A kind of conclusion: a search for order among multifactorial processes and multifarious interactions

Abstract

In a sense the Masters of Washington and Heidelberg were right: granite is born of basalt, but only by a complex series of partial meltings, first of mantle material to provide a basaltic underplate, then of the latter to supply the primitive granitic melts that are subsequently differentiated by the diverse processes we have studied. And concerning the mantle source there is a veritable deluge of modern literature discussing the possible processes by which basaltic melts are generated and extracted within the various global tectonic niches, either during subduction at plate margins or whenever the crust is in extension above the hot spots or ridges in the mantle. Without embarking on a thesis well outside my experience I accept that basalt is so produced and does underplate the continental crust, though often transformed into its metamorphic form of amphibolite (e.g. Bergantz, 1989). Certainly the experimental evidence for an origin by partial melting is convincing, though the fluid mechanics involved in the extraction is still a matter of research and discussion (e.g. Spiegelman, 1993).