How are silicic volcanic and plutonic systems related? Part 1: A review of geological and geophysical observations, and insights from igneous rock chemistry

Silicic volcanic and plutonic rocks (SiO2 >= 66 wt%) are generally viewed as being either compositional equiv-alents or complementary to each other. However, consideration of the geological, geophysical and chemical evidence suggests that silicic volcanic and plutonic rocks are not generally direct compositional equivalents. Also, volumetric and compositional relationships between the two groups suggest that silicic plutonic rocks are not generally cumulate mushes left behind after withdrawal of eruptible magma fractions. Reservoirs from which large-volumes of rhyolite have erupted should, in theory, contain residual, unevacuated magma fractions. However, the fine-grained and generally porphyritic rhyolites do not have the same compositions as the large volumes of silicic magmas that solidify to form batholiths. From these data we conclude that important genetic differences may exist in the modes of formation and evolution of silicic magmas that become rhyolites and those that become coarse-grained granitic rocks. In the second part of this study, we use phase-equilibrium and chemical modelling approaches to further investigate these preliminary conclusions. The aim is to shed light on the question of how the major classes of silicic magmas chemically evolve and whether there are fundamental differences between volcanic and plutonic silicic magmas.