TECTONIC EVOLUTION OF PALEOPROTEROZOIC ROCKS IN THE GRAND-CANYON - INSIGHTS INTO MIDDLE-CRUSTAL PROCESSES

Citation
Br. Ilg et al., TECTONIC EVOLUTION OF PALEOPROTEROZOIC ROCKS IN THE GRAND-CANYON - INSIGHTS INTO MIDDLE-CRUSTAL PROCESSES, Geological Society of America bulletin, 108(9), 1996, pp. 1149-1166
Citations number
66
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
0016-7606
Volume
108
Issue
9
Year of publication
1996
Pages
1149 - 1166
Database
ISI
SICI code
0016-7606(1996)108:9<1149:TEOPRI>2.0.ZU;2-V
Abstract
Paleoproterozoic supracrustal rocks of the Upper Granite Gorge are div ided into three mappable units: the Rama, Brahma, and Vishnu Schists, here collectively named the ''Granite Gorge Metamorphic Suite.'' The B rahma Schist consists of mafic to intermediate-composition metavolcani c 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 t he 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-a re-related, submarine volcanic rocks. The metavolcanic units are local ly overlain by the metamorphosed, are basin, submarine sedimentary roc ks of the Vishnu Schist. The term ''Zoroaster Plutonic Complex,'' prev iously used for all Paleoproterozoic and Mesoproterozoic plutonic rock s in the Grand Canyon, is abandoned in favor of use of names for indiv idual plutons. Plutons are classified into two genetically and tempora lly separate intrusive suites: (1) 1740-1713 Ma plutons that are typic ally concordant to foliation, range in composition from gabbro to gran odiorite, have calc-alkalic chemistry, and are interpreted to be are-r elated plutons and (2) 1698-1662 Ma granite plutons, stocks, and pegma tite and aplite dikes that have textures and intrusive geometries indi cating they were emplaced synchronously with peak metamorphism and nor thwest-southeast shortening. The dominant tectonic fabric Is a northea st-striking, subvertical, variably developed foliation (S-2) that is a xial 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 r epetition of rock units across the transect. Partitioning of deformati on in space and perhaps in time was strongly influenced by crustal het erogeneity. For example, plutons, pegmatite networks, and fold-hinge z ones acted as buttresses, and S-2 high-strain domains and shear zones are localized at the margins of these features. Generally northeast-st riking, S-2-related shear zones segment the transect into blocks. Shea r zones last moved late during shortening and postdate the metamorphic peak. One of these, the Crystal shear zone, may be a fundamental crus tal province boundary. Paleoproterozoic rocks in the Grand Canyon reco rd the accretion of 1750-1713 Ma island-are rocks and their assembly t o Laurentia via dynamic interaction of deformation, metamorphism, and magmatism over approximate to 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 coincid ed with changes in styles of magmatism. The 1740-1730 Ma arc-related m afic to intermediate-composition plutons preserve strong S-1 foliation . The 1717-1713 Ma are-related plutons preserve weak S-1 tectonic laye ring and locally strong S-2 strain. The 1698-1685 Ma granitic plutonis m locally preceded S-2 and F-2 strain and was regionally synchronous w ith peak S-2 and F-2 strain (1700-1685 Ma) and peak metamorphism (1706 -1697 Ma). More limited deformation and plutonism continued until at l east 1662 Ma.