Early crustal evolution: Insights obtained from the compositions of Archaean (meta)sedimentary rocks
- Washington Univ., St. Louis, MO (United States). Dept of Earth and Planetary Sciences
The question of how the composition of the Earth's continental crust has changed over the course of geological time represents a challenging research problem. One approach to this class of problems has been to examine the chemical compositions of (meta)sedimentary rocks, because sediments are derived from broad source areas, and presumably can provide information on the average composition of rock types in those sources. The compositions of Archaean sediments have been modeled as mixtures of mafic and felsic end-members, implying a bimodal Archaean crust. The perspective of the writer, based on investigations of the West Greenland Archaean gneiss complex, is the following. Firstly, a bimodal Archaean crust cannot explain the compositions of the sedimentary rocks, as a great diversity of sediment types implies the existence of an equally diverse range of source rocks. The author accounts for this diversity by appealing to local derivation without extensive mixing and sediment recycling. Secondly, there is abundant evidence as far back as the beginning of the rock record (i.e., before 3.8 Ga) for the existence of thick continental crust and evolved continental rocks, including ones with substantial negative Eu-anomalies. The author concludes that continent formation, rather than being synchronous globally, more likely began in isolated areas that underwent evolution from oceanic arc, through amalgamation of oceanic arcs into micro-continents, followed by cratonization. In West Greenland, this process was completed by [approximately]3.6 Ga but in some areas (e.g., Guiana), this progressive evolution of sial continued into the mid-Proterozoic.
- OSTI ID:
- 5011918
- Report Number(s):
- CONF-9303210-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:3; Conference: 27. annual Geological Society of America (GSA) North-Central Section meeting, Rolla, MO (United States), 29-30 Mar 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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