Eclogites and Eclogites: Their Differences and Similarities

Research Article| May 01, 1965 Eclogites and Eclogites: Their Differences and Similarities R. G COLEMAN; R. G COLEMAN U. S. Geological Survey, Menlo Park, Calif. Search for other works by this author on: GSW Google Scholar D. E LEE; D. E LEE U. S. Geological Survey, Menlo Park, Calif. Search for other works by this author on: GSW Google Scholar L. B BEATTY; L. B BEATTY U. S. Geological Survey, Menlo Park, Calif. Search for other works by this author on: GSW Google Scholar W. W BRANNOCK W. W BRANNOCK U. S. Geological Survey, Menlo Park, Calif. Search for other works by this author on: GSW Google Scholar GSA Bulletin (1965) 76 (5): 483–508. https://doi.org/10.1130/0016-7606(1965)76483:EAETDA2.0.CO;2 Article history received: 18 May 1964 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation R. G COLEMAN, D. E LEE, L. B BEATTY, W. W BRANNOCK; Eclogites and Eclogites: Their Differences and Similarities. GSA Bulletin 1965;; 76 (5): 483–508. doi: https://doi.org/10.1130/0016-7606(1965)76483:EAETDA2.0.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 Eclogites are divisible into three groups based on mode of occurrence: Group A, inclusions in kimberlites, basalts, or layers in ultramafic rocks; Group B, bands or lenses within migmatite gneissic terrains; Group C, bands or lenses within alpine-type metamorphic rocks. The compositions range from olivine basalt for Group A to tholeiitic basalts for Group C. New analytical data on six eclogites from glaucophane schist terrains in California and New Caledonia now permit comparisons among the three eclogite types. The pyrope content of the garnets is distinctive for each group as follows: Group A, greater than 55 per cent py; Group B, 30–55 per cent py; Group C, less than 30 percent py. Pyroxenes coexisting with these garnets also reflect a compositional change related to their occurrence. The jadeite content progressively increases from Group A through Group B, whereas the diopside content decreases. A comparison of eclogites from different geologic occurrences but with similar bulk compositions demonstrates variation in Ca-Mg partition between coexisting garnet and pyroxene. The Ca/Mg ratio increases in garnet and decreases in pyroxene from Group A through Group B eclogites. This obvious difference in the Ca-Mg partition between coexisting garnet-pyroxene in eclogites of the same bulk composition indicates a broad range of pressure-temperature conditions obtained during crystallization. Experimental synthesis of eclogite-like material at high pressures and temperatures demonstrates that some eclogites may form in the earth's mantle, but naturally occurring Group C eclogites have coexisting garnet-pyroxene with distinct Ca/Mg ratios when compared to Group A or B eclogites of similar bulk composition. This difference in the Ca/Mg ratio must reflect the pressure-temperature conditions characterizing the glaucophane schist facies.The formation of eclogites within different metamorphic facies is strong evidence of the divergent pressure-temperature conditions that allow basalts to recrystalhze into garnet-pryoxene rocks. In view of the rather compelling field evidence, it would seem advisable to discontinue the concept of an eclogite metamorphic facies. 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.