Provenance study of detrital garnets and rutiles from basaltic pyroclastic rocks of Southern Slovakia (Western Carpathians)

Detrital garnets and rutiles have been recovered from basaltic pyroclastic rocks in the northern part of the Pannonian Basin and characterized using electron probe microanalysis and imaging. All garnets are dominated by the almandine component, except for one sample dominated by spessartine. A total of three garnet groups have been distinguished according to the increased contents of grossular (Group I), pyrope (Group II) and spessartine components (Group III). Compositions of the group I and II garnets with fluctuating Ca- and relatively low Mg contents are consistent with low- to medium-grade metasediments and/or metabasites. Locally increased Mg contents could indicate higher P–T metamorphic overprint. The dominantly metamorphic origin of the Group I and II garnets (composed of >99 % of samples) is also corroborated by chlorite, tourmaline, staurolite, ilmenite and andalusite inclusions. Spessartine-rich garnets (Group III composed of <1 % of samples) could be genetically linked with granitoids. Detrital rutiles invariably plot within the field of metasediments metamorphosed under amphibolite-facies conditions. Possible proximal (subjacent basement sampled by ascending lava) or distal sources (catchment sediments from uplifted Central Carpathian basement) of heavy mineral assemblages are discussed

REE Minerals as Geochemical Proxies of Late-Tertiary Alkalic Silicate ± Carbonatite Intrusions Beneath Carpathian Back-Arc Basin

The accessory mineral assemblage (AMA) of igneous cumulate xenoliths in volcanoclastic deposits and lava flows in the Carpathian back-arc basin testifies to the composition of intrusive complexes sampled by Upper Miocene-Pliocene basalt volcanoes. The magmatic reservoir beneath Pinciná maar is composed of gabbro, moderately alkalic to alkali-calcic syenite, and calcic orthopyroxene granite (pincinite). The intrusive complex beneath the wider area around Fiľakovo and Hajnáčka maars contains mafic cumulates, alkalic syenite, carbonatite, and calc-alkalic granite. Both reservoirs originated during the basaltic magma underplating, differentiation, and interaction with the surrounding mantle and crust. The AMA of syenites is characterized by yttrialite-Y, britholite-Y, britholite-Ce, chevkinite-Ce, monazite-Ce, and rhabdophane(?). Baddeleyite and REE-zirconolite are typical of alkalic syenite associated with carbonatite. Pyrochlore, columbite-Mn, and Ca-niobates occur in calc-alkalic granites with strong peralkalic affinity. Nb-rutile, niobian ilmenite, and fergusonite-Y are crystallized from mildly alkalic syenite and calc-alkalic granite. Zircons with increased Hf/Zr and Th/U ratios occur in all felsic-to-intermediate rock-types. If rock fragments are absent in the volcanic ejecta, the composition of the sub-volcanic reservoir can be reconstructed from the specific AMA and zircon xenocrysts–xenolith relics disintegrated during the basaltic magma fragmentation and explosion

Niobium Mineralogy of Pliocene A1-Type Granite of the Carpathian Back-Arc Basin, Central Europe

A1-type granite xenoliths occur in alkali basalts erupted during Pliocene–Pleistocene continental rifting of Carpathian back-arc basin (Central Europe). The Pliocene (5.2 Ma) peraluminous calc-alkalic granite contains unusually high concentrations of critical metals bound in Nb, Ta, REE, U, Th-oxides typical for silica-undersaturated alkalic granites, and syenites: columbite-Mn, fergusonite-Y, oxycalciopyrochlore, Nb-rutile, and Ca-niobate (fersmite or viggezite). In contrast, it does not contain allanite and monazite—the main REE-carriers in calc-alkalic granites. The crystallization of REE-bearing Nb-oxides instead of OH-silicates and phosphates was probably caused by strong water deficiency and low phosphorus content in the parental magma. Increased Nb and Ta concentrations have been inherited from the mafic parental magma derived from the metasomatized mantle. The strong Al- and Ca-enrichment probably reflects the specific composition of the mantle wedge modified by fluids, alkalic, and carbonatitic melts liberated from the subducted slab of oceanic crust prior to the Pliocene-Pleistocene rifting

Formation of Esseneite and Kushiroite in Tschermakite-Bearing Calc-Silicate Xenoliths Ejected in Alkali Basalt

Skarnoid calc-silicate xenoliths composed of anorthite, clinopyroxene and Mg-Al spinel occur in alkali basalts of the Pliocene-Pleistocene intra-plate magmatic province in the northern part of the Pannonian Basin. Randomly oriented and elongated pseudomorphs are tschermakite crystals replaced by olivine, spinel and plagioclase. The relict amphibole within the pseudomorphs is characterized by high VIAl, between 1.95 and 2.1, and very low occupancy of the A-site (3+Al)AlSiO6 endmember with an equal proportion of VIAl3+ and Fe3+. Concentrations of kushiroite CaAlAlSiO6 endmember, up to 47.5 mol%, are the highest recorded in terrestrial samples. The AlFe3+-rich pyroxenes originated at the expense of diopside-augite during the interaction with carbonate-aluminosilicate melt. Forsterite (Fo72–83) and hemoilmenite with up to 32 mol% geikielite (9.3 wt% MgO) also crystallized from the melt, leaving behind the residual calcic carbonate with minor MgO (1–3 wt%). Columnar habit of neoformed olivine growing across diopside-augite layers indicates rapid crystallization from eutectic liquid. Euhedral aragonite and apatite embedded in fine-grained calcite or aragonite groundmass indicate slow crystallization of the residual carbonatite around the calcite-aragonite stability boundary. Corundum exsolutions in rock-forming anorthite are products of superimposed low-pressure pyrometamorphic reworking during transport in alkali basalt. Concomitant alkali metasomatism produced neoformed interstitial sodalite, nepheline, sanidine, albite, biotite, Mg-poor ilmenite (10–18 mol% MgTiO3), Ti-magnetite and fluorapatite. Olivine-ilmenite-aragonite-calcite thermobarometry returned temperatures of 770–860 °C and pressures of 1.8–2.1 GPa, whereas plagioclase-amphibole thermobarometer yielded 781 ± 13 °C and 2.05 ± 0.03 GPa. The calculated pressures correspond to depths of 60–70 km. The calc-silicate xenoliths are most likely metamorphosed marbles; however, a magmatic protolith (metagabbro, metaanorthosite) cannot be ruled out owing to high Cr contents in spinels (up to 30 mol% chromite) and abundant Cu-sulfides

Origin and provenance of 2 Ma–2 Ga zircons ejected by phreatomagmatic eruptions of Pliocene basalts in southern Slovakia

International audienceU–Pb ages of zircons recovered from Pliocene pyroclastic deposits in northern part of the Cenozoic intra-Carpathian back-arc basin (Pannonian Basin) span the interval from Pliocene (2.2 Ma) to Paleoproterozoic (Orosirian–Rhyacian, 1850–2115 Ma). The scattered U–Pb ages reflect eruption ages of the host basaltic volcanic centres, two episodes of post-Eocene magmatic crustal growth, and the possible tectonic affiliation, provenance and age of the subjacent basement or the sedimentary basin detritus sampled by the basaltic magma. The youngest zircons define the maximum ages of phreatomagmatic eruptions during the Late Miocene–Pliocene extension. These zircons are distinguished from older zircons by Zr/Hf (40–90) and Th/U ratios (0.5–4.5) as well as super-chondritic εHf(t) values ranging from + 7 to + 14, indicating mantle-derived parental magmas. The locally increased Th/U ratios (up to 8) accompanied by Zr/Hf > 60 are diagnostic of evolved phonolite parental melt. Hence, the youngest zircons can be interpreted as antecrysts, originating from evolved melts cogenetic with the host alkali basalts. In contrast, older zircons represent xenocrysts scavenged by the uprising basalt from surrounding rocks. Subordinate Eocene–Early Oligocene (29–38 Ma) sub-group of zircon xenocrysts is coincidental with the magmatism and volcanism along the Periadriatic lineament and the middle-Hungarian zone. The Early Miocene (18 Ma) cluster is coeval with the deposition of the Bükk Mountains felsic ignimbrite correlated with the onset of the back-arc extension that triggered Miocene sedimentation within the Pannonian Basin. The Eocene–Early Oligocene zircons have been likely scavenged from pyroclastic and ash-fall deposits of the Palaeogene retroarc basin subjacent to the Miocene basin infilling. Sub-chondritic εHf(t) values between − 2.5 and − 8 in the Eocene–Early Miocene zircons attest their crystallization from subduction-related felsic-to-intermediate melts containing large amounts of recycled crustal material. Palaeozoic–Proterozoic zircons create a heterogeneous population with variable trace element abundances and εHf(t) values. The determined age clusters are reminiscent of some basement units cropping out recently in Central Western Carpathians. Zircon Hf isotope data indicate recycling of up to 3.4 Ga old mafic crust and also the presence of 2 Ga old juvenile mafic crust. These units had either underlain the northern part of the Pannonian Basin during Pliocene or had been exposed during the deposition of Miocene clastic sediments. The absence of Mesoproterozoic, Grenvillian zircons (0.9–1.8 Ga) in the pre-Cenozoic population of zircon xenocrysts is provisionally interpreted as indicating the evolution of the zircon source area within the west-African Craton