The Cleaverville Group in the West Pilbara Coastal Granitoid-Greenstone Terrain of Western Australia: an example of a Mid-Archaean immature oceanic island-arc succession

Abstract

The 4400 m-thick volcanosedimentary succession that forms the 3200-3000 yr-old Cleaverville Group is well-preserved along the western coastline of the Pilbara in northwestern Western Australia. Two lithological assemblages. namely the Volcanic Rock Assemblage and the Chemical-Volcanosedimentary Rock Assemblage, are identified in the Cleaverville Group. The Volcanic Rock Assemblage is approximately 3300 m-thick, and consists mainly of pillow basalt and rhyolitic flow rocks. The Chemical-Volcanosedimentary Rock Assemblage, which is at least 1100 m-thick, comprises chemogenic sedimentary rocks (chert and banded iron formation) and volcanosedimentary rocks (coarse and fine grained tuffs). Three volcanosedimentary cycles have been recognized from stratigraphic patterns displayed by the Cleaverville Group. Each cycle involved an initial episode of basaltic volcanism that was followed by rhyolitic volcanism, and then by chemogenic sedimentation. Boundaries between each cyclic package of basalt, rhyolite and chemogenic sedimentary rocks are conformable surfaces. The thickness of volcanic rocks, relative to chemogenic sedimentary rocks, within each package, decreases upwards within the succession. The upward decrease in the proportion of volcanic rocks in each package, suggests that bimodal volcanic activity, at the depositional site, decreased in intensity through time. Basalts are primarily non-vesicular, whereas chemogenic sedimentary rocks are very finely laminated and lack evidence of continental derivation. These features imply that the Cleaverville Group was deposited in an offshore, probably deep-sea environment. The Lizard Hills Formation lies above an angular unconformity on the Cleaverville Group. The formation records the first appearance, in the West Pilbara Coastal Granitoid-Greenstone Terrain, of sedimentary rocks that are of a continental origin. The cyclic record of bimodal volcanism, and accompanying volcanogenic sedimentation, within an oceanic setting suggests that the Cleaverville Group originated in a tectonic environment similar to a modern immature oceanic island-arc. Furthermore, it is possible that continentally derived sedimentary rocks of the Lizard Hills Formation record an arc-continent collision event.