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Precambrian History

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									Precambrian History
      The Archean
       Chapter 8
              Geologic Column
Precambrian
      Archean and Proterozoic eons
      Eoarchean era, ….
       Eoarchean (4.6-3.6 Ga)
   Earth was hot
   Bombarded by meteorites
   Lots of volcanic activity
   Hard to make and keep crust
   1st rocks were ultramafic
The Eoarchean Earth?
       Eoarchean (4.6-3.6 Ga)
   Melt ultramafic rock, make basalt
   Melt basalt a few times to make
    granite/rhyolite
                                         In search of the world’s
                                              oldest mineral
                                                                                                     WYNDHAM
                                                                                                       KUNUNURRA


                                                                                                        ARGYLE

                         500 km                                                 DERBY
                                                                      BROOME

                                                                                                 HALLS CREEK
                                                                             FIT ZROY CROSSING




                                  PORT HEDLAND
                           DAMPIER
           BARROW ISLAND             KARRATHA


   NORT H WEST CAPE                                                  T ELFER

                                   WIT TENOOM
                                            T OM PRICE

                            PARABURDOO          NEWMAN




                CARNARVON




                                  MEEKATHARRA            WILUNA


                                            CUE

                                MT MAGNET            AGNEW

            GERALDTON                                               LAVERT ON
                                                               LEONORA
               DONGARA
                ENEABBA



                                                               KALGOORLIE
                                                  COOLGARDIE
                                     SOUT HERN CROSS              KAMBALDA
                                              MERREDIN
                                      NORT HAM
                        PERTH
                                                  NORSEMAN                                                     EUCLA
                              JARRAHDALE
                                 BODDINGT ON
                                    NARROGIN
                                              LAKE GRACE
                      BUNBURY COLLIE
                            CAPEL     RAVENST HORPE
                               GREENBUSHES
                                                   ESPERANCE
               CAPE LEEUWIN

                                             ALBANY
AW H10
Metamorphic sandstone and
Conglomerate: 3 Ga
           World’s oldest mineral
   This zircon is 4.404
    billion years old ±4
    million years
   From a granite
            SHRIMP II

                        4350± Ma
                             3
                                              Cameca 4f Ion Microprobe
                  4
             4363± Ma
 4267± Ma
      5
                                              University of Edinburgh


                                                     d18O=5.0‰
                                  3
                             4339± Ma

                                                                         0.6 ppm La
                                             5.5 ppm La
                            4
              4353± Ma 4326± Ma
                   8

               Surface 1
            4321± Ma 4355± Ma
                 3        2                                                   0.3 ppm
     4
4283± Ma

                                   3
                              4364± Ma

                                  3
                             4284± Ma



                                                          13.6 ppm La 9.2 ppm La
                                         d18O=7.4‰
            4319± Ma
                 3              4
                           4288± Ma
      4
 4404± Ma
                                                                Peck et al., 2001
A Cool Early Earth ?
                  Eoarchean
   World’s oldest rocks,
    Acasta Gneiss
   Slave Craton of
    Canada
   ~4.0 Ga
                 Archean
   Making and keeping crustal fragments
   Arcs collide due to
subduction to form larger
       land mass
Land masses continue to
        grow
     Precambrian shields and
            cratons
   Shield: area of exposed Precambrian
    rocks



           Qui ckTi me™ and a TIFF (Un comp re ssed) decompresso r are nee ded to see th is pi cture.
     Precambrian shields and
            cratons
   Shield: area of exposed Precambrian
    rocks
   Craton: buried and exposed Precambrian
    rocks
    Superior Craton of Canada
   Small fragments
    before 3.0 billion
   Explosion of crustal
    growth 3.0-2.5 billion
        Model for assembly of
               cratons
   Continental
    accretion: plates
    collide with volcanic
    arcs and other plates.
   Continents grow
    along margins
             Archean rocks
   Granite-greenstone belts
   Granite often gneiss
Archean greenstone
Archean greenstone pillows
Archean greenstone pillows
Archean greenstone pillows
           Archean rock record
   Evidence of volcanoes
       greenstones
   Evidence of water
       Pillow basalts
Archean conglomerate: more
    evidence for water
Archean sediment layers:
more evidence for water
Canadian Shield Gneiss
Volcanic arcs in oceans
       Precambrian accretion
   Lots of crust 3.0-2.5 Ga
          Implications of accretion
                    model
   Plate tectonics active in Archean
   Differences
       Earth hotter
   Hotter Earth:
     Easy to make
      ultramafic rocks
     Plates move faster
     More volcanic
      activity
    Radiogenic heat production
          through time
   By end of Archean
       Earth cooler
       Easier to keep crust
       Easier to make and
        keep granite
       No more ultramafic
        rocks

								
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