Proterozoic Igneous Rocks of the Grand Canyon

 

I have been fortunate to work with a consortium of several workers who have spent the past decade exploring the Proterozoic structural, metamorphic, and tectonic history of the Grand Canyon.The core of this group are Karl Karlstrom of the University of New Mexico, Sam Bowring of MIT, and Mike Williams of the University of Massachusetts.Raft-based research trips take place about twice per year, focusing on the Upper Granite Gorge, or the Lower Granite Gorge of the Grand Canyon (sometimes both in the same trip).Tens of graduate students of Karlstrom, Williams, and Bowring have produced outstanding dissertations (e.g. Brad Ilg, Dave Hawkins, Carol Dehler) and masterís theses (Katie Robinson, Mike Timmons) on Grand Canyon topics over the past ten years.The projects that Iím focused on in the Grand Canyon are described in the following sections.

 

Ultramafic Rocks of the Grand Canyon The Grand Canyon of northern Arizona cuts through one of the major Proterozoic sutures in North America, along which the Yavapai province was accreted to the Mojave province (and the rest of North America) around 1700 million years ago.Small (to about 1 km2) exposures of ultramafic rock occur near the eastern boundary of the suture zone.The ultramafic rocks are wehrlites (mostly cpx + olivine) and lherzolites (cpx + opx + ol).The best exposures have well-preserved primary minerals and cumulate texture.Dunite inclusions and basalt inclusions occur within the cumulate layers. The dunite inclusions may be dismembered remnants of dunite dikes.Paul Low, who is completing a masterís thesis on these rocks, has identified a few pyroxene inclusions in olivine crystals in the adjacent cumulates, consistent with the possibility that the dunite dike magmas were out of equilibrium with pyroxene, and dissolved it on their way upward, and precipitated olivine in its place.

 

Granites of the Grand Canyon Three pulses of Proterozoic granitic magmatism are associated with the 1700 Ma Yavapai orogeny.The first pulse (1740-1710 Ma) is represented by the intrusion of large sheet-like plutons that are characterized by obvious mingling of mafic and felsic magmas.The felsic end-members are rarely as siliceous as true granites.Plutons of this group include the Ruby pluton, the Diamond Creek pluton, and the Trinity pluton.Granites of the second pulse of plutonism (1700-1680 Ma) are smaller, high-silica granitic plutons and pegmatite pods and dikes.Research on granitoids of the Grand Canyon is focused on identifying the source materials that melted to produce these magmas, and to integrate the timing of melting and availability of source rocks to the emerging Proterozoic tectonic history of the Grand Canyon.