Evolution of Mafic Alkaline Melts Crystallized in the Uppermost Lithospheric Mantle: a Melt Inclusion Study of Olivine-Clinopyroxenite Xenoliths, Northern Hungary |
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Authors: | Zajacz Zoltan; Kovacs Istvan; Szabo Csaba; Halter Werner; Pettke Thomas |
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Institution: | 1Department of Earth Sciences, Institute of Isotope Geochemistry and Mineral Resources, Eth Zürich, 8092 Zürich, Switzerland
2Research School of Earth Sciences, Building 61 Mills Road, the Australian National University, Canberra, Act 0200, Australia
3Lithosphere Fluid Research Laboratory, Institute of Geography and Earth Sciences, Eötvös University Budapest (Elte), Pazmany Setany 1/c, Budapest, Hungary, H-1117
4University of Bern, Institute of Geological Sciences, Baltzerstrasse 13, Ch-3012 Bern, Switzerland |
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Abstract: | Olivine-clinopyroxenite xenoliths exhumed in alkali basalts(sensu lato) in the NógrádGömörVolcanic Field (NGVF), northern Hungary, contain abundant silicatemelt inclusions. Geothermobarometric calculations indicate thatthese xenoliths crystallized as cumulates in the upper mantlenear the Moho. These cumulate xenoliths are considered to representa period of Moho underplating by mafic alkaline magmas priorto the onset of Late Tertiary alkaline volcanism in the CarpathianPannonianregion. The major and trace element compositions of silicatemelt inclusions in olivine display an evolutionary trend characterizedby a strong decrease in CaO/Al2O3. The parental melt of thecumulates was a basanite formed by low-degree ( 2%) partialmelting of a garnet peridotite source. The compositional trendof the silicate melt inclusions, textural features, and modellingwith pMELTS show that the parental melt evolved by major clinopyroxeneand minor olivine crystallization followed by the appearanceof amphibole simultaneously with significant resorption of theearlier clinopyroxene and olivine. The resulting residual meltwas highly enriched in Al2O3, alkalis and most incompatibletrace elements. This type of melt is likely to infiltrate andreact with surrounding mantle peridotite as a metasomatic agent.It might also form high-pressure pegmatite-like bodies in themantle that might be the source of the amphibole and sanidinemegacrysts also found in the alkali basalts of the NGVF. Preferentialremelting of the later-formed (i.e. lower temperature) mineralassemblage (amphibole, sanidine, residual glass) might havesignificantly contaminated the host alkaline mafic lavas, increasingtheir Al2O3 and total alkali contents and, therefore, reducingtheir MgO, FeO and CaO content. KEY WORDS: silicate melt inclusions; geochemistry; petrogenesis; NógrádGömör Volcanic Field; Pannonian Basin |
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Keywords: | : silicate melt inclusions geochemistry petrogenesis Nó grá d Gö mö r Volcanic Field Pannonian Basin |
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