Precambrian Geology

 Comprises 88% of geologic time  Precambrian has 2 Eons  Geology hard to Study...  Preserved rocks are metamorphosed  Very few fossils present  Relative timing and correlation impossible

Earth billion years ago

Pre-Archean Crustal Evolution  Interior cooled from molten magma  Early crust = mafic – Form thin oceanic crust  Recycled oceanic crust led to continental crust – Released felsic components through partial melting – Subduction formed andesitic island arcs

Continental Crust  Oldest crust about 4 b.y.  Rocks in Greenland  3.8 b.y. – Metamorphosed – so older – Rocks in S. Africa, Minnesota – same age  Zircon grains in sed rocks  crust about b.y. – Oldest evidence for liquid water on Earth Greenland, 3.8 b.y Itsaq gneiss Zircon

Shields and Cratons  Each present-day continent has Precambrian shield  Shield  exposed Precambrian rock  Platform  buried Precambrian rock outward from shield  Shield + Platform = “Craton”

North American Canadian Shield  Cratons are relatively stable, immobile parts of continent  In N. America, includes Canada, Greenland, & Lake Superior Precambrian Rocks, Ontario, Canada

Archean Rocks  Two main types  Greenstone belts – Metamorphosed volcanic and sedimentary rocks – Green color due to chlorite  Granite-gneiss complexes – Metamorphosed granites and gabbros Granite-Gneiss Greenstone Belt

Greenstone Stratigraphic Column  3 Major Rock Units  Upper unit is sedimentary rocks – Mostly graywackes and conglomerates – Shallow marine deposits  Middle units dominated mafic volcanics – Pillow lavas common – Indicate underwater eruption  Lower units are ultramafic volcanics – Surface temperature 1,600ºC  Shows sequential transition from ultramafic to felsic volcanics at top Precambrian Pillow Basalts, Michigan

Greenstones-Pillow Basalt

Greenstone Belt Structure  Belts have synclinal form  Greenstone belts found between granite-gneiss complexes

Tectonic Evolution of Greenstone Belt  Backarc Basin Model  Magma intrudes continents, formed by subduction processes  Convection beneath backarc causes extension and forms basin  Volcanics and sediments collect in basin  Accretion results in metamorphism  Formation of syncline  Belts found between protocontinents

Age of Archean Rocks  Most greenstones  2.5 b.y. old  Australia Belt  3.0 b.y. old  Pongolo Supergroup  3.0 b.y. old, SE Africa  Witwaterrand overlie Pongolo, SE Africa  b.y. old  Non-marine Precambrian Rocks, Canada

Archean Greenstone Belts  Canadian Shield Slave and Superior Craton Superior Craton

Formation of Superior Craton  Successive collision of arc with craton produced greenstone belts

Proterozoic Eon

 42% of geologic time is Proterozoic

Archean vs. Proterozoic  Most Archean rocks have been metamorphosed and deformed – Mostly metamorphosed greenstone belts and granite-gneiss complexes  Proterozoic rocks have changed little – Widespread sedimentary rocks on passive margins  Continents were larger – Due to accretion onto ancient Archean craton  Plate Tectonics similar to today – Ophiolites preserved – Quartzite-carbonate-shale assemblage – Widespread glaciation

Proterozoic Eon  Early Proterozoic ( Ga) – Deposition of most banded iron formation (BIF) – Oldest well-preserved complete ophiolite – Amalgamation of Laurentia – Oldest known red beds – Origin of Central Plains, Yavapai & Mazantal Province  Middle Proterozoic (1.6 Ga-900 Ma) – Igneous activity – Midcontinent rifting  Late Proterozoic ( Ma) – Widespread glaciation

Early Proterozoic Banded Iron Formation (BIF)  Red Bands  silica-rich = chert  Black Bands  Iron oxide – Deposited Ga – Record major oxygenation event

Early Proterozoic  Oldest known complete ophiolite (slice of oceanic crust plastered to continent) sequence – Plate tectonics similar to present day – Shows areas of ancient subduction zone Jormua Complex, Finland

Early Proterozoic Amalgamation of Laurentia  Laurentia – consisted of North America, Greenland, parts of NW Scotland, and Baltic Shield – Archean rocks formed nuclei around which Proterozoic crust accreted – Therefore, much larger landmasses – Archean and Proterozoic cratons collided and sutured producing deformation belts – orogen  Many orogens followed

Early Proterozoic Amalgamation of Laurentia  Trans-Hudson Orogen – Record initial rifting – Development of ocean basin – Led to origin of subduction zone and island arc  Wopmay Orogen – Oldest known mountain-building event – Central Plains, Yavapai, & Mazantal Orogen – Accretion along southern border

Early Proterozoic Oldest Known Red Bed (1.5 Ga)  Oceanic oxygen saturated, so free oxygen building up in atmosphere

Mid Proterozoic Igneous Activity  No major growth of Laurentia  Extensive igneous activity – Mostly granitic plutons Precambrian granite

Mid Proterozoic Midcontinent Orogeny & Rifting  Grenville Orogeny – Final episode of Proterozoic accretion – Collision of 2 cratons – Formation of supercontinent – Crystalline rocks in New York and Texas  Midcontinent Rifting – Rift filled with thick basaltic lava and quartzite- carbonate-shale assemblage Grenville Rocks, NY

Late Proterozoic Widespread Glaciation  Poorly-sorted, unstratified sediment

Proterozoic Supercontinent Rodinia  Possible configuration of Late Proterozoic  North American and Green-land part of supercontinent  Proterozoic-Phanerozoic transition – Laurentia, Basaltica separated from super continent (800 Ma) – Failed rifts (aulacogen) led to N. America development §Developed passive margin §Shallow H2O LS, SS, & MS