Distribution and Age of High-Grade Blueschists, Associated Eclogites, and Amphibolites from Oregon and California

Abstract

Research Article| September 01, 1971 Distribution and Age of High-Grade Blueschists, Associated Eclogites, and Amphibolites from Oregon and California R.G COLEMAN; R.G COLEMAN U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar M.A LANPHERE M.A LANPHERE U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1971) 82 (9): 2397–2412. https://doi.org/10.1130/0016-7606(1971)82[2397:DAAOHB]2.0.CO;2 Article history received: 01 Feb 1971 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R.G COLEMAN, M.A LANPHERE; Distribution and Age of High-Grade Blueschists, Associated Eclogites, and Amphibolites from Oregon and California. GSA Bulletin 1971;; 82 (9): 2397–2412. doi: https://doi.org/10.1130/0016-7606(1971)82[2397:DAAOHB]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 Isolated blocks of high-grade blueschist and amphibolite facies metamorphic rocks occur within the Jurassic and Cretaceous eugeosynclinal deposits of the Coast Ranges of southwestern Oregon and California. The blocks range in size from individual rock masses commonly 5 to 1,000 ft in diameter to a few larger masses as much as 7 mi long and 2 mi wide. The high-grade blocks are predominantly basaltic in composition and include glaucophane schists, eclogites, and gneissic rocks of the amphibolite facies. Field relationships indicate that the blocks are closely associated with serpentine, that high-grade blueschist and amphibolite blocks, lower grade blueschists, volcanic rocks, and cherts occupy disturbed zones that may be related to thrusting, and that there is no exposed in situ provenance for the high-grade blueschists, eclogites, and amphibolites. Potassium-argon mineral ages of white mica and actinolite from the blueschists and of hornblende from the amphibolites indicate that these minerals crystallized approximately 150 m.y. ago, but the ages measured on glaucophane from the blueschist blocks are commonly younger. These data suggest that the high-grade blue-schist and amphibolite blocks represent fragments of a cryptic metamorphic terrane of pre-Tithonian age that have been tectonically mixed with younger rocks of the Franciscan Formation in California and Otter Point Formation in Oregon. The younger ages for glaucophane probably reflect metamorphic episodes in which lower grade in situ blueschist facies mineral assemblages were developed in the blocks after their emplacement within the Franciscan Formation. This pre-Tithonian cryptic metamorphic terrane probably developed as a result of interaction between oceanic and continental plates. The occurrence of tectonic blocks of this terrane within mélange zones in Oregon and California may be related to later plate interaction. 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.