Tectonic evolution of Paleoproterozoic rocks in the Grand Canyon: Insights into middle-crustal processes

Abstract

Paleoproterozoic supracrustal rocks of the Upper Granite Gorge are divided into three mappable units: the Rama, Brahma, and Vishnu Schists, here collectively named the “Granite Gorge Metamorphic Suite.” The Brahma Schist consists of mafic to intermediate-composition metavolcanic rocks that have yielded an age of 1750 Ma. The Rama Schist consists of felsic metavolcanic rocks that have yielded an age of 1742 Ma. On the basis of the presence of relict pillow structures, interlayering of metavolcanic units, and the large volumes of metavolcanic rocks, the combined volcanic package is interpreted to be metamorphosed, island-arc-related, submarine volcanic rocks. The metavolcanic units are locally overlain by the metamorphosed, arc-basin, submarine sedimentary rocks of the Vishnu Schist. The term “Zoroaster Plutonic Complex,” previously used for all Paleoproterozoic and Mesoproterozoic plutonic rocks in the Grand Canyon, is abandoned in favor of use of names for individual plutons. Plutons are classified into two genetically and temporally separate intrusive suites: (1) 1740–1713 Ma plutons that are typically concordant to foliation, range in composition from gabbro to granodiorite, have calc-alkalic chemistry, and are interpreted to be arc-related plutons and (2) 1698–1662 Ma granite plutons, stocks, and pegmatite and aplite dikes that have textures and intrusive geometries indicating they were emplaced synchronously with peak metamorphism and northwest-southeast shortening. The dominant tectonic fabric is a northeast-striking, subvertical, variably developed foliation (S 2 ) that is axial planar to plunging, open to isoclinal, upright, asymmetric folds (F 2 ) of an early, penetrative, apparently initially northwest-striking foliation (S 1 ). F 2 folds have wavelengths of kilometres and cause repetition of rock units across the transect. Partitioning of deformation in space and perhaps in time was strongly influenced by crustal heterogeneity. For example, plutons, pegmatite networks, and fold-hinge zones acted as buttresses, and S 2 high-strain domains and shear zones are localized at the margins of these features. Generally northeast-striking, S 2 -related shear zones segment the transect into blocks. Shear zones last moved late during shortening and postdate the metamorphic peak. One of these, the Crystal shear zone, may be a fundamental crustal province boundary. Paleoproterozoic rocks in the Grand Canyon record the accretion of 1750–1713 Ma island-arc rocks and their assembly to Laurentia via dynamic interaction of deformation, metamorphism, and magmatism over ≈20 m.y. Changes in styles of deformation from F 1 -related thrusting and penetrative S 1 fabric development to F 2 -related upright folding and variable S 2 fabric development coincided with changes in styles of magmatism. The 1740–1730 Ma arc-related mafic to intermediate-composition plutons preserve strong S 1 foliation. The 1717–1713 Ma arc-related plutons preserve weak S 1 tectonic layering and locally strong S 2 strain. The 1698–1685 Ma granitic plutonism locally preceded S 2 and F 2 strain and was regionally synchronous with peak S 2 and F 2 strain (1700–1685 Ma) and peak metamorphism (1706–1697 Ma). More limited deformation and plutonism continued until at least 1662 Ma.