Partial melting accompanied high-pressure amphibolitization of eclogitic blocks and metasomatism of blocks and their peridotite matrix in a unit of the Catalina Schist, a subduction zone metamorphic terrane. Migmatitic blocks contain leucocratic zones, veins, and pods with albitic plagioclase ? quartz ? muscovite. Mineralogically similar dikes and veins cut the ultra-mafic matrix; a few veins can be traced back into migmatitic blocks. Uniform phase proportions and igneous textures indicate that the dikes and veins crystallized from silicate melts. Field relations, mineral and bulk compositions, and comparisons with experimental data suggest that the migmatites and dikes are partial melts of variably metasomatized amphibolites. Element-partitioning, phase-equilibrium, and fluid-inclusion data are compatible with partial melting of the amphibolites at P = ∼8–11 kbar and T = ∼640–750 °C, in the presence of a low-salinity aqueous fluid. Although the Catalina metamorphism occurred at much shallower levels than those inferred for subduction-related magmatism, the process envisioned is one of multistage, slab-derived metasomatic contamination of the mantle in the “hanging wall” of a subduction zone.
Abstract Allanite is present in most samples of the tonalitic Bell Island Pluton, with an average mode near 0.05 wt.%. Allanite occurs as cores in igneous epidote-clinozoisite and exhibits characteristic and consistent zoning patterns. REE -rich cores (All 40-70 ) grade out towards epidote-clinozoisite with REE below electron microprobe detection limits. La, Ce and Pr contents are highest in the REE-rich cores of zoned crystals. Nd and Sm contents both initially increase as total REE decreases and are highest in intermediate zones. Y contents are generally low throughout, but tend to be highest in analyses with A11 5-20. The zoning behaviour exhibited by the allanite, specifically the rimward increases in Nd, Sm, and Y, cannot be accounted for by simple fractionation and are best explained by increases in allanite/ melt partition coefficients ( K d values) for these elements during crystallization. We propose that the variation in K d values reflects modification of the allanite structure with changing REE content. These modifications are manifested by changes in colour, extinction, and pleochroism within the zoned crystals and include changes in unit-cell volume and dimensions. The changes in K d values are large enough to result in crossing REE patterns within single allanite crystals. Fractional crystallization of zoned allanite can have noticeable effects on LREE contents and La/Sm (and almost certainly La/Lu) in magmas. In the Bell Island pluton, 80% of La, but <3% of Y is contained in allanite. Although some of the variation in the LREE chemistry of the pluton is attributable to statistical sampling error, much of it appears to reflect petrogenetic processes that controlled LREE abundance and, ultimately, allanite mode. One sample of Bell Island tonalite is depleted in LREE and has low La/Lu and La/Sm. These chemical features can be modelled by fractionation of zoned allanite.
Research Article| January 01, 2004 Two high-pressure–low-temperature serpentinite-matrix mélange belts, Motagua fault zone, Guatemala: A record of Aptian and Maastrichtian collisions George E. Harlow; George E. Harlow 1Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024-5192, USA Search for other works by this author on: GSW Google Scholar Sidney R. Hemming; Sidney R. Hemming 2Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964-8000, USA Search for other works by this author on: GSW Google Scholar Hans G. Avé Lallemant; Hans G. Avé Lallemant 3Department of Earth Science, MS-126, Rice University, Houston, Texas 77005-1892, USA Search for other works by this author on: GSW Google Scholar Virginia B. Sisson; Virginia B. Sisson 3Department of Earth Science, MS-126, Rice University, Houston, Texas 77005-1892, USA Search for other works by this author on: GSW Google Scholar Sorena S. Sorensen Sorena S. Sorensen 4Department of Mineral Sciences, NHB-119, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119, USA Search for other works by this author on: GSW Google Scholar Geology (2004) 32 (1): 17–20. https://doi.org/10.1130/G19990.1 Article history received: 01 Jul 2003 rev-recd: 15 Sep 2003 accepted: 16 Sep 2003 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation George E. Harlow, Sidney R. Hemming, Hans G. Avé Lallemant, Virginia B. Sisson, Sorena S. Sorensen; Two high-pressure–low-temperature serpentinite-matrix mélange belts, Motagua fault zone, Guatemala: A record of Aptian and Maastrichtian collisions. Geology 2004;; 32 (1): 17–20. doi: https://doi.org/10.1130/G19990.1 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 Left-lateral motion along the North American–Caribbean plate boundary has juxtaposed two high-pressure–low-temperature (HP-LT) belts from separate Cretaceous collisions. These two belts have quite different ages and different suites of high-pressure assemblages, yet they both contain jadeitite, a relatively rare rock type. This part of the plate boundary zone follows the Motagua River Valley in Guatemala, where it separates the Maya block (North American plate) from the Chortís block (Caribbean plate). On both sides of the bounding Motagua fault, tectonic slices of serpentinite-matrix mélange host the HP-LT rocks. South of the fault, the mélange slices contain eclogite, lawsonite eclogite, glaucophane eclogite, and blueschist blocks. North of the fault, the mélange slices contain omphacite metabasite, albitite, and garnet amphibolite blocks, but lack intact eclogite. In addition to the dissimilar rock assemblages, 40Ar/39Ar geochronology of phengitic micas yields 77–65 Ma for northern and 125–113 Ma for southern blocks. These data suggest that the southern belt formed during Early Cretaceous (Aptian), northeastward-dipping subduction of the Farallon plate and collision of the Chortís block with western Mexico. The block was then displaced southeastward along this suture. In contrast, the northern belt records subduction related to the Maastrichtian collision of an extension of the Chortís block, perhaps the Nicaraguan Rise, with the Maya block. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Mr. President, Officers, Councilors and Members of the Mineralogical Society of America, ladies and gentlemen:
It is a distinct honor and my great pleasure to introduce to you Richard S. Fiske, the year 2000 recipient of MSA’s Public Service Medal. Why is this an honor? Well, one is seldom placed in a position to publicly praise the guy who hired you. And no, Dick is not getting the Public Service Medal for having created, back in 1984, an entirely new position for a metamorphic petrologist! It is my pleasure to be able to tell you about Dick’s contributions to a broader audience’s understanding of volcanoes, especially Hawaiian volcanoes.
These contributions have been made over a career as a research geologist, first at the United States Geological Survey, and now at the Smithsonian Institution, a career that has sustained two significant interruptions. Sitting before you is a veritable volcanological Lazarus, a man who has returned from a senior administrative …
Allanite, which is a common accessory mineral in a wide variety of rock types, typically contains high concentrations of Th and U; thus, an in-situ method of U-Th-Pb dating of this phase would have broad application. We describe a method to permit Th-Pb ages of allanite to be determined with approximately ±10% accuracy using a high-resolution ion microprobe. Knowledge of the composition and substitution mechanisms of this complex mineral is key to understanding the relative ionization efficiencies of Th+ and Pb+. The chemical compositions of three allanite samples used as age standards (Cima d'Asta Pluton, 275.5 ± 1.5 Ma; Atesina Volcanic Complex, 276.3 ± 2.2 Ma; La Posta Pluton, 94 ± 2 Ma) were determined using an electron microprobe, permitting an assessment of matrix effects on ionization. An ion-microprobe calibration curve involving elemental and oxide species of Th and Pb (i.e., 208Pb*/Th+ vs. ThO2+/Th+) yields highly scattered apparent ages when allanite age standards with different Fe contents are used. However, a three-dimensional plot of 208Pb*/Th+ vs. ThO2+/Th+ vs. FeO+/SiO+ improves the accuracy of the calibration to about ±10%. Even though this level of uncertainty is substantially greater than that expected for U-Th-Pb ionmicroprobe analyses of zircon or monazite, Th-Pb ages of allanite can still be used to address important geologic questions.
Research Article| July 01, 2000 Steep tilting of metavolcanic rocks by multiple mechanisms, central Sierra Nevada, California Othmar T. Tobisch; Othmar T. Tobisch 1Department of Earth Sciences, A232 Earth and Marine Science Building, University of California, Santa Cruz, California 95064, USA Search for other works by this author on: GSW Google Scholar Richard S. Fiske; Richard S. Fiske 2Department of Mineral Sciences, Smithsonian Institution, NHB-119, National Museum of Natural History, Washington, D.C. 20560, USA Search for other works by this author on: GSW Google Scholar Jason B. Saleeby; Jason B. Saleeby 3Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 94112, USA Search for other works by this author on: GSW Google Scholar Elizabeth Holt; Elizabeth Holt 3Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 94112, USA Search for other works by this author on: GSW Google Scholar Sorena S. Sorensen Sorena S. Sorensen 2Department of Mineral Sciences, Smithsonian Institution, NHB-119, National Museum of Natural History, Washington, D.C. 20560, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Othmar T. Tobisch 1Department of Earth Sciences, A232 Earth and Marine Science Building, University of California, Santa Cruz, California 95064, USA Richard S. Fiske 2Department of Mineral Sciences, Smithsonian Institution, NHB-119, National Museum of Natural History, Washington, D.C. 20560, USA Jason B. Saleeby 3Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 94112, USA Elizabeth Holt 3Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 94112, USA Sorena S. Sorensen 2Department of Mineral Sciences, Smithsonian Institution, NHB-119, National Museum of Natural History, Washington, D.C. 20560, USA Publisher: Geological Society of America Received: 02 Oct 1998 Revision Received: 20 May 1999 Accepted: 30 Jul 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (7): 1043–1058. https://doi.org/10.1130/0016-7606(2000)112<1043:STOMRB>2.0.CO;2 Article history Received: 02 Oct 1998 Revision Received: 20 May 1999 Accepted: 30 Jul 1999 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 Othmar T. Tobisch, Richard S. Fiske, Jason B. Saleeby, Elizabeth Holt, Sorena S. Sorensen; Steep tilting of metavolcanic rocks by multiple mechanisms, central Sierra Nevada, California. GSA Bulletin 2000;; 112 (7): 1043–1058. doi: https://doi.org/10.1130/0016-7606(2000)112<1043:STOMRB>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 For ∼200 km along the eastern Sierra Nevada continental magmatic arc, Mesozoic metavolcanogenic rocks dip steeply to the southwest (∼80°), a feature that must reflect fundamental processes in magmatic-arc construction. Although tight folds can account for such steep bedding tilts, folds in the metavolcanogenic sections are sparse and small scale. We propose that the high bedding tilts were produced by a combination of thrusting, downward displacement, and ductile deformation of the beds. The last two processes accompanied emplacement of the Sierra Nevada batholith.The Ritter Range pendant lies within this ∼200 km belt and provides a relatively large and well exposed Mesozoic volcanic section ranging in age from Late Triassic to mid–Cretaceous. Detailed mapping and ages from U-Pb zircon dates and fossils within the volcanic section reveal five structural blocks (I–V) that are separated by bedding-parallel thrusts, some of which are cryptic. To explain the present difference in bedding orientations between blocks III and IV, we suggest that the thrusting may have had a duplex geometry, which produced a maximum bedding dip of ∼45° in some blocks. Downward displacement of wall rock and ductile strain account for the remaining ∼35° of the observed average bedding dip (∼80°SW).The exact time of thrusting and duplex formation of Late Triassic to Early Jurassic rocks in blocks I–IV is uncertain, but these structures developed either (1) between 105 and 164 Ma, well before the other rotational processes were active, or (2) mostly around 105 Ma, and closer to the time when other rotational processes were active. Much of the subsequent (ca. 91–76 Ma) bedding tilting is related to downward displacement of beds associated with the emplacement of voluminous Late Cretaceous plutons, and to regional ductile deformation of the wall rocks during that period: the majority of the tilting probably took place between ca. 91 and 86 Ma. Bedding tilts of early to mid-Cretaceous rocks in blocks IV and V is bracketed between ca. 98 and ca. 90 Ma.Comparisons with metavolcanic sections to the northwest near Tioga Pass and to the southeast in the Mount Morrison, Mount Goddard and Oak Creek pendants, suggest that bedding rotation by thrusting(?), downward displacement and ductile strain of wall rock may explain the steep dips along this entire ∼200 km segment of the continental arc. Similar mechanisms may operate at midcrustal levels in other continental arcs. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
