The crust beneath the Polish Sudetes: evidence from a gneiss xenolith in Tertiary basanite from Paszowice

Abstract

Xenoliths found in basaltic lavas provide useful information about the materials of the Earth crust and mantle sampled by the lava on its way to the surface, and they help to extend our interpretations based on geophysical data. A garnet-biotite-hornblende-gneiss xenolith (min. ca. 40 cm across) was drilled at a depth of ca. 18 m in a Tertiary basanite flow at Paszowice village near Jawor, in the easternmost part of the Kaczawa Mountains, central Sudetes (SW Poland). Based on petrography, geochemistry and thermobarometry, the xenolith is interpreted to have been formed at deep crustal levels from a metasedimentary rock. The maximum PT conditions recorded in the gneiss (T 700-740 o C, P ca. 10 kbar) indicate that the main metamorphic event took place at a depth of 25-30 km, thus corresponding to the present-day lower crustal level defined on geophysical constraints. The gneiss xenolith contains four zircon populations, as revealed by U-Pb SHRIMP analyses: (1) 1.7 -2.1 Ga, (2) 547 -623 Ma, (3) 473 -494 Ma, and (4) 354 -437 Ma. The zircons of the youngest group (4) are interpreted to have originated during a metamorphic episode, at ca. 374±13 Ma, corresponding well with the HT-MP major metamorphic event in the neighboring Góry Sowie gneisses and migmatites. A range of features (e.g. the good preservation of the xenolith, lack of significant chemical and thermal interaction with the lava) suggest that the gneiss was sampled by the rising lava on the final path of the magma to the surface, probably from a depth of less than ca. 10 km. That means that the upper crust below this region (underlying the low-grade metamorphic Kaczawa Complex) contains gneisses originally derived from deep-crustal levels, and similar to those locally exposed in the neighborhood (e.g. in the Góry Sowie Massif to the south-east). This interpretation extends our knowledge about the structure and composition of the crust in the eastern part of the European Variscides, which otherwise is inferred only from geophysical data.

Pre-Variscan evolution of the Western Tatra Mountains: new insights from U-Pb zircon dating

“…The 2.0–2.2 Ga old zircons represent abundant inherited components in the Variscan crust (e.g. Gebauer et al 1989 ; Zeck and Williams 2001 ; Oberc-Dziedzic et al 2009 ). A series of magmatic events at the end of the Archean between 2.5 and 2.6 Ga has also been documented for various rocks in the Variscan fold belt (Gebauer et al 1989 ; Friedl et al 2004 ; Kryza and Zalasiewicz 2008 and refs.…”

Timing of post-collisional volcanism in the eastern part of the Variscan Belt: constraints from SHRIMP zircon dating of Permian rhyolites in the North-Sudetic Basin (SW Poland)

“…33–35 km thick Earth's crust of the region. Petrological and SHRIMP studies of a gneiss xenolith in Cenozoic basanite lava erupted through the Kaczawa Complex suggest that the deeper part of the upper crust of that region contains gneisses similar to those exposed in the Góry Sowie Block to the SE (Oberc-Dziedzic, Kryza & Pin, 2009). The structurally complex middle to lower crustal levels are presumably composed of various crystalline complexes of the Variscan orogenic wedge (Grad et al .…”

Variscan granitoids related to shear zones and faults: examples from the Central Sudetes (Bohemian Massif) and the Middle Odra Fault Zone

“…The observed low Th/Nb (0.59-1.9) and high La/Nb (1.6-4.6) ratios reflect depletion of Nb relative to other incompatible elements and might indicate a crustal origin of the granitoids (Hofmann 1988;McLennan et al 2006;Oberc-Dziedzic et al 2009a). However, several features are at odd with this interpretation, including the overall mafic major element composition and the very high contents in Sr and Ba, which cannot be accounted for by crustal sources, but instead require a substantial contribution of mafic magmas, that were extracted from enriched-mantle reservoirs.…”