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ABSTRACT

Intense post-depositional alteration has profoundly affected sandstones in the volcanic portions of Early Archaean (3·5–3·3 Ga) greenstone belts. The mineralogy and bulk compositions of most grains have been completely destroyed by pervasive metasomatism, but grain textures are commonly well preserved. Consequently, microtextural information coupled with present alteration compositions as determined petrographically can be used to estimate original framework modes.

Silicified Early Archaean volcaniclastic sandstones assigned to the Panorama Formation and Duffer Formation, Warrawoona Group, eastern Pilbara Block, Western Australia, were originally composed of volcanic (VRF) and sedimentary (SRF) rock fragments, volcanic quartz, feldspar, traces of ferromagnesian minerals and pumice. Only volcanic megaquartz remained stable during alteration. All other primary components were replaced by granular microcrystalline quartz (GMC) and sericite. In most areas, the sandstones were composed of dacitic to rhyolitic VRFs, now totally replaced by sericite-poor GMC and recognized by preserved microporphyritic textures. In a few areas, quartz-poor dacitic to andesitic(?) VRFs dominated the detrital assemblage. Minor SRFs and mafic VRFs, now replaced by GMC, are recognized on the basis of colour, internal structures, and internal textures, including skeletal, possible spinifex textures. Detrital feldspar is represented by blocky, sericite-rich grain pseudomorphs.

A semi-quantitative point-count scheme, developed for the analysis of heavily altered sandstones, indicates the following primary detrital-mode ranges for Panorama arenites: quartz, 0–28%; feldspar, 0–28%, VRFs, 58–86%, and SRFs 0–25%. In about half the point-counted samples, feldspar could not be distinguished from rock fragments. In such cases, both were counted as one grain type, Lv', which makes up from 84 to 100% of the framework modes of these rocks.

These sands were derived from a terrane composed largely of fresh felsic volcanic rocks and sediments, but locally including minor mafic, ultramafic, and sedimentary rocks. Much, but not all, of the felsic volcaniclastic sand represents reworked pyroclastic debris. There is no evidence for contributions from plutonic or metamorphic sources. The Panorama modal assemblage represents a provenance that is lithologically more restricted than that of Archaean greywackes and other siliciclastic units common in the sedimentary portions of these same Early Archaean greenstone belts and younger greenstone belts (3·0–2·7 Ga).