NOMARSKI STUDY OF ZONED PLAGIOCLASES FROM GRANITOIDS OF THE SEVILLE RANGE BATHOLITH, SW SPAIN - PETROGENETIC IMPLICATIONS

The application of Nomarski imagery to the study of etched plagioclase s from intermediate plutonic rocks, reveals many features not observab le through conventional polarizing microscopy. The most salient featur es are resorption surfaces indicative of dramatic changes in condition s during magma consolidation. The distribution of these resorption sur faces across the crystals is related to changes in the anorthite conte nt of the melt in an open system induced by magma mixing. These featur es are typical of hybrid granitoids, such as the granodiorites and ton alites of the Seville Range batholith, Spain. Tn these rocks, previous geochemical and isotopic studies indicate mixing between a felsic end -member and a fractionated mantle-derived magma at a shallow depth in the crust. The cyclicity of resorption processes, repeated several tim es during plagioclase growth, is consistent with some kind of self-org anized process related to convection induced by mafic-magma input in t he composite magma chamber. Alternatively, cyclicity may be related to repeated pulses of mafic magma.