AbstractLate Cenozoic basalts of the Bazman volcanic field, Makran volcanic arc of
southern Iran, contain two types of plagioclase feldspar phenocrysts with
significant textural and compositional differences. The most common type is
rather homogeneous with only weak zoning and maximum An content of 83 mol.%.
The less common type of phenocryst exhibits complex zoning and, other than
rims, is close in composition and similar in texture to those of associated
andesites. This type of plagioclase phenocryst is characterized by an
engulfed core with oscillatory zoning, which is overgrown by sieve-textured,
moderately zoned mantle, and a relatively narrow rim. In both rock types,
the An content of the core is between 40 and 63 mol.% with abrupt
fluctuations. No significant correlation between An content and MgO, FeO,
SrO and BaO is apparent in the core of phenocrysts in basalts. Anorthite
content of the core of phenocrysts in andesites inversely correlates with
SrO and BaO. The mantle of plagioclase phenocrysts in both rock types is
characterized by sharp increases of An (up to 41 mol.%), MgO, and FeO, in
the contact with the core. Anorthite correlates positively with MgO and FeO
in the mantle, but correlation between An and SrO and BaO is not evident. It
is assumed that plagioclase phenocrysts originally crystallizing from the
host andesitic magma were interrupted by mixing with a hotter, juvenile
basaltic magma. The resulting changes in temperature, composition, and
H2O content of the surrounding melt caused compositional
zonation, and the development of resorption in the cores and sieve texture
in the mantles. As the An contents of the rims of the phenocrysts resemble
the average An content of the groundmass plagioclases in both rock types, it
is thought that the two involved magmas gained their independent physical
identity before the formation of compositionally-distinct rims of
plagioclase phenocrysts.