Devonian deep-crustal metamorphism and exhumation in the Variscan Orogen: evidence from SHRIMP zircon ages from the HT-HP granulites and migmatites of the Góry Sowie (Polish Sudetes)

Abstract

The high-temperature and high-pressure granulites in the internal zone of the Variscan belt are witnesses of deep crustal processes and subsequent exhumation of deeper lithospheric fragments. The Góry Sowie Block granulites in SW Poland and the surrounding gneisses and migmatites contain different inherited zircon age spectra, testifying different sources of their protoliths: mainly Cadomian (ca. 580 Ma) igneous rocks, and early Ordovician (ca. 500 Ma) granitoids (or their reworked products), respectively. The metamorphic spheric and oval zircons in the granulites give two statistically distinct ages of c. 395 and 380 Ma, in excess of experimental uncertainty. These ages may correspond to the high-temperature (HT) and high-pressure (HP) granulite facies metamorphism and subsequent amphibolite facies retrogression. However, essentially isochemical reconstitution of zircons and inheritance of radiogenic Pb cannot yet be excluded. The retrogression in the granulites coincided with the amphibolite facies metamorphism in the surrounding gneisses evidenced by a range of various isotopic ages between 384 and 370 Ma. The new SHRIMP zircon data provide new evidence of the presence of "old" (c. 400-395 Ma) HT-HP granulites in the central European part of the Variscides. The granulites were subsequently exhumed to mid-crustal levels, and tectonically interleaved with gneisses. Continuing uplift and decompression caused migmatization in the surrounding gneisses and partial re-equilibration of the granulites, at around 380 Ma. This high-temperature and medium-pressure (HT-MP) event was followed by rapid uplift and exhumation at c. 360 Ma due to Eo-Variscan orogenic movements.

Last stage of Variscan granitoid magmatism in the Strzelin Massif (SW Poland): petrology and age of the biotite-muscovite granites

“…Sim i lar 206 Pb/ 238 U zir con ages of 373-383 Ma were also re ported from the fine-grained bi o tite gran ite from the Strzelin I Quarry (Oberc-Dziedzic et al, 2013). Such ages were also found in the Sowie Góry migmatites (Kryza and Fan ning, 2007) and in xe no lith gneiss sam pled in the Paszowice ba salt from the crust un der ly ing the low-grade meta mor phic Kaczawa Unit (Oberc-Dziedzic et al, ). These ages doc u ment an im por tant meta morphic/anatectic event in the Cen tral Sudetes.…”

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

“…7). As often observed in the Góry Sowie gneisses and migmatites, this compositional trend indicates decreasing metamorphic pressures on the P-T paths of these rocks [18].…”

“…This concept is essentially consistent with much geochronological data obtained over the past twenty years in the Góry Sowie Massif. The granulite-facies metamorphism dated at 401.5±0.9 Ma [16], or 395±3 Ma [18] was soon followed in that area by a rapid decompression to amphibolite facies conditions. The latter temperature-dominated metamorphism took place between ca.…”

“…Finally, the significant population of Th-poor grains of Early Devonian age is interpreted to reflect metamorphic zircon (over) growth during the major episode, well documented e.g. in the Góry Sowie Massif, south of the study area [18].…”

“…Summing up, the critically reviewed geochronological data [18] indicate that we are dealing with the following well recognized thermal events in the Góry Sowie Massif: -ca. 500 Ma: magmatic crystallization (granitoids or rather their reworked products as protoliths for the gneisses and migmatites), -ca.…”