Vishnu Basement Rocks

Telling Time at Grand Canyon

This article is part of a series adapted from the Natural Resources Report "Telling Time at Grand Canyon: 2020 Update". All of the articles, including numeric ages for rocks exposed at Grand Canyon, are available on the [article series] index page.

The inner gorge of the Grand Canyon and the Colorado River.

NPS Photo by Michael Quinn.

Introduction

Rock Type

Metamorphic and igneous rocks with vertical folds and foliation

Environment

Originally in volcanic island chains that collided with ancestral North America to form the southwest United States; rocks were metamorphosed and invaded by magmas in the deep crust.

Age (Ma; mega annum = million years ago)

Paleo- and Mesoproterozoic Eras (Precambrian)
1375–1840 Ma

The Vishnu Basement Rocks contains all the ancient crystalline rocks at the bottom of the Grand Canyon. Vishnu Basement Rocks is an informal term but is useful because the formal stratigraphic nomenclature does not encompass all of Grand Canyon’s metamorphic units and all individual igneous plutons.

The terminology uses “Vishnu” because the public is familiar with the Vishnu Schist and “basement” to indicate the type of rock assemblage and its position. These rocks span from 1,840 to 1,375 Ma (Table 2), a duration of 465 million years. A nominal age of about one and three quarter billion years ago (1.75 Ga) focuses attention on the tectonic collisions that added this tract onto the North American continent and is a general number for the age of the Vishnu Basement Rocks.

^{Figure 26. Tectonic evolution of the continent during the formation of the Vishnu Basement Rocks. A) The volcanic and sedimentary precursors of the Granite Gorge Metamorphic Suite were deposited on the flanks of volcanic island chains, and the granodiorite plutons formed as magma chambers underneath the islands. B) Later granite and pegmatite intrusions formed as the volcanic islands were added to the Wyoming Province, part of the growing North American continent.}

The basement rocks are divided into five informal groupings reflecting distinct time periods and tectonic histories (Table 2). Of these groupings, only the Granite Gorge Metamorphic Suite and the Zoroaster Plutonic Complex have been formally defined in the geologic literature.

Best numeric ages of the Vishnu Basement Rocks

Group	Formation	Stratigraphic Age	Numeric Age (Ma)	Precision (Ma)
Youngest granite	Quartermaster granite	Mesoproterozoic	1,375	± 2
Later granites / dike swarms	Phantom granite	Paleoproterozoic	1,662	± 1
	Cremation pegmatite	Paleoproterozoic	1,698	± 1
Zoroaster Plutonic Complex	Horn Creek granite	Paleoproterozoic	1,713	± 2
	Ruby gabbro	Paleoproterozoic	1,716	± 0.5
	Trinity granite	Paleoproterozoic	1,730	± 93
	Diamond Creek granite	Paleoproterozoic	1,736	± 1
	Zoroaster granite	Paleoproterozoic	1,740	± 2
Granite Gorge Metamorphic Suite	Vishnu Schist	Paleoproterozoic	1,750	± 2
	Brahma Schist	Paleoproterozoic	1,750	± 2
	Rama Schist	Paleoproterozoic	1,751	± 2
Oldest basement	Elves Chasm pluton	Paleoproterozoic	1,840	± 1

Table 2.
Ma = mega annum = million years ago

Elves Chasm Pluton

Tectonic Environment

An older crustal pluton (magma chamber) in one of the microcontinents that gradually united to form North America

Age (Ma; mega annum = million years ago)

1,840 Ma

The Elves Chasm pluton (1,840 Ma) is the oldest basement identified in Grand Canyon. It is only exposed in the vicinity of Elves Chasm near River Mile 115. The age of this rock reflects its crystallization age from magma. Since it is an intrusive rock, it must have intruded older rocks, but these rocks have not yet been identified.

Granite Gorge Metamorphic Suite

Tectonic Environment

Volcanic and sedimentary rocks were deposited on the Elves Chasm pluton; they were metamorphosed as they were buried up to 12-mile (20-km) depths during the Yavapai orogeny. This event was similar to the collision of Italy with Europe to form the Alps. The Yavapai orogeny added new continental crust onto the then-southern margin of North America.

Age (Ma; mega annum = million years ago)

1,750–1,751 Ma

An unconformity of 90 million years separates the Elves Chasm pluton from the overlying Granite Gorge Metamorphic Suite, which consists of the 1,751 Ma Rama Schist and the 1,750 Ma Vishnu and Brahma schists. The Brahma and Rama schists originated as volcanic rocks, and the Vishnu Schist as sedimentary rocks. These rocks were deposited in volcanic island arcs that were later welded to the growing continent in the Yavapai orogeny approximately 1,700 Ma. They were metamorphosed during the mountain building event when the folds and vertical foliation that characterize these units was developed.

These rocks were intruded at great depths by two main types of magma: an early group of granodiorite plutons, and a later group of granite plutons and pegmatite dikes.

Numeric Ages of the Granite Gorge Metamorphic Suite

Formation	Stratigraphic Age	Numeric Age(Ma)	Precision(Ma)
Vishnu Schist	Paleoproterozoic	1,750	± 2
Brahma Schist	Paleoproterozoic	1,750	± 2
Rama Schist	Paleoproterozoic	1,751	± 2

Ma = mega annum = million years ago

Zoroaster Plutonic Complex

Tectonic Environment

Granodiorite magma chambers similar to modern ones under the Cascade volcanoes of the Pacific Northwest. The Zoroaster magmas formed during plate tectonic collisions during the Yavapai orogeny.

Age (Ma; mega annum = million years ago)

1,713–1,740 Ma

Most of the granodiorite intrusions in the Vishnu Basement Rocks are part of the Zoroaster Plutonic Complex. These plutons formed during the Yavapai orogeny and were likely related to the magma chambers that fed the volcanic arcs above subduction zones and range in age from 1,740 to 1,713 Ma.

Numeric Ages of the Zoroaster Plutonic Complex

Formation	Stratigraphic Age	Numeric Age (Ma)	Precision (Ma)
Horn Creek granite	Paleoproterozoic	1,713	± 2
Ruby gabbro	Paleoproterozoic	1,716	± 0.5
Trinity granite	Paleoproterozoic	1,730	± 93
Diamond Creek granite	Paleoproterozoic	1,736	± 1
Zoroaster granite	Paleoproterozoic	1,740	± 2

Ma = mega annum = million years ago

Later Granites and Dike Swarms

Tectonic Environment

The last crustal melts from the waning Yavapai orogeny

Age (Ma; mega annum = million years ago)

1,662–1,698 Ma

Later granites and dike swarms (Figure 27 and 28) formed more than 12 mile (20 km) below the surface due to crustal melting during the plate collisions that thickened and metamorphosed the crust during the Yavapai orogeny. These plutons and dikes can be differentiated from plutons in the Zoroaster Plutonic Complex by cross-cutting relationships and differences in composition. They are also slightly younger in age, ranging from 1,698–1,662 Ma.

Numeric Ages of the Later Granites and Dike Swarms

Formation	Stratigraphic Age	Numeric Age(Ma)	Precision(Ma)
Phantom granite	Paleoproterozoic	1,662	± 1
Cremation pegmatite	Paleoproterozoic	1,698	± 1

Ma = mega annum = million years ago

^{Figure 27. Vishnu Basement Rocks include both metamorphic rocks of the Rama, Brahma, and Vishnu schists (dark in this photo) and granitic intrusions of several types and ages (lighter colors). This picture shows the Cremation pegmatite swarm near Phantom Ranch; one of these intrusions has been dated as 1,698 ± 1 million years old.
Photo by Laurie Crossey.}

^{Figure 28. Granite intrusions of different sizes and compositions intruded the metavolcanic and metasedimentary rocks at great depths in the Vishnu mountains. This image shows granite from the later magmas filling a fracture network in the schist.
Photo by Laurie Crossey.}

Youngest Granite

Tectonic Environment

Younger granites derived from crustal melting were widespread in the Southwest US, probably formed as other continental fragments collided with the growing North American continent far to the south of what is now the Grand Canyon region.

Age (Ma; mega annum = million years ago)

1,375 Ma

The Quartermaster pluton in the western Grand Canyon is substantially younger than the rest of the Vishnu Basement Rocks at 1,375 Ma. It is part of a belt of similar-age plutons that extends from southern California to Labrador that formed during a period of crustal melting due to later plate collisions.

Numeric Ages

The many reliable radiometric age determinations of the Vishnu Basement Rocks were obtained using the U-Pb method on the mineral zircon.

These radiometric age determinations are all highly precise, with analytical errors that are within ± 2 Ma, except for the 1,730 Ma Trinity Granite, which has an error of ± 93 Ma (Table 2). This imprecise age is due to mixed zircon populations in this granite that, with newer methods, could now be improved.

The dates we report for the Vishnu Basement Rocks have not been revised much in the past two decades. Additional research efforts are needed to further unravel their complex history. For example, the Vishnu Schist contains zircon grains whose ages were not reset during metamorphism, and can be used to determine the age of the sediment source. The oldest zircon grains are as old as 3.8 Ga, with many grains dated at about 2.4 and 1.8 Ga. A next step is to pinpoint the potential source regions outside Grand Canyon from which these grains were derived.

The igneous and metamorphic rocks of Vishnu Basement formed beneath the now-eroded Vishnu mountains. The erosional demise of those mountains to sea level was complete by the time the Grand Canyon Supergroup was deposited atop the Great Nonconformity erosion surface.

Learn More

To learn more about the age of Grand Canyon’s rocks, please see:

Karlstrom, K., L. Crossey, A. Mathis, and C. Bowman. 2021. Telling time at Grand Canyon National Park: 2020 update. Natural Resource Report NPS/GRCA/NRR—2021/2246. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/nrr-2285173. [IRMA Portal]

Meet the Authors

Authors

• Dr. Karl Karlstrom is a Distinguished Professor of Geology at the University of New Mexico with a specialty in tectonics; he has researched Grand Canyon rocks of all ages over the past 35 years.
• Dr.Laurie Crossey is a Professor of Geology and Geochemistry at the University of New Mexico who has worked on Grand Canyon rocks and water issues over the past 20 years.
• Allyson Mathis is a Research Associate with the Northern Rockies Conservation Cooperative, and a geologist by training. Allyson worked for the National Park Service at Grand Canyon National Park from 1999 to 2013.
• Carl Bowman is a retired air quality specialist, formerly of Grand Canyon National Park, where he worked in various positions between 1980 and 2013.

References Cited in Full Report

References

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Glossary of Geologic Terms

Glossary

• Absolute age: a numeric age in years. Numeric age is the preferred term.

• Accuracy: measure of how close a numeric date is to the rock’s real age.

• Angular unconformity: a type of unconformity or a gap in the rock record where horizontal sedimentary layers (above) were deposited on tilted layers (below). At Grand Canyon, horizontal layers of the Layered Paleozoic Rocks lie on top of the tilted rocks of the Grand Canyon Supergroup.

• Basalt: a dark, fine-grained volcanic (extrusive igneous) rock with low silica (SiO2) content.

• Biochron: length of time represented by a fossil biozone.

• Carbonate: sedimentary rock such as limestone or dolostone largely composed of minerals containing carbonate (CO3^-2) ions.

• Contact: boundary between two bodies of rock or strata.

• Daughter isotope: the product of decay of a radioactive parent isotope.

• Detrital: pertaining to grains eroded from a rock that were transported and redeposited in another.

• Dike: a wall-like (planar) igneous intrusion that cuts across pre-existing layering.

• Diabase: a dark igneous rock similar in composition to basalt but with coarser (larger) grain size.

• Disconformity: a type of unconformity or gap in the rock record between two sedimentary layers caused by erosion or nondeposition where the layers are parallel to one another.

• Dolomite: the mineral calcium magnesium carbonate CaMg(CO3)2 that usually forms when magnesium-rich water alters calcium carbonate (CaCO3).

• Dolostone: a rock predominantly made of dolomite.

• Eon: longest subdivision of geologic time in the Geologic Timescale; for example, the Proterozoic Eon.

• Era: second-longest subdivision of geologic time below eon in the Geologic Timescale; for example, the Paleozoic Era.

• Epoch: fourth-longest subdivision of geologic time, shorter than a period and longer than a stage in the Geologic Timescale; for example, the Pleistocene Epoch.

• Faunal succession: the change in fossil assemblages through time which has a specific, reliable order.

• Foliation: tectonic layering in metamorphic rocks caused by parallel alignment of minerals due to compression.

• Formation: the fundamental unit in stratigraphy and geologic mapping that consists of a set of strata with distinctive rock characteristics. Formations may consist of a single rock type (e.g., Tapeats Sandstone or Redwall Limestone), or a mixture of rock types (e.g. Hermit Formation, which includes sandstone, mudstone, and shale).

• Fossil: evidence of life in a geologic context usually consisting of the remains or traces of ancient life.

• Fossil biozone: stratigraphic unit defined by a distinctive assemblage of fossils.

• Ga: giga annum: billion years; in this paper, our usage implies billion years before present (or ago) when used for numeric ages.

• Gneiss: a high-grade metamorphic rock with strong foliation and light and dark bands of minerals.

• Granite: a high silica (SiO2) pink to white intrusive igneous rock composed mainly of feldspar and quartz.

• Granodiorite: a gray intrusive igneous rock composed of feldspar, quartz, biotite, and hornblende with less silica (SiO2) than granite.

• Group: a sequence of two or more related formations, with a stratigraphic rank higher than formation; for example, the Chuar Group is made up of the Nankoweap, Galeros, and Kwagunt formations.

• Igneous rock: a rock that solidified from molten material (magma or lava), either within the Earth (as an intrusive or plutonic rock) or after eruption onto the Earth’s surface (as an extrusive or volcanic rock).

• Inclusion: a fragment of an older rock within a younger rock.

• Index fossil: a fossil or assemblage of fossils that is diagnostic of a particular time in Earth history.

• Intrusion: an igneous rock body that crystallized underground. Intrusions may have any size or shape; large ones are known as plutons, thin ones parallel to layering are known as sills, and thin ones that cut across layering are called dikes.

• Isotope: one of the forms of a chemical element (with the same atomic number) that contains a different number of neutrons.

• Lateral continuity: a geologic principle that sedimentary rocks extend laterally, and that if they are now separated due to erosion, they were once laterally continuous; for example, the Kaibab Formation on the South Rim is laterally continuous with the Kaibab Formation on the North Rim.

• Lava: molten rock erupted onto the Earth’s surface.

• Ma: mega annum: million years; in this paper, our usage implies million years before present (or ago) when used for numeric ages.

• Magma: molten or partially molten rock material formed within the Earth.

• Member: a subdivision of a formation, usually on the basis of a different rock type or fossil content; for example, the Hotauta Conglomerate is a member of the Bass Formation.

• Metamorphic rock: a rock formed by recrystallization under intense heat and/or pressure, generally in the deep crust.

• Monadnock: a bedrock island that sticks above the general erosion level.

• Nonconformity: an unconformity or gap in the rock record where sedimentary layers directly overlie older and eroded igneous or metamorphic rocks.

• Numeric age: age of a rock in years (sometimes called absolute age).

• Numeric age determination: measurement of the age of a rock in years, often through the use of radiometricdating techniques.

• Orogeny: mountain building event, usually in a collisional tectonic environment.

• Parent isotope: the radioactive isotope that decays to a daughter isotope.

• Pegmatite: a type of intrusive igneous rock usually of granitic composition with large crystal size.

• Period: third-longest subdivision of geologic time shorter than an era and longer than an epoch in the Geologic Timescale; for example, the Permian Period.

• Plate tectonics: theory that describes the Earth’s outer shell as being composed of rigid plates that move relative to each other causing earthquakes, volcanism, and mountain building at their boundaries.

• Pluton: large intrusion of magma that solidified beneath the Earth’s surface.

• Precambrian: the period of time before the Cambrian Period that includes the Proterozoic, Archean, and Hadean eons and represents approximately 88% of geologic time.

• Precision: measure of the analytical uncertainty or reproducibility of an age determination.

• Proterozoic: geologic eon dominated by single-celled life extending from 2,500 to 541 million years ago; divided into the Paleoproterozoic (1,600–2,500 Ma), Mesoproterozoic (1,000–1,600 Ma), and Neoproterozoic (541–1,000 Ma) eras.

• Radioactive decay: the process by which the nuclei of an unstable (radioactive) isotope lose energy (or decay) by spontaneous changes in their composition which occurs at a known rate for each isotope (expressed as a half life); for example, the parent uranium (238U) isotope decays to the daughter lead (206Pb) isotope with a half life of 4.5 billion years.

• Radiometric dating: age determination method that uses the decay rate of radioactive isotopes and compares the ratio of parent and daughter isotopes within a mineral or rock to calculate when the rock or mineral formed.

• Regression: geologic process that occurs when the sea level drops relative to the land level; for example, by sea level fall and/or uplift of the land, causing the withdrawal of a seaway from a land area.

• Relative time: the chronological ordering of a series of events.

• Rift basin: a basin formed by stretching (extension) of the Earth’s crust. Rift basins are linear, fault-bounded basins that can become filled with sediments and/or volcanic rocks.

• Rodinia: a Neoproterozoic supercontinent that was assembled about 1.0 Ga (during Unkar Group time) and rifted about 750 Ma (during Chuar Group time).

• Sedimentary rock: a rock composed of sediments such as fragments of pre-existing rock (such as sand grains), fossils, and/or chemical precipitates such as calcium carbonate (CaCO3).

• Schist: a metamorphic rock with platy minerals such as micas that have a strong layering known as foliation or schistosity.

• Silica: silicon dioxide (SiO2), a common chemical “building block” of most major rock-forming minerals, either alone (i.e., as quartz) or in combination with other elements (in clays, feldspars, micas, etc.).

• Sill: a sheet-like igneous intrusion that is parallel to pre-existing layering.

• Snowball Earth: a hypothesis that the Earth’s surface became completely or mostly frozen between 717 and 635 million years ago.

• Stage: a short subdivision of geologic time in the Geologic Timescale often corresponding to the duration of a fossil assemblage.

• Stratigraphic age: the era, period, epoch, or stage a rock is assigned to based on its fossil biozones or numeric age.

• Stratigraphy: the study of layered rocks (strata), which usually consist of sedimentary rock layers, but may also include lava flows and other layered deposits.

• Stromatolite: a fossil form constructed of alternating layers (mats) of microbes (algal or bacterial) and finegrained sediment.

• Subduction zone: a plate boundary where two plates converge and one sinks (subducts) beneath the other.

• Supergroup: a sequence of related groups, with a higher stratigraphic rank than group; for example, the Grand Canyon Supergroup consists of the Unkar and Chuar groups.

• Superposition: principle of geology that the oldest layer in a stratigraphic sequence is at the bottom, and the layers get progressively younger upwards.

• Tectonics: large-scale processes of rock deformation that determine the structure of Earth’s crust and mantle.

• Trace fossil: a sign or evidence of past life, commonly consisting of fossil trackways or burrows.

• Transgression: a movement of the seaway across a land area, flooding that land area because of a relative sea level rise and/or land subsidence.

• Travertine: calcium carbonate (CaCO3) precipitated by a spring; most travertine deposits also contain some silica.

• Unconformity: a rock contact across which there is a time gap in the rock record formed by periods of erosion and/or nondeposition.

• Volcanic ash: small particles of rock, minerals, and volcanic glass expelled from a volcano during explosive eruptions. Volcanic ash may be deposited great distances (even hundreds of miles or kilometers) from the volcano in especially large eruptions.

• Yavapai orogeny: mountain building period that occurred approximately 1,700 million years ago when the Yavapai volcanic island arc collided with proto-North America.

• Zircon: a silicate mineral (ZrSiO4) that often forms in granite and other igneous rocks and incorporates uranium atoms, making it useful for radiometric dating.

Photos and Illustrations

Related Links

• Grand Canyon National Park (GRCA), Arizona—[GRCA Geodiversity Atlas] [GRCA Park Home] [GRCA npshistory.org]

• NPS Geology—Geologic Time Scale

• NPS—Geologic Resources Division