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Journal of Petrology Advance Access originally published online on January 19, 2008
Journal of Petrology 2008 49(3):421-439; doi:10.1093/petrology/egm086
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© The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

A Quartz-bearing Orthopyroxene-rich Websterite Xenolith from the Pannonian Basin, Western Hungary: Evidence for Release of Quartz-saturated Melts from a Subducted Slab

E. Bali1,*, Z. Zajacz1,2, I. Kovács1,3, CS. Szabó1, W. Halter2, O. Vaselli4, K. Török1,5 and R. J. Bodnar6

1lithosphere Fluid Research Lab, Department of Petrology & Geochemistry, Institute of Geography and Earth Sciences, Eötvös University Budapest (ELTE), Pazmany Setany 1/c, Budapest, H-1117, Hungary
2Institute of Isotope Geochemistry and Mineral Resources, ETH Zürich, Sonneggstrasse 5, ETH Zentrum, 8092 Zürich, Switzerland
3Research School of Earth Sciences, Building 61 Mills Road, the Australian National University, Canberra, Act 0200, Australia
4Department of Earth Sciences, University of Florence, Via G. LA Pira, 4, Florence, 50121, Italy
5Eötvös Loránd Geophysical Institute of Hungary, Budapest, Columbus U. 17–23, H-1145, Hungary
6Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA

RECEIVED JANUARY 31, 2007; ACCEPTED DECEMBER 19, 2007


  Abstract

An unusual quartz-bearing orthopyroxene-rich websterite xenolith has been found in an alkali basaltic tuff at Szigliget, Bakony–Balaton Highland Volcanic Field (BBHVF), western Hungary. Ortho- and clinopyroxenes are enriched in light rare earth elements (LREE), middle REE and Ni, and depleted in Nb, Ta, Sr and Ti compared with ortho- and clinopyroxenes occurring in either peridotite or lower crustal granulite xenoliths from the BBHVF. Both ortho- and clinopyroxenes in the xenolith contain primary and secondary silicate melt inclusions, and needle-shaped or rounded quartz inclusions. The melt inclusions are rich in SiO2 and alkalis and poor in MgO, FeO and CaO. They are strongly enriched in LREE and large ion lithophile elements, and display negative Nb, Ta and Sr anomalies, and slightly positive Pb anomalies. The xenolith is interpreted to represent a fragment of an orthopyroxene-rich body that crystallized in the upper mantle from a hybrid melt that formed by interaction of mantle peridotite with a quartz-saturated silicate melt that was released from a subducted oceanic slab. Although the exact composition of the slab melt cannot be determined, model calculations on major and trace elements suggest involvement of a metasedimentary component.

KEY WORDS: quartz; mantle; silicate melt inclusion; SiO2-rich melt; subduction; Carpathian-Pannonian Region

*Corresponding author. Present address: Bayerisches Geoinstitut, Universität Bayreuth, Universitätsstraße 30, D-95447 Bayreuth, Germany. Telephone: +49 (0)921 155-3729. Fax: +49 (0)921 55-3769. E-mail: Eniko.Bali{at}uni-bayreuth.de


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