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Precambrian Evolution of the Spanish Peaks Area, Montana

1975; Geological Society of America; Volume: 86; Issue: 6 Linguagem: Inglês

10.1130/0016-7606(1975)86 2.0.co;2

1943-2674

Edgar Winston Spencer, S. J. Kozak,

Paleontology and Stratigraphy of Fossils

Research Article| June 01, 1975 Precambrian Evolution of the Spanish Peaks Area, Montana EDGAR W. SPENCER; EDGAR W. SPENCER 1Department of Geology, Washington and Lee University, Lexington, Virginia 24450 Search for other works by this author on: GSW Google Scholar SAMUEL J. KOZAK SAMUEL J. KOZAK 1Department of Geology, Washington and Lee University, Lexington, Virginia 24450 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1975) 86 (6): 785–792. https://doi.org/10.1130/0016-7606(1975)86 2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation EDGAR W. SPENCER, SAMUEL J. KOZAK; Precambrian Evolution of the Spanish Peaks Area, Montana. GSA Bulletin 1975;; 86 (6): 785–792. doi: https://doi.org/10.1130/0016-7606(1975)86 2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Precambrian crystalline rocks in the Spanish Peaks area of southwestern Montana are composed primarily of granitic and amphibolitic gneiss, quartzite, and amphibolite. Sillimanite gneiss, ultrabasic intrusions, marble, and post-Precambrian basalt are also present. Almandine-amphibolite metamorphic facies and sillimanite-almandine-orthoclase subfacies are widely distributed in the area.Two major Precambrian orogenic events followed accumulation of a thick sedimentary sequence. An early Precambrian orogenic deformation produced isoclinal folds, folded pegmatites, and amphibolite boudins. These structural features were deformed in a second orogeny, which was characterized by large, open northeast-trending folds that developed under metamorphic conditions favorable for ductile deformation and a second generation of pegmatites. Cross folding and flexure of the second generation folds and development of kinks accompanied or followed the second orogeny. Mafic dikes and sills were emplaced following the second orogenic event and subsequently metamorphosed. They cut across the second-generation folds. Young, probably post-Precambrian quartz veins, plus felsic porphyry dikes, and basalt dikes cut all earlier structures.Alternate correlation of thermal events whose ages have been determined radiometrically at 2.6, 1.9, and 1.6 b.y. with metamorphic and structural events are made as follows:Amphibolitization of unfolded dikes and sills: I, 1.9 b.y; II, 1.6 b.y.Open, subhorizontal folds: I, 2.6 b.y.; II, 1.9 b.y.Isoclinal folds: I, age uncertain; II, 2.6 b.y. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.