A Metamorphosed Early Cambrian Crust-Mantle Transition in the Eastern Alps, Austria

Abstract

In the Speik Complex (Eastern Alps, Austria), highly melt-depleted, metamorphosed harzburgites with abundant pods and layers of chromitite are interlayered with a suite of metamorphosed orthopyroxenites, clinopyroxenites and gabbros. Coarse-grained orthopyroxenites occur as centimetre- to metre-wide veinlets and pods, but also as intrusive plugs several tens of metres wide. Intimately associated metaclinopyroxenite and metagabbro are present as bodies up to several metres thick at a distinct stratigraphic level within the complex. In the ultramafic rocks, relict magmatic olivine, orthopyroxene, clinopyroxene and spinel have been overprinted by a metamorphic assemblage of forsterite, diopside, tremolite, anthophyllite, chlorite, serpentine, talc and Cr–Fe-rich spinel. Hornblende, epidote, zoisite and chlorite dominate the metamorphic paragenesis in metagabbros, in addition to rare relicts of clinopyroxene and two phases of Ca-rich garnet. The polymetamorphic evolution of the Speik Complex includes rarely preserved pre-Variscan (400Ma) eclogite-facies conditions, Variscan ( � 330Ma) amphibolite-facies conditions (600–700 � C, >5kbar) and Eoalpine ( � 100Ma) greenschist- to amphibolite-facies conditions reaching 550 � C and 7–10kbar. Orthopyroxenites are characterized by high concentrations of SiO2, MgO and Cr, and by U-shaped chondrite-normalized rare earth element (REE) patterns similar to those of their harzburgite hosts. The REE patterns of the clinopyroxenites are flat to slightly enriched in light REE. Metagabbro compositions are variable, but generally characterized by low SiO2 and high mg-numbers (61–78). Their REE patterns all have GdN/YbN > 1; some samples have large positive Eu anomalies implying the original presence of cumulus plagioclase. In the orthopyroxenites, clinopyroxenites and some peridotites, Pt, Pd and Re are distinctly enriched compared with Os, Ir and Ru, whereas most harzburgites have unfractionated to slightly fractionated platinum-group element (PGE) patterns with respect to average upper mantle. The Re–Os isotope compositions of the pyroxenites define an errorchron at 550 � 17Ma and a supra-chondritic 187 Os/ 188 Os of 0� 179 � 0� 003. An isochron age of 554 � 37Ma with eNd(i) þ0� 7 is indicated by the Sm–Nd isotope compositions of whole-rock pyroxenite and gabbro samples, whereas the harzburgites plot on an errorchron of 745 � 45Ma and eNd(i) þ6. The pyroxenites and gabbros probably represent a cogenetic suite of magmatic dykes intruded into uppermost, highly depleted, suboceanic mantle below the crust–mantle transition zone in an oceanic basin close to the northwestern margin of Gondwana.