The Polish Lowlands, located southwest of the Teisseyre-Tornquist Zone, within Trans-European Suture Zone, were affected by bimodal, but dominantly rhyolitic, magmatism during the Late Paleozoic. Thanks to the inherited zircon they contain, these rhyolitic rocks provide a direct source of information about the pre-Permian rocks underlying the Polish Lowland. This paper presents zircon U-Pb geochronology and Hf and O isotopic results from five drill core samples representing four rhyolites and one granite. Based on the ratio of inherited vs. autocrystic zircon, the rhyolites can be divided into two groups: northern rhyolites, where autocrystic zircon is more abundant and southern rhyolites, where inherited zircon dominates. We suggest that the magma sources and the processes responsible for generating high silica magmas differ between the northern and southern rhyolites. Isotopically distinct sources were available during formation of northern rhyolites, as the Hf and O isotopes in magmatic zircon differ between the two analysed localities of northern rhyolites. A mixing between magmas formed from Baltica-derived mudstone-siltstone sediments and Avalonian basement or mantle can explain the diversity between the zircon compositions from the northern localities Daszewo and Wysoka Kamieńska. Conversely, the southern rhyolites from our two localities contain zircon with similar compositions, and these units can be further correlated with results from the North East German Basin, suggesting uniform source rocks over this larger region. Based on the ages of inherited zircon and the isotopic composition of magmatic ones, we suggest that the dominant source of the southern rhyolites is Variscan foreland sediments mixed with Baltica/Avalonia-derived sediments.
Abstract. The origin of large miarolitic (also known as
“chamber”) pegmatites is not fully understood although they may have great
economic value. The formation of cavities in magmatic bodies is related to
melt degassing and gas or fluid flow through partially solidified magma. In
this paper, the origin of the Volyn pegmatite field, located in the
Palaeoproterozoic Korosten anorthosite–mangerite–charnockite–granite (AMCG) complex, North-Western region of the Ukrainian Shield, is discussed. Pegmatites of
the field host deposits of piezoelectric quartz that is accompanied by gem-quality beryl and topaz. The Volyn pegmatite field is confined to granites
located in the south-western part of the Korosten complex and extends for 22 km along the contact with the anorthosite massif within the Korosten
plutonic complex. Geological data indicate hybridization of basic melts and
partly crystallized granites, as well as direct impact of fluids derived from basic
melts on the chamber pegmatites. The new U–Pb zircon ages obtained for granites and pegmatites of the
Korosten complex confirm that the rock assemblage in the northern part of
the complex crystallized between 1800 and 1780 Ma, whereas rocks in the
southern part intruded mainly between 1768 and 1755 Ma. U–Pb zircon ages for
granites from the south-western part of the Korosten complex indicate that
granites were emplaced at 1770–1765 Ma, a few million years prior to the intrusion of
the gabbro–anorthosite massif (1762–1758 Ma), while chamber pegmatites in
these granites crystallized at 1760 ± 3 Ma, coevally with the basic
rocks. Ultimately, the formation of the chamber pegmatites was related to
the reheating of the semi-crystallized granitic intrusion and to fluids
migrating from the underlying gabbro–anorthosite massif.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.