Zoisite-Quartz Segregations in Garnet-Amphibolites and Their Significance for Fluid Flow

A. Brunsmann Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany


G. Franz Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany

J. Erzinger GeoForschungsZentrum Potsdam, Telegrafenberg A17, D-14473 Potsdam, Germany

Segregations are important indicators of fluid flow and brittle behaviour in rocks at metamorphic conditions. We report an example of zoisite-quartz segregations from the south central Lower Schieferhülle unit, Tauern Window, Austria. Data for the prealpine evolution of this part of the Lower Schieferhülle indicate an early (t = 400-500 Ma) high-pressure metamorphism at P = 12-16 kbar, T ª 500 °C, followed by a hercynian stage (P < 5 kbar; T > 630 °C), which produced contact metamorphism and local anatexis. The alpine P-T-t path is characterized by an eoalpine (ª 60 Ma) high-pressure event at P = 10-12 kbar; T ª 450°C, followed by the Tauerncrystallization (20-35 Ma) at amphibolite facies conditions ( P ª 6 kbar; T = 520-550 °C).

The host rocks of the segragations are m- to 100 m-sized boudins of garnet amphibolites with alkali basaltic to tholeiitic composition (within gneissic country rocks). Symplectites and relict omphacites indicate the prealpine eclogite event. The segregations are dm-sized and contain up to 10 cm large zoisite crystals in a matrix of quartz and/or calcite. Three stages of evolution can be recognized: Stage 1 is represented by the paragenesis zoisite+quartz+calcite. In stage 2 some of the segregations were deformed (kink-bands in zoisite, shearing, rotation into the foliation of the host rock). Stage 3 is the breakdown of the primary paragenesis (especially of zoisite) into clinozoisite+paragonite+phengite+albite An2-7+calcite+clinochlore. This breakdown is strongly controlled by deformation induced (100)-cleavage of zoisite, and is less prominent or absent in undeformed crystals.

The primary zoisite crystals display an (often periodic) growth-zonation with Fe3+ poor cores (8-11 mol% Al2Fe) and 17-21 mol% in the rims. Clinozoisite (stage 3) shows maximum contents of 40-50 mol% Al2Fe. Whole rock and mineral chemical composition along profiles from the host rock to the segregation shows that major elements do not change significantly, but minor elements (such as TiO2 and P2O5) and trace elements (Nb, LREE, Th, Rb, Cu) strongly decrease within approximately 10 cm towards the segregation. LREE contents in the zoisites and clinozoisites within the segregation are high (ª 10 to 100 x chondritic). However, as estimated from the modal amount of minerals, they are in the whole segregation still much lower than in the host rock. No Na-mineral is present in the primary mineral assemblage.

We propose the following model for the generation of the segregations: They were formed at the first pre-Alpine high-pressure event from an H2O-dominated (XCO2 <0.2) fluid. Dissolved species in the fluid, as recorded by the solid phases, must include Si, Al, Ca and minor Fe3+. Other species probably present in equilibrium with an alkali-basaltic eclogite, such as Na and K and also a significant part of the REE, Nb, Rb, Th and Cu were not precipitated in the segregation. Many of the segregations survived the later overprint (stage 3), but whereever this is recorded by decomposition of the primary mineral assemblage, the presence of albite and chlorite shows the introduction of a fluid with Na and Mg into the segregation. Calculation of the phase equilibria during stage 3 indicates that this was the eoalpine high pressure event where the fluid was also H2O-dominated (XCO2 <0.2). In both stages, however, the segregations with sharp contacts to the host rock recorded brittle behaviour of the host rocks at near-eclogite facies conditions.