Origin and ascent history of unusually crystal-rich alkaline basaltic magmas from theudwestern Pannonian Basin

摘要

The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Fieldud(western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-richudalkaline basalts which are unique among the alkaline basalts of the Carpathian-udPannonian Region. Similar alkaline basalts are only rarely known in other volcanicudfields of the world. These special basaltic magmas fed the eruptions of two closelyudlocated volcanic centres: the Bondoró-hegy and the Füzes-tó scoria cone. Theiruduncommon enrichment in diverse crystals produced unique rock textures and modifiedudoriginal magma compositions (13.1-14.2 wt.% MgO, 459-657 ppm Cr, 455-564 ppm Niudcontents).udDetailed mineral-scale textural and chemical analyses revealed that the Bondoró-hegyudand Füzes-tó alkaline basaltic magmas have a complex ascent history, and that mostudof their minerals (~30 vol.% of the rocks) represent foreign crystals derived fromuddifferent levels of the underlying lithosphere. The most abundant xenocrysts, olivine,udorthopyroxene, clinopyroxene and spinel, were incorporated from different regions andudrock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene andudspinel could have originated from pegmatitic veins / sills which probably representudmagmas crystallized near the crust-mantle boundary. Green clinopyroxene xenocrystsudcould have been derived from lower crustal mafic granulites. Minerals that crystallizedudin situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene,udplagioclase, Fe-Ti oxides) are only represented by microphenocrysts and overgrowthsudon the foreign crystals. The vast amount of peridotitic (most common) and maficudgranulitic materials indicates a highly effective interaction between the ascendingudmagmas and wall rocks at lithospheric mantle and lower crustal levels. However,udfragments from the middle and upper crust are absent from the studied basalts,udsuggesting a change in the style (and possibly rate) of magma ascent in the crust.udThese xenocryst- and xenolith-rich basalts yield divers tools for estimating magmaudascent rate that is important for hazard forecasting in monogenetic volcanic fields.udAccording to the estimated ascent rates, the Bondoró-hegy and Füzes-tó alkalineudbasaltic magmas could have reached the surface within hours to few days, similarly toudthe estimates for other eruptive centres in the Pannonian Basin which were fed byud"normal" (crystal- and xenolith-poor) alkaline basalts.