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Archean Rocks

Archean Rocks. Best, Chapter 15A. Archean Terranes. >2.5 Gy old Younger supracrustal sequences Greenstone belts Calc-alkaline metavolcanic rocks Older gneiss complexes Quartzo-feldspathic rocks Tonalites and migmatites intruded by granites. West Greenland.

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Archean Rocks

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  1. Archean Rocks Best, Chapter 15A

  2. Archean Terranes • >2.5 Gy old • Younger supracrustal sequences • Greenstone belts • Calc-alkaline metavolcanic rocks • Older gneiss complexes • Quartzo-feldspathic rocks • Tonalites and migmatites intruded by granites

  3. West Greenland • Contains the oldest rocks on Earth • Isua supracrustal sequence • Dated at 3.71 to 3.77 Gy • Amphibolites and pyroxenites • Tholeiitic to komatiitic affinities

  4. Southeastern Africa • Rhodesian Craton • Kaapvaal Craton

  5. Kaapvaal Craton • Ancient gneiss complex • Overlying Barberton Greenstone Belt

  6. Ancient Gneiss Complex • Alternating sequence • Trondjehmite-tonalite (70% SiO2) • Amphibolite (55% SiO2) • Younger tonalite gneiss • 3.4 Gy • Youngest migmatites and gneisses • 3.2 Gy

  7. Barberton Greenstone Belt • Upper part of the Kaapvaal Craton • Alternating meta peridotite & pillow lavas • Overlain by thin tuffs, cherts, and shales

  8. Barberton Greenstone Belt • Lower part is peridotite & pillow lavas ~11 km thick • Middle part is a calc-alkaline volcanic group • Upper part is a sedimentary sequence

  9. Superior Province, Canada • Basement of gneiss sequence • Younger greenstone belt • Upper clastic sequence

  10. Gneiss Sequence • >3.0 Gy tonalitic gneisses • 2.8 to 2.7 tonalite-granite intrusions

  11. Greenstone Belt • 2.7 to 2.5 Gy old • Lower part is tholeiite lava and peridotite • 5 to 8 km thick • Upper part is intermediate and felsic • Calc-alkaline metavolcanics • 4 to 7 km thick

  12. Clastic Sequence • Capping units • Quartzo-feldspathic clastics • 10 km thick

  13. Petrology of Archean Rocks • Gneiss complexes • Komatiite

  14. Characteristics of Gneisses • Contain feldspar, quarts, mica, hornblende • Early tonalitic gneisses are Na rich • They probably originated by partial melting of tholeiitic basaltic sources • Later granitic gneisses are K rich • Their source could have been greywackes or calc-alkaline volcanics

  15. Metamorphism in Gneisses • Ranges from amphibolite to granulite facies • P = 0.7 to 1.2 GPa • T = 700° to 1200°C • Charnockites are common • Suggests intermediate P/T facies series

  16. Komatiites • Composed of peridotite magma • Occur only in Archean terrane • Emplaced as lavas • Pillows, breccias, glassy rinds • Spinafex texture • Very high temperature melts (>1600° C)

  17. Spinafex Texture • Criss-crossing blades of feathery olivine or clinopyroxene • Intergrown devitrified glass and skeletal crystals of clinopyroxene and chromite • This texture indicates rapid under cooling of magmas (entirely liquid at first)

  18. Evolution of Archean Crust • Different or similar to modern petrologic evolution? • Thermal evolution • Oldest crust? • Plate tectonic models

  19. Thermal Considerations • Early Earth was much hotter • Geothermal gradients were much higher • More convective flow within the Earth • Conditions (thin crust, meteorite bombardment) favored the destruction of very old crust • Oldest crust is ~ 3.8 Gy

  20. What Was the Oldest Crust? • Model 1 - no early sialic crust identified • Gneisses were intruded into a more mafic scum at the surface • Model 2 – sialic crust formed early • Mafic rocks are intrusive into this early crust • Model 3 – modern plate tectonic processes apply • Small island arcs formed of sialic material

  21. Model 1 for Oldest Crust • No early sialic crust - West Greenland case • Early basaltic lavas pushed into imbricate piles over descending mantle currents • Deeper parts of the piles changed to amphibolites • Partial melting produced tonalitic magmas • Gneisses intrude a mafic framework

  22. Model 2 for Oldest Crust • Sialic crust formed early – Canada example • Sialic partial melts collected and froze above the protomantle • Thickness constrained by the 750°C isotherm • Early crust would have been thin and may have covered much of the globe • This crust would not have been easily subducted • Mantle-derived basaltic magmas then intruded this crust

  23. Uniformatarian Model 3 • Small sialic arcs and microcontinents • Collision produced greenstone belts in marginal basins • Barberton example supports an ocean ridge model • Sheeted dike complex • Flanking pillow lavas • Thick sedimentary capping • Both of these models are on a smaller scale

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