EOE 3104 Archaean and Proterozoic Geosystems

Ronov (1968) - the rock record is evidence for the Archaean Earth being radically different.

Ronov - certain rock types are restricted to a specific period of geol 747q1622h ogical time

These are keys to understanding planetary evolution.

They are also very difficult to interpret because they are not amenable to simplistic uniformitarian analysis.

The study of the early Earth is an end in itself but it also provides vital insights to long term global change and to the evolution and interaction of different global systems.

The Archaean/Proterozoic transition marks a period of major change in most geosystems and this reflects the interconnected nature of those systems.

Cloud's (1976) model and Lovelock's Gaia hypothesis are attempts to describe and explain this. Both are valuable, both are flawed.

Archaean Rock Record

There are 3 major types of rock sequence which provide evidence for the nature of the Archaean Earth:

Greenstone Belts (Granite/Greenstone Belts)

Supracrustal sequences exhibiting lower grades of metamorphism (greenschist) and deformation. Not found after 2.5Ga.

High Grade Gneiss terrains

Highly deformed and metamorphosed (granulite/amphibolite) relics of the lower parts of ancient continental crust

Late Archaean Sedimentary sequences

These provide the basis of our understanding of the tectonic evolution of the Earth.

Evidence from the study of other planets shows that Earth tectonic systems are virtually unique in the Solar System.

Equally the Archaean record shows that Archaean tectonic systems were different.

Archaean Tectonic Issues

Plate Tectonics?

Origin of Continental Crust?

Crustal/Global Evolution

Uniformitarianism?

Archaean Greenstone Belts

Barberton Mountain Land - South Africa

Bulawayan-Shamvaian - Zimbabwe

Abitibi, Yellowknife, Superior, Slave Canada

Dharwar - India

Yilgarn, Pilbara - W Australia

'Classic' Greenstone belt Form

10-25 km wide, 100-300 km long.

Originally thought to have a simple synclinal structure but.

Tectonic slices of interthrust rocks- imbricated volcanics, gneisses and granites.

Overall variety encompasses a wide range of shape and origin for different greenstone belts especially the later examples.

Geochronology

Early belts- Barberton, Pilbara date back to 3.5 Ga but a really intense magmatic period world-wide seems to be 3.0-2.6 Ga.

In the Zimbabwean and Yilgarn provinces there were 2-3 phases of greenbelt formation from early to late Archaean.

The Dharwar belts are the youngest 2.6-2.5 Ga.

Recent geochron data show that 2.7-2.6 Ga granites were intruding stable cratonic cover sequences, like the Pongola Supergroup, at the same time as the Ventersdorp greenstone volcanics were being extruded

Archaean High Grade Gneiss Belts

Slave Province

Labrador Canada

West Greenland

Limpopo Belt

Baltic,

Anabar, Aldan - Russia

Hebei Province NE China

Beartooth Mtns, Montana

Enderby Land, Antarctica

Age range 4.0-2.5 Ga. Many experienced major plutonic-supracrustal activity 3.1-2.8 Ga

Dominant rock type (80/90%) - quartzo-feldspathic gneisses of granulite to amphibolite grade.

Foliated, banded, quartz, feldspar + biotite, hornblende, hypersthene or diopside.

Other rock types : amphibolites, mica-schists, quartzite, marble, banded iron formation, meta-anorthosite and meta- amphibolite dykes.

Limited proportion of the belts represents supracrustals

The bulk of the gneisses probably represent metamorphosed and deformed calc-alkaline plutonic rocks such as tonalites and granodiorites.

Magmas that were intruded and underplated into deep layers of Archaean continental crust culminating in granulite -facies metamorphism.

Tonalite-Trondhjiemite-Granodiorite (TTG) associations some are geochemically similar to I-type granites derived from the mantle.

Some gneiss belts have examples of S-type granites intruded into metamorphosed supracrustal rocks.

Patterns of deformation and metamorphism indicate crustal thickening by trust intercalation and magma injection.

Present granulite belts are currently underlain by 30-35 km of crust suggesting that original thicknesses of 60-75 km are likely; similar to modern Himalayas-a collisional belt.

Late Archaean Basins

Evidence of increasing crustal stability?

Pongola S/group SE Kaapvaal 2.9 Ga Rift Basin? Fluvial and tidal shelf clastics, diamictites, volcanics

Witwatersrand S/group 3.0 Ga

Ventersdorp S/group 2.7 Ga

Transvaal S/group 2.6 Ga Graben sequences? Conglomerates, fluvial clastics, greywackes, shales, bimodal volcanics, Au

Pilbara Block, W Australia 2.7 Ga Pull apart/rift basins

Oraniemi Sequence, Finland 2.8 Ga Rift basin? Alluvial fan to tidal shelf sequence,   incl arkosic red beds.

Supercontinents

Archaean/Proterozoic boundary

Mid-Proterozoic

Late Proterozoic - 'Rodinia'

Problems with the interpretation of PreC palaeomagnetic record

Archaean Crustal Evolution - issues and questions

Thermal budget

Komatiites: evidence of generally higher heat flow or hot-spot related?

Estimates of Archaean mantle temp 200-300C hotter but lower estimates exist.

ArchaeanOceanic Lithosphere

Thicker ocean crust? 16-50 km

Faster spreading? 20-40 cm yr

Komatiitic ocean crust?

More hotspot activity? -more oceanic plateaux?

Archaean Subduction

Subduction of younger ocean lithosphere

Slab melting vs slab dehydration

Archaean Ophiolites?

Thermal and Tectonic Evolution of Continents

Higher heat flow? -oceanic loss

Pre-3.9 Ga tectonics?

Origin of earliest continental crust?

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