Type-II xenoliths and related metasomatism from the No ´gra ´d-Gfmfr Volcanic Field, Carpathian-Pannonian region (northern Hungary–southern Slovakia) I. Kova ´cs, Z. Zajacz, C. Szabo ´ * Lithosphere Fluid Research Lab, Department of Petrology and Geochemistry, Eotvos University, Budapest, H-1117, Pazmany Peter setany 1/C, Hungary Accepted 21 April 2004 Available online 25 September 2004 Abstract Xenoliths from the upper mantle and lower crust are abundant in Plio–Pleistocene alkali basalts of the No ´gra ´d-Gfmfr Volcanic Field (NGVF; northern Pannonian Basin, northern Hungary/southern Slovakia), representing a valuable dprobe’ of lithospheric structures and processes. Ultramafic xenoliths have been divided into two groups: (1) Type-I, composed mostly of olivine with subsidiary orthopyroxene, clinopyroxene and spinel, and (2) Type-II, containing mostly Al- and Ti-rich clinopyroxene with subordinate olivine, spinel and plagioclase. Both types often contain amphibole and, to a lesser extent, mica. The refractory character of Type-I xenoliths suggests they represent mantle depleted by prior episodes of partial melting. In contrast, Type-II series (wehrlites, olivine clinopyroxenites, clinopyroxenites and plagioclase-bearing ultramafic lithologies), on the basis of their textural features, thermobarometric histories and major and trace element variation, appear to have formed as magmatic cumulates. Petrologic and geochemical studies of Type-II xenoliths from No ´ gra ´d-Gfmfr suggest they crystallized from basaltic melts emplaced within the lithospheric mantle and lower crust, prior to the onset of Plio–Pleistocene volcanic activity. After their consolidation, metasomatic agents reacted with the anhydrous cumulate phases producing amphiboles and micas at the expense of olivine and clinopyroxene. The metasomatic agents appear to have been adakitic rather than basaltic in composition, possibly linked to a retreating arc–forearc system. Large-scale contamination of the lithospheric mantle can therefore be attributed to fluid and melt fractions related to subduction beneath the outer Carpathian arc. D 2004 Elsevier B.V. All rights reserved. Keywords: Carpathian-Pannonian region; No ´gra ´d-Gfmfr Volcanic Field; Cumulate xenolith; Underplating; Metasomatism; Trace element geochemistry 1. Introduction Assemblages of ultramafic xenoliths hosted by undersaturated basaltic and kimberlitic magmas worldwide have been extensively studied. These 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.07.032 * Corresponding author. Tel.: +36 1 209 0555x8338; fax: +36 1 381 2108. E-mail address: cszabo@cerberus.elte.hu (C. Szabo ´). Tectonophysics 393 (2004) 139– 161 www.elsevier.com/locate/tecto