GEOLOGY OF THE OLDER PRECAMBRIAN ROCKS OF THE GRAND CANYON .1. PETROLOGY AND STRUCTURE OF THE VISHNU COMPLEX

Rocks comprising the older Precambrian of the Grand Canyon are subdivided into the Vishnu Complex, the Trinity and Elves Chasm orthogneisses and the Zoroaster Plutonic Complex. Lithologies of the Vishnu Complex include metamorphosed sandstones, shales, impure carbonate sediments and basic volcanic rocks. The metasandstones and shales greatly predominate. Two major metamorphic/deformational events and one minor event are recorded in the structures of the Vishnu Complex. A penetrative schistosity and associated upright macroscopic folds mark the first deformation (D1). This was accompanied by lower amphibolite facies metamorphism. The second metamorphic episode (D2) to a large extent overprinted and obliterated earlier features, and produced most of the minerals and structures seen today in the Vishnu Complex. S2 foliation ranges from strain-slip cleavage to penetrative schistosity; L2 lineation is marked by crenulations and mineral alignment; F2 folds are open to isoclinal and steeply plunging. Mineral assemblages of the second metamorphism are of mid- to upper-amphibolite facies and indicate maximum temperatures of approximately 650°C at pressures of 3-4 Kb. The age of this event is in the range of 1650-1700 M.a. A third, relatively weak, metamorphic deformational event (D3) produced local shear zones and in these areas caused retrogressive metamorphism of high-grade rocks to the greenschist facies. © 1979.