Abstract
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.
Highlights
Direct information about the composition of the deep lithosphere is encoded in rock fragments ejected by basaltic volcanoes
The vast majority of smaller rock fragments scavenged from the walls of volcanic conduits is resorbed in the hot basalt, whereas the larger xenoliths are disintegrated and ground during the magma fragmentation and phreatomagmatic eruption triggered by the magma-water interaction and explosive release of rapidly expanding gas bubbles
Upper Miocene–Quaternary basaltic pyroclastic deposits in the northern part of the Pannonian Basin (Carpathian back-arc basin) contain a unique assemblage of igneous xenoliths [3], and significantly more abundant spinel peridotite and wehrlite xenoliths transported from the metasomatized upper mantle [4]
Summary
Direct information about the composition of the deep lithosphere is encoded in rock fragments (xenoliths) ejected by basaltic volcanoes. Only specific accessory mineral assemblage (AMA) is preserved, consisting of phenocrysts crystallized from the magma and xenocrysts derived from the disintegrated xenoliths. Trace elements and radiogenic isotopes define the whole suite of igneous xenoliths as a cogenetic assemblage of an Upper Miocene–Pliocene A1-type intra-plate continental magmatism [9]. We demonstrate that the composition of sub-volcanic magmatic intrusive complexes can be inferred from the specific AMA recovered from pyroclastic deposits. The information obtained from AMA can be supplemented by the chemical composition of crystal-melt inclusions enclosed in zircon xenocrysts, which represent refractory relics of igneous xenoliths disintegrated during explosion
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