Hybrid Rocks

rocks whose material composition and structure do not correspond to the derivatives of normal magmas. Hybrid rocks have inhomogeneous textures and structures and contain anomalous mineral parageneses and xenoliths of local and abyssal origin. They form during assimilation without preservation of the characteristics of the absorbed fragments and during contamination, in which the traits of the assimilated fragments are preserved. Hybrid rock formation is also aided by the shattered condition of surrounding rock, an abundance of volatile substances in the magma, and contrast in the composition of surrounding rock and magmas. For granite intrusives with assimilation of lava material of basic composition, a series of hybrid rocks linked by transformations (from the edges of the intrusives to their central portions) is typical: gabbro-gabbro-diorites-diorites-quartz diorites-granite diorites-granites. The content of calcium, magnesium, and iron (the materials of the surrounding rocks) diminishes toward the granites in this series, and the role of potassium, sodium, and silicon (the granite portion) increases. Hybrid phenomena are also known for basalt lavas, when they acquire an andesitic composition as a result of the assimilation of metamorphic and other rocks.