Abstract
Amphiboles from mafic fades within a Late Proterozoic granodioritic complex pluton show a variety of textures similar to those reported for amphiboles from porphyry-copper granodiorites. Complexly zoned phenocrysts of hornblende have cores of pargasite, cli-nopyroxene, or actinolite and actinolitic hornblende, and some have rims of actinolitic hornblende. Hornblende shows patchy compositional domains with gradational boundaries and also occurs as crystal aggregates. Actinolite occurs as small euhedral crystals in patches of actinolitic hornblende. Electron-microprobe analyses show clusters of data defining four compositional types of amphibole—pargasite, hornblende, actinolitic hornblende, and actinolite. Of these, actinolite shows coupled substitutions different from those of hornblende and pargasite, and variability in actinolitic hornblende falls on either trend. Early minerals crystallized under conditions of low oxygen fugacity, indicated by low Mg content of amphiboles and common sulfide inclusions. Later minerals crystallized under conditions of increasingly higher oxygen fugacity, indicated by high Mg content of amphiboles and abundance of opaque oxides. There is no evidence that the development of actinolite is associated with a discrete late hydrothermal event; rather, it is the result of subsolidus alteration of clinopyroxene. The development of these amphibole textures appears more dependent on the evolution of volatiles than the chemical composition of the magma.
