GEOLOGY OF THE OLDER PRECAMBRIAN ROCKS OF THE GRAND CANYON .3. PETROLOGY OF MAFIC SCHISTS AND AMPHIBOLITES

The amphibolites and mafic schists in the Precambrian metamorphic rocks of the Grand Canyon are divided into five major groups on the basis of field occurrence and mineral composition: (1) anthophyllite- and cordierite-anthophyllite-bearing rocks, (2) early (plagioclase-hornblende) amphibolites, (3) Granite Park mafic complex, (4) hornblende-bearing dikes, (5) tremolite-bearing dikes. Cordierite-anthophyllite rocks and associated schists represent sedimentary types which attained their bulk composition prior to metamorphism. They were derived from basic igneous materials which had suffered intense alteration and loss of lime and alkalis, probably by reaction with sea water. The early amphibolites were originally a series of lavas and tuffs of approximately basaltic composition. These meta-volcanic rocks have a tholeiitic character, and trace element abundances suggest that they may have formed in an island arc environment. Field and chemical data for the Granite Park mafic complex strongly indicate that it originated as a layered series of plagioclase-rich cumulates. Rocks of the complex show calc-alkaline chemical trends but with some tholeiitic affinities. Hornblende-bearing dikes range from ultramafic types composed only of hornblende plus opaques, to more normal plagioclase-hornblende amphibolites. They are strongly tholeiitic and may represent either ocean floor or island arc intrusives according to trace element data. Tremolite-bearing dikes are highly magnesian in composition, plot as early differentiates, and belong to either the calc-alkali or the tholeiitic series. © 1979.