Discussion
Several petrographical features in the observed plagioclase phenocrysts from the studied rocks such as resorbed appearances, strongly sieve texture, dusty rim or core and the maximum compositional variations from core to rim suggest that these phenocrysts are in disequilibrium, with compositions inconsistent with the current magma.
Moreover, the reverse zoning, the development of reaction rims and the opacite pseudomorph after amphiboles are all features supporting the disequilibrium conditions in dacitic and rhyodacitic amphiboles. On the other hand, some amphiboles are magnesio-hornblende in composition. Magnesio-hornblende in volcanic rocks has been interpreted by some researchers as a result of mixing between basaltic and rhyolitic magma (Heiken and Eichelberger, 1980; Nakada, 1991). The presence of reverse zoning in mafic minerals can be due to increasing PH2O which caused the precipitation of phases with enriched Mg rims in order to reach equilibrium (Sakuyama, 1981; Halsor and Rose, 1991); or more mafic magma invaded the magma chamber (Luhr and Charmichael, 1980; Halsor and Rose, 1991). The reaction textures imply that the magmatic water content for stability of hydrous minerals (such as amphibole and biotite) is too low because of decompression, or introduction and mixing of water-poor melt (Rutherford and Hill, 1993). These mafic mineral features reflect the disequilibrium crystallization with the parent magma.
Based on the geochemistry and mineralogical evidences, it seems that the Plio-Quaternary dacite and rhyodacite from Central Iran formed by melting of continental crust. The disequilibrium textures, mineral compositions and the presence of vesicular magmatic enclaves are all in support of interaction and mingling of dacite and more mafic magma. Following the partial melting of the local continental crust, dacitic magma penetrated the shallow level magma chamber. Amphibole, plagioclase, biotite and quartz crystallized prior to the injection of mafic magma. The presence of reaction rims on the mafic minerals (i.e amphibole, biotite), the sieved textured plagioclase and the rounded and embeyed felsic minerals are due to intrusion of small volume of mafic magma into the magma chamber. The low temperature silicic magma affect on the mafic magma caused quenching of mafic magma and accumulation of some bubbles at the silicic-mafic magma interface. Then mafic magma foamed up and plumes are found. In this entrainment, exchange of xenocrysts (calcic plagioclase and pargasite amphibole) might have occurred and plumes are detached to form the rounded mafic enclaves.