Zajacz, Z.

English

Zajacz Z.

Kovács I.

Falus Gy.

Hidas Károly

Szabó Csaba

Ntaflos T.

University of Szeged

Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 CUMULATE XENOLITHS IN THE ALKALINE BASALTS OF NÓGRÁD-GÖMÖR VOLCANIC FIELD (N-HUNGARY/S-SLOVAKIA): A MELT INCLUSION STUDY ZAJACZ. Z. ' , KOVÁCS, I.1, FALUS, G Y . ' , HIDAS, K.1, SZABÓ, CS.1 , N T A F L O S , T.2 1 Eötvös University, Pázmány Péter sétány 1/c., Budapest, Hungary, H - l 117. i n s t i t u t e of Geological Sciences, University of Vienna, Althan str. 14, A-1090, Vienna , Austria. E-mail: zajaczz@freemail .hu The Nógrád-Gömör Volcanic Field (NGVF) , built up by Plio-Pleistocene post-extensional alkali basalts and minor Badenien calc-alkaline andesites, is located at northern edge of the Pannonian Basin. T h e Pannonian Basin is an extensional back arc basin formed by two evolutional stages in the Neogene t imes related to the southward and later westward subduct ion of an thinned lithosphere beneath the Carpathian orogen belt. Most of the recent studies showed that the extension was related to roll-back effect of subducting lithosphere and followed by subsequent small scale upwell ing of as thenosphere dur ing the second stage of rifting (Huismans, 2001). Three different types of upper mantle and lower crustal xenoliths can be found in the alkaline basalts of the N G V F sampling different domains of deep lithosphere. 1) Peridotites (mostly spinel lherzolites), 2) Cumulates (pyroxeni tes in composition), and 3) granulites. The cumulate xenoliths reperesent underplated mafic melts (Kovács and Szabó, 2003) crystallized at the mantle-crust boundary. These xenoliths are composed mostly of cl inopyroxene, olivine and minor spinel. The ratio of ol ivine and clinopyroxene varies in a wide range. A noticeable part of these xenoliths show metasomatic alteration, which resulted in the the presence of amphiboles. Both the cumulat ing process and the metasomatic alteration prevents assessing the initial parent melt composit ion of the cumulate bodies using their bulk chemical composit ion. Four cumulate xenoliths selected contain large amount of primary partially crystallized silicate melt inclusions. These inclusions occur in mostly olivines and subordinately in cl inopyroxenes. Two different type of melt inclusions can be distinguished: 1) Type A., and 2) Type B. The type A silicate melt inclusions are mostly isometric and rounded in shape. They Figure 1: Mutiphase type A inclusion in olivine. Figure 2: Type B inclusion in olivine. occur both in olivine and cl inopyroxene showing similar phase ration and composi t ion (Fig. 1). Size of these inclusions ranges f rom 5 n m to 70 | im. The main daughter phase is cl inopyroxene, however rhonite and spinel are also c o m m o n consti tuent. Amphibole might occur a portion of the silicate melt inclusions. In addition, in some inclusions sulfide blebs and apatite can be also recognized. All the type A silicate melt inclusions studied contain gas bubble about one f if th in diameter of the host inclusion. The volume ratio of solid phases are in the range 25-35 vol% of clinopyroxene, 15-25 vol% of rhonite, 3-7 vol% of spinel, 20-50 vol% of glass, and 5-10 vol% of amphibole (if any), and other phases The type B of silicate melt inclusions occurs only in olivine of one particular olivine cl inopyroxenite xenolith (Fig. 2). These inclusions consists of Cr-spinel and glass. The ratio of these phases varies in a very wide range conf i rming the contemporaneous enclosure. Gas bubble is not typical (or absent) of this type of inclusions. The shape and the size of these inclusions are also very diverse, showing f rom isometric to elongated shape, and size in the range of 10-500 pim. The chemical composit ion of phases in silicate melt inclusions are similar to the composi t ion observed in c o m m o n silicate melt inclusions enclosed in olivine phenocrysts of alkali basalts. The cl inopyroxene has high Ca content (Wo 5 4 Fig. 3) and Ti content (up to 5.7 wt%); the mg# ranges f rom 68 to 84. The spinel composi t ions vary along the spinel-hercinite line and are 232 Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 close to the M g A l 2 0 4 end member (Fig. 4), however, in some silicate melt inclusions hosted in cl inopyroxene t i tanomagneti te also occurs, whereas the cr# is very low: f rom 0.1 to 6. Rhonite shows low silica content ( S i 0 2 ~ 2 7 wt%), low Na and high Ti content (cation number of Na ranges f rom 0.3 to 0.6 and of Ti ranges f rom 0.6 to 1.2, respectively). This composit ion is similar to those observed in the melt inclusions of olivine phenocrysts of alkaline basalts (Sharygin et al., 2003). The amphibole has pargasitic and kaersutitic composit ions with cation numbers 5.7<Si<6.4, 0.13<Ti<0.91, respectively, and the mg# ranges f rom 68 to 76. Figure 3: Composition of clinopyroxenes plotted on Figure 4: Composition of spinel in Type A inclusions. Enstatite-Ferrosillite-Wollastonite diagram. High temperature microthermometric data observed using Linkam-stage indicate melting temperature ranges for different phases of the silicate melt inclusions: glass: 1090-1190 °C, cl inopyroxene: 1190-1240 °C, rhonite: 1220-1265 °C, spinel: 1260-1320 °C. Homogenizat ion has been reached by the total melting of spinel, however gas phase in most cases does not disappear even during heating up to 1350 °C. Occurrence of amphiboles suggests relatively high volatile content (in this case OH-) for initial silicate mel. References: HUISMANS, R. S. (2001): Dynamic modeling of the transition f rom passive to active rifting, application to the Panno-nian basin. Tectonics, 20. No. 6: 1021-1039. KOVÁCS, I. & SZABÓ, CS. (2003): History of the lithosphere beneath the Nógrad-Gömör Volcanic Field, Carpathian Pannonian Region (N-Hungary/S-Slovakia) submitted to Tectonophysics KUNZMANN, T. (1999): The aenigmatite-rhönite mineral group. Eur. J. Mineral. , 11: 743-756. 233 

Cumulate xenoliths in the alkaline basalts of Nógrád-Gömör volcanic field (N-Hungary/S-Slovakia) : a melt inclusion study

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Cumulate xenoliths in the alkaline basalts of Nógrád-Gömör volcanic field (N-Hungary/S-Slovakia): : A melt inclusion study

Cumulate xenoliths in the alkaline basalts of Nógrád-Gömör volcanic field (N-Hungary/S-Slovakia): : A melt inclusion study

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