Vishnu Basement Rocks

Vishnu Basement Rocks Stratigraphic range: Early Paleoproterozoic
Type	Geological formation
Sub-units	Elves Chasm pluton, Granite Gorge Metamorphic Suite, and younger intrusive plutons, and dike swarms.
Underlies	Unkar Group and, as part of the Great Unconformity, the Tapeats Sandstone
Thickness	unknown
Lithology
Primary	schist and granite
Other	granodiorite, tonalite, pegmatite, and ultramafic rocks
Location
Country	United States of America
Type section
Named for	"Vishnu's Temple" butte
Named by	Charles Doolittle Walcott
Year defined	1894

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 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, granitic pegmatite, aplite, and granodiorite that have intruded these metamorphic rocks, and also, intrusive Early Paleoproterozoic ultramafic rocks.

In the past, the term Zoroaster Plutonic Complex, which has been used for all Paleoproterozoic granitic and grandioritic plutonic rocks in the Grand Canyon, has been abandoned. This term has been abandoned and 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, Elves Chasm Gneiss, likely represent 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 they were metamorphosed.

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