Petrological Characteristics of Archean Graywackes

Abstract

ABSTRACT In terms of QFR and alkali element (K, Na) relationships, most Archean graywackes are similar to Phanerozoic quartz-intermediate graywackes. However, a number of important differences preclude direct comparisons. Felsic volcanic rock fragments greatly outweigh andesitic rock fragments in Archean graywackes, whereas the reverse is seen in Phanerozoic examples. In comparison to Phanerozoic quartz-intermediate graywackes, Archean graywackes tend to have about 5% lower silica contents. A dominantly volcanogenic provenance can be documented for several Archean graywacke suites. In almost all examples, plagioclase dominates over K-feldspar, but this alone cannot be taken as evidence for a volcanic source because early Archean granitic rocks are typically sodium-rich (tonalites, trondhjemite ). The abundance of quartz in most Archean graywackes (Q typically > 40) is indicative of a major granitic component. Archean graywackes tend to have high FeO + MgO values (typically > 8.0%) and, in this respect, are similar to quartz-poor graywackes or modern sands deposited in fore-arc basins. It is suggested that these high values represent moderate amounts of mafic volcanic debris which have degraded to form part of the matrix. Thus, many Archean graywackes appear to have been derived from the well-established igneous bimodal suite (mafic volcanics--Na-granites, felsic volcanics) characteristic of many Archean terranes. The petrological differences between Archean graywackes and their younger counterparts preclude direct interpretations of tectonic setting, but the petrology of many Archean graywackes is consistent with deposition at the edge of continents.