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Three major groups (A-C) of Central European Variscan granitoids can be distinguished based on petrologic and chemical data: A. Low-K, high-Na, calc-alkaline, alkali-calcic, met/peraluminous, I- and S-type, trondhjemitic (slightly granodioritic)-suite, mostly magnesian granitoids formed at the early stage of continent-continent collision (353-356 Ma). These types of granitoids occur in the northern part of the Central Bohemian Plutons (CBP) in the Moldanubian Zone or in the Austroalpine Zone of the outer belt of the Western Carpathian Plutons (WCP). The granitoid melt source originated from partially melted oceanic and continental crusts due to compression. B. High-K, high-Mg, calc-alkaline, metaluminous, I-type, monzonitic-suite granitoids with lamprophyre-derived small, ultrapotassic, Mg-rich intrusions formed in the post-collision zone, where the melts originated from the partially melted, uplifted, LIL-rich mantle and continental crust, due to extension (334-354 Ma) after earlier compression. These intrusions can be found in the southern part of the CBP, the eastern part of Southern Bohemian Plutons (SBP) and northwestern part of the Tisia Terrain. Later on (314-303 Ma), as a result of interaction of melts originated from depleted mantle and partial melted lower crust, low-K, high-Na, calc-alkaline-type granitoids formed during extension, occurring in the inner part of the WCP. C. Peraluminous, S- or S/A-type granodioritic-suite, small intrusions with some K-subalkaline and alkaline characters, formed in post-orogenic or probably rifting settings at the main tectonic zones, e.g. the Periadriatic-Balaton Lineament (Velence Mts, Gemericum) during the Lower Permian (274±1.7 Ma). The most unstable part of the Variscan orogenic belt was where the high K, Mg-calc-alkaline granitoid and ultrapotassic intrusions occur (Massif Central, Vosges, Black Forest, the southern part of the CBP, the eastern part of the SBP and the northwestern part of the Tisia Terrain). They formed the innermost part of the Variscan Belt; only the Tisia Terrain occurs in allochthonous positions, which originated from the Moldanubian Zone and were completely separated from it since mid-Cretaceous times.

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Abstract

The ophiolitic rocks of the easternmost Penninic unit, the Kõszeg-Rechnitz series, were analyzed for their H, C and O stable isotope compositions. Serpentinite, gabbro, blueschist, talc deposits, ophicarbonates, as well as calcite and inclusion fluids from quartz segregation veins were analyzed in order to determine the effects of different metamorphic events on the stable isotope compositions.

The oxygen isotope compositions have a wide range depending on rock type and locality. Gabbro and serpentinite of Bienenhütte (Bernstein Window) have preserved mantle-like δ 18O values (5.9 to 6.3‰; all values are in ‰ relative to V-SMOW), whereas the serpentinite of Glashütten and Rumpersdorf (Kõszeg-Rechnitz Window) and the silicate minerals of the ophicarbonate rocks show a strong 18O-enrichment (up to 16.2‰). The 18O-enrichment may have been induced by low-temperature serpentinization or interaction with 18O-rich fluids that had been in equilibrium with sedimentary rocks. Contrary to the O isotope compositions, the H isotope compositions seem to be homogeneous in the entire series, with D values of −63 ± 7‰. Only some serpentinite rocks were depleted in D (down to −106‰), usually regarded as a result of interaction with meteoric water infiltrating during late-stage metamorphism. The meteoric water infiltration was rather limited, as even samples taken directly from slickensides within serpentinite bodies preserved isotopic compositions close to those of the bulk series. H and O isotope compositions of fluids mobilized in the metasedimentary rocks of the Penninic unit during the main metamorphic stage were determined by analyzing inclusion fluids and calcites in quartz-carbonate veins. The isotope compositions indicate interaction between these fluids and the ophiolite series, although relative deuterium enrichment has been preserved in the ophiolitic rocks.

The strong D-enrichment characteristic for oceanic crust that has experienced high-temperature interaction with seawater was not detected. However, the H isotope compositions obtained for the Kõszeg-Rechnitz series indicate that subduction of the Penninic oceanic crust and the associated devolatilization may have been potentially responsible for mantle metasomatism, resulting in H isotope compositions of about −40‰, similar to the range determined from mantle-derived amphibole megacrysts (Demény et al. 2005). To conclude, the present dataset is discussed in the light of earlier studies on the formation of the Sopron leucophyllite.

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Acta Geologica Hungarica
Authors:
Réka Lukács
,
György Czuppon
,
Szabolcs Harangi
,
Csaba Szabó
,
Theodor Ntaflos
, and
Friedrich Koller

Silicate melt inclusions are frequent in the phenocryst phases (quartz, plagioclase, orthopyroxene, ilmenite and accessory minerals) of the Miocene silicic pyroclastic rocks of the Bükkalja Volcanic Field, Northern Hungary. These melt inclusions were trapped at different stages of magma evolution; therefore, they provide important information on the petrogenetic processes. The melt inclusions in the Bükkalja pyroclastic rocks show various textures such as (1) wholly enclosed type; (2) hourglass inclusions and (3) reentrant or embayment glass. Among the wholly enclosed type melt inclusions further textural subgroups can be distinguished based on their shape: negative crystal, rounded, elongated and irregular shaped. These various textures reflect differences in the time of entrapment prior to eruption and in the post-entrapment condition in the magma chamber. The largest textural variation was found in the quartz-hosted melt inclusions. However, the major element compositions of these melt inclusions do not differ from one another in the same unit. In general, compositions of the melt inclusions are similar to the chemistry of the glass shards. Comparing the composition of the quartz-hosted melt inclusions from three main ignimbrite units (Lower, Middle and Harsány Ignimbrite Units), slight differences have been recognized, suggesting distinct erupted host magmas. Melt inclusions from the andesitic lithic clast of the Lower Ignimbrite Unit could represent heterogeneous interstitial melt in the crystal mush zone at the magma chamber wall. The largest geochemical variation was found in the melt inclusion of the Middle Ignimbrite Unit, even in single samples. This compositional variation overlaps that of the rhyolitic juvenile clasts, but does not match that of the glasses of scoria clasts. We suggest that syn-eruptive magma mixing (mingling) occurred in a compositionally heterogeneous magma chamber of the Middle Ignimbrite Unit.

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