Subduction of a rifted passive continental margin: the Pohorje case of Eastern Alps - constraints from geochronology and geochemistry

A-type subduction is considered to occur at the final stage of continent-continent collision. In many cases, the UHP/HP metamorphic conditions are well known but data on the type of subducted continental crust is lacking. In terms of end members, the type of subducted crust is either (1) normal thick continental crust or (2) the crust from the center of a rift zone, which is influenced by strong extension, high-temperature metamorphism due to thinning of even the continental mantle lithosphere and strong magmatism. To resolve these alternative scenarios, we investigated the southernmost part of the Eclogite-Gneiss Unit (EGU) of Cretaceous metamorphic age exposed in the Pohorje Mountains in Eastern Alps. There, UHP eclogites and ultramafic mantle rocks are exposed in a matrix of paragneiss and hitherto undated granitic orthogneises (Kirst et al., 2010). This study presents, for the first time, geochronological and geochemical data from newly discovered Permian granitic orthogneisses in this area. LA-ICP-MS zircon U–Pb ages of the orthogneisses are 255±2.2 Ma and 260±0.81 Ma, which are interpreted as the age of zircon crystallization in a magma. In contrast, all rounded zircons from paragneissic rocks give Cretaceous ages (89.34±0.69 Ma and 90.8±1.2 Ma), considered as the age of UHP/HP metamorphism. These zircons overgrew older zircons of Permian and rare older ages tentatively indicating that the metasedimentary could be not older than latest Permian. Zircon εHf(t) values of the four ortho- and paragneisses with (¹⁷⁶Hf/¹⁷⁷Hf) initial from 0.282201 to 0.282562, T_DM2 are Proterozoic (1390~1970 Ma). The granitic orthogneisses show the geochemical features (high (La/Lu) _N ratios (160.3–307.3), strong negative Eu anomalies) of an evolved granite molten from continental crust. This type of orthogneisses could be considered as the source magma of seemingly rootless Late Permian to Triassic pegmatites (Knoll et al., 2018) widespread within the EGU further to the north. The paragneisses are heterogeneously composed and are associated with eclogites and ultramafic cumulates of oceanic affinity (De Hoog et al., 2011). We argue that the Permian granitic orthogneisses might be derived from partial melting of lower crust in a rift zone. We consider, therefore, this segment of the EGU as part of the distal Late Permian rift zone, which finally led to the opening of the Meliata Ocean during Middle Triassic times. If true, the new data also imply that the stretched continental crust was potentially not much wider than ca. 100 km, was subducted and then rapidly exhumed during early Late Cretaceous times.

 

References

De Hoog, J.C.M., Janák, M., Vrabec, M., Hatton, K.H., 2011. In: Dobrzhinetskaya, L., Faryad, S.W., Wallis, S., Cuthbert, S. (Eds.), Ultrahigh-pressure Metamorphism: 25 Years After the Discovery of Coesite and Diamond. Elsevier Insights, pp. 399–439.

Janák, M., Froitzheim, N., Yoshida, K., Sasinková, V., Nosko, M., Kobayashi, T., Hirajima, T., Vrabec, M., 2015. Journal of Metamorphic Geology 33, 495–512.

Kirst, F., Sandmann. S., Nagel. T., et al. 2010. Geologica Carpathica 61(6), 451-461.

Knoll, T., Schuster, R., Huet, B., Mali, H., Onuk, P., Horschinegg, M., Ertl, A., Giester, G., 2018. Canadian Mineralogist 56, 489-528.