GEOLOGY OF THE OLDER PRECAMBRIAN ROCKS OF THE GRAND CANYON .2. ZOROASTER PLUTONIC COMPLEX AND RELATED ROCKS

The older Precambrian rocks of the Grand Canyon consist in large part of granitic rocks, including foliated and unfoliated plutons, numerous dikes and sills, and some distinctive bodies of well-foliated gneiss. The plutons plus the dikes and sills, which are of obvious intrusive origin, comprise the Zoroaster Plutonic Complex. In the Upper Granite Gorge eleven plutons have been recognized, with seven more located in the Lower Granite Gorge. The plutons range in composition from alkali-granite to tonalite and from unfoliated to strongly gneissose. The dikes and sills include fine- to medium-grained granites as well as granitic pegmatites and aplites. The dikes and sills are undeformed except for a few quartz-rich types which have undergone metamorphic recrystallization. Four bodies of granitic gneiss have been distinguished from the Zoroaster Complex on the basis of differences in field relationships and composition. The Trinity Gneiss Complex is a granodioritic to tonalitic orthogneiss that may be at least partly supracrustal. The Elves Chasm Gneiss Complex shows evidence for an origin as a tonalitic pluton which was emplaced prior to the main stage of Vishnu metamorphism. The origin of the 229 mile gneiss is uncertain, but features suggestive of extensive metasomatic replacement are present. The 245 mile gneiss occurs only as blocks engulfed by the later Surprise pluton, but its contact relationships indicate an intrusive origin. On the basis of chemical and isotopic data, the granitic rocks of the Grand Canyon can be subdivided into four groups: 1. (1) Older plutons and orthogneisses which apparently were emplaced prior to the main stage of Vishnu metamorphism. 2. (2) Younger plutons which apparently were intruded during or after the main stage of Vishnu metamorphism. 3. (3) Granitic and pegmatite/aplite dikes and sills which are mostly younger than the main stage of Vishnu metamorphism. 4. (4) Anomalous plutons and orthogneisses which are characterized by unusual chemical or mineralogical composition. The younger plutons are granitic to granodioritic while the older plutons are granodioritic to tonalitic. The younger plutons also are generally lower in Na2O/K2O ratios, higher in Ba/Sr ratios, and higher in K2O, Rb, Zr, La and total alkali content. The general difference in composition between the older and younger plutons probably reflects changes in the source material and/or the mechanism of melting. One possibility is a shift from mantle to crustal derivation of magmas, associated with a fundamental change in the tectonic environment of the region at a time corresponding roughly to the main stage of Vishny metamorphism. © 1979.