Field occurrence, geochemistry and petrogenesis of the Archean Mid-Oceanic Ridge Basalts (AMORBs) of the Cleaverville area, Pilbara Craton, Western Australia

A 3.1-3.3 Ga Mid-Archean accretionary complex has been identified in the Cleaverville area, Pilbara granite-greenstone terrain, Western Australia, by using a well-defined duplex structure and by reconstructing the oceanic plate stratigraphy. Archean mid-oceanic ridge basalts (AMORBs) which are part of this sequence were selected for detailed petrochemical analysis to infer Archean divergent plate tectonic process. The Cleaverville AMORBs are low-K tholeiites which tend to be richer in FeO * than modern MORBs. Abundance ratios of most incompatible elements such as REE, Ti, Y and Zr are chondritic. Neither Nb/Zr nor P/Zr suggest partitioning into the metallic core before the middle Archean. Estimated Mg * values of the mantle source is about 85, and is lower than that of 89-92 modern MORBs. The Cleaverville AMORBs are considered to have been produced by partial melting at a pressure and temperature of 2.5 GPa and 1425°C of a Fe-rich mantle peridotite (Mg * value = 85.0). Potential mantle temperature (PMT) of the Earth of the mid-oceanic ridge at this time 3.1-3.3 Ga was estimated from the above constraints on the Cleaverville AMORBs to be 1400°C. If the Cleaverville example is representative of Archean plate boundary processes at spreading ridges, the PMT was about 120°C higher than today and extensive partial melting of adiabatically rising mantle material would have been initiated at 90 km depth. Net production of partial melt at the AMOR axis was equivalent to 15-20 km thickness of oceanic crust (roughly 2-3 times thicker than today).