Vishnu Basement Rocks

The Vishnu Basement Rocks is the name recommended for all Early Proterozoic crystalline rocks (metamorphic and igneous) exposed in the Grand Canyon region. They form the crystalline basement rocks that underlie the Bass Limestone of the Unkar Group of the Grand Canyon Supergroup and the Tapeats Sandstone of the Tonto Group. These basement rocks have also been called the Vishnu Complex and 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, granitic pegmatite, aplite, and granodiorite that have intruded these metamorphic rocks, and also intrusive Early Paleoproterozoic ultramafic rocks. The term Zoroaster Plutonic Complex is used for all Paleoproterozoic granitic and grandioritic plutonic rocks in the Grand Canyon. Specific names have been assigned to individual plutons and dike swarms because the plutons and swarms differ greatly in their age, origin, and tectonic significance. 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 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 their metamorphosis. It was named after a natural rock structure in the Colorado River valley which was named "Temple of Vishnu" from its appearance.

Granite Gorge Metamorphic Suite The Granite Gorge Metamorphic Suite consists of lithologic units, the Brahma, Rama, and Vishnu schists, that have been mapped within the Upper, Middle, and Lower Granite Gorges of the Grand Canyon. The Vishnu Schist consists of quartz-mica schist, pelitic schist, and meta-arenites. They exhibit relict sedimentary structures and textures that demonstrate that they are metamorphosed submarine sedimentary rocks. The Brahma Schist consists of amphibolite, hornblende-biotite-plagioclase schist, biotite-plagioclase schist, orthoamphibole-bearing schist and gneiss, and metamorphosed sulfide deposits. As inferred from relict structures and textures, the Brahma Schist is composed of mafic to felsic-composition metavolcanic rocks. The Rama Schist consists of massive, fine-grained quartzofeldspathic schist and gneiss that 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 metasedimentary rocks were originally composed of particles of quartz, clay, and volcanic rock fragments that have become metamorphosed into various schists. The Vishnu Schist exhibits relict graded bedding 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). 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. 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 rock debris eroded from older plutonic rocks.

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