Age and geotectonic setting of metavolcanic rocks in the eastern Saxothuringian margin: whole rock geochemistry, zircon trace element geochemistry and U-Pb geochronology of the Staré Město Belt (Czech Republic, Poland)

Late Proterozoic to Early Palaeozoic metavolcano-sedimentary successions are important components of the Variscan massifs of Europe. Felsic and mafic metavolcanic rocks with Cambro-Ordovician protolith ages also occurs in the Staré Město Belt (SMB) in the Central Sudetes (Czech Republic, Poland) (e.g. Kröner et al. 2000). The SMB is the NNW-trending fold-and-thrust belt that forms the eastern margin of the Saxothuringian Zone of the Bohemian Massif. To constrain timing and geodynamic setting of the volcanism recorded in that part of the Saxothuringia, the whole rock geochemistry, zircon trace element geochemistry and U-Pb zircon geochronology of metabasalts, metagabbros and acid metavolcanites of the SMB were carried out.

Field and petrographic studies show that bimodal association in the SMB is mainly expressed by alternating layers of fine-grained amphibolites composed of Amp, Pl and Px and fine- and medium-grained acid metavolcanites composed of Qz, Pl, Kfs, Grt, Bt and Ms. Such close relationships between felsic and mafic meta-volcanic rocks suggest their common origin. Whole-rock geochemistry data suggest, however, a diversity both in the chemical composition and tectonic environments of formation of their igneous protoliths. Magmatic precursors of the amphibolites were tholeiitic and calc-alkaline basalts, andesitic basalts and andesites that were derived either from MORB, BABB, volcanic arc or within-plate magmas. The acid metavolcanites originated from rhyolites and dacites belonging to tholeiite, calc and calc-alkaline series. Geotectonic diagrams suggest that the felsic magmas were formed most likely in island arc or continental arc environments.

New LA-ICPMS zircon dating of two metadetrital rocks of the SMB revealed the predominance of Neoproterozoic-Cambrian and Palaeoproterozoic age clusters, characteristic for rocks of the Saxothuringian Zone. Zircon dating of four samples of acid metavolcanites, two samples of metabasalts and one sample of metagabbro confirmed that their igneous protoliths crystalized at the same time, at ca. 495-500 Ma. Trace elements in zircons were analyzed in all metavolcanic samples. Range of values of Nb/Yb = 0.001-0.1, U/Yb = 0.1-10 and Y = 25-6993 ppm are observed in both types of rocks and together indicate a contribution of continental crust in the SMB volcanites. Their values plotted on geotectonic classification diagrams of Grimes et al. (2015) suggest a continental arc setting for the whole Late Cambrian bimodal volcanism in the easternmost part of the Saxothuringian Zone.

The research was financed from the grant of the National Science Center, Poland No. 2018/29/B/ST10/01120.

 

References:

Grimes, C.B., Wooden, J.L., Cheadle, M.J., John, B.E., 2015.  “Fingerprinting” tectono-magmatic provenance using trace elements in igneous zircon. Contrib Mineral Petrol 170, 46.

Kröner, A., Štipská, P., Schulmann, K., Jaeckel, P., 2000. Chronological constraints on the pre-Variscan evolution of the northeastern margin of the Bohemian Massif, Czech Republic. In: Franke, W., Haak, V., Oncken, O., Tanner, D. (Eds.), Orogenic Processes: Quantification and Modelling in the Variscan Belt. Geological Society, London, Special Publications 179, pp. 175–197.