Archean sedimentary rocks

Nance WB, Taylor SR (1976) Rare earth element patterns and crustal evolution—I. Australian post-Archean sedimentary rocks. Geochim Cosmochim Acta 40 1539-1551 Nishio Y, Nakai S (2002) Accurate and precise lithium isotopic determinations of igneous rock samples using multi-collector inductively coupled plasma mass spectrometry. Anal Chim Acta 456 271-281 Nishio Y, Nakai S, Hirose K, Ishii T, Sano Y (2002) Li isotopic systematics of volcanic rocks in marginal basins. Geochim Cosmochim Acta 66 A556... 

The composition of the Archean upper crust as revealed in the sedimentary record stands in marked contrast to that of the Post-Archean crust. A significant difference is shown by the REE patterns in the Archean sedimentary rocks, which relative to those of the Post-Archean crust, typically show no Eu anomalies and lower enrichment in the LREE. 

Terrestrial igneous and metamorphic 10.3. Archean sedimentary rocks 561... 

The second common lanthanide abundance pattern which is uniform and which has widespread geochemical significance, is that observed in most post-Archean sedimentary rocks such as shales. This pattern, as discussed later, is generally taken to represent that of the upper continental crust exposed to weathering and erosion, so that it forms a suitable base for comparison of terrestrial surface processes affecting the lanthanides. Two different sets of shale abundances have been used for normalisation. The first is the North American Shale Composite (NASC, Haskin et... 

The most remarkable feature of the lanthanide abundance patterns in post-Archean sedimentary rocks is their uniformity. Figure 44 shows patterns for... 

Fig. 49. Typical lanthanide patterns for Archean sedimentary rocks from greenstone belts showing light lanthanide enrichment and lack of Eu anomalies. Such a pattern can be explained by derivation of the greywackes either from andesitic sources or from a bimodal basic-felsic igneous source. Several lines of evidence indicate that the latter model is most likely. (Data are from table 30.)...

Fig. 50. Both steep (KH21, YKl) and flatter (C-3, 8781) lanthanide abundance patterns are observed in Archean sedimentary rocks, and reflect derivation from felsic and basic igneous rocks, and lend weight to the suggestion that a tamodal mixing model best explains the provenance of Archean sedimentary rocks in greenstone belts. (Data are from table 30.)...

Lanthanide abundances for the present upper continental crust are given in table 32 and are derived from the sedimentary rock data. Lanthanide abundances for the Archean upper crust can also be estimated from the Archean sedimentary rock data, although such estimates are less securely based, on account of the inherently greater variability in lanthanide patterns found in most Archean examples. The most recent estimate from Taylor and McLennan (1985) is also given in table 32. 

Chert is defined here, following Folk (1980, p. 79), as a chemically precipitated sedimentary rock, essentially monomineralic and composed chiefly of microcrystalline and/or chalcedonic quartz, with subordinate megaquartz and minor amounts of impurities. Chalcedony, which consists of sheath-like bundles of thin, radiating fibers of SiOi, is rare in most Precambrian chert. Precambrian chert occurs as distinct beds or lenses, particularly in Archean rocks, as nodules or silicified laminae in carbonate rocks, or as a sihceous end-member, as granules or cements in iron formation. 

The outcrops of very old Archean rocks are few and thus may not be representative of the original sediment compositions deposited. Nevertheless, it appears that carbonate rocks are relatively rare in the Archean. Based on data from the limited outcrops, Veizer (1973) concluded that Archean carbonate rocks are predominantly limestones. During the early Proterozoic, the abundance of carbonates increases markedly, and for most of this Era the preserved carbonate rock mass is typified by the ubiquity of early diagenetic, and perhaps primary, dolostones (Veizer, 1973 Grotzinger and James, 2000). In the Phanerozoic, carbonates constitute 30% of the total sedimentary mass, with sandstones and shales accounting for the rest. The Phanerozoic record of carbonates will be elaborated upon in the subsequent text. 

Elsewhere in the Barberton Mountain Land is a wide array of mid-Archean volcanic and sedimentary rocks, ranging up to >3.5 Ga old. Some material is clearly biogenic (Westall et al., 2001), with highly fractionated carbon isotopes —27%o), but may be of non-Archean age. Thus the case for mid-Archean biotic material in Barberton remains open. 

Figure 8 Archean distribution of banded iron formations, with short reoccurrence associated with widespread glaciation in the Neoproterozoic, and the Proterozoic and Phanerozoic distribution of sedimentary rocks containing ferric-iron cements (red beds). The end of banded iron formation and beginning of red bed deposition at —2.2 Ga has been taken as evidence for a major oxygenation event in Earth s atmosphere.

Lowe, D. R. 1994. Archean greenstone-related sedimentary rocks. In Condie, K. C. (ed.) Archean Crustal Evolution. Developments in Precambrian Geology, 11, 121-169. 

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