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 master’s theses
(Katie Robinson, Mike Timmons) on Grand Canyon topics
over the past ten years. The projects
that I’m 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 master’s 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.