Heavy minerals from Urgonian limestone pebbles of the Northern Calcareous Alps (Austria, Bavaria): further evidence for an intra- Austroalpine suture zone

Shallow-water limestones of the Lower Cretaceous occur as allochthonous detritus within several Cretaceous and Tertiary formations of the Northern Calcareous Alps (NCA). Allodapic limestones and Urgonian limestone pebbles of a presumed NCA-internal origin are characterized by heavy mineral assemblages rich in chrome spinel. Microprobe analysis indicates the predominance of chrome spinels with relatively high Cr-contents, suggesting harzburgite-bearing ophiolite sequences in the source area. A similar geochemistry is known from detrital chrome spinels observed in the deep-water clastics of the Rossfeld Formation, which are derived from a southern, intra-Austroalpine source. On a chrome spinel-free base significant amounts of garnet and staurolite characterize the heavy mineral associations of both, the allochthonous Urgonian limestones as well as the Rossfeld Formation. Detrital chrome spinels in Urgonian limestone pebbles, therefore, represent further evidence for the existence of a Cretaceous intra-Austroalpine suture zone south of the Northern Calcareous Alps and its continuation into the Carpathian-Pannonian region. -from Authors