Die Meta-Basalte der Iberger Klippen

Abstract

The meta-basalts are part of a typical ophiolite assemblage and occur as exotic thrust slices of several meters thickness within of the nappe pile of the «Iberg Klippen» (for a detailed description see Trümpy 2006, this volume). The outcrop conditions are very poor due to forest and dense vegetation. The volcanics consist of pillow lava, pillow breccia, hyaloclastite, as well as of compact diabase sills interlayered with the extrusive pillow sequences. We use the classification scheme for volcanites in case of these slightly metamorphosed basalts since all structures and micro-fabrics are mostly still well preserved. All rock types (pillow lava, -breccia, hyaloclastite and diabase-sills) have been transformed to lower greenschist facies assemblages (pumpellyite-zone; Dietrich et al., 1974). Analcime may still be present according to an X-ray spectrum obtained on one sample. The mineral composition of the pillow lava consists of chlorite (ripidolite), sericite (muscovite), pumpellyite, sphene, hematite, albite, calcite and quartz. A high proportion of hematite and muscovite is present in reddish pillow lava and breccia. Meta-basaltic sills contain the minerals chlorite (ripidolite), albite, pumpellyite, sphene and calcite. Generally, all Iberg volcanics may be interpreted as basalts derived from a mid-oceanic ridge (analogous to «MORB» Middle Oceanic Ridge Basalt). They show a typical tholeiitic differentiation trend, whereas diabase, in particular sample Ob 5 (table 1) exhibits a quite undifferentiated olivine-basaltic character: MgO = 11.68 wt.%, Ni = 221 ppm and Cr = 426 ppm, as well as low Ti, Zr, Y, V, Sc, low K2O and Rb. In contrast, the hematitic pillow lava is fairly differentiated: MgO varies between 6.7 and 3.5 wt.%, Cr between 277–264 ppm, Ni between 134–82 ppm, and higher concentrations in Fe, Ti, Zr, Y, V, and Sc. The unusual high and variable concentrations of Na2O, K2O and Rb are interpreted to have resulted from oceanic alteration and hydrothermal processes as well as from weak regional Alpine metamorphism. However, despite postmagmatic metasomatic effects, the basaltic composition exhibits primary characteristics of slightly enriched (transitional) MORB. All data on the Iberg meta-basalts, including their stratigraphical contacts with the overlying upper Jurassic radiolarian cherts and calcareous limestones («Aptychenkalke», respectively «Calpionella limestones») and their chemical composition, indicate a close relationship to the «Arosa zone» and the «Upper Platta nappe thrust slices» of the southern Oberhalbstein valley (Grisons), Dietrich, 1969. Therefore it can be assumed, that these ophiolitic volcanics were part of an oceanic crust, cut by numerous transform faults and part of a rather small «Piemont ocean», representing the northern extension of the «Ligurian Tethys» located between the Eurasian plate and a «Southern» continental margin consisting of the South-Alpine, Adriatic and Austroalpine micro-plates.