Complex Growth Textures in a Polymetamorphic Metabasite from the Kraubath Massif (Eastern Alps)

Abstract

Zoned garnet and amphibole occur in metabasites of the Kraubath Massif, Eastern Alps, that contain relic magmatic clinopyroxene. The amphibole composition gradually changes from core (XMg 1⁄4 0 83) to rim (XMg 1⁄4 0 6–0 7). A number of compositional varieties of garnet occur in the metabasite. An older porphyroblastic garnet (Py23–27, Alm41–43, Grs29–33) has two different compositional domains, one relatively rich in Mg (Py27–30) and the other rich in Ca (Grs35–38) with a low Mg (Py20–25) content. The youngest variety, which forms rims on, or microveins in, the porphyroblastic garnet, has high Ca and low Mg (Grs40–57, Py2–7, Alm46–51). The amphibole cores and garnet porphyroblasts are interpreted to represent minerals formed during Variscan regional metamorphism under amphibolite-facies conditions. Alpine metamorphism is represented by the most recent Ca-rich and Mg-poor variety of garnet that coexists with the amphibole rims, epidote and chlorite. Fracturing in the porphyroblastic garnet probably originated during retrogression of the Variscan amphibolite-facies assemblages. Textural relations suggest that the garnet in the microveins formed by dehydration of hydrous phases during an Alpine metamorphic overprint that reached P–T conditions of 550–583 C at 1 0 GPa.