MAFIC METAVOLCANIC ROCKS FROM THE PRECAMBRIAN OF THE GRAND CANYON: EVIDENCE FOR MULTIPLE EVENTS OF MAGMATISM

Paleoproterozoic rocks exposed in the Upper Granite Gorge, Grand Canyon are composed of three interlayered rock units: the 1740 ± 2 Ma Rama Schist, the 1750 ± 1 Ma Brahma Schist, and the Vishnu Schist. These units comprise felsic metavolcanic rocks, mafic metavolcanic rocks, and metaturbidites, respectively. The tectonic history of the Upper Granite Gorge is marked by multistage deformation and metamorphism associated with the 1.72-1.68 Ga Yavapai Orogeny. The Yavapai terrane of the Upper Granite Gorge is separated from the Mojave terrane by the Crystal Shear Zone. The mafic metavolcanic rocks of the Grand Canyon are interpreted to have formed in a marginal marine arc environment or as slices of submarine basalts imbricated during the Yavapai orogeny. Defining the tectonic environment of the protoliths has significant implications for the Paleoproterozoic development of Laurentia and correlations with other continental crust. Mafic samples were collected during river trips through the Grand Canyon to investigate their setting. Based on trace-element geochemistry, three types of metamafic rocks were identified. The rocks from Group 1 are amphibolites of mafic composition and tholeiitic affinity. They exhibit a flat pattern in chondrite-normalized REE plots and a near-flat pattern with Nb-Ta negative anomalies in primitive mantle-normalized spidergrams, a signature typical of back-arc basin basalts. Group 2 comprises quartz-rich amphibolites of mafic to intermediate composition and transitional character. These rocks are characterized by moderate LREE enrichment and negative Eu anomalies in REE plots, as well as LILE enrichment and negative anomalies of Nb-Ta, Ti, and P in spidergrams. This geochemical signature, together with a zircon U-Pb age of 1842 ± 3.72 Ma, suggests that Group 2 rocks are associated with the Elves Chasm gneiss. The rocks from Group 3 are foliated amphibolites and schists of mafic to intermediate composition and calc-alkaline affinity. They exhibit strong LREE and LILE enrichment and negative anomalies of Nb-Ta, Ti, and P, a signature typical of continental arc basalts. Thus, data of the mafic metavolcanic rocks show affinities with more than one tectonic setting in both the Yavapai and Mojave Provinces. Ongoing attempts to date the mafic metavolcanic rocks may reveal multiple generations of mafic crust within the Vishnu Basement rocks.