The Vishnu Basement Rocks is the name recommended for all Early

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

crystalline rock A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...

s ( metamorphic and

igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

) exposed in the Grand Canyon region. They form the crystalline

basement rocks In geology, basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The baseme ...

that underlie the

Bass Limestone The Bass Formation, also known as the Bass Limestone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The Bass Formation erodes as either cliffs or stair-stepped cliffs. In the case of the ...

of the Unkar Group of the

Grand Canyon Supergroup The Grand Canyon Supergroup is a Mesoproterozoic to a Neoproterozoic sequence of sedimentary strata, partially exposed in the eastern Grand Canyon of Arizona. This group comprises the Unkar Group, Nankoweap Formation, Chuar Group and the Sixtym ...

and the

Tapeats Sandstone Except where underlain by the Sixtymile Formation, Cambrian Tapeats Sandstone is typically the lowest geologic unit, about thick, at its maximum, of the 5-member Tonto Group. It is famous for being the highly-resistant mostly-horizontal unit abo ...

of the

Tonto Group The Tonto Group is a name for an assemblage of related sedimentary strata, collectively known by geologists as a ''Group'', that comprises the basal sequence Paleozoic strata exposed in the sides of the Grand Canyon. As currently defined, the T ...

. These basement rocks have also been called either the Vishnu Complex or Vishnu Metamorphic Complex. These Early Proterozoic crystalline rocks consist of metamorphic rocks that are collectively known as the ''Granite Gorge Metamorphic Suite''; sections of the Vishnu Basement Rocks contain Early Paleoproterozoic

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

, granitic

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...

, aplite, and granodiorite that have intruded these metamorphic rocks, and also, intrusive Early Paleoproterozoic ultramafic rocks.Anonymous (nd
''Vishnu Basement Rocks.''
U. S. Geological Survey, Reston, Virginia.Karlstrom, KE, BR Ilg, Bradley, D Hawkins, ML Williams, G Dumond, KK. Mahan, and SA Bowring, Samuel (2012) ''Vishnu basement rocks of the Upper Granite Gorge: Continent formation 1.84 to 1.66 billion years ago.'' In JM Timmons and KE Karlstrom, eds., pp. 7–24, Grand Canyon geology: Two billion years of earth's history. Special Paper no 294, Geological Society of America, Boulder, Colorado. The term ''Zoroaster Plutonic Complex '' is used for all Paleoproterozoic granitic and grandioritic

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

ic rocks in the Grand Canyon. Specific names have been assigned to individual

s and

dike Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes, ...

swarms because the plutons and swarms differ greatly in their age, origin, and tectonic significance. The oldest of these plutonic complexes, Elves Chasm Gneiss, likely represent a small fragment of ''basement'' upon which the meta

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

rocks that comprise the Granite Gorge Metamorphic Suite accumulated. The remainder of the Early Paleoproterozoic granites, granitic pegmatites, aplites, and granodiorites – are parts of either younger plutons or dike swarms, that have intruded the Granite Gorge Metamorphic Suite, either contemporaneously with, or after they were metamorphosed.Karlstrom, KE, BR Ilg, ML Williams, DP Hawkins, SA Bowring, and SJ Seaman (2003) ''Paleoproterozoic rocks of the Granite Gorges.'' In SS Beus and M Morales, eds., pp. 9–38, Grand Canyon Geology, 2nd ed. Oxford University Press, New York.Ilg, BR, KE Karlstrom, and ML Williams (1996) ''Tectonic evolution of Paleoproterozoic rocks in the Grand Canyon – Insights into middle-crustal processes.'' Geological Society of America Bulletin. 108(9):1149–66. It was named after a natural rock structure in the Colorado River valley which was named " Temple of Vishnu" from its appearance. From p. 148: "The finest butte of the chasm is situated near the upper end of the Kaibab division; but it is not visible from Point Sublime. It is more than 5,000 feet high, and has a surprising resemblance to an Oriental pagoda. We named it Vishnu's temple."

Granite Gorge Metamorphic Suite

The Granite Gorge Metamorphic Suite consists of lithologic units, the Brahma, Rama, and Vishnu

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...

s, that have been mapped within the Upper, Middle, and Lower Granite Gorges of the Grand Canyon. The Vishnu Schist consists of

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

schist,

pelitic A pelite (Greek: ''pelos'', "clay") or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock, ...

schist, and meta- arenites. They exhibit relict sedimentary structures and textures that demonstrate that they are metamorphosed

submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...

sedimentary rocks Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles t ...

. The Brahma Schist consists of

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky ...

, hornblende-

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

schist, biotite-plagioclase schist, ortho

amphibole Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...

-bearing schist and

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

, and metamorphosed sulfide deposits. As inferred from relict structures and textures, the Brahma Schist is composed of

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

felsic In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, whi ...

-composition metavolcanic rocks. The Rama Schist consists of massive, fine-grained quartzofeldspathic schist and gneiss that likely are probable felsic metavolcanic rocks. On the basis of the presence of relict pillow structures, interlayering of metavolcanic strata, and the large volumes of metavolcanic rocks, the Brahma and Rama schists are interpreted to consist of metamorphosed, volcanic island-arc and associated submarine volcanic rocks. These metavolcanic rocks are locally overlain by the metamorphosed submarine sedimentary rocks of the Vishnu Schist that are interpreted to have accumulated in oceanic trenches. These meta

were originally composed of particles of quartz,

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

, and volcanic rock fragments that have become metamorphosed into various schists. The Vishnu Schist exhibits relict

graded bedding In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into pro ...

and structures indicative of turbidite deposits that accumulated in oceanic trenches and other relatively deep-marine settings. The Brahma Schist has been dated to about 1.75 billion years ago. The felsic metavolcanic rocks that comprise the Rama Schist have yielded an age of 1.742 billion years ago.

Early Paleoproterozoic basement

The oldest rocks that are part of the Vishnu Basement Rocks is the Elves Chasm pluton. It consists of metamorphosed mafic (hornblende-biotite tonalite) and intermediate-composition plutonic rocks (quartz

diorite Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-silic ...

). Within it, there are tabular amphibolite bodies that might be dikes, that have been dated at about 1.84 billion years ago. It is regarded to be an older granodioritic pluton that was exposed by erosion prior to being buried by the original volcanic and submarine sedimentary rocks of the Granite Gorge Metamorphic Suite. The Elves Chasm pluton is likely part of the basement rocks on which the original volcanic rocks and sediments of the Granite Gorge Metamorphic Suite were deposited.Hawkins, DP, SA Bowring, BR Ilg, KE Karlstrom, and ML Williams (1996) ''U-Pb geochronologic constraints on the Paleoproterozoic crustal evolution of the Upper Granite Gorge, Grand Canyon, Arizona.'' Geological Society of America Bulletin. 108(9):1167–81. The highly tectonized contact between Elves Chasm pluton and the Granite Gorge Metamorphic Suite is exposed near Waltenberg Canyon, in 115-Mile Canyon, near Blacktail Canyon, and in the Middle Granite Gorge. This contact is characterized by a high-grade orthoamphibole-bearing gneiss. This gneiss is interpreted to be a highly metamorphosed and sheared paleosol and associated regolith that originally consisted of several meters of

weathered ''Weathered'' is the third studio album by American rock band Creed, released on November 20, 2001. It was the last Creed album to be released until '' Full Circle'' came out in October 2009, with Creed disbanding in June 2004. It is the only Cr ...

rock debris eroded from older plutonic rocks.

Younger intrusive igneous rocks

On the basis of rock type, type of intrusion, chemistry, and age of rocks, two main groups of younger Early Paleoproterozoic igneous intrusive (plutonic) rocks have been distinguished within the Vishnu Basement Rocks. One group, which dates between 1.74 and 1.71 billion years ago, consists of large plutons such as the Zoroaster pluton, the Ruby pluton, and the Diamond Creek pluton. There is no noticeable baking and metamorphism of the

country rock Country rock is a genre of music which fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal s ...

adjacent to them. Because of this, they were likely shallowly emplaced beneath the

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...

in which the metavolcanics and metasediments of the Granite Gorge Metamorphic Suite accumulated. In addition, these intrusive rocks have undergone all the deformation that has also affected their adjacent country rock. This further indicates that they are just slightly younger than the metavolcanic and metasedimentary rocks they intrude. This and their calc-alkaline granitic composition, which is similar to plutons forming in modern '

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

zone related' volcanic arcs, indicates that they are remnants of early volcanic arc systems associated with Early Paleoproterozoic subduction zones. Comparable volcanic arc systems, which are associated with subduction zones, are active today in the

Aleutian Islands The Aleutian Islands (; ; ale, Unangam Tanangin,”Land of the Aleuts", possibly from Chukchi language, Chukchi ''aliat'', "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a cha ...

and

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...

(e.g.,

Sumatra Sumatra is one of the Sunda Islands of western Indonesia. It is the largest island that is fully within Indonesian territory, as well as the sixth-largest island in the world at 473,481 km2 (182,812 mi.2), not including adjacent i ...

and

Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...

). The second group of younger Early Paleoproterozoic igneous intrusive rocks is quite different in style, age, and significance. These igneous intrusive rocks consist of granitic and pegmatitic dike swarms, i.e. the Cottonwood, Cremation, Sapphire, and Garnet pegmatite complexes, that cut the Granite Gorge Metamorphic Suite from east to west. They formed as granite

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

, and related pegmatite fluids, filled crack-systems as magma migrated through the crust. The chemical composition of the granite and pegmatite comprising these dike swarms is indicative of the partial melting of the metasedimentary and metavolcanic rocks of the Granite Gorge Metamorphic Suite both in-place and at greater depth, in the crust. These dikes exhibit a wide variability in the degree that they have been deformed from straight and nearly undeformed – to varying degrees of folding, stretching, and shearing. The variable degree of the deformation of these structures is interpreted to indicate that these dike swarms were emplaced during a period of significant mountain building and crustal thickening that was possibly associated with continental collision.

Ultramafic rocks

Also present within the Vishnu Basement Rocks, are thin, discontinuous, and unnamed lenses of

ultramafic rock Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...

s. They are found in several places within the Inner Gorge, such as at River Miles 81, 83, and 91; Salt Creek; Granite Park; and Diamond Creek. These ultramafic rocks occur typically as tectonic fault-bounded slivers, which are often associated with tectonic shear zones and exhibit coarse-grained relict cumulate textures. These rocks are interpreted to be the tectonically dismembered parts of the bases of large 1.74 and 1.71 billion years ago plutons that have intruded the Granite Gorge Metamorphic Suite. This interpretation is based upon the abundance of phlogopite and geochemistry of light rare-earth elements that imply a geochemical contribution from subducting slab material. The composition of these ultramafic rocks is consistent with their origin by simple fractional crystallization within a pluton.Seaman, SJ, KE Karlstrom, ML Williams, and AJ Petruski (1997) ''Proterozoic Ultramafic Bodies in the Grand Canyon.'' Geological Society of America, Abstracts with Programs. 29(6):A-89.

Upper contact

The upper contact of the Vishnu Basement Rocks is a major unconformity between it and either the Tonto Group or Unkar Group that resulted from uplift and the deep erosion, by at least , of the Vishnu Basement Rocks and any overlying strata. In the case of the unconformity between the Vishnu Basement Rocks and the Unkar Group, studies of the underlying Vishnu Basement Rocks indicate they were uplifted from a depth of about to a depth of about , between 1.75 and 1.66 billion years ago, and from a depth of about to the weathered surface on which the Bass Formation of the Unkar Group accumulated – between 1.66 and 1.25 billion years ago.Timmons, JM, KE Karlstrom, MT Heizler, SA Bowring, GE Gehrels, and LJ Crossey (2005) ''Tectonic inferences from the ca. 1254–1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis''. Geological Society of America Bulletin. 117(11/12):1573–95.Timmons, JM, J. Bloch, K. Fletcher, KE Karlstrom, M Heizler, and LJ Crossey (2012) ''The Grand Canyon Unkar Group: Mesoproterozoic basin formation in the continental interior during supercontinent assembly.'' In JM Timmons and KE Karlstrom, eds., pp. 25–47, Grand Canyon geology: Two billion years of earth's history. Special Paper no 294, Geological Society of America, Boulder, Colorado.

References

{{reflist

External links

* Anonymous (nd
''Vishnu Basement Rocks.''
U. S. Geological Survey, Reston, Virginia. * Babcock, R.S., E.H. Brown, M.D. Clark and D.E. Livingston (1979). ''Geology of the Older Precambrian Rocks of the Grand Canyon:'' PART I. Petrology and Structure of the Vishnu Complex. PART II. The Zoroaster Plutonic Complex and Related Rocks. PART III. Petrology of Mafic Schists and Amphibolites. Precambrian Research, 8, pp. 219–302. * Babcock, R.S. (1990). ''The Precambrian Crystalline Core'' ''(of the Grand Canyon)''. In S.S. Beus and M. Morales, eds., Grand Canyon Geology (1st ed.), pp. 11–28. Oxford University Press/Museum of Northern Arizona. * Billingsley, G.H., W.J. Breed, M.D. Clark, P.W. Huntoon and others (1976). ''Geologic Map of the Grand Canyon National Park, Arizona''. Museum of Northern Arizona and Grand Canyon Natural History Association. * Billingsley, GH (2000
''Geologic Map of the Grand Canyon 30' by 60' Quadrangle, Coconino and Mohave Counties, Northwestern Arizona.''
Geologic Investigation Series no I-2688, U.S. Geological Survey, Reston, Virginia. * Mathis, A, and C Bowman (2007
''The Grand Age of Rocks: The Numeric Ages for Rocks Exposed within Grand Canyon''Grand Canyon National Park, Arizona
National Park Service, Grand Canyon National Park, Arizona. * Noble, FL (1914

ttp://ngmdb.usgs.gov/ngmdb/ngmdb_home.html National Geologic Map Database U.S. Geological Survey, Reston, Virginia. * Share, J (2102a
''The Great Unconformity of the Grand Canyon and the Late Proterozoic-Cambrian Time Interval: Part I – Defining It.''
last accessed September 22, 2013. * Share, J (2102a

last accessed September 22, 2013. * Timmons, JM, MK Karlstrom, and C Dehler (1999) ttp://www.gcrg.org/bqr/12-1/supergroup.html ''Grand Canyon Supergroup Six Unconformities Make One Great Unconformity A Record of Supercontinent Assembly and Disassembly''br>Boatman's Quarterly Review. vol. 12, no. 1
pp. 29–32. * Timmons, S.S (2003

National Park Service, Grand Canyon National Park, Arizona. Natural history of the Grand Canyon Geologic formations of Arizona Geologic formations of Nevada Precambrian Arizona Paleoproterozoic geology Metamorphic complexes