DOCSLIB.ORG

Bull. Geol. Soc. Am., Vol. 50 Osborn, Pl. 1

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Bull. Geol. Soc. Am., Vol. 50 Osborn, Pl. 1 BULL. GEOL. SOC. AM., VOL. 50 OSBORN, PL. 1 F i g u r e 1 . G a b b r o a t C o n t a c t w i t h T o n a l i t e Poikilitic crystals of hornblende (dark) enclose andesine. W ithout analyzer (x 25). F i g u r e 2 . T o n a l i t e Tongue-like projections of biotite cut andesine. Crossed nicols (x 80). PHOTOMICROGRAPHS OF GABBRO AND TONALITE Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/50/6/921/3415803/BUL50_6-0921.pdf by California Inst of Technology user on 21 November 2019 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA STRUCTURAL PETROLOGY OF THE YAL YERDE TONALITE, SOUTHERN CALIFORNIA BY E. F. OSBORN CONTENTS Page Abstract...................................................................................................................................... 922 Introduction.............................................................................................................................. 922 Acknowledgments.................................................................................................................... 923 Earlier publications................................................................................................................. 923 Location of district.................................................................................................................. 923 General geologic setting......................................................................................................... 924 General statement........................................................................................................... 924 Metamorphic rocks......................................................................................................... 924 Igneous rocks.................................................................................................................... 925 General statement................................................................................................... 925 Gabbro....................................................................................................................... 925 Tonalite..................................................................................................................... 926 Inclusions in tonalite.............................................................................................. 927 Granite...................................................................................................................... 927 D ikes.......................................................................................................................... 928 Age and correlation................................................................................................ 928 Structural geology........................................................................................................... 928 Micrometric and radioactivity measurements.................................................................. 929 General statement........................................................................................................... 929 Errors in sampling Val Verde tonalite....................................................................... 929 Variation in mineralogic composition of tonalite..................................................... 933 Correlation of mineralogic data with radioactivity determinations..................... 933 Radioactivity determinations............................................................................... 933 Earlier studies of radioactivity of rocks............................................................ 934 Source of radioactivity.......................................................................................... 935 Origin of dark inclusions in tonalite........................................................................... 935 Petrofabric analyses................................................................................................................ 937 General statement..................................................................................)........................ 937 Petrofabric analysis of schist....................................................................................... 938 Axes of reference..................................................................................................... 938 Fabric diagrams....................................................................................................... 938 Joints and faults...................................................................................................... 938 Interpretation.......................................................................................................... 938 Petrofabric analysis of tonalite.................................................................................... 939 Axes of reference..................................................................................................... 939 Fabric diagrams....................................................................................................... 942 Interpretation........................................................................................................... 944 Conclusions....................................................................................................................... 948 (921) Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/50/6/921/3415803/BUL50_6-0921.pdf by California Inst of Technology user on 21 November 2019 922 E. F . OSBORN----PETROLOGY OF VAL VERDE TONALITE ILLUSTRATIONS Figure Page 1. Map showing location of Yal Verde district............................................................. 924 2. Geologic map of the Val Verde district..................................................................... 925 3. Graph of variation in mineral composition oftonalité............................................ 932 4. Graphs of albite content of plagioclase and radioactivity of tonalité................ 934 5. Diagrams illustrating relation of axes of reference to dimensional orientation of grains in tonalité.......................................................................................................... 939 6. Fabric diagrams of schist.............................................................................................. 940 7. Fabric diagrams of tonalité.......................................................................................... 941 8. Fabric diagrams of tonalité.......................................................................................... 942 9. Fabric diagrams of tonalité and an inclusion........................................................... 943 Plate Facing page 1. Photomicrographs of gabbro and tonalité................................................................. 921 2. Photomicrographs of tonalité....................................................................................... 926 3. Hand specimen oriented with respect to axes of reference...................................... 927 ABSTRACT Micrometric analyses of the Val Verde tonalité were made on samples taken along a line extending from the border of the intrusive to a point 5 miles out in the intrusive. An attempt was made to evaluate sampling errors and to deter­ mine variation in minéralogie composition and radioactivity. The border of the intrusive was found to be more sodic and higher in radioactivity than the central part, and this is thought to be due to assimilation of quartz-biotite schist. Inclusions in the tonalité are believed to be xenoliths of gabbro and quartz- biotite schist. Petrofabric analyses of the schist country rock and the tonalité indicate that : (1) the schist was deformed primarily by rotation of mineral grains about a hori­ zontal axis; (2) during intrusion of the tonalité a gneissoid structure developed in it parallel to the contact with the country rock with a linear element lying in the plane of foliation and paralleling the dip; (3) later stresses directed parallel to the strike of the foliation caused rotation of mineral grains in the tonalité about an axis parallel to the dip. The orientation of the minerals in the tonalité is thus believed to be due to a combination of magmatic flow and post-magmatic deforma­ tion. INTRODUCTION Two phases of the problem of batholiths and their origin are of special interest. The first concerns variation in composition of intrusive bodies, and the second deals with the origin and interpretation of primary flow structure. A unique opportunity to investigate these problems as applied to one intrusive was furnished by the construction of the Yal Yerde tunnel, one of a series composing the Colorado River Aqueduct which will carry water from Parker Dam on the Colorado River to Los Angeles and vicinity. For slightly over 5 miles this tunnel passes through an apparently uni­ form tonalité intrusive at nearly right angles to the strike of the flow structure in the mass, and crosses the contact between the tonalité and quartz-biotite schist. The tonalité was examined along the line of the Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/50/6/921/3415803/BUL50_6-0921.pdf by California Inst of Technology user on 21 November 2019 INTRODUCTION 923 tunnel for variations in mineralogic composition and radioactivity; petro- fabric data were obtained from oriented specimens in an attempt to determine the origin of the gneissoid structure. ACKNOWLEDGMENTS
Load more

Recommended publications
  • Some Geologic and Exploration Characteristics of Porphyry Copper Deposits in a Volcanic Environment, Sonora, Mexico
    Some geologic and exploration characteristics of porphyry copper deposits in a volcanic environment, Sonora, Mexico Item Type text; Thesis-Reproduction (electronic); maps Authors Solano Rico, Baltazar, 1946- Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 30/09/2021 03:12:43 Link to Item http://hdl.handle.net/10150/566651 SOME GEOLOGIC AND EXPLORATION CHARACTERISTICS OF PORPHYRY COPPER DEPOSITS IN A VOLCANIC ENVIRONMENT, SONORA, MEXICO by Baltazar Solano Rico A Thesis Submitted to the Faculty of the DEPARTMENT OF MINING AND GEOLOGICAL ENGINEERING In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE WITH A MAJOR IN GEOLOGICAL ENGINEERING In the Graduate College THE UNIVERSITY OF ARIZONA 19 7 5 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of re­ quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg­ ment the proposed use of the material is in the interests of scholar­ ship.
    [Show full text]
  • The Commingling of Diverse Magma Types in the Fwgstaff Ltlke Igneous Complex
    Maine Geological Survey Studies in Maine Geology: Volume 3 1989 The Commingling of Diverse Magma Types in the Fwgstaff Ltlke Igneous Complex Roger L. Nielsen1 Emily S. Landis Vincent M . Ceci Cynthia}. Post01i2 Department ofGeo logy University of Maryland College Park, Maryland 20742 1Present address: College of Oceanography Oregon State University Corvallis, Oregon 97331 2 Present address: Building 222, Room A/21 National Bureau ofStandards Gaithersburg, Maryland 20899 ABSTRACT The Flagstaff Lake Igneous Complex is a member of an array of Acadian plutons which extend from the Katahdin batholith in the northeast to New Hampshire in the southwest. It is characterized by four major rock types: (1) gabbro, (2) granite, (3) garnet tonalite, and (4) trondhjemite. Field, petrographic, and geochemical evidence support the hypothesis that these diverse rocks existed as contemporaneous liquids, and that extensive mixing occurred at the time of emplacement. The dominant mafic mineral pairs in the gabbros are olivine-clinopyroxene, two pyroxenes, and pyroxene­ hornblende. The high Fe, low Ni and Cr contents of most of the gab bro sampled imply that it underwent substantial fractional crystallization prior to and during emplacement. The granitic rocks of the Flagstaff Lake Igneous Complex are heterogeneous, ranging from two-mica granites with equal amounts (-30%) of orthoclase, plagioclase, and quartz, to mafic diorites. These K-feldspar-bearing silicic rocks are exposed over -40% of the areal extent of the complex. Trondhjemite containing biotite, plagioclase, cordierite, and quartz, with minor muscovite and K-feldspar, is located along the margins of the intrusion, particularly in areas where the contact between the gabbro and granite intersects aluminous metapelitic wall rocks.
    [Show full text]
  • Petrogenesis of Slab-Derived Trondhjemite-Tonalite-Dacite/ Adakite Magmas M
    Transactions of the Royal Society of Edinburgh: Earth Sciences, 87, 205-215, 1996 Petrogenesis of slab-derived trondhjemite-tonalite-dacite/ adakite magmas M. S. Drummond, M. J. Defant and P. K. Kepezhinskas ABSTRACT: The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic process. Experimental partial melting data for basalt over a wide range of pressures (1-32 kbar) and temperatures (700-1150=C) have shown that melt compositions are primarily trondhjemite- tonalite-dacite (TTD). High-Al (> 15% A12O3 at the 70% SiO2 level) TTD melts are produced by high-pressure 015 kbar) partial melting of basalt, leaving a restite assemblage of garnet + clinopyroxe'ne ± hornblende. A specific Cenozoic high-Al TTD (adakite) contains lower Y, Yb and Sc and higher Sr, Sr/Y, La'/Yb and.Zr/Sm relative to other TTD types and is interpreted to represent a slab melt under garnet amphibolite to eclogite conditions. High-Al TTD with an adakite-like geochemical character is prevalent in the Archean as the result of a higher geotherm that facilitated slab melting. Cenozoic adakite localities are commonly associated with the subduction of young (<25Ma), hot oceanic crust, which may provide a slab geotherm (*9-10=C km"1) conducive for slab dehydration melting. Viable alternative or supporting tectonic effects that may enhance slab melting include highly oblique convergence and resultant high shear stresses and incipient subduction into a pristine hot mantle wedge.
    [Show full text]
  • General Disclaimer One Or More of the Following Statements May Affect
    General Disclaimer One or more of the Following Statements may affect this Document This document has been reproduced from the best copy furnished by the organizational source. It is being released in the interest of making available as much information as possible. This document may contain data, which exceeds the sheet parameters. It was furnished in this condition by the organizational source and is the best copy available. This document may contain tone-on-tone or color graphs, charts and/or pictures, which have been reproduced in black and white. This document is paginated as submitted by the original source. Portions of this document are not fully legible due to the historical nature of some of the material. However, it is the best reproduction available from the original submission. Produced by the NASA Center for Aerospace Information (CASI) 4f Final Report X85-1. 4025 THE SOUTH-CENTRAL UNITED STATES MAGNETIC ANOMALY (Ld5- I 25 NASA -Lb-174048) 1tB NtfS-1143d ,oulki- l-GN'1nAA. uN11BU 5'1bfE j MAGNbTIC ANCCALY F11141 neirOLt it? UL l.UU U11A.V tb p HL &O5IlMF AU 1 LSLI. UtiG uaclas G3/43 UOC25 Prepared by Pa'.rick J. Starich Geophysics Section Oepartment of Geosciences Purdue University West Lafayette, IN 47907 for the National Aeronautics & Space Administration Greenbelt, MD 20771 Grant No. NAG-5-231. Original Photo ^*a t from EROS Data may h9 PurC;,,. Sioux Fella Oanter S11 ___ 67198 Principal Investigators William J. Hin7e and Lawrence W. Braile Department of Geosciences Purdue University West Lafayette, IN 47907 June, 1984 _,j THE SOUTH—CENTRAL UNITED STATES MAGNETIC ANOMALY A Thesis Submitted to the Faculty of Purdue University by s ;^ Patrick James Starich In Partial Fulfillment of the Requirements for the Degree i of 'i Master of Science August 1984 rf rA~ > ii ACKNOWLEDGEMENTS I would like to express my admiration and gratif.,ude to Dr.
    [Show full text]
  • Precambrian Geology of the Elk Mountain-Spring Mountain Area
    Precambrian Geology of the Elk Mountain - Spring Mountain Area, San Miguel County, New Mexico by Ingrid Klich, Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in Geology New Mexico Institute of Miningand Technology Socorro, New Mexico February, 1983 i ABSTRACT Precambrian rocks of the Elk Mountain - Spring Mountain area, San Miguel County, New Mexico consist primarily of a bimodal (mafic-felsic) metavolcanic assemblage, plus locally abundant metasedimentary rocks (predominantly of volcanic provenance, plus ironstone, shales, and limestone) , and a complex of intrusives including meta- diabase and gabbro, tonal.f.te , granite , syenite and pegmatites. The metavolcanic suite is largely comprised of subaqueous basalts (with well-preserved pillow-structures), plus lesser amounts of felsic volcanic and volcaniclastic rocks ranging from rhyolites to dacites. The supracrustal succession has been subjected to at least two periodsof tight to isoclinal.folding, accompanied byintense tectonic transposition such that present orientations of 1ithol.ogic units no longer represent their original stratigraphy. Thus, delineation of a stratigraphic section for the supracrustal rocks within the study area is not yet possible. Rocks of the supracrustal succession have been variably metamorphosed to lower amphibolite facies with slight retrograde metamorphism to greenschist facies. Chemical analyses verify the compositional bimodality of the supracrustal assemblage. The mafic rocks are characterizedmainly as subaJ.kaline, low-K tholeiites, ii whereas the felsic rocks appear to be both tholeiitic and calc-al.kaline.Tectonic discrimination diagrams suggest formation of: these rocks at an island arc - destructive plate boundary. The submarine environment, bimodal volcanic suite, and relationship to associated sedimentary rocks are all compatible with deposition ina marginal.
    [Show full text]
  • Magma Traps and Driving Pressure: Consequences for Pluton Shape and Emplacement in an Extensional Regime
    Missouri University of Science and Technology Scholars' Mine Geosciences and Geological and Petroleum Geosciences and Geological and Petroleum Engineering Faculty Research & Creative Works Engineering 01 Sep 1998 Magma Traps and Driving Pressure: Consequences for Pluton Shape and Emplacement in an Extensional Regime John Patrick Hogan Missouri University of Science and Technology, jhogan@mst.edu Jonathan D. Price M. Charles Gilbert Follow this and additional works at: https://scholarsmine.mst.edu/geosci_geo_peteng_facwork Part of the Geology Commons, and the Petroleum Engineering Commons Recommended Citation J. P. Hogan et al., "Magma Traps and Driving Pressure: Consequences for Pluton Shape and Emplacement in an Extensional Regime," Journal of Structural Geology, vol. 20, no. 9-10, pp. 1155-1168, Elsevier, Sep 1998. The definitive version is available at https://doi.org/10.1016/S0191-8141(98)00063-7 This Article - Journal is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in Geosciences and Geological and Petroleum Engineering Faculty Research & Creative Works by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu. Journal of Structural Geology, Vol. 20, No. 9/10, pp. 1155 to 1168, 1998 # 1998 Elsevier Science Ltd. All rights reserved PII: S0191-8141(98)00063-7 0191-8141/98/$ - see front matter Magma traps and driving pressure: consequences for pluton shape and emplacement in an extensional regime JOHN P. HOGAN*, JONATHAN D. PRICE and M. CHARLES GILBERT University of Oklahoma, School of Geology and Geophysics, Sarkey's Energy Center, Rm 810, Norman, OK, 73019-0628, U.S.A.
    [Show full text]
  • RESEARCH a Non–Plate Tectonic
    RESEARCH A non–plate tectonic model for the Eoarchean Isua supracrustal belt A. Alexander G. Webb1,*, Thomas Müller2, Jiawei Zuo1, Peter J. Haproff3, and Anthony Ramírez-Salazar2 1DEPARTMENT OF EARTH SCIENCES AND LABORATORY FOR SPACE RESEARCH, UNIVERSITY OF HONG KONG, POKFULAM ROAD, HONG KONG, CHINA 2SCHOOL OF EARTH AND ENVIRONMENT, UNIVERSITY OF LEEDS, MATHS/EARTH AND ENVIRONMENT BUILDING, LEEDS LS2 9JT, UK 3DEPARTMENT OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF NORTH CAROLINA, WILMINGTON, NORTH CAROLINA 28403, USA ABSTRACT The ca. 3.8–3.6-b.y.-old Isua supracrustal belt of SW Greenland is Earth’s only site older than 3.2 Ga that is exclusively interpreted via plate- tectonic theory. The belt is divided into ca. 3.8 Ga and ca. 3.7 Ga halves, and these are interpreted as plate fragments that collided by ca. 3.6 Ga. However, such models are based on idiosyncratic interpretations of field observations and U-Pb zircon data, resulting in intricate, conflicting stratigraphic and structural interpretations. We reanalyzed published geochronological work and associated field constraints previously interpreted to show multiple plate-tectonic events and conducted field-based exploration of metamorphic and structural gra- dients previously interpreted to show heterogeneities recording plate-tectonic processes. Simpler interpretations are viable, i.e., the belt may have experienced nearly homogeneous metamorphic conditions and strain during a single deformation event prior to intrusion of ca. 3.5 Ga mafic dikes. Curtain and sheath folds occur at multiple scales throughout the belt, with the entire belt potentially representing Earth’s largest a-type fold. Integrating these findings, we present a new model in which two cycles of volcanic burial and resultant melt- ing and tonalite-trondhjemite-granodiorite (TTG) intrusion produced first the ca.
    [Show full text]
  • Epidote in Calc-Alkaline Magmas: an Experimental Study of Stability, Phase Relationships, and the Role of Epidote in Magmatic Evolution
    American Mineralogist, Volume 81, pages 462-474, 1996 Epidote in calc-alkaline magmas: An experimental study of stability, phase relationships, and the role of epidote in magmatic evolution MAX W. SCHMIDT1,2 ANDALAN B. THOMPSON3 'Departement Geologie, Universite Blaise Pascal, CNRS-URAIO, 5 rue Kessler, 63038 Clermont-Ferrand, France 2Bayerisches Geoinstitut, Universitat Bayreuth, D-95440 Bayreuth, Germany 3lnstitut flir Mineralogie and Petrographie, Sonneggstrasse 5, Eidgenossische Technische Hochschule, CH-8092 Zurich, Switzerland ABSTRACT Experiments on tonalite and granodiorite were performed at conditions ranging from 2.1 to 18 kbar and 550 to 850°C to establish the magmatic stability field of epidote as a function of P, T, and f02. At water-saturated conditions and f02 buffered by NNO, epidote has a wide magmatic stability field in tonalite. At 13 kbar, this field extends from the wet solidus at 630 to 790 0c. The low-pressure intersection of the magmatic epidote crystallization reaction and the H20-saturated tonalite solidus, and hence the minimum pressure for magmatic epidote, occurs at about 5 kbar. The Clapeyron slopes of epidote melting reactions are moderately positive in P-T space at pressures below the intersection of the epidote melting and the gamet-in reactions at 13 kbar, 790°C. At this intersection, epidote reaches its maximal thermal stability (790°C) in tonalite-H20. In the presence of garnet, that is above 14 kbar, epidote melting reactions have steep negative Clapeyron slopes in P-T space. At P-T conditions near the low-pressure intersection of the epidote melting reaction with the water-saturated solidus, experiments were also performed with f02 buffered by HM.
    [Show full text]
  • Tonalite-Dominated Magmatism in the Abitibi Subprovince, Canada, and Significance for Cu-Au Magmatic-Hydrothermal Systems
    minerals Article Tonalite-Dominated Magmatism in the Abitibi Subprovince, Canada, and Significance for Cu-Au Magmatic-Hydrothermal Systems Lucie Mathieu 1,* , Alexandre Crépon 1 and Daniel J. Kontak 2 1 Centre D’études sur les Ressources Minérales (CERM), Département des Sciences Appliquées, Université du Québec à Chicoutimi (UQAC), 555 blvd. de l’université, Chicoutimi, QC G7H 2B1, Canada; alexandre.crepon1@uqac.ca 2 Mineral Exploration Research Centre, Harquail School of Earth Sciences, Goodman School of Mines, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6, Canada; dkontak@laurentian.ca * Correspondence: lucie1.mathieu@uqac.ca; Tel.: +1-418-545-5011 (ext. 2538) Received: 8 February 2020; Accepted: 3 March 2020; Published: 7 March 2020 Abstract: In Archean greenstone belts, magmatism is dominated by intrusive and volcanic rocks with tholeiitic affinities, as well as tonalite- and granodiorite-dominated large-volume batholiths, i.e., tonalite–trondhjemite–granodiorite (TTG) suites. These intrusions are associated with poorly documented mineralization (Cu-Au porphyries) that, in the Neoarchean Abitibi Subprovince (>2.79 to ~2.65 Ga), Superior Province, Canada, are associated with diorite bearing plutons, i.e., tonalite–trondhjemite–diorite (TTD) suites. The importance of TTG versus TTD suites in the evolution of greenstone belts and of their magmatic-hydrothermal systems and related mineralization is unconstrained. The aim of this study was to portray the chemistry and distribution of these suites in the Abitibi Subprovince. The study used data compiled by the geological surveys of Québec and Ontario to evaluate the chemistry of TTG and TTD suites and uncovered two coeval magmas that significantly differentiated (fractional crystallization mostly): 1) a heavy rare earth elements (HREE)-depleted tonalitic magma from high pressure melting of an hydrated basalt source; and 2) a hybrid HREE-undepleted magma that may be a mixture of mantle-derived (tholeiite) and tonalitic melts.
    [Show full text]
  • Geology, Petrology and PGE-Cu-Ni Mineralization of the Neoarchean Entwine Lake Intrusion, an Intermediate to Mafic Igneous Intrusion
    Geology, Petrology and PGE-Cu-Ni Mineralization of the NeoArchean Entwine Lake Intrusion, an Intermediate to Mafic Igneous Intrusion J.P. Arnold1, R.S. James1 and G. Beakhouse2 1Department of Earth Sciences and Mineral Exploration Research Centre, Laurentian University, Ontario, Canada. P3E 2R9 2Ontario Geological Survey, MNDM, Willet Green Miller Centre, Ramsay Lake Rd, Sudbury, Ontario, Canada, P3E 6B5 e-mail: jasonarnold@hotmail.com Introduction Regional Geology The Entwine Lake intrusion (ELI) is an In Ontario, the Superior Province of the east west trending, tadpole-shaped, neo-Archean Canadian Shield is divided into thirteen intrusion located 90 km south of Dryden, Ontario. subprovinces based on recently identified tectonic The ELI is approximately 35 km long with a near boundaries (Thurston et al., 1991). Diagnostic to circular, 10 km diameter western portion and a most subprovinces, including the Wabigoon, are narrow tail extending to the east (Stone, 1999). The granite-greenstone assemblages consisting of pluton has recently attracted attention due to PGE- metavolcanic and subordinate metasedimentary Cu-Ni mineralization located in the northern units interleaved and cut by granite batholiths and portion of the intrusion by Champion Bear late intrusions. The Wabigoon Subprovince, a 3.0- Resources. The presence of PGE mineralization is 2.69 Ga, 900km long and 150km wide, northeast of particular interest as this intrusion has trending granite-greenstone belt, has been divided characteristics similar to that of a suite of mantle- into eastern, central and western regions (Blackburn derived monzodioritic intrusions (termed sanukitoid et al., 1991). The Entwine Lake intrusion occurs by Shirey and Hanson, 1984) that have not adjacent to the western boundary of the central previously been examined for their PGE potential.
    [Show full text]
  • Evidence for 3.3-Billion-Year-Old Oceanic Crust in the Barberton Greenstone Belt, South Africa
    Evidence for 3.3-billion-year-old oceanic crust in the Barberton greenstone belt, South Africa Eugene G. Grosch1* and Jiri Slama2 1Geology Department, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa 2Institute of Geology of the Czech Academy of Science, Rosvojova 269, 16500 Prague, Czech Republic ABSTRACT of the BGB and test for a continental versus juvenile oceanic setting. We Recognition of oceanic crust in Archean greenstone belts has develop a multi-pronged approach combining field observations, scientific remained a controversial and unresolved issue, with implications drill core, U-Pb detrital zircon ages, and geochemistry of metabasalts to for understanding early Earth geodynamics. In the search for early evaluate whether the sequence erupted on top of continental tonalite- Archean oceanic crust, we present a multi-pronged approach to test trondhjemite-granodiorite (TTG) crust, or whether it represents a pre- for the presence of an ophiolite-type sequence preserved in the Paleo- served remnant of accreted juvenile oceanic crust. The extent to which archean Barberton greenstone belt (BGB) of South Africa. New field the Kromberg type section potentially represents an Archean ophiolite observations are combined with detrital U-Pb zircon geochronology is assessed, with important implications for the nature of geodynamic and geochemistry on fresh drill-core material from the Kromberg processes on the early Archean Earth. type-section sequence of mafic-ultramafic rocks in the 3.56–3.33 Ga Onverwacht Group of the BGB. Trace element geochemistry indi- GEOLOGY AND SAMPLING cates that the Kromberg metabasalts were derived from the primi- The geographical location of the BGB on the border between Swaziland tive mantle.
    [Show full text]
  • Significance of Highly Discordant Radioisotope Dates for Precambrian Amphibolites in Grand Canyon, USA
    The Proceedings of the International Conference on Creationism Volume 6 Print Reference: Pages 407-424 Article 34 2008 Significance of Highly Discordant Radioisotope Dates for Precambrian Amphibolites in Grand Canyon, USA Andrew A. Snelling Answers in Genesis Follow this and additional works at: https://digitalcommons.cedarville.edu/icc_proceedings DigitalCommons@Cedarville provides a publication platform for fully open access journals, which means that all articles are available on the Internet to all users immediately upon publication. However, the opinions and sentiments expressed by the authors of articles published in our journals do not necessarily indicate the endorsement or reflect the views of DigitalCommons@Cedarville, the Centennial Library, or Cedarville University and its employees. The authors are solely responsible for the content of their work. Please address questions to dc@cedarville.edu. Browse the contents of this volume of The Proceedings of the International Conference on Creationism. Recommended Citation Snelling, Andrew A. (2008) "Significance of Highly Discordant Radioisotope Dates for Precambrian Amphibolites in Grand Canyon, USA," The Proceedings of the International Conference on Creationism: Vol. 6 , Article 34. Available at: https://digitalcommons.cedarville.edu/icc_proceedings/vol6/iss1/34 In A. A. Snelling (Ed.) (2008). Proceedings of the Sixth International Conference on Creationism (pp. 407–424). Pittsburgh, PA: Creation Science Fellowship and Dallas, TX: Institute for Creation Research. Significance of Highly Discordant Radioisotope Dates for Precambrian Amphibolites in Grand Canyon, USA Andrew A. Snelling, Ph.D., Director of Research, Answers in Genesis, PO Box 510, Hebron, KY, 41048 Abstract The Brahma amphibolites of the Precambrian crystalline basement of Grand Canyon were originally erupted as basalt lavas and subsequently suffered high-grade regional metamorphism.
    [Show full text]