The Origin of Co-Magmatic Andesite Enclaves in the Rabor Miocene Pyroxene Andesites, SE Kerman, Iran: Insights from Textural and Mineralogical Composition

Abstract

The Miocene pyroxene andesite lava flows are exposed in southeastern edge of Urumieh Dokhtar Magmatic assemblage in Iran. The hypocrystalline andesite in parts contain conspicuous co-magmatic igneous enclaves which are dark grey and occur mostly as spherical and occasionally as ribbon shapes with some showing chilled margins. Petrographic study shows that the ribbon type enclaves have been formed by merging of rounded blobs. Mineralogically the enclaves consist of plagioclase, clinopyroxene, orthopyroxene and magnetite as phenocrysts and microphenocrysts in a glassy microlitic groundmass of the same minerals. The host andesite contains the same mineral assemblage, but with high amount of glassy groundmass. The plagioclase shows a relatively large range of anorthite content (An43-91), whereas pyroxenes generally show a uniform chemical composition and are classified mainly as hypersthene (En59.67 to En73.30) and augite (Wo40.88-46.00, Fs11.08-17.07, En36.69-47.85). Enclaves are cogenetic with the host andesites and reveal the existence of disequlibrium crystallization in the magma chamber. The enclaves are resulted from a magmatic emulsion with suspended droplets of one magma in another. The rounded enclaves are likely to represent quenched blebs of intruding new andesite magma with higher liquidus that have been incorporated into the host andesitic magma in a liquid or near liquid state. Their small sizes reflect a small viscosity contrast and some turbulence during mixing. The intrusion of fresh magma also resulted convection in the magma chamber which along with the temperature differences has permitted buoyant uprising of enclaves and incorporating some of the phenocrysts from the host andesitic magma.