The Origin of Proterozoic Ultramafic Rocks in the Grand Canyon

Aaron Petruski
University of Masssachusetts

Three Proterozoic ultramafic bodies have been identified in the Grand Canyon. They are located at river miles 83, 91, and 98. These ultramafic bodies range in area from 3 km2 to 1 km2. The purpose of this work is to constrain the origin of the ultramafic bodies. The ultramafic bodies are associated with the broad (100+ km) transitional zone between the older Mojave (1.8-2.5 Ga) and the younger Yavapai (1.7-1.75 Ga) crustal provinces. In this tectonic context, the ultramafic bodies are likely to represent either ophiolite fragments or cumulates at the base of Yavapai-related island arc plutons.. The 1 km2 body at mile 91 has been examined in most detail. The entire cumulate sequence of the mile 91 body consists of coarse-grained (2-6 cm-diameter crystals) of wehrlite (olivine, aluminum-rich diopside, and phlogopite). Spherical to elliptical, aphanitic bodies to 5 cm diameter occur as inclusions in phaneritic ultramafic hosts. These inclusions consist essentially entirely of olivine.

Major, trace, and rare earth element analyses suggest a petrogenetic relationship between the three ultramafic bodies. These bodies may have originally been a single cumulate sequence with mineral layers related by fractional crystallization. The sequence was presumably dismembered during deformation associated with either the Yavapai or the Mazatzal orogeny. The ultramafic rocks are enriched in large ion lithophile elements such as K, Rb, and Ba, and depleted in Nb, Ti, and Zr, typical of rocks associated with subduction zone volcanism. The hydrous nature of the rocks, represented by the abundance of phlogopite, further suggests an origin as part of a subduction-related plutonic complex.