Magmatic remnants in plutonic rocks

Direct evidence of magmatic activity in plutonic rocks is generally obliterated by hydrothermal alteration. Systematic studies on shallow intrusives, especially those emplaced in water-rich environments, led to the common belief that the transition from magmatic to submagmatic stages is not visible. However, remnants of magmatic inclusions, obvious in effusive rocks, have been preserved in some deeper rocks, especially when they cooled slowly in a dry environment. Such remnants, under the name of ''stone cavities'' have been described by Sorby in 1858 in a number of granites, notably from Cornwall and Scotland (St. Austell granite). The interpretation of magmatic remnants in plutonic rocks is not easy because of advanced evolution. The following investigation steps are proposed and discussed : (1) identification of magmatic textures in rock-forming minerals and selection of favourable samples ; (2) detailed petrographic study of possible traces of former melt inclusion and characterization of post-trapping inclusion evolution, including melt crystallization and eventual interaction with late aqueous fluids; (3) microthermometric studies at high temperatures (800-1000-degrees-C) and electron-microprobe analyses on quenched inclusions; chemical compositions should be compared with bulk rock analyses. More than magmatic trends, melt inclusions may record magmatic immiscibility phenomena (silicate melt/brines, notably), which cannot be detected by any other method. Two examples (Mount Genis granite, SE Sardinia, Italy and Sybille monzosyenite, Laramie anorthosite complex, Wyoming, USA.) are briefly reported and discussed