Geochemistry and tectonic environment of Ordovician meta-igneous rocks in the Rudawy Janowickie Complex, SW Poland

Abstract

A bimodal suite of Ordovician metavolcanic rocks in the Rudawy Janowickie Mountains and the Lasocki Ridge (Sudetic region, SW Poland) is dominated by voluminous epidote amphibolites and greenschists derived from basalt, dolerite and gabbro precursors. Less abundant felsic components comprise both mylonitized granitic sheets that intrude the mafic rocks, and minor interlaminated volcanic rocks ranging from andesite to rhyolite in composition. These rocks are collectively referred to as the Rudawy Janowickie Complex. In chemical terms the mafic volcanic rocks define three north-south-trending provinces (western, central and eastern) which are separated tectonically by broad major mylonite zones. The western province contains all the alkali basalts in the study area and minor tholeiites, all exhibiting within-plate features: specifically high Zr/Y (>4) and enriched chondrite-normalized incompatible element patterns. The central province contains two fractionated tholeiitic suites (Leszczyniec and Okraj groups) with low Zr/Y ratios (2.0-3.5), depleted to flat REE patterns, and incompatible element compositions ranging from normal MORB to slightly enriched MORB. The eastern province has some features in common with the central province, notably metabasalts with normalized flat REE patterns, although it is also characterized by metagabbro bodies with enriched light REE patterns ((La/Yb) N 4-7), but depleted high field strength element contents. In tectonic terms the Rudawy Janowickie complex is interpreted as a former intracratonic rift floored by oceanic crust and bounded to the west by an attenuated continental margin. Existing data do not allow a definitive conclusion about the exact nature of the eastern margin of the basin: it might, in part reflect the presence of a distant subduction zone.