Late-orogenic magmatism in the Central European Variscides: SHRIMP U-Pb zircon age constraints from the Żeleźniak intrusion, Kaczawa Mountains, West Sudetes

Katarzyna Machowiak¹, Richard Armstrong², Ryszard Kryza³ & Andrzej Muszyński⁴

¹ Institute of Civil Engineering, Poznań University of Technology, Piotrowo 5, 61-138 Poznań, Poland, e-mail: kamachow@amu.edu.pl
² Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia, e-mail: Richard.Armstrong@anu.edu.au
³ Uniwersytet Wrocławski, Instytut Nauk Geologicznych, ul.Cybulskiego 30, 50-205 Wrocław, Poland, e-mail: ryszard.kryza@ing.uni.wroc.pl
⁴ Institute of Geology, Adam Mickiewicz University, Maków Polnych 16, 61-606 Poznań, Poland, e-mail: anmu@amu.edu.pl

Key words:  Variscan igneous activity, geochemistry, zircon geochronology, Żeleźniak intrusion, West Sudetes.

Abstract
We present new U-Pb isotope data obtained using the sensitive high mass-resolution ion microprobe (SHRIMP) technique on zircon crystals from the Żeleźniak subvolcanic intrusion in the Kaczawa Mountains, West Sudetes, SW Poland. The intrusion comprises shallow-level unmetamorphosed and undeformed fine-grained rhyolites, rhyodacites, and trachyandesites and deep-level medium-grained monzogranites and granodiorites. The surrounding country rocks, thought to be fragments of a Variscan accretionary prism, are blueschist- to subsequent greenschist facies metavolcanic and metasedimentary rocks of the Kaczawa Complex. The Żeleźniak intrusion has been correlated with other late- to post-tectonic Variscan volcanic and plutonic bodies in the region, such as the Karkonosze Granite, but the scarcity and often problematic quality of age constraints and of geochemical data have made such correlations speculative. Our new SHRIMP zircon ages of ~315-316 Ma from the Żeleźniak intrusion probably corresponds to the main magmatic stage. However, a younger age of ~269 Ma, derived from some zircon rims, is more difficult to interpret but might reflect either a much younger igneous event or a hydrothermal episode. The new date of ~315-316 Ma for the undeformed Żeleźniak intrusion also provides an upper age limit for deeper-level tectonic and metamorphic processes in the Kaczawa accretionary prism. Furthermore, the new SHRIMP zircon ages are among the oldest obtained from the volcanic rocks within the Variscan Belt in Central Europe and may correspond to the final stages of the exhumation of the blueschist facies rocks in this part of the orogen.

Geologia Sudetica, 40: 19-31.

Lower Carboniferous bentonites in the Bardo Structural Unit (central Sudetes): geological context, petrology and palaeotectonic setting

Ryszard Kryza, Jolanta Muszer, Czesław August, Joanna Haydukiewicz & Marta Jurasik

Wrocław University, Institute of Geological Sciences, Department of Mineralogy and Petrology, ul. Cybulskiego 30, 50–205 Wrocław, Poland; e-mails: ryszard.kryza@ing.uni.wroc.pl;  jolanta.muszer@ing.uni.wroc.pl;  czeslaw.august@ing.uni.wroc.pl;  joanna.haydukiewicz@ing.uni.wroc.pl

Key words:  bentonite, zircon, SHRIMP geochronology, Viséan, Bardo Unit, Sudetes.

Abstract
The Lower Carboniferous Paprotnia beds of the Bardo Structural Unit in the central Sudetes, composed predominantly of mudstones with Upper Viséan fossils, include several bentonite layers. The bentonites, composed mainly of kaolinite, illite/smectite and smectite, with minor amounts of quartz, calcite and iron hydroxides, also contain abundant zircons, the features of which indicate their volcanic derivation. The main population of the zircons yielded a SHRIMP U-Pb age of ~ 334 Ma corresponding with, and numerically constraining, the biostratigraphic data. The field evidence, biostrati- graphic and geochronological results, together with mineralogical data from the bentonites, indicate continental margin-type sedimentation and contemporaneous volcanic (andesitic-rhyolitic) activity in the neighbouring region during the ongoing Variscan orogeny in central Europe in Late Viséan times.

Geologia Sudetica, 40: 33-50.

A new Lower Coniacian fauna from the Jerzmanice Zdrój region of the North Sudetic Basin, SW Poland

Alina Chrząstek

Institute of Geological Sciences, Wrocław University, Maksa Borna 9, 50-204 Wrocław, Poland, e-mail: alina.chrzastek@ing.uni.wroc.pl

Key words:  Lower Coniacian, Upper Cretaceous, North Sudetic Basin, inoceramids, bivalves, palaeoenvironment, West Sudetes.

Abstract
This paper describes and interprets a newly discovered Lower Coniacian (lower Upper Cretaceous) macro- and micro- fossil fauna (vertebrate and invertebrate remains) from sedimentary rocks of the Jerzmanice Zdrój region of the North Sudetic Basin of SW Poland. Several inoceramid bivalve taxa that previously were only known from other parts of the North Sudetic Basin were recovered from light grey, marly sandstones of Early Coniacian age. A fragment of ammonite was also discovered, as was a shark's tooth from the family Cretoxyrhinidae: this may be ?Cretoxyrhina mantelli Agassiz, 1843, a species not hitherto known from the Lower Coniacian (Emscherian sensu Scupin (1912-13)) of the North Sudetic Basin. Abundant foraminifers were observed in thin sections. The newly discovered inoceramid bivalves - Cremnoceramus deformis erectus Meek, 1877, Cremnoceramus waltersdorfensis waltersdorfensis Andert, 1911 and Inoceramus lusatiae Andert, 1911 - fit into the current biostratigraphic scheme for the region. The inoceramids can all be assigned to the Cremnoceramus deformis erectus Zone, which correlates with the Gavelinella moniliformis foraminiferal Zone and thereby confirms an Early Coniacian age. The Turonian-Coniacian boundary in the North Sudetic Basin can now be placed between the respective inoceramid zones of Inoceramus costellatus Woods, 1912 (actually Mytiloides costellatus Woods, 1912) and Inoceramus schloenbachi Böhm, 1911 (actually Cremnoceramus crassus crassus Petrascheck, 1903). The macrofossils found in the Jerzmanice section suggest that the host sediments were laid down in a Late Cretaceous epicontinental basin, under the North Sudetic Sea, that had deepened during the Early Coniacian. This interpretation agrees with the global bathymetric curve for the Late Cretaceous in Europe.

Geologia Sudetica, 40: 51-63.

Tectonic evolution of the late Cretaceous Nysa Kłodzka Graben, Sudetes, SW Poland

Jerzy Don & Roman Gotowała

Institute of Geological Sciences, Wrocław University, Maksa Borna 9, 50-204 Wrocław, Poland, e-mail: roman.gotowala@ing.uni.wroc.pl

Key words:  fault, turbidite, fan delta, Idzików beds, Orlica-Śnieżnik Dome, Bohemian Massif, Cenozoic.

Abstract
The Nysa Kłodzka Graben, located in the Sudetes of SW Poland, developed as a result of Coniacian (middle Upper Cretaceous) N-trending faulting of the Variscan crystalline basement rocks that comprise the crest of the Orlica-Śnieżnik Dome. The graben was transgressed by a late Cretaceous sea that encroached during the Cenomanian from the northwest. Up to 700 m of Coniacian shales, sandstones and conglomerates were deposited in the graben, with shales (the ~500 m thick Idzików 'clays') dominating the graben's central section. On the western side of the graben, shales grade upwards to greywackes in a style that resembles a turbidite sequence; on the eastern side, shales are overlain by sandstones and conglomerates (the Idzików conglomerates) that represent extensive late Cretaceous fan deltas. These within-graben fan deltas date the onset of fault-block movements that uplifted the Sudetes region during the late Cretaceous-Cenozoic. By the end of the Cretaceous, both the sedimentary infill and the underlying Cenomanian and Turonian strata were steepened at the graben margins and were gently folded, the fold axes paralleling the graben's marginal faults. Subsequent Cretaceous-Paleocene ('Laramian') deformations resulted in NW-trending reverse faulting, which restructured the earlier N-S template of the graben, and in transcurrent faults, which cut the N-trending folds, modified the north and south ends of the graben and strongly affected the graben's western walls. The total thickness of the Upper Cretaceous strata of the Nysa Kłodzka Graben is 3 times that of the Intra-Sudetic Synclinorium, implying that the two units developed independently.