DOCSLIB.ORG

Archean Komatiite Volcanism Controlled by the Evolution of Early Continents

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Archean Komatiite Volcanism Controlled by the Evolution of Early Continents Archean komatiite volcanism controlled by the evolution of early continents David R. Molea,b,1,2, Marco L. Fiorentinia, Nicolas Thebauda, Kevin F. Cassidya, T. Campbell McCuaiga, Christopher L. Kirklandc, Sandra S. Romanoc, Michael P. Doubliera,c, Elena A. Belousovad, Stephen J. Barnese, and John Millera aCentre for Exploration Targeting, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems, School of Earth and Environment, University of Western Australia, Perth, WA 6009, Australia; bDepartment of Applied Geology, Curtin University, Bentley, WA 6102, Australia; cGeological Survey of Western Australia, Department of Mines and Petroleum, East Perth, WA 6004, Australia; dKey Centre for the Geochemical Evolution and Metallogeny of Continents, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems, Macquarie University, North Ryde NSW 2109, Australia; and eEarth Science and Resource Engineering, Commonwealth Scientific and Industrial Research Organization (CSIRO), Kensington, Perth, WA 6151, Australia Edited by Norman H. Sleep, Stanford University, Stanford, CA, and approved April 14, 2014 (received for review January 7, 2014) The generation and evolution of Earth’s continental crust has In the Yilgarn Craton of Western Australia (Fig. 1), two major played a fundamental role in the development of the planet. Its pulses of komatiite activity occurred at ∼2.9 Ga (southern Youanmi formation modified the composition of the mantle, contributed to Terrane; refs. 17–19) and 2.7 Ga (Kalgoorlie Terrane, Eastern the establishment of the atmosphere, and led to the creation of Goldfields Superterrane; refs. 5, 10). These represent two ecological niches important for early life. Here we show that in the separate plume events that impinged onto preexisting continental Archean, the formation and stabilization of continents also con- crust (20–23), with the resulting magmas heterogeneously dis- trolled the location, geochemistry, and volcanology of the hottest tributed across the craton (8, 10, 17–19, 23, 24). In this study, preserved lavas on Earth: komatiites. These magmas typically rep- we provide the first evidence of a fundamental relationship resent 50–30% partial melting of the mantle and subsequently between the spatiotemporal variation in komatiite abundance, record important information on the thermal and chemical evolu- geochemistry, and volcanology and the evolution of an Archean tion of the Archean–Proterozoic Earth. As a result, it is vital to microcontinent, reflected in the changing isotopic composition of constrain and understand the processes that govern their localiza- the crust. tion and emplacement. Here, we combined Lu-Hf isotopes and We used Lu-Hf and U-Pb isotopic techniques on multiple U-Pb geochronology to map the four-dimensional evolution of the magmatic and inherited zircon populations from granitoid rocks Yilgarn Craton, Western Australia, and reveal the progressive de- and felsic volcanic units, which represent the exposed Archean velopment of an Archean microcontinent. Our results show that in crust of the Yilgarn Craton. All zircon grains were dated using the early Earth, relatively small crustal blocks, analogous to modern the sensitive high-resolution ion microprobe (SHRIMP), before microplates, progressively amalgamated to form larger continen- in situ laser ablation inductively coupled plasma mass-spectrometry tal masses, and eventually the first cratons. This cratonization pro- (LA-ICP-MS) analysis for Lu-Hf isotopes. The Lu-Hf isotopic cess drove the hottest and most voluminous komatiite eruptions data are expressed as eHf, which denotes the derivation of the to the edge of established continental blocks. The dynamic evolu- 176Hf/177Hf ratio of the sample from the contemporaneous ratio tion of the early continents thus directly influenced the addition of deep mantle material to the Archean crust, oceans, and atmo- Significance sphere, while also providing a fundamental control on the distri- bution of major magmatic ore deposits. Komatiites are rare, ultra-high-temperature (∼1,600 °C) lavas that were erupted in large volumes 3.5–1.5 bya but only very crustal evolution | lithosphere | architecture | mantle plumes | rarely since. They are the signature rock type of a hotter early EARTH, ATMOSPHERIC, Ni-Cu-PGE deposits Earth. However, the hottest, most extensive komatiites have AND PLANETARY SCIENCES a very restricted distribution in particular linear belts within olcanism on Earth is the dynamic surface expression of our preserved Archean crust. This study used a combination of dif- Vplanet’s thermal cycle, with heat created from radioactive ferent radiogenic isotopes to map the boundaries of Archean decay and lost through mantle convection (1). In the Archean microcontinents in space and time, identifying the microplates eon (>2.5 bya), Earth’s heat flux was significantly higher than that form the building blocks of Precambrian cratons. Isotopic that observed today (1, 2) due to the combined effects of a more mapping demonstrates that the major komatiite belts are lo- radioactive mantle (1, 3) and residual heat from planetary ac- cated along these crustal boundaries. Subsequently, the evo- cretion (4). This resulted in the eruption of komatiites: ultra-high lution of the early continents controlled the location and temperature, low-viscosity lavas with MgO >18% and eruption extent of major volcanic events, crustal heat flow, and major temperatures >1,600 °C (5) formed from mantle plumes (1, 2, 6). ore deposit provinces. These rare, ancient magmas are dominantly restricted to the early history of the planet (3.5–1.5 Ga; ref. 7) and represent the Author contributions: D.R.M., M.L.F., and J.M. designed the research project; D.R.M., N.T., remnants of huge volcanic flow fields (8) consisting of the hottest S.S.R., and M.P.D. performed research; D.R.M., M.L.F., K.F.C., T.C.M., C.L.K., and S.J.B. analyzed data; D.R.M. wrote the paper; N.T., K.F.C., T.C.M., C.L.K., and J.M. performed lavas preserved on Earth (5, 9, 10). These now-extinct volcanic regional geological analysis; S.S.R. and M.P.D. performed regional geological analysis and systems and flow complexes had the potential to cover significant mapping; E.A.B. provided assistance with operation of analytical equipment and data portions of the early continents, and were likely analogous to reduction; and S.J.B. provided access to the CSIRO komatiite database. large igneous provinces in size and magma volume (11, 12). The authors declare no conflict of interest. Komatiites are vital to our understanding of Earth’s thermal This article is a PNAS Direct Submission. – – evolution (1 3, 7, 13 16), and represent a window into the dy- 1Present address: Earth Science and Resource Engineering, Commonwealth Scientific and namic secular development of the mantle throughout the early Industrial Research Organization (CSIRO), Kensington, Perth, WA 6151, Australia. history of our planet (5). Subsequently, understanding the physical 2To whom correspondence should be addressed. E-mail: [email protected]. and chemical processes that govern their localization, volcanology, This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. and geochemistry is vital in deciphering this information. 1073/pnas.1400273111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1400273111 PNAS | July 15, 2014 | vol. 111 | no. 28 | 10083–10088 Downloaded by guest on September 29, 2021 displayed in Fig. S4]. In these maps, point data representing the median eHf value of granites and felsic volcanics are plotted as contour maps that show the spatial extent of “blocks” of specific Lu- Hf isotopic character and their evolution through time. This method is based on previous isotopic mapping of the Yilgarn Craton using the analogous Sm-Nd system (27). Importantly, the Lu-Hf data presented here replicate the features of the Sm-Nd work (27, 28), with the added ability to look further back in time due to the in situ analysis of abundant inherited zircons (21). The variable isotopic signatures of the crust (Figs. 2–4) can be interpreted as proxies for lithospheric thickness (Figs. 5 and 6; ref. 29), where young, juvenile eHf values (eHf > 0) indicate relatively thin lithosphere and old, reworked values (eHf < 0) reflect thicker lithosphere (29, 30); a pattern observed in the modern-day western United States (29–32). Here, this informa- tion is combined to document the four-dimensional lithospheric architecture of the Yilgarn Craton and development of an Archean microcontinent. Results The first time slice (T1 − 3,050–2,820 Ma; Fig. 2) shows the litho- Fig. 1. Map of the Archean Yilgarn Craton showing the basic granite- spheric architecture at the time of ∼2.9 Ga komatiite emplacement greenstone bedrock geology and location of the ∼2.9 and 2.7 Ga komatiite in the southern Youanmi Terrane. The Lu-Hf mapping identifies localities. Individual terranes/domains (39, 40) are labeled. Greenstone belts three lithospheric blocks: Marda, Hyden, and Lake Johnston. The are labeled as follows: MD, Marda–Diemals; SC, Southern Cross; FO, For- Marda and Hyden blocks dominantly comprise reworked older – restania; LJ, Lake Johnston; RAV, Ravensthorpe; AW, Agnew Wiluna; and crust, with eHf −6.0 and −4.0, respectively. In contrast, the Lake KAL, Kalgoorlie/Kambalda. Komatiite localities are from Barnes and Fior- entini (10) (Table S4). Johnston block comprises younger, more juvenile material, with eHf +2.0. This protocratonic lithospheric
Load more

Recommended publications
  • Palynomorph and Palynofacies Assemblages of Neutral-Alkaline and Acid Lakes South of Norseman, Southern Western Australia
    Scholars' Mine Masters Theses Student Theses and Dissertations Summer 2012 Palynomorph and palynofacies assemblages of neutral-alkaline and acid lakes south of Norseman, Southern Western Australia Lutfia Grabel Follow this and additional works at: https://scholarsmine.mst.edu/masters_theses Part of the Geology Commons Department: Recommended Citation Grabel, Lutfia, "Palynomorph and palynofacies assemblages of neutral-alkaline and acid lakes south of Norseman, Southern Western Australia" (2012). Masters Theses. 5209. https://scholarsmine.mst.edu/masters_theses/5209 This thesis is brought to you by Scholars' Mine, a service of the Missouri S&T Library and Learning Resources. 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 [email protected]. PALYNOMORPH AND PALYNOFACIES ASSEMBLAGES OF NEUTRAL- ALKALINE AND ACID LAKES SOUTH OF NORSEMAN, SOUTHERN WESTERN AUSTRALIA by LUTFIA GRABEL A THESIS Presented to the Faculty of the Graduate School of the MISSOURI UNIVERSITY OF SCIENCE AND TECHNOLOGY In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE IN GEOLOGY 2012 Approved by Francisca E. Oboh-Ikuenobe, Advisor Wan Yang Melanie R. Mormile iii ABSTRACT The Yilgarn Craton in Western Australia hosts hundreds of shallow ephemeral hypersaline lakes, the majority of which have acid to neutral pH values. As part of a multidisciplinary study of the evolution of hypersalinity and acidity in the region, three drill cores were studied for their palynofacies and palynomorph contents in order to characterize palynofloral response to environmental changes. Drill cores from two acid lakes, Prado Lake (PL1-09 and PL2-09) and Twin Lake West (TLW1-09), and the neutral-alkaline Gastropod Lake (GLE1-09) south of Norseman recovered Miocene to Holocene sediments of the Revenge and Polar Bear formations.
    [Show full text]
  • Bedrock Geology Glossary from the Roadside Geology of Minnesota, Richard W
    Minnesota Bedrock Geology Glossary From the Roadside Geology of Minnesota, Richard W. Ojakangas Sedimentary Rock Types in Minnesota Rocks that formed from the consolidation of loose sediment Conglomerate: A coarse-grained sedimentary rock composed of pebbles, cobbles, or boul- ders set in a fine-grained matrix of silt and sand. Dolostone: A sedimentary rock composed of the mineral dolomite, a calcium magnesium car- bonate. Graywacke: A sedimentary rock made primarily of mud and sand, often deposited by turbidi- ty currents. Iron-formation: A thinly bedded sedimentary rock containing more than 15 percent iron. Limestone: A sedimentary rock composed of calcium carbonate. Mudstone: A sedimentary rock composed of mud. Sandstone: A sedimentary rock made primarily of sand. Shale: A deposit of clay, silt, or mud solidified into more or less a solid rock. Siltstone: A sedimentary rock made primarily of sand. Igneous and Volcanic Rock Types in Minnesota Rocks that solidified from cooling of molten magma Basalt: A black or dark grey volcanic rock that consists mainly of microscopic crystals of pla- gioclase feldspar, pyroxene, and perhaps olivine. Diorite: A plutonic igneous rock intermediate in composition between granite and gabbro. Gabbro: A dark igneous rock consisting mainly of plagioclase and pyroxene in crystals large enough to see with a simple magnifier. Gabbro has the same composition as basalt but contains much larger mineral grains because it cooled at depth over a longer period of time. Granite: An igneous rock composed mostly of orthoclase feldspar and quartz in grains large enough to see without using a magnifier. Most granites also contain mica and amphibole Rhyolite: A felsic (light-colored) volcanic rock, the extrusive equivalent of granite.
    [Show full text]
  • Neuro-Fuzzy Classification of Felsic Lava Geomorphology at Alarcon Rise, Mexico Christina Hefron Maschmeyer University of South Carolina
    University of South Carolina Scholar Commons Theses and Dissertations 2016 Neuro-Fuzzy Classification of Felsic Lava Geomorphology at Alarcon Rise, Mexico Christina Hefron Maschmeyer University of South Carolina Follow this and additional works at: https://scholarcommons.sc.edu/etd Part of the Geology Commons Recommended Citation Maschmeyer, C. H.(2016). Neuro-Fuzzy Classification of Felsic Lava Geomorphology at Alarcon Rise, Mexico. (Master's thesis). Retrieved from https://scholarcommons.sc.edu/etd/3566 This Open Access Thesis is brought to you by Scholar Commons. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. NEURO-FUZZY CLASSIFICATION OF FELSIC LAVA GEOMORPHOLOGY AT ALARCON RISE, MEXICO by Christina Hefron Maschmeyer Bachelor of Science College of Charleston, 2014 Bachelor of Arts College of Charleston, 2014 Submitted in Partial Fulfillment of the Requirements For the Degree of Master of Science in Geological Sciences College of Arts and Sciences University of South Carolina 2016 Accepted by: Scott White, Director of Thesis Michael Bizimis, Reader Brian Dreyer, Reader Lacy Ford, Senior Vice Provost and Dean of Graduate Studies © Copyright by Christina Hefron Maschmeyer, 2016 All Rights Reserved. ii DEDICATION This thesis is dedicated to Dr. Jim Carew for making me go to graduate school. iii ACKNOWLEDGEMENTS Data for this study were collected during cruises in 2012 aboard the R/V Zephyr and R/V Western Flyer and during 2015 on the R/V Rachel Carson and R/V Western Flyer from the Monterey Bay Aquarium Research Institute. I want to thank the captains, crews, ROV pilots and science parties for their work during these expeditions.
    [Show full text]
  • Chapter 2 Alaska’S Igneous Rocks
    Chapter 2 Alaska’s Igneous Rocks Resources • Alaska Department of Natural Resources, 2010, Division of Geological and Geophysical Surveys, Alaska Geologic Materials Center website, accessed May 27, 2010, at http://www.dggs.dnr.state.ak.us/?link=gmc_overview&menu_link=gmc. • Alaska Resource Education: Alaska Resource Education website, accessed February 22, 2011, at http://www.akresource.org/. • Barton, K.E., Howell, D.G., and Vigil, J.F., 2003, The North America tapestry of time and terrain: U.S. Geological Survey Geologic Investigations Series I-2781, 1 sheet. (Also available at http://pubs.usgs.gov/imap/i2781/.) • Danaher, Hugh, 2006, Mineral identification project website, accessed May 27, 2010, at http://www.fremontica.com/minerals/. • Digital Library for Earth System Education, [n.d.], Find a resource—Bowens reaction series: Digital Library for Earth System Education website, accessed June 10, 2010, at http://www.dlese.org/library/query.do?q=Bowens%20reaction%20series&s=0. • Edwards, L.E., and Pojeta, J., Jr., 1997, Fossils, rocks, and time: U.S. Geological Survey website. (Available at http://pubs.usgs.gov/gip/fossils/contents.html.) • Garden Buildings Direct, 2010, Rocks and minerals: Garden Buildings Direct website, accessed June 4, 2010, at http://www.gardenbuildingsdirect.co.uk/Article/rocks-and- minerals. • Illinois State Museum, 2003, Geology online–GeoGallery: Illinois State Museum Society database, accessed May 27, 2010 at http://geologyonline.museum.state.il.us/geogallery/. • Knecht, Elizebeth, designer, Pearson, R.W., and Hermans, Majorie, eds., 1998, Alaska in maps—A thematic atlas: Alaska Geographic Society, 100 p. Lillie, R.J., 2005, Parks and plates—The geology of our National parks, monuments, and seashores: New York, W.W.
    [Show full text]
  • Geologic Influences on Apache Trout Habitat in the White Mountains of Arizona
    GEOLOGIC INFLUENCES ON APACHE TROUT HABITAT IN THE WHITE MOUNTAINS OF ARIZONA JONATHAN W. LONG, ALVIN L. MEDINA, Rocky Mountain Research Station, U.S. Forest Service, 2500 S. Pine Knoll Dr, Flagstaff, AZ 86001; and AREGAI TECLE, Northern Arizona University, PO Box 15108, Flagstaff, AZ 86011 ABSTRACT Geologic variation has important influences on habitat quality for species of concern, but it can be difficult to evaluate due to subtle variations, complex terminology, and inadequate maps. To better understand habitat of the Apache trout (Onchorhynchus apache or O. gilae apache Miller), a threatened endemic species of the White Mountains of east- central Arizona, we reviewed existing geologic research to prepare composite geologic maps of the region at intermediate and fine scales. We projected these maps onto digital elevation models to visualize combinations of lithology and topog- raphy, or lithotopo types, in three-dimensions. Then we examined habitat studies of the Apache trout to evaluate how intermediate-scale geologic variation could influence habitat quality for the species. Analysis of data from six stream gages in the White Mountains indicates that base flows are sustained better in streams draining Mount Baldy. Felsic parent material and extensive epiclastic deposits account for greater abundance of gravels and boulders in Mount Baldy streams relative to those on adjacent mafic plateaus. Other important factors that are likely to differ between these lithotopo types include temperature, large woody debris, and water chemistry. Habitat analyses and conservation plans that do not account for geologic variation could mislead conservation efforts for the Apache trout by failing to recognize inherent differences in habitat quality and potential.
    [Show full text]
  • Depth and Degree of Melting of Komatiites
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. B4, PAGES 4521-4540, APRIL 10, 1992 Depth and Degree of Melting of Komatiites CLAUDE HERZBERG Departmentof GeologicalSciences, Rutgers University,New Brunswick,New Jersey Mineral PhysicsInstitute, State Universityof New York, StonyBrook, New York High pressuremelting experimentsßhove ." v .......... new constraintsto be placedon the depthand degreeof partial melting of komatiites. Komatiitesfrom GorgonaIsland were formed by relatively low degreesof pseudoinvariantmelting(< 30 %)involving L + O1 + Opx + Cpx + Gt on the solidusat 40 kbar, about 130 km depth. Munro-typekomatiites were separatedfrom a harzburgiteresidue (L + O1 + Opx) at pressuresthat are poorly constrained,but were probablyaround 50 kbar, about 165 km depth;the degreeof partial melting was <40%. Komatiites from the BarbertonMountain Land were formed by high degrees(-50 %) of pseudoinvariantmelting (L + O1 + Gt + Cpx) of fertile mantleperidotitc in the 80- to 100-kbarrange, about 260- to 330- km depth. Secularvariations in the geochemistryof komatiitescould have formed in response to a reductionin the temperatureand pressureof meltingwith time. The 3.5 Ga Barbertonkomatiites and the 2.7 Ga Munro-typekomatiites could have formedin plumesthat were hotterthan the present-daymantle by 500ø and 30(Y',respectively. When excesstemperatures are this size, melting is deeperand volcanismchanges from basalticto komatiitic. The komatiitesfrom Gorgona Island, which are Mesozoic in age, may be representativeof komatiitesthat are predictedto occur in oceanicplateaus of Cretaceousage throughoutthe Pacific [Storey et al., 1991]. 1. INTRODUCTION range of CaO and A1203contents in the 80- to 160-kbar range. A calibration has been made of the effect of pressure on Komatiites are high MgO volcanic rocks that can be CaO/(CaO + A1203)and MgO in komatiiticliquids formed on roughly explained by high degrees of melting of mantle the solidus, and an examinationhas been made of the effect of peridotitc,typically 50 to 100 % [e.g., Vi.ljoenand Vi.ljoen, FeO.
    [Show full text]
  • The Science Behind Volcanoes
    The Science Behind Volcanoes A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from the magma chamber below the surface. Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust in the interiors of plates, e.g., in the East African Rift, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "Plate hypothesis" volcanism. Volcanism away from plate boundaries has also been explained as mantle plumes. These so- called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth. Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. Volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere.
    [Show full text]
  • Lecture 8: Volcanism
    Lecture 8: Volcanism EAS 2200 Introduction to the Earth System Today’s Plan Introduction Melting in the Earth mid-ocean ridges subduction zones mantle plumes Crystallization of igneous rocks Volcanic eruptions Introduction Volcanic eruptions are among the most spectacular natural phenomena. Where does the magma come from? Why does most volcanism occur only in certain areas? What causes eruptions to sometimes be catastrophic and sometimes quiescent? Why is there such a variety of igneous rocks? Where does magma come from? Early ideas: Hot vapors produce melting Burning coals layers provide heat for melting Global layer of molten rock at depth Modern ideas: Decompression melting Flux melting Intrusions of magma into the crust (but this begs the question of the origin of the original magma). Deep burial of low melting point material (rare). Melting of Rock Complex (“multi-phase”) substances progressively melt over a range of temperatures. The lowest temperature at which melt exists (temperature at which melting begins) is known as the solidus. The highest temperature at which solid persists (temperature at which melting is complete) is known as the liquidus. The melting range for most rocks (diference in solidus and liquidus) is several hundred degrees C. In essentially all cases, melting in the Earth is believed to be partial (i.e., liquidus temperature Volcanoes are like Clouds Decompression Melting Solidus temperature of rock decreases with decreasing pressure. Temperature of rising mantle rock also decreases with pressure (adiabatic decompression). Adiabat is steeper than solidus, so that rising mantle rock eventually reaches solidus and Melting and Mantle Convection We can expect melting to occur within hot, rising mantle convection cells.
    [Show full text]
  • GY 112 Lecture Notes D
    GY 112 Lecture Notes D. Haywick (2006) 1 GY 112 Lecture Notes Archean Geology Lecture Goals: A) Time frame (the Archean and earlier) B) Rocks and tectonic elements (shield/platform/craton) C) Tectonics and paleogeography Textbook reference: Levin 7th edition (2003), Chapter 6; Levin 8th edition (2006), Chapter 8 A) Time frame Up until comparatively recently (start of the 20th century), most geologists focused their discussion of geological time on two eons; the “Precambrian” and the Phanerozoic. We now officially recognize 4 eons and the Precambrian is just used as a come-all term for all time before the Phanerozoic: Eon Time Phanerozoic 550 MA to 0 MA Proterozoic 2.5 GA to 550 MA Archean 3.96 GA to 2.5 GA Hadean 4.6 GA to 3.96 GA The “youngest” of these two new eons, the Archean, was first introduced by field geologists working in areas where very old rocks cropped out. One of these geologists was Sir William Logan (pictured at left; ess.nrcan.gc.ca ) one of the most respected geologists working with the Geological Survey of Canada. His study area was the Canadian Shield. Logan was able to identify two major types of rocks; 1) layered sedimentary and volcanic rocks and 2) highly metamorphosed granitic gneisses (see image at the top of the next page from http://www.trailcanada.com/images/canadian-shield.jpg). Logan found evidence that the gneisses mostly underlayed the layered rocks and hence, they had to be older than the layered rocks. The layered rocks were known to be Proterozoic in age.
    [Show full text]
  • Northwestern Superior Craton Margin, Manitoba: an Overview of Archean
    GS-7 Northwestern Superior craton margin, Manitoba: an overview of Archean and Proterozoic episodes of crustal growth, erosion and orogenesis (parts of NTS 54D and 64A) by R.P. Hartlaub1, C.O. Böhm, L.M. Heaman2, and A. Simonetti2 Hartlaub, R.P., Böhm, C.O., Heaman, L.M. and Simonetti, A. 2005: Northwestern Superior craton margin, Manitoba: an overview of Archean and Proterozoic episodes of crustal growth, erosion, and orogenesis (parts of NTS 54D and 64A); in Report of Activities 2005, Manitoba Industry, Economic Development and Mines, Manitoba Geological Survey, p. 54–60. Summary xenocrystic zircon, and in the The northwestern margin of the Superior Province in isotopic signature of Neoarchean Manitoba represents a dynamic boundary zone with good granite bodies (Böhm et al., 2000; potential for magmatic, sedimentary-hosted, and structur- Hartlaub et al., in press). The ALCC extends along the ally controlled mineral deposits. The region has a history Superior margin for at least 50 km, and may have a com- that commences in the early Archean with the formation mon history with other early Archean crustal fragments of the Assean Lake Crustal Complex. This fragment of in northern Quebec and Greenland (Hartlaub et al., in early to middle Archean crust was likely accreted to the press). Superior Province between 2.7 and 2.6 Ga, a major period South of the ALCC, the Split Lake Block repre- of Superior Province amalgamation. Sediments derived sents a variably retrogressed and shear zone–bounded from this amalgamation process were deposited at granulite terrain that is dominated by plutonic rocks and numerous locations along the northwestern margin of mafic granulite (Hartlaub et al., 2003, 2004).
    [Show full text]
  • Part 629 – Glossary of Landform and Geologic Terms
    Title 430 – National Soil Survey Handbook Part 629 – Glossary of Landform and Geologic Terms Subpart A – General Information 629.0 Definition and Purpose This glossary provides the NCSS soil survey program, soil scientists, and natural resource specialists with landform, geologic, and related terms and their definitions to— (1) Improve soil landscape description with a standard, single source landform and geologic glossary. (2) Enhance geomorphic content and clarity of soil map unit descriptions by use of accurate, defined terms. (3) Establish consistent geomorphic term usage in soil science and the National Cooperative Soil Survey (NCSS). (4) Provide standard geomorphic definitions for databases and soil survey technical publications. (5) Train soil scientists and related professionals in soils as landscape and geomorphic entities. 629.1 Responsibilities This glossary serves as the official NCSS reference for landform, geologic, and related terms. The staff of the National Soil Survey Center, located in Lincoln, NE, is responsible for maintaining and updating this glossary. Soil Science Division staff and NCSS participants are encouraged to propose additions and changes to the glossary for use in pedon descriptions, soil map unit descriptions, and soil survey publications. The Glossary of Geology (GG, 2005) serves as a major source for many glossary terms. The American Geologic Institute (AGI) granted the USDA Natural Resources Conservation Service (formerly the Soil Conservation Service) permission (in letters dated September 11, 1985, and September 22, 1993) to use existing definitions. Sources of, and modifications to, original definitions are explained immediately below. 629.2 Definitions A. Reference Codes Sources from which definitions were taken, whole or in part, are identified by a code (e.g., GG) following each definition.
    [Show full text]
  • Pan-African Orogeny 1
    Encyclopedia 0f Geology (2004), vol. 1, Elsevier, Amsterdam AFRICA/Pan-African Orogeny 1 Contents Pan-African Orogeny North African Phanerozoic Rift Valley Within the Pan-African domains, two broad types of Pan-African Orogeny orogenic or mobile belts can be distinguished. One type consists predominantly of Neoproterozoic supracrustal and magmatic assemblages, many of juvenile (mantle- A Kröner, Universität Mainz, Mainz, Germany R J Stern, University of Texas-Dallas, Richardson derived) origin, with structural and metamorphic his- TX, USA tories that are similar to those in Phanerozoic collision and accretion belts. These belts expose upper to middle O 2005, Elsevier Ltd. All Rights Reserved. crustal levels and contain diagnostic features such as ophiolites, subduction- or collision-related granitoids, lntroduction island-arc or passive continental margin assemblages as well as exotic terranes that permit reconstruction of The term 'Pan-African' was coined by WQ Kennedy in their evolution in Phanerozoic-style plate tectonic scen- 1964 on the basis of an assessment of available Rb-Sr arios. Such belts include the Arabian-Nubian shield of and K-Ar ages in Africa. The Pan-African was inter- Arabia and north-east Africa (Figure 2), the Damara- preted as a tectono-thermal event, some 500 Ma ago, Kaoko-Gariep Belt and Lufilian Arc of south-central during which a number of mobile belts formed, sur- and south-western Africa, the West Congo Belt of rounding older cratons. The concept was then extended Angola and Congo Republic, the Trans-Sahara Belt of to the Gondwana continents (Figure 1) although West Africa, and the Rokelide and Mauretanian belts regional names were proposed such as Brasiliano along the western Part of the West African Craton for South America, Adelaidean for Australia, and (Figure 1).
    [Show full text]