Magmatic processes and mixing origin of andesite: Miocene Karamagara volcanics, central Anatolia, Turkey

Abstract

The Miocene Karamagara volcanics (KMV) crop out in the Saraykent region (Yozgat) of Central Anatolia. The KMV include four principal magmatic components based on their petrography and compositional features: basaltic andesites (KMB); enclaves (KME); andesites (KMA); and dacites (KMD). Rounded and ellipsoidal enclaves occur in the andesites, ranging in diameter from a few millimetres to ten centimetres. A non-cognate origin for the enclaves is suggested due to their mineralogical dissimilarity to the enclosing andesites. The enclaves range in composition from basaltic andesite to andesite. Major and trace element data and primitive mantle-normalized rare-earth element (REE) patterns of the KMV exhibit the effects of fractional crystallization on the evolution of the KME which are the product of mantle-derived magma. The KMA contain a wide variety of phenocrysts, including plagioclase, clinopyroxene, orthopyroxene, hornblende and opaque minerals. Comparison of textures indicates that many of the hornblende phenocrysts within the KMA were derived from basaltic andesites (KMB) and are not primary crystallization products of the KMA. Evidence of disequilibrium in the hybrid andesite includes the presence of reacted hornblendes, clinopyroxene mantled by orthopyroxene and vice versa, and sieve-texture and inclusion zones within plagioclase. The KMV exhibit a complex history, including fractional crystallization, magma mixing and mingling processes between mantle and crust-derived melts. Textural and geochemical characteristics of the enclaves and their hosts require that mantle-derived basic magma intruded the deep continental crust followed by fractional crystallization and generation of silicic melts from the continental material. Hybridization between basic and silicic melts subsequently occurred in a shallow magma chamber. Modelling of major element geochemistry suggests that the hybrid andesite represents a 62:38 mix of dacite and basaltic andesite. The implication of this process is that calc-alkaline intermediate volcanic rocks in the Saraykent region represent hybrids resulting from mixing between basic magma derived from the mantle and silicic magma derived from the continental crust. Copyright 2005 John Wiley & Sons, Ltd.