Journal of the Virtual Explorer

Abstract

The Plio-Quaternary calc-alkaline dacitic and rhyodacitic lava flows and domes are located in Yazd province within the central Iranian magmatic belt. These rocks are composed of plagioclase, hornblende, biotite, and quartz set in a glassy to felsity groundmass. The subrounded to vesicular magmatic enclaves, ranging in size from a few millimeters to a few centimeters and similar mineral assemblage to their host rocks, are recognized in the studied rocks.

The variety of disequilibrium features which are observed in all phases can be taken as an evidence of magma mingling. The plagioclase phenocrysts exhibit sieved- and normal-type populations, dusty core or rim, a wide range in rim compositions and oscillatory zoning. Hornblende and biotite phenocrysts have reaction rims and are replaced by Fe-Ti oxides indicating disequilibrium crystallization with magma. Both hornblende and biotite crystals show reverse zoning and the Mg/Mg+Fe ratio increases from core to rim. Quartz phenocrysts are corroded and embayed.

Although the existence of sieve texture in plagioclase and reaction rims on mafic minerals may be considered to be the result of decompression, this cannot explain the observed compositional changes and the co-existence of normal types of plagioclase in the same sample. In fact, other evidence such as resorbed and embayed phenocrysts indicate that the mafic magma is intruded into felsic magma and magma mingling was a viable process in the generation of these volcanics.