28 research outputs found
Origin and ascent history of unusually crystal-rich alkaline basaltic magmas from the western Pannonian Basin
The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field
(western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich
alkaline basalts which are unique among the alkaline basalts of the Carpathian-
Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic
fields of the world. These special basaltic magmas fed the eruptions of two closely
located volcanic centres: the Bondoró-hegy and the Füzes-tó scoria cone. Their
uncommon enrichment in diverse crystals produced unique rock textures and modified
original magma compositions (13.1-14.2 wt.% MgO, 459-657 ppm Cr, 455-564 ppm Ni
contents).
Detailed mineral-scale textural and chemical analyses revealed that the Bondoró-hegy
and Füzes-tó alkaline basaltic magmas have a complex ascent history, and that most
of their minerals (~30 vol.% of the rocks) represent foreign crystals derived from
different levels of the underlying lithosphere. The most abundant xenocrysts, olivine,
orthopyroxene, clinopyroxene and spinel, were incorporated from different regions and
rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and
spinel could have originated from pegmatitic veins / sills which probably represent
magmas crystallized near the crust-mantle boundary. Green clinopyroxene xenocrysts
could have been derived from lower crustal mafic granulites. Minerals that crystallized
in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene,
plagioclase, Fe-Ti oxides) are only represented by microphenocrysts and overgrowths
on the foreign crystals. The vast amount of peridotitic (most common) and mafic
granulitic materials indicates a highly effective interaction between the ascending
magmas and wall rocks at lithospheric mantle and lower crustal levels. However,
fragments from the middle and upper crust are absent from the studied basalts,
suggesting a change in the style (and possibly rate) of magma ascent in the crust.
These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma
ascent rate that is important for hazard forecasting in monogenetic volcanic fields.
According to the estimated ascent rates, the Bondoró-hegy and Füzes-tó alkaline
basaltic magmas could have reached the surface within hours to few days, similarly to
the estimates for other eruptive centres in the Pannonian Basin which were fed by
"normal" (crystal- and xenolith-poor) alkaline basalts
Relative contributions of crust and mantle to generation of Campanian high-K calc-alkaline I-type granitoids in a subduction setting, with special reference to the Harsit Pluton, Eastern Turkey
We present elemental and Sr-Nd-Pb isotopic data for the magmatic suite (similar to 79 Ma) of the Harsit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO(2) = 59.43-76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO(2) = 54.95-56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb)(n) = 2.40-12.44] and display weak Eu anomalies (Eu/Eu* = 0.30-0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harsit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr-Nd isotopic data for all of the samples display I(Sr) = 0.70676-0.70708, epsilon(Nd)(79 Ma) = -4.4 to -3.3, with T(DM) = 1.09-1.36 Ga. The lead isotopic ratios are ((206)Pb/(204)pb) = 18.79-18.87, ((207)Pb/(204)Pb) = 15.59-15.61 and ((208)Pb/(204)Pb) = 38.71-38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir-Ankara-Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr-Nd isotope modeling shows that the generation of these magmas involved similar to 65-75% of the lower crustal-derived melt and similar to 25-35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar-Ar plateau age on hornblende, combined with regional studies, imply that the Harsit pluton formed in a subduction setting and that the back-arc extensional period started by least similar to 79 Ma in the Eastern Pontides.Geochemistry & GeophysicsMineralogySCI(E)33ARTICLE4467-48716
Paleogeographic evolution of the Southern Pannonian Basin: 40Ar/39Ar age constraints on the Miocene continental series of notthern Croatia
The Pannonian Basin, originating during the
Early Miocene, is a large extensional basin incorporated
between Alpine, Carpathian and Dinaride fold-thrust belts.
Back-arc extensional tectonics triggered deposition of up to
500-m-thick continental fluvio-lacustrine deposits distributed
in numerous sub-basins of the Southern Pannonian
Basin. Extensive andesitic and dacitic volcanism accompanied
the syn-rift deposition and caused a number of
pyroclastic intercalations. Here, we analyze two volcanic
ash layers located at the base and top of the continental
series. The lowermost ash from Mt. Kalnik yielded an
40Ar/39Ar age of 18.07 ± 0.07 Ma. This indicates that the
marine-continental transition in the Slovenia-Zagorje
Basin, coinciding with the onset of rifting tectonics in the
Southern Pannonian Basin, occurs roughly at the Eggenburgian/
Ottnangian boundary of the regional Paratethys
time scale. This age proves the synchronicity of initial
rifting in the Southern Pannonian Basin with the beginning
of sedimentation in the Dinaride Lake System. Beside
geodynamic evolution, the two regions also share a biotic
evolutionary history: both belong to the same ecoregion,
which we designate here as the Illyrian Bioprovince. The
youngest volcanic ash level is sampled at the Glina and
Karlovac sub-depressions, and both sites yield the same
40Ar/39Ar age of 15.91 ± 0.06 and 16.03 ± 0.06 Ma,
respectively. This indicates that lacustrine sedimentation in
the Southern Pannonian Basin continued at least until the
earliest Badenian. The present results provide not only
important bench marks on duration of initial synrift in the
Pannonian Basin System, but also deliver substantial
backbone data for paleogeographic reconstructions in
Central and Southeastern Europe around the Early–Middle
Miocene transition