Feldsparphyric basalts commonly occur as a distinctive 'marker horizon' 300 to 500 m below the felsic volcanic units of many Archean greenstone belts. Within the Knee Lake greenstone belt, northern Manitoba, glomeroporphyritic basalts, characterized by large irregularly-shaped aggregates of plagioclase phenocrysts, occupy this stratigraphic position within the two lower-most volcanic cycles. A major chemical and/or tectonic break does not occur within the volcanic cycles, but rather, the chemical affinities of the Knee Lake volcanic rocks changes gradually (at a silica content of approximately 55%) from tholeiitic to calc-alkaline with increasing stratigraphic height. The accumulation of plagioclase phenocrysts suggests extensive crystal fractionation of the Knee Lake magmas. The gradual transition in affinity of the Knee Lake lavas from tholeiitic to calc-alkaline can be explained by near-surface fractionation under constant fO 2 conditions.
Research Article| October 01, 1981 Geology and petrology of Quaternary volcanic rocks, Garibaldi Lake area, southwestern British Columbia: Summary NATHAN L. GREEN NATHAN L. GREEN 1Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information NATHAN L. GREEN 1Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1981) 92 (10): 697–702. https://doi.org/10.1130/0016-7606(1981)92<697:GAPOQV>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 Email Permissions Search Site Citation NATHAN L. GREEN; Geology and petrology of Quaternary volcanic rocks, Garibaldi Lake area, southwestern British Columbia: Summary. GSA Bulletin 1981;; 92 (10): 697–702. doi: https://doi.org/10.1130/0016-7606(1981)92<697:GAPOQV>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 No Abstract Available. 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.
At least twelve Pleistocene-Holocene calc-alkaline eruptive complexes were formed in the Mount Garibaldi and Garibaldi Lake volcanic fields during the intervals 1.1-1.3 Ma, 0.4-0.7 Ma, 0.2-0.3 Ma, and 0.10 Ma to present. Mildly alkalic basalts, which resemble extensional magma types, were erupted only during the last 100,000 years.
Evolution of basaltic andesite and andesite magmas can be explained by polybaric crystal fractionation of more mafic parental magmas. The dacitic and rhyodacitic magmas probably originated by continued fractionation of andesitic liquids accompanied by extensive assimilation of heterogeneous crustal contaminants. Erupted basalts and calc-alkaline rocks, however, cannot be related by fractionation processes.
A suite of hy-normative hawaiites, ne-normative mugearite, and calc-alkaline andesitic rocks from the Garibaldi Lake area exhibits fractionated, slightly concave-upward REE patterns (Ce N /Yb N = 4.5–15), heavy REE contents about 5–10 times the chondritic abundances, and no Eu anomalies. It is unlikely that the REE patterns provide information concerning partial melting conditions beneath southwestern British Columbia because they have probably been modified substantially by upper crustal processes including crustal contamination and (or) crystal fractionation. The REE contents of the Garibaldi Lake lavas are not incompatible with previous interpretations that (1) the hawaiites have undergone considerable fractionation of olivine, plagioclase, and clinopyroxene; and (2) the individual andesitic suites were derived from separate batches of chemically distinct magma that evolved along different high-level crystallization trends. In general, however, the andesites are characterized by lower light REE contents than the basaltic andesites. These differences in LREE abundances may reflect different amounts of LREE-rich accessory phases, such as apatite, sphene, or allanite, assimilated from the underlying quartz diorites.
Lode gold mineralization in the Blue Ridge of the southernmost Appalachians is hosted by metavolcanic rocks (e.g., Anna Howe mine, AL; Royal Vindicator mine, GA), metaplutonic rocks (e.g., Hog Mountain mine, AL), and metasedimentary rocks (e.g., Lowe, Tallapoosa, and Jones Vein mines, AL). Most gold occurs in synkinematic quartz + or - plagioclase + or - pyrite + or - pyrrhotite + or - chlorite veins localized along polydeformational faults that juxtapose rocks with significantly different peak metamorphic mineral assemblages.Mineralogy, chemistry, and O and H isotope studies suggest that the three types of host rocks have undergone differing amounts and types of alteration during mineralization. Limited wall-rock alteration in metavolcanic- and metasediment-hosted deposits, and relatively extensive wall-rock alteration in granitoid-hosted deposits, suggests that most deposits formed from fluids that were close to equilibrium with metavolcanic and metasedimentary rocks. Stable isotope compositions of the fluids calculated from vein minerals and vein selvages are consistent with a predominantly metasedimentary fluid source, but vary from deposit to deposit (-22 to -47ppm delta D, 4-5ppm, delta 18 O, and 5-7ppm delta 34 S at Anna Howe and Royal Vindicator; -48 to -50ppm delta D, 9-13ppm delta 18 O, and ca. 19ppm delta 34 S at Lowe and Jones Vein; and -22 to -23ppm delta D, 8-11ppm delta 18 O, 9-10ppm delta 34 S, and -6 delta 13 C at Hog Mountain). Silicate mineral thermobarometry of vein, vein selvage, and wall-rock mineral assemblages indicate that mineralization and regional metamorphism occured at greenschist to amphibolite facies (480 degrees + or - 75 degrees C at Anna Howe, 535 degrees + or - 50 degrees C at 6.4 + or - 1 kbars at Lowe, 530 degrees + or - 50 degrees C at 6.9 + or - 1 kbars at Tallapoosa, and 460 degrees + or - 50 degrees C at 5.5 + or - 1 kbars at Hog Mountain). Oxygen isotope fractionation between vein minerals and selvage minerals consistently records equilibration temperatures that are similar to or slightly lower than those estimated from silicate thermometry.Auriferous veins contain numerous fluid inclusions that were emplaced in several stages and can be subdivided into five compositional types based on salt and CO 2 concentrations. Fluid inclusion isochores for early formed inclusions from these veins intercept the pressure and temperature conditions estimated from silicate mineral thermobarometry and stable isotope thermometry, and are compatible with entrapment at those conditions. These fluids exhibit significant variation in salinity (X (sub NaCl equiv ) ) = 0.0-0.2) and CO 2 (X (sub CO 2 ) = 0.0-0.2), suggesting variation in fluid-wall-rock interaction that accompanied gold deposition during declining temperatures. Less abundant and later fluids within the veins are dominantly CO 2 .The association of gold mineralization with structurally controlled concordant and discordant quartz sulfide veins, and the temperatures and pressures of wall-rock alteration and regional metamorphism indicate that the present distribution of gold is a result of metamorphism during progressive D 2 -D 3 deformation. Isotopic data for alteration envelopes date this event as Alleghanian: 279 + or - 14 Ma (K-Ar whole rock) and 343 + or - 18 Ma (K-Ar biotite) at Lowe; and 315 + or - 18 Ma (Rb-Sr whole-rock isochron; 87 Sr/ 86 Sr i = 0.7061 + or - 0.0008) and 294 + or - 16 Ma (K-Ar whole-rock) at Hog Mountain. Available data are compatible with development of the lodes during early Alleghanian overthrusting of allochthons over sedimentary rocks of the autochthonous North American margin. The implication is that the fluids were derived from metasedimentary and/or metavolcanic formations in the lower parts of the crystalline thrust stack (or possibly from underlying autochthonous sedimentary formations), ascended along permeable fault zones, and were emplaced as veins into dilatent areas in and adjacent to the fault zones.
Research Article| February 01, 1994 Mount St. Helens: Potential example of the partial melting of the subducted lithosphere in a volcanic arc: Comments and Reply Ralph L. Dawes; Ralph L. Dawes 1Department of Geological Sciences, AJ-20, University of Washington, Seattle, Washington 98195 Search for other works by this author on: GSW Google Scholar Nathan L. Green; Nathan L. Green 2Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487-0338 Search for other works by this author on: GSW Google Scholar Marc J. Defant; Marc J. Defant 3Department of Geology, University of South Florida, Tampa, Florida 33620 Search for other works by this author on: GSW Google Scholar Mark S. Drummond Mark S. Drummond 4Department of Geology, University of Alabama, Birmingham, Alabama 35294 Search for other works by this author on: GSW Google Scholar Geology (1994) 22 (2): 187–190. https://doi.org/10.1130/0091-7613(1994)022<0187:MSHPEO>2.3.CO;2 Article history first online: 02 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 Ralph L. Dawes, Nathan L. Green, Marc J. Defant, Mark S. Drummond; Mount St. Helens: Potential example of the partial melting of the subducted lithosphere in a volcanic arc: Comments and Reply. Geology 1994;; 22 (2): 187–190. doi: https://doi.org/10.1130/0091-7613(1994)022<0187:MSHPEO>2.3.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 SocietyGeology Search Advanced Search Abstract No abstract available 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.
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