Origin of sulfide inclusions in cumulate xenoliths from Nógrád–Gömör Volcanic Field, Pannonian Basin (north Hungary/south Slovakia)

Abstract

We provide new information about the evolution of the lithosphere beneath the Nógrád–Gömör Volcanic Field (NGVF, northern Pannonian Basin) based on sulfide inclusions in cumulate-origin ultramafic xenoliths. The clinopyroxene-rich cumulate xenoliths, representing the lower crust and upper mantle, underwent metasomatic alteration, which resulted in formation of amphiboles. We have carried out a detailed petrographic observation on sulfide inclusions using reflected light microscope and analyzed numerous back-scattered electron images of the most typical sulfide blebs. Based on the petrographic study, only rounded, elongated or negative crystal-shaped single inclusions, occurring randomly in clinopyroxene and amphibole and rarely in olivine and spinel, have been selected for detailed electron microprobe analysis. The size of these single inclusions ranges from 3 to 75 μm in diameter. The sulfide blebs consist mostly of pyrrhotite and minor chalcopyrite, pentlandite and cubanite. Pyrrhotite, which is the major phase in all the inclusions, is Ni poor (max 6.1 wt.%). Chalcopyrite is deficient in Cu content (≥28.9 wt.%). Pentlandite and cubanite show regular compositions and were identified only in one xenolith. The bulk compositions of sulfide blebs show a tight compositional range and are rich in Fe compared to those in Type-I peridotite xenoliths from the same volcanic field [Geochim. Cosmochim. Acta 59 (1995) 3917; Falus, Gy., 2000. Geochemical significance of sulfide inclusions of Cr-diopsidic xenoliths of alkaline basalts occurring in the Carpathian–Pannonian Region. MSc thesis, Department of Petrology and Geochemistry, Eötvös University, Budapest]. The sulfide blebs studied likely formed from a sulfide melt coexisting with a silicate melt. The latter melt was the source of host clinopyroxene-rich cumulates. The sulfide blebs (mineralogically pyrrhotite) experienced a high- and low-temperature evolution, producing further sulfide phases (chalcopyrite, pentlandite, cubanite) present in the blebs.