Mineralogy, geochemistry and paleomagnetism of Variscan diastrophic sediments in the Bohemian Massif: provenance and paleotectonic interpretation (P. Pruner, F. Patočka, J. Hladil, O. Man, D. Venhodová, J. Slepičková, M. Burian, J. Kadlec, P. Štorch, J. Otava & L. Maštera, Czech Geological Institute, Brno)

The Moravo-Silesian Zone (MSZ) rims the Bohemian Massif, in the eastern branch of the Variscan Belt of Europe. The Variscan diastrophic sediments of the MSZ (Culm facies, Devonian?-E. Carboniferous) are considered to be equivalent of the Rhenohercynian Zone. The detached segments of the MSZ were thrust over the Neoproterozoic basement. The palaeomagnetically evidenced clockwise rotation of these detached segments was 100 to 190 degree relative to cratons on the Northeast. A strong Late Carboniferous shear zone contributed to a conspicuously narrow shape of the Variscan thrust belt in this area. The geochemical data on the MSZ siliciclastic sediments, mostly greywackes/arkoses to greywackes/lithic sandstones, provided significant implications on provenance and palaeotectonic setting of the rock origin. The westernmost basin fills are mostly of early Visean age with as old as Givetian(?) basal part. The siliciclastics were derived from young continental island arc showing also some features of oceanic island arc. The arc was situated on an old passive continental margin (of the Brunovistulian microplate?). A wide variety of basic to acid (meta)igneous rocks contributed to the siliciclastics. The ages of the folded and detached segments (formations) diminish towards the E. The Middle to late Visean basin fills were sedimented in a setting of maturing CIA with scarcely preserved features of OIA. Mostly acidic arc involving a wide spectrum of rocks of granitic (rhyolitic-dacitic) geochemistry was a source of the siliciclastics. Somewhat more condensed spectrum of the same rock types characterized a tectonic setting of mature CIA (evolving to active continental margin) where a subsequent (Late Visean) sequences of the MSZ sedimentary fills were accumulated. The youngest siliciclastics of the latest Visean to early Namurian(?) age were deposited in a CIA to ACM setting, and were provided by a homogeneous acidic arc source. A narrow spectrum of rocks geochemically equivalent to acid magmatites produced uniformity of the sediments. The succession from CIA(+/- OIA) to CIA to ACM sources recorded in geochemical composition of the MSZ diastrophic siliciclastics seems to correspond to a series of events from the early to culminating stages of orogenic collision, and subsequent uplift and exhumation of tectonic slices. The source development may reflect the docking of the Tepla-Barrandian microplate on the Brunovistulicum during the final accretion of the Bohemian Massif terrane mosaic.

Variscan diastrophic sediments (DS) of the Moravo-Silesian Zone are mostly greywackes, sandstones and siltstones, with subordinate layers of conglomerates and shales (Culm facies). These thick and tectonically multiplied strata-sets were deformed into wedges and nappes, with scattered slices of crystalline and Devonian rocks on the margins (a mosaic of terrane segments). The main DS volumes were deposited during the Visean to Visean/Namurian-A, but the oldest DS formation on the Northwest is in close contact with the Givetian trilobite shale and can be also Devonian(?). The time-span between the oldest and youngest DS is ca. 50-60 Ma. The palaeomagnetic studies in separate segments (formations) of the DS were concentrated on the separation of primary magnetisation components, with special emphasis on the directions, which correspond to synsedimentary and early diagenetic history of the rocks. The shales from the oldest DS segment on the Northwest (Mohelnice Fm., at Slavonov) yield mean directions of remanent magnetism with D = 143 and I = -26 degree. The virtual-pole position is close to pole positions of the pre-flysh Late Devonian in Moravian Karst (e.g., at Krtiny). The clockwise tectonic rotation, if compared with the Devonian "stable" Europe NE of TESZ, is approx. 100 degr., i.e., 120 degr. against the cratonic Permian cover. However, the tectonic-rotation values for the main Visean Culm group of formations are even higher than at Slavonov (difference ca. +70 degr.). This next side-developed group of formations consists of the Andelska Hora Fm., Protivanov Fm., and Rozstani Fm., where the relevant mean directions D/I are 210 / -13; 218 / -22; and 216 / -14, respectively. These clockwise tectonic rotations of evidently Visean DS are higher than above. The primary remanent-magnetisation components, all on greywackes, shales and siltstones, differ form the early Carboniferous on the "stable Europe" about 170-190 degr. clockwise, i.e., approx. 180-200 degr. against the cratonic Permian cover. These data clearly separate the oldest DS segment on the Northwest and, practically, co-indicate its different age. The stepwise evolution of the clockwise tectonic rotation was irregular, with a great gap between the oldest DS and the Late Visean group of the DS formations. Within the latter group, the declination values are closely grouped (similar deformation history), but the different values of inclination suggest different pre-deformation positions of the basins. The youngest Myslejovice Fm. on the SE was remagnetised with typical Carboniferous/Permian directions.