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St. Paul’s Rocks are an upper mantle-derived ultrabasic intrusion emplaced largely in the solid state. They consist of three major groups of mylonitized rocks which originally were peridotites, brown hornblende-rich alkaline plutonic rocks, and gabbros. Major, minor, and trace element compositions appear to rule out any simple single stage genetic relation between these three groups. Hornblendes are an important constituent of these rocks, and range from pargasitic in the peridotites to kaersutitic in the brown hornblende mylonites. The subgroups in the peridotites include hornblende peridotite, plagioclase peridotite, and aluminous pyroxene peridotites. Coexistence of these assemblages is attributed to (1) inhomogeneous distribution of water pressure in the intrusive mass, (2) mixing of assemblages which possibly equilibrated at different mantle depths and were subsequently mixed during emplacement, and (3) rapid ascent and emplacement, preventing re-equilibration. Maximum temperatures and pressures for the primary assemblages appear to be around 1,100° C, and 15 kbars. The rocks were mylonitized during emplacement at temperatures higher than 500° C.

Rb-Sr measurements indicate that the peridotites may have been emplaced less than 100 m.y. ago, an age consistent with the sea-floor spreading model ages for St. Paul’s Rocks. Remarkably, a hornblende separate from Southeast Islet gives an apparent age of 835 m.y. by the K-Ar method. This age does not appear to be due to excess radiogenic argon. The intrusion appears to have incorporated older material, but it is not clear how such older material could have been left behind during sea-floor spreading.

St. Paul’s Rocks cannot be representative of undifferentiated (chondritic) oceanic upper mantle, but they may be representative of the kind of mantle rock which yields alkali olivine basalts by partial fusion.

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