S-type granites and their probable absence in southwestern North America.

A review of the vast volume of S-type granites in the Lachlan fold belt of SE Australia shows that these granites are near-surface, batholithic and commonly associated with S-type volcanics but not with regional metamorphic rocks and migmatites. They are strongly peraluminous, as shown by the presence of cordierite but rare muscovite. All are low in Na, Ca and Sr, resulting from chemical weathering during formation of the sedimentary sources. Peraluminous granites of various ages in SW North America are distinctly different: cordierite is rare, occasional Fe-Mn-rich garnet evolving instead. They are dominantly two-mica granites, indicating crystallization at high water fugacities and greater depths; no associated cordierite-bearing volcanics are reported. Sodium is generally high and some granites have trondhjemitic affinities. The parent magmas are likely to have been produced by partial melting of altered basaltic rocks and some may be contaminated by high-level I-types; no compelling evidence for these rocks being S-type has been presented.-L.C.H.