The microcline/sanidine transformation isograd in metamorphic regions: Western Tauern Window and Merano–Mules–Anterselva Complex (Eastern Alps)

Lattice parameters and optic axial angles of alkali feldspars from the Penninic augen gneisses of the western Tauem Window and from the Austroalpine Merano–Mules–Anterselva unit south of it have been determined along eight N–S profiles.

Parallel and somewhat below the 500°C isotherm of the syn- to post-kinematic metamorphism of Tertiary age the K feldspars show an abrupt change in their optic axial angle and their Al–Si order. This line separates a northern area showing exclusively low microclines (2V_X ≥ 76° and 0.95 ≤ (t₁O–t₁m) ≤ 1.0) from a southern area showing “high microclines” (2V_X ≤ 76° and 0.0 ≤ (t₁O–t₁m) ≤ 0.55). The border line represents the isotherm at which the diffusive phase transformation microcline/sanidine occurred during prograde Alpine metamorphism.

The Merano–Mules–Anterselva complex is characterized by low grade Hercynian metamorphism below 500°C. In spite of the fact that north of the Defereggen–Anterselva–Valles line the Alpine metamorphism seems to have exceeded 500°C all investigated K feldspars of the complex are low microclines.