FIELD OCCURRENCE, GEOCHEMISTRY AND PETROGENESIS OF THE ARCHEAN MIDOCEANIC 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 t he Cleaverville area, Pilbara granite-greenstone terrain, Western Aust ralia, by using a well-defined duplex structure and by reconstructing the oceanic plate stratigraphy. Archean mid-oceanic ridge basalts (AMO RBs) which are part of this sequence were selected for detailed petroc hemical analysis to infer Archean divergent plate tectonic process. Th e Cleaverville AMORBs are low-K tholeiites which tend to be richer in FeO than modem MORBs. Abundance ratios of most incompatible elements such as REE, Ti, Y and Zr are chondritic. Neither Nb/Zr nor P/Zr sugge st partitioning into the metallic core before the middle Archean. Esti mated Mg values of the mantle source is about 85, and is lower than t hat of 89-92 modem MORBs, The Cleaverville AMORBs are considered to ha ve been produced by partial melting at a pressure and temperature of 2 .5 GPa and 1425 degrees C of a Fe-rich mantle peridotite (Mg value = 85.0). Potential mantle temperature (PMT) of the Earth of the mid-ocea nic ridge at this time 3.1-3.3 Ga was estimated from the above constra ints on the Cleaverville AMORBs to be 1400 degrees C. If the Cleavervi lle example is representative of Archean plate boundary processes at s preading ridges, the PMT was about 120 degrees C higher than today and extensive partial melting of adiabatically rising mantle material wou ld have been initiated at 90 km depth. Net production of partial melt at the AMOR axis was equivalent to 15-20 ran thickness of oceanic crus t (roughly 2-3 times thicker than today).