New constraints on the evolution of the Mallina Basin, and their bearing on relationships between the contrasting eastern and western granite–greenstone terranes of the Archaean Pilbara Craton, Western Australia

Abstract

The northeast trending Mallina Basin, in the central part of the Archaean Pilbara Craton, separates western and eastern granite–greenstone terranes with significantly different structural histories and age distribution patterns. The basin, which is up to 70 km wide and has a strike length of more than 150 km, is mainly composed of metamorphosed turbidites assigned to the De Grey Group; in the north these rocks are in faulted contact with metamorphosed volcanosedimentary rocks of the Whim Creek Group. New SHRIMP U–Pb zircon dates show that the turbidite sequence and the Whim Creek Group were deposited within the same period, between ca. 3010 and 2940 Ma, and that the basement to the Mallina Basin is as young as ca. 3015 Ma. Geochronological, structural and stratigraphic data now mitigate against previous models which suggested that the Whim Creek Group was significantly younger than the turbidite sequence, and that the Whim Creek Group evolved in a late ensialic pull-apart basin. The Mallina Basin, reinterpreted to include at least part of the Whim Creek Group, is now believed to be an extensional basin containing an overlap sequence. An ensialic setting is preferred, in which the basement to the basin consisted of ca. 3170–3020 Ma granites and greenstones rifted by major northeast trending faults. A continental block to the south and east, of which the presently exposed east Pilbara granite–greenstone terrane is a remnant, influenced sedimentation in most of the basin. Previous tectonostratigraphic models that correlated across and beneath the Mallina Basin, and interpreted the entire granite–greenstone terrane of the Pilbara Craton in terms of evolution during two cycles of arc-accretion, are not compatible with the new geochronological data and stratigraphic interpretations.