Facies and provenance analysis of Upper Carboniferous to Lower Permian fan sequences at a convergent plate margin, using phyllosilicates, heavy minerals and rock fragments (Erbendorf Trough, F.R.G.)

Abstract

The Erbendorf Trough is a narrow Permo-Carboniferous graben in the northeast Bavarian crystalline basement at the Saxothuringian-Moldanubian plate edge. Its elastic and volcanic rocks reflect a complex story of alluvial fan progradation and retreat. Unit I (Precambrian gneissic bedrock), is followed by units II to V, a wet/mixed fan (debris-stream flows, braided streams, peat swamps, organic lake). Units VI to X reflect a retrogradational dry fan (braided streams, calcretes, flood plain deposition with tuffaceous interbeds), units XI to XII are acid extrusive rocks (the uppermost part hydrothermally altered) and units XIII to XIV denote a progradational dry fan. The wet fan at the base is characterized by a predominance of illite, stable to intermediate heavy mineral associations (zircon and garnet) and a clast community pointing to medium pressure metamorphic rocks in the Variscan nappe. The following dry fan contains various 14 Å phyllosilicates besides white mica. Its heavy mineral suite contains more labile constituents (apatite and amphibole). Gneissic pebbles accompany those from acid volcanic and plutonic rocks. The volcanogenic rocks contain expandable 14 Å phyllosilicates, formed by devitrification. Anatase and brookite dominate the heavy minerals. TiO 2 associated with phosphate indicates hydrothermal alteration of host rock, whereas detrital TiO 2 and a high ZTR ( = zircon-tourmaline-rutile) index reflect paleosol formation as shown at the wet-dry fan boundary. Fan progradation occurred under semi-arid conditions, with kaolinization and formation of illite (partly detrital), and a moderately stable heavy mineral association (apatite-dominated). Owing to the kaolinization only quartz-bearing gneisses were preserved. The facies succession reflect the evolution from a fault-bordered basin, with various fan sections (proximal vs. distal) preserved, into a caldera-like volcanotectonic depression. Regionally the grain size intervals reflect a passage from “recycled orogen” into a “transitional arc”, which accords with the late Variscan horizontal gliding of the Moldanubian plate over the Saxothuringian plate and basement uplift proximal to this suture.