The metabasites from the Texel Unit (Austroalpine nappe stack): markers of Cretaceous intracontinental subduction and subsequent collision

Abstract

Abstract Eclogites in the Texel Unit (Austroalpine nappe stack; Eastern Alps, South Tyrol, Italy) represent the westernmost out-crops of the E-W striking Eoalpine High-Pressure Belt (EHB). East of the Tauern Window, the EHB forms part of a Cretaceous intracontinental south-dipping subduction/collision zone. The impetus of this contribution is to extend the geothermo-barometric data of the eclogites from the EHB using conventional geothermobarometry and multi-equilibrium calculations as well as Zr-in-rutile/titanite geothermometry and to put the P-T results of eclogites and amphibolites from the Texel Unit into the geodynamic framework. The investigated samples of this study are from the Spronser- and the Saltaus valleys (S-Tyrol) in the Austroalpine Texel Unit. The Texel Unit is composed mainly of paragneisses with minor intercalations of micaschists, orthogneisses, amphibolites and subordinately eclogites. The amphibolites from the Spronser valley contain the mineral assemblage amphibole + plagioclase + garnet + clinozoisite/epidote + quartz + titanite ± ilmenite ± rutile ± apatite ± calcite. Chemical zoning in plagioclase and amphibole shows two main growth stages: an older P-dominated stage (e.g. albite and barroisite cores) and a younger amphibolite-facies stage. The core of the amphiboles shows barroisite composition, the rim can be chemically classified as hornblende, edenite, tschermakite and pargasite. Geothermobarometric calculations with multi-equilibrium geothermobarometry (THERMOCALC v.3.21) yield temperatures of 600–654°C and pressures of 0.98–1.17 GPa for the same samples. The eclogites from the Saltaus valley contain the peak mineral assemblage omphacite + amphibole + garnet + clinozoisite/epidote + muscovite + quartz + titanite ± ilmenite ± rutile. Thermobarometric calculations, were performed in the system CaO-FeO-MgO-Na2O-Al2O3-FeO-SiO2-H2O with the assemblage clinopyroxene + garnet + amphibole + clinozoisite/epidote + muscovite + quartz ± H2O. The calculations involved an H2O-absent invariant point (mode-1 calculation) as well as two types of average P-T mode-2 calculations. The obtained average P-T H2O-absent mode 1 P-T conditions are 1.89 ± 0.18 GPa and 578 ± 60°C. Using the average P-T mode-2 two types of calculations were done: (1) calculations without amphiboles but with H2O present, which yield mean P-T conditions of 1.95 ± 0.28 GPa and 601 ± 55°C and (2) calculations with amphiboles but without H2O, which yield mean P-T conditions of 1.95 ± 0.26 GPa and 666 ± 77°C. Calculations using THERMOCALC v.3.33 yield similar results with slightly higher pressures of 0.3 GPa. Based on the present geothermobarometric data in conjunction with available mineral ages the eclogites represent the Eoalpine intracontinental subduction stage whereas the amphibolites reflect the subsequent, P-accentuated stage of decompression associated with the subsequent Eoalpine collisional stage.