Vishnu Basement Rocks

The oldest of these plutonic complexes, the Elves Chasm Gneiss, likely represents a small fragment of basement upon which the metavolcanic rocks that comprise the Granite Gorge Metamorphic Suite accumulated.The Vishnu Schist exhibits relict graded bedding and structures indicative of turbidite deposits that accumulated in oceanic trenches and other relatively deep-marine settings.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.Comparable volcanic arc systems, which are associated with subduction zones, are active today in the Aleutian Islands and Indonesia (e.g., Sumatra and Java).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.These rocks are interpreted to be the tectonically dismembered parts of the bases of large 1.74-to-1.71-billion-years-old 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.
Gray and reddish rock face with rough surface adjacent to a river.
The Vishnu Basement Rocks were deposited as mafic and felsic volcanic rocks and sediments but were later metamorphosed and intruded by igneous rock.
Figure 1. A geologic cross section of the Grand Canyon. [ 7 ]
Stratigraphic rangeProterozoicArcheanGeological formationUnkar GroupGreat UnconformityTapeats SandstoneArizonaGrand CanyonCharles Doolittle Walcottcrystalline rocksmetamorphicigneousbasement rocksBass LimestoneGrand Canyon SupergroupTonto Groupgranitepegmatiteaplitegranodioriteultramafic rocksplutonicplutonsvolcanicTemple of Vishnuschistsquartzpeliticarenitessedimentary structuressubmarinesedimentary rocksamphibolitehornblendebiotiteplagioclaseamphibolegneissfelsicpillow structuresvolcanic island-arcoceanic trenchesgraded beddingturbiditecross sectiontonalitedioritepaleosolregolithweatherederodedcountry rockvolcanic arcsubductionAleutian IslandsIndonesiaSumatracumulate texturesphlogopiterare-earth elementsGeology of the Grand Canyon areaMuseum of Northern Arizona