates several of these inclusions and their host basalts collected from the Toroweep, Ponderosa, and Lower Gray Ledge Flows at sites located 179.4, 181.2, and 184.7 river miles downstream from Lee’s Ferry using whole rock major and trace element analysis (ICP-OES) and careful microanalysis of mineral composition and texture using optical petrography, and SEM-EDS. Samples from the Toroweep Flow contain dunites and olivine-rich lherzolites, one gneiss xenoliths in basalt, the Ponderosa Flow contains inclusions of felsic, dunites (some of which have been extensively weathered), and one clinopyroxenite in basalts with varying degrees of vesicularity, and the Lower Gray Ledge Flow contains dunites, gneisses, and felsic inclusions. Olivine grains from the dunites are moderately primitive (Fo₈₉) with low minor element abundances. Preliminary analysis of mineral composition as well as the presence of well-preserved cumulate layering suggests that at least some of the dunite and olivine-rich lherzolite enclaves could be early fractionates of their host basalts (i.e. autoliths). Inclusion-host geochemical relationships will be further examined using thermodynamic modeling software and experimental techniques. On the other hand, olivine grains in the clinopyroxenite xenolith have a much more evolved composition (Fo₇₈), and diopside grains in the clinopyroxenite have the aluminous compositions associated with crystallization at high pressure and have calculated equilibration temperatures of around 600ºC (versus 1000ºC for the diopside grains in the dunites). These observations preclude any genetic relation between the clinopyroxenite and its host basalt and indicate an origin in the lower crust. Several quartzofeldspathic gneiss xenoliths are crustal in origin and could be related to local Precambrian paragneisses and be among the oldest rocks in the Grand Canyon. The origin of the other felsic xenoliths is somewhat more enigmatic. The diverse suite of rocks investigated here provides an understanding of fractionation in igneous systems as well as a greater understanding of the conditions and materials present deep within the Earth.