Origin and global tectonic significance of Early Archean cherts from the Marble Bar greenstone belt, Pilbara Craton, Western Australia

Five sections of bedded chert in mafic-ultramafic rocks of the Archean Warrawoona Group in the Marble Bar greenstone belt, Pilbara Craton, were analyzed in order to understand their depositional environment and to provide some constraints on Early Archean tectonics. The sections are divisible into two types based on their field occurrence, mineralogy and geochemistry; thicker ones (A and B) that overlie Fe-rich, low-K tholeiites and thinner ones (C1, C2, and C3) overlying komatiitic basalts. The thickest, ferruginous section (A; 45 m thick) is the Marble Bar Chert of the Towers Formation, and is interpreted to have been an in situ precipitate derived from a high-T hydrothermal solution emanating from a mid-oceanic ridge (MOR). The following geochemical features are similar to those of modem hydrothermal iron-rich sediments at a MOR: (i) P, V, Zn, and Y are positively correlated with Fe, (ii) a positive Eu anomaly (normalized to chondrite) decreases from 6.6 to 1.3 and the magnitude of a negative Ce anomaly decreases from 0.6 to 1.0 as SigmaREE and LREE/HREE increase. The 13 m thick B-section in the Apex Basalt, dominated by SiO2 and containing significant amounts of Ba (up to 4330 ppm), originated from a low-T MOR hydrothermal solution. This section is characterized by an association with massive black/gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center. Geochemical evidence from the greenstones underlying the B-section indicates that they are of MORB origin. In contrast to A and B, C1-, C2-, and C3-sections are intercalated conformably with komatiitic basalts of the Apex Basalt and are 3-6 m thick. The geochemical signatures of these three sections suggest that they were most likely formed by low-T, weak hydrothermal activity that may have been associated with hot-spot volcanism. They show strong enrichment of Cr and Ni, reflecting a significant contribution of komatiitic basaltic detritus during sedimentation. The presence of volcaniclastic chert in their uppermost beds indicates a decrease of hydrothermal silica precipitation due to waning hydrothermal activity. Among the three sections, the uppermost (C3) exhibits greater enrichment in Zr, Nb, Hf, and Th, and higher Th/Sc and (La/Yb)(N) relative to the lower C1- and C2-sections. This could mean that the depositional environment of C3 was relatively closer to a continent or to island arcs composed of granitoids and/or their volcanic equivalents. Siliceous mudstones and sandstones of the uppermost elastic rocks (T-section; Panorama Formation) have geochemical signatures analogous to those of felsic plutonic/volcanic rocks. SigmaREE in some siliceous mudstones from this unit imply that differentiated High Th/Sc, negative Eu anomalies, and high Y granitoids (or volcanic equivalents) were already exposed (erupted), at least locally, in the Pilbara at this time. This study shows these Warrawoona Group cherts were deposited in a variety of environments ranging from a mid-oceanic spreading center to a convergent plate boundary via a hot-spot. This variation was most likely due to horizontal plate motions which accordingly support the operation of plate tectonics in the Early Archean. (C) 2003 Elsevier Science B.V. All rights reserved.