The geology and petrochemistry of the precambrian metamorphic rocks of the Grand Canyon, Arizona.

The amphibolites and mafic schists which occur in the Older Precambrian 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 body (4) hornblende-bearing dykes (5) tremolite-bearing- dykes. Cordierite-anthophyllite rocks and associated mafic schists represent sedimentary types which attained their bulk composition prior to metamorphism. They were derived in varying degrees from basic igneous materials which had suffered intense alteration and loss of lime and alkalis, probably by reactions 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 considerations indicate that they may have formed in an island arc environment. Field and chemical data for the Granite Park mafic body strongly suggest that it originated as a layered series of plagioclase-rich cumulates. The body shows calc-alkaline chemical trends but with some affinity to tholeiitic rocks. Hornblende-bearing dykes range from ultramafic types composed only of hornblende plus opaques, to more normal plagioclase- hornblende amphibolites; the original composition of the majority of the dykes was probably in the basalt to basaltic-andesite range. They are strongly tholeiitic and may represent either ocean floor or island arc intrusives according to trace eluent data. Tremolite-bearing dykes are highly magnesian in composition, plot as early differentiates, and belong to either the calc-alkali or the tholeiitic series.