Petrogenesis and geotectonic setting of ultramafic rocks in the Eastern Alps: constraints from geochemistry

Abstract

In the Eastern Alps, a heterogeneous pile of Pre-Mesozoic and Mesozoic nappes was emplaced during several distinct orogenic events. Ultramafic rocks are preserved in: (1) Pre-Mesozoic basement units of the Penninic Tauern Window and in Austroalpine basement complexes, and (2) in Mesozoic units within Penninic windows and in overlying Austroalpine segments. Geochemical data (major, trace and rare earth elements, REE, of bulk rock samples) of ultramafic rocks from 18 regions covering all the major tectonic units are presented. Metamorphic olivine-rich and clinopyroxene-rich ultramafic rocks and serpentinites from the Cambro-Ordovician Lower Schist cover units of the Tauern Window (Stubach Group, “Greiner Series”) are grouped into peridotites (#Mg values=molecular 100×Mg/(Mg+Fe tot), 87–92; <2 wt.% Al 2O 3) and clinopyroxene-rich pyroxenites (#Mg=80–92; 1–3 wt.% Al 2O 3). Major and trace element characteristics indicate a residual origin of most peridotites as restites after 10–20% partial melting. Most probably, the pyroxenites are crystal segregates from a basaltic melt in a mantle transition zone. Small lenses of completely serpentinised ultramafic rocks in the Cambrian Habach Group (#Mg=90–92) also have residual character, although they are strongly metasomatised. The Stubach and Habach Groups developed in a back arc and volcanic arc setting, respectively, along the northern margin of Gondwana. Metamorphic harzburgite and dunite (#Mg=89–90; 0.5–2.4 wt.% Al 2O 3) veined by metagabbroic dikes were sampled in the Austroalpine Silvretta nappe (Hochnörderer). The harzburgites represent restites after 12% partial melting in suboceanic mantle, most probably during the Cambrian. In the Austroalpine basement east of the Tauern Window, metamorphosed ultramafic rocks occur in association with amphibolite and eclogite in the Proterozoic (about 750 Ma) Speik Complex. Harzburgite and dunite (#Mg=89–92; 0.1–0.8 wt.% Al 2O 3) are highly depleted in incompatible elements (2.6–35 ppb Yb) and are interlayered with small amounts of more fertile peridotite (amphibole-rich harzburgite and lherzolite, #Mg=89; up to 3.0 wt.% Al 2O 3), metaclinopyroxenite (#Mg∼85) and hornblendite. Coarse-grained orthopyroxenite (#Mg=89–92; 0.5–0.7 wt.% Al 2O 3; 19–36 ppb Yb) is present as veins and stocks in the harzburgite–dunite sequence. Chondrite-normalised rare earth element patterns of harzburgite–dunite and orthopyroxenite are characteristically U-shaped. The geochemical data indicate that harzburgites and some dunites are products of multiple melting events. They can be modelled as restites after 20–30% partial melting from already depleted mantle in a suprasubduction zone geotectonic setting. Orthopyroxenites crystallised from high-(Si, Mg) melts with a significant crustal component, probably during a Cambrian subduction event. In Mesozoic units of the Penninic windows (Lower Engadine, Tauern and Rechnitz windows), serpentinised ultramafic rocks of harzburgitic composition (#Mg=90–92; 1.1–2.0 wt.% Al 2O 3) are associated with metagabbro, metabasalt, radiolarite and ophicarbonate. Furthermore, serpentinised lherzolites (#Mg=88–90; 2.9–4.6 wt.% Al 2O 3) are exposed in the Matrei Zone (a tectonic mélange zone between Penninic and Austroalpine nappes), in the Lower Austroalpine Reckner Complex and in a body of unclear tectonic affiliation at the southern margin of the Lower Engadine Window at Nauders. The harzburgites are residual mantle left behind after 10–15% partial melting of a fertile source, and form part of the Mesozoic (Jurassic) incomplete ophiolite sequences of the Ligurian–Piemontais ocean. The lherzolites, having experienced less than 10% partial melting, are either attributable to an early (Permian) rifting episode during break-up of Pangaea, or represent subcontinental mantle from the Adria plate tectonically incorporated into the Neo-Tethys.