The initiation and thermal diversity of granite magmatism

Abstract

Notions of batholith magma generation in crustal thermal environments are countered by the consanguinity of intrusive and extrusive magmas at destructive plate margins, their overlapping mantle-type initial strontium isotope ratios and by their contribution to observed crustal thickening in the absence of significant shortening. Conductive heat modelling produces geotherms which do not intersect the field of crustal fusion. However, crustal scavenging by ascending melts, initiated in the mantle, is a distinct possibility in most tectonic environments. Scavenging occurs more effectively at modern plate margins as activity continues to increase crustal thicknesses, temperatures and acidity of magmas. However, evidence from British Caledonian granites, an older Cordilleran suite, shows the opposite crustal cooling trend probably linked to younger granite formation after subduction processes ceased. Mantle derived Cordilleran magmas contribute to contemporary crustal growth at 0.1—0.5 km 3 a -1 - a decreasing rate, proportional to the Earth’s decaying thermal output, which has controlled the changing style of tectonics and granitic activity during geological history.