Vishnu Basement Rocks
Vishnu Basement Rocks is a name recommended for all Early Proterozoic crystalline rocks (metamorphic and igneous) in the Grand Canyon region.
Vishnu Basement Rocks are exposed in the Grand Canyon's Inner Gorge (or Granite Gorge as named by John Wesley Powell). This view is near the Colorado River crossings at Phantom Ranch.
Age:
Precambrian*
Proterozoic, Early*
Early Proterozoic - 1.680 to 1840 billion years
(age used by National Park Service;
Mathis and Bowman, 2005)
Southern Rocky Mountain region*
AZ(n)*
Not compiled to date.
Age modified (Elston, 1989b). Overview (Elston, 1989b).
EARLY PROTEROZOIC CRYSTALLINE ROCKS
Intrusive and metamorphic rocks as defined and mapped by Ilg and others (1996), Hawkins and others (1996), and Karlstrom and others (2001).
Intrusive rocks
Young granite and pegmatite—Granite plutons, stocks, dikes, and pegmatite. About 1.4 billion years old.
Granite, granitic pegmatite, and aplite—Granite plutons, stocks, and pegmatite and aplite dikes emplaced synchronously with peak metamorphism. About 1.7–1.66 billion years old.
Granodiorite complexes—Gabbro-diorite-granodiorite complexes of probable volcanic-arc origin. About 1.74–1.71 billion years old.
Ultramafic rocks—Probable cumulate origin as supracrustal rocks.
Metamorphic rocks
Vishnu Schist—Quartz-mica schist, pelitic schist, and meta-arenites of metamorphosed, arc-basin, submarine sedimentary rocks. About 1.75 billion years old. Locally interlayered with Brahma schist and Rama Schist.
Brahma Schist—Consists of amphibolite, hornblende-biotite-plagioclase schist, biotite- plagioclase schist, orthoamphibole-bearing schist and gneiss, and metamorphosed sulfide deposits. Mafic to intermediate-composition metavolcanic rocks. About 1.75 billion years old. Locally interlayered with Rama Schist and Vishnu Schist.
Rama Schist—Massive, fine-grained quartzofeldspathic schist and gneiss of probable felsic metavolcanic rocks. About 1.75 billion years old. Locally interlayered with Brahma Schist and Vishnu Schist.
Orthoamphibole-bearing gneiss—Regolith. An interval several meters thick of weathered detritus eroded from older plutonic rocks. Metamorphic monazite from pelitic schist containing garnet, kyanite, gedrite, sillimanite, and cordierite. Very small outcrops along Colorado River within Granite Gorge.
Elves Chasm pluton—Oldest plutonic rocks, possible “basement” substrate. Contains mafic (hornblende-biotite tonalite) and intermediate-composition plutonic units (quartz diorite), including tabular amphibolite bodies that may be dikes. About 1.84 billion years old.
Click here to learn more about Precambrian geology of the Colorado Plateau.
References
Elston, D.P., 1989, Grand Canyon Supergroup, northern Arizona; stratigraphic summary and preliminary paleomagnetic correlations with parts of other North American Proterozoic successions, IN Jenney, J.P., and Reynolds, S.J., editors, Geologic evolution of Arizona: Arizona Geological Society Digest, v. 17, p. 259-272.
Babcock, R.S., 1990, Precambrian Crystalline Core. In: Beus, S.S., Morales,
M. (eds), Grand Canyon Geology, Oxford University Press, New York, p.
11-28.
Ilg, B.R., Karlstrom, K.E., and Williams, M.L., 1996, Tectonic evolution of Paleoproterozoic rocks in the Grand Canyon -Insights into middle-crustal processes: Geological Society of America Bulletin, v. 108, no. 9, p. 1149-1166.
Hawkins, D.P., Bowring, S.A., Ilg, B.R., Karlstrom, K.E., and Williams, M.L., 1996, UPb geochronologic constraints on the Paleoproterozoic crustal evolution of the Upper Granite Gorge, Grand Canyon, Arizona: Geological Society of America Bulletin, v. 108, no. 9, p. 1167-1181.
Karlstrom, K.E., Ilg, B.R., Williams, M.L., Hawkins, D.P., Bowring, S.A., and Seaman, S.J., 2001, Chapter 2; Precambrian crystalline core; Paleoproterozoic rocks of the Granite Gorges, in Beus, S.S., and Morales, Michael, editors, Grand Canyon geology: Oxford and New York , Oxford University Press, and Flagstaff , Ariz. , Museum of Northern Arizona Press , second edition.
Billingsley, George H., 2000, Geologic Map of the Grand Canyon 30' by 60' Quadrangle, Coconino and Mohave Counties, Northwestern Arizona: U.S. Geological Survey Geologic Investigation Series I-2688, Available on-line at: http://pubs.usgs.gov/imap/i-2688/.).
Mathis, A. and Bowman, C., 2005, What's in a number? Numeric ages for rocks exposed within the Grand Canyon, Part 2: Nature Notes ( Grand Canyon National Park ), v. 21, no. 2, p. 1-5.
