Structural petrology of the garnet-peridotite of Alpe Arami (Ticino, Switzerland)

The decomposition of pyrope-rich garnet into spinel-amphibole symplektite and the alteration of the latter into chlorite indicate the presence of three successive mineral parageneses in the peridotite of Alpe Arami: a garnet-peridotite, a spinelamphibole-peridotite, and a chlorite-peridotite association. Titanclinohumite may be a relict mineral from an older assemblage. The peridotite lens lies between gneisses of the root zone of the Pennine nappes of the Alps. The main phase of metamorphism (F1) of these gneisses is of Alpine age. They have a steeply southward-dipping schistosity (S1). In the peridotite, chlorite nodules deriving from altered garnets are flattened parallel to the schistosity planes of the surrounding gneisses; the formation of chlorite was contemporaneous with F1. S1-planes are also present as cleavage planes. Due to quantitative differences in the mineralogical composition of the peridotite, a layering plane (S1) can be observed; it may show a mineral lineation (10). In the southern part of the peridotite lens, SL is subparallel to S1 and 10 pitches steeply to the west, whereas in the northern part SL dips moderately to the north, with 10 plunging to the north. Petrofabric analysis shows that the orientation of the garnet-peridotite minerals is dependent on the orientation of SL: \u03b3-olivine and \u03b2-orthopyroxene are perpendicular to SL; the orientation of olivine is weaker than the orthopyroxene orientation, \u03b3-orthopyroxene and c-clinopyroxene are parallel to 10 (= 1’0). The alteration of garnet-peridotite into spinel-amphiboleperidotite is related to schistosity planes S0 that are mostly parallel to SL. SL/S0-fold axes are parallel to 10-lineations. In the spinel-amphibole-peridotites, \u03b3-olivine is perpendicular to S0, and \u03b3-orthopyroxene and c-amphibole are parallel to 10 (= 1""0). The fabrics of olivine, orthopyroxene, and amphibole are not affected by the transformation of spinel-amphibole- into chloriteperidotite, except for the olivine fabric of a chlorite-peridotite-mylonite. The garnet-peridotite fabric is inconsistent with a magmatic genesis. It may have been brought about by plastic deformation followed by recrystallization. The formation of the garnet-peridotite and its alteration into spinel-amphibole-peridotite took place before the emplacement of the peridotite into its present position, which occurred during the Alpine orogeny. The garnet-peridotite may represent a fragment of old basement material, or it may have been formed by a phase of high-pressure metamorphism of Alpine age, which antedated F1. Petrofabric analysis of biaxial minerals should preferably be done with a five-axis universal stage. Fabric diagrams can be prepared with a computer. Calculation of statistical parameters removes uncertainties from the interpretation of the diagrams.