Gneisses in the Orlica-Śnieżnik Dome, West Sudetes: a single batholitic protolith or a more complex origin?

Abstract

Detailed structural and petrographic studies confirmed the presence of two major units of ca. 515–480 Ma gneisses in the Orlica-Śnieżnik Dome (NE Bohemian Massif) and enabled the distinction of two formations which differ in their mineral composition (modal and chemical) and structural records. An intrusive contact between rocks of the two formations was observed. The Gierałtów Gneiss Formation is composed of rocks having at least two sets of folded metamorphic foliations, with relics of compositional banding and records of early shearing prior to migmatization and metablastesis which produced quartzofeldspathic segregations (D1–D2 events). Such aggregates, even if isometric and shared (D3) may, but must not be mistaken for original augens (porphyroclasts in the original granite). Modal contents of the feldspars differ widely (20–40% of plagioclase feldspar, 16–40% of alkali feldspar) as well as their composition (Ab_0-90, An_6-38); the biotites can be either poor or enormously enriched in Al (0.26–1.07 Al^Vi). Such heterogeneities are consistent with the inferred metamorphic transformations of originally diversified sedimentary-volcanogenic protoliths. in contrast, the Śnieżnik Gneiss Formation is composed of metagranites, dynamically metamorphosed into the augen gneisses. They possess only one set of mylonitic foliation and one rodding lineation, both developed during a regional shear event (D3). Nearly equal modes of feldspars and quartz, uniform composition of plagioclase feldspar (An_6-23) and a rather stable amount of Al (0.3–08 Al^Vi) in the biotites are indicative of homogenization of a granitic protolith. Anatectic provenance of the gneisses is evidenced by enclaves. Felsic microgranular enclaves are chilled fragments of the parental intrusion, while xenoliths and surmicaceous enclaves are akin to rocks of the Gierałtów Formation, thus the latter or equivalent rocks formed a migmatic envelope of the Śnieżnik granite pluton.