1995/12/01 by James S. Scoates, Kevin R. Chamberlain · 1 citation
Earth and Planetary Sciences · Computer Science · Chemistry · #Geological and Geochemical Analysis #Geochemistry and Geologic Mapping #Radioactive element chemistry and processing
paper · doi:10.2138/am-1995-11-1222
Other| December 01, 1995 Baddeleyite (ZrO2) and zircon (ZrSiO4) from anorthositic rocks of the Laramie anorthosite complex, Wyoming: Petrologic consequences and U-Pb ages James S. Scoates; James S. Scoates University of Wyoming, Department of Geology and Geophysics, Laramie, WY, United States Search for other works by this author on: GSW Google Scholar Kevin R. Chamberlain Kevin R. Chamberlain Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (11-12): 1317–1327. https://doi.org/10.2138/am-1995-11-1222 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation James S. Scoates, Kevin R. Chamberlain; Baddeleyite (ZrO2) and zircon (ZrSiO4) from anorthositic rocks of the Laramie anorthosite complex, Wyoming: Petrologic consequences and U-Pb ages. American Mineralogist 1995;; 80 (11-12): 1317–1327. doi: https://doi.org/10.2138/am-1995-11-1222 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 SocietyAmerican Mineralogist Search Advanced Search Abstract The Zr-bearing minerals baddeleyite (ZrO2) and zircon (ZrSiO4) occur within plagio-clase-rich (61–95% plagioclase) cumulates of the Laramie anorthosite complex (LAC), southeastern Wyoming. In each of the examined samples, zircon is present as relatively coarse (1–2 mm) interstitial grains, and baddeleyite occurs as small (0.05 mm) inclusions within cumulus plagioclase. Zircon crystallized between cumulus plagioclase crystals near solidus temperatures from highly fractionated, Zr-saturated liquids. The resultant shape of zircon was controlled by the form of the remaining pore space. The origin of baddeleyite in the anorthositic rocks of the LAC is less well constrained. It may have crystallized early from the anorthositic parental magmas at relatively low silica activities; however, this would require baddeleyite saturation at extremely low Zr concentrations in the parental magmas (≪s 100 ppm).Baddeleyite and zircon U-Pb ages reveal that several petrologically distinct intrusions were emplaced and crystallized in the LAC over a relatively restricted 1–3 m.y. interval at ca. 1434 Ma. The 207Pb/206Pb ages obtained for the baddeleyite and zircon in each sample are identical within error (±1–3 m.y.), and U concentrations are uniformly low (<240 ppm), supporting a genetically related origin for the minerals. Two anorthositic layered cumulates and a crosscutting, oxide-rich troctolite from the Poe Mountain anorthosite have crystallization ages that are identical within error: 1434.4 ± 0.6, 1434.5 ± 0.6, and 1434.1 ± 0.7 Ma, respectively. The only earlier period of anorthositic magmatism that can be identified from the Poe Mountain anorthosite is represented by a leucogabbroic xenolith (1436.2 ± 0.6 Ma), which settled onto the floor of the magma chamber that produced the layered cumulates. A sample composed almost entirely of zoned, iridescent, plagioclase megacrysts from the Chugwater anorthosite yields a baddeleyite age of 1435.4 ± 0.5 Ma, intermediate between the ages of the xenolith and the layered anorthositic rocks.All the ca. 1.4–1.5 Ga anorthosites in North America, including the LAC, are located near or on Paleoproterozoic boundaries between Archean cratons and accreted Proterozoic island arc terranes. These preexisting crustal structures appear to play a major role in the origin and ascent of anorthositic magmas. The evidence for anorthositic magmatism at 1.43 Ga in the LAC suggests that there may be a strong genetic link between these high-temperature mafic magmas and the regional production of anorogenic granites in the western and southwestern U.S., many of which have similar crystallization ages in the interval of 1.43–1.44 Ga. 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.