In the SW-Schobergruppe basic eclogites and eclogitic amphibolites occur as lenses within a sequence of polymetamorphic paragneisses and micaschists. Microclinegneisses are accompanying the eclogites that frequently contain concordant intercalations (dm - m scale) of metasediments. Sm-Nd and Rb-Sr mineralisochrons and whole rocks of eclogites point to Eo-Alpine high-pressure metamorphism (around 100 Ma) and subsequent uplift and cooling in the Upper Cretaceous (90-70 Ma). Rb-Sr data of microclinegneisses and micaschists show that muscovites do not preserve relics of an older metamorphic event. Biotite, on the other hand, does not record any post-Cretaceous influence.
The eclogites interlayered by metasediments have a geochemical composition similar to MORB (Schulz, 1995). They show flat chondrite-normalized REE patterns with LaN/YbN=1.5 at 20 times chondrite abundance. Eclogites without intercalations of metasediments show pronounced banding, defined by layering with garnet-, omphacite- and amphibole-rich layers. Their chemical composition suggests layered gabbros as precursors.
The paragneisses and micaschists alternating with the eclogites exhibit unusual isotopic and chemical compositions. With only slightly higher 147Sm/144Nd-ratios if compared with "normal" basement they exhibit much less negative present-day eNd-value of -2.5 (micaschists) to -4.1 (paragneisses). In contrast, metasediments of the surrounding basement have a common present-day eNd-value of -14.5 and their Nd model age (TDM=1.6 Ga) and Sm-Nd isotope systematics fit well with those of metasediments of the greater Austroalpine basement unit (Thöni, 1994). Also, the 87Sr/86Sr-ratios are significantly lower in interlayered metasediments than in paragneisses from outside the eclogite sequence. Additionally, the chondrite-normalized REE pattern of interlayered micaschists tends to have lower LREE abundances but slightly higher abundances of HREE (LaN/YbN=3.6) than the surrounding metasediments (LaN/YbN=13.3).
These isotopic and geochemical data of the metasediments occurring as layer within eclogites and eclogitic amphibolites is interpreted to give evidence that the whole sequence reflects a primary association of basalts and sediments, where in situ decomposition of basalts may have significantly influenced the REE composition and isotopic signatures of the sediments.
Schulz, B., Schweiz. Mineral. Petrogr. Mitt. 75, 225-239 (1995).
Thöni, M., Mitt. Ges. Geol.-Bergbaustud.Österr. 39/40, 187-209 (1994).