Magmatic inclusions in silicic and intermediate volcanic rocks

Abstract

Fine‐grained ellipsoidal inclusions from a few millimeters to over l m in size are present in many intermediate to silicic lava flows and domes. Only recently has it become widely accepted that such inclusions are chilled blobs of magma. Their magmatic origin is manifested by vesicularity and high groundmass porosity, by ellipsoidal shapes, by mingling at contacts with the host, and by textural evidence for groundmass crystallization in an undercooled state. Groundmass textures reflect the degree of undercooling, which is a function of thermal contrast (ΔT) between inclusions and host magmas before mingling. Compositions of inclusions are invariably more mafic than their hosts and indicate that the magmatic systems were compositionally zoned. Many inclusions were formed from hybrid magmas. Four examples are given in order to document effects of decreasing ΔC (compositional contrast) and ΔT, as hosts for mafic andesitic to andesitic inclusions vary from high‐silica rhyolite (HSR) to silicic andesite. In the HSR example from the Coso volcanic field, California, inclusions track the evolution of zonation in a long‐lived magmatic system. At the other extreme, andesitic inclusions in silicic andesite from Crater Lake, Oregon, demonstrate zonation of a system of low ΔT and ΔC and show effects of in situ differentiation by gas filter‐pressing of residual liquid. Inclusions form when a relatively small proportion of mafic magma comes into contact with a larger proportion of silicic magma. Several mechanisms by which the magmas mix have been proposed: buoyant rise of vesiculated mafic magma or convective stirring at the interface between mafic and overlying silicic magma, forcible injection of mafic magma, diking of fractured silicic magma, and mixing during ascent in a conduit. No single mechanism explains all field observations. Many inclusions in plutons are analogous to magmatic inclusions in volcanic rocks but are likely to have suffered greater chemical and textural change during comparatively slow postentrapment cooling. Magmatic inclusions are evidence of open system behavior of the differentiated parts of magma bodies, and the inclusions themselves may behave as open systems where ΔT is low.