A geophysical study of the granite problem

Abstract

Summary Following an introductory account of granite petrography, hypotheses for source and emplacement of granite are classified in a form suitable for geophysical assessment. The evidence of the negative gravity anomalies associated with post-tectonic granites is summarized and considered in its various aspects of space, mass and density. For comparison with observations, gravity fields are calculated for the types of mass distribution expected for the various hypotheses. The seismological evidence of Gutenberg's low-velocity lithosphere channel is also considered. The gravity results indicate that batholiths have considerable extent in depth and hence forcible intrusion is unlikely to be the main mechanism in post-tectonic granite emplacement. The negative anomalies demonstrate that the emplacement involves a considerable over-all removal of mass from the regions now occupied by the granite, but there is no support from the gravity anomalies for the hypothesis that the bulk of this now resides in the country rocks as a basic front. The evidence suggests that both downward diffusion of surplus mass and stoping are possible primary mechanisms. Geophysical, geochemical and geological observations conspire to support the idea that a true granitic layer, underlying the metasedimentary rocks, provides the source material for granite. Syn- and post-emplacement uplift of granite batholiths and the origin of mantled gneiss domes are attributed to isostatic-type phenomena depending on the granite mass deficiency.