Atypical Ultrapotassic Rock occurrence in Romania Gătaia ...rjes.igr.ro/wp-content/uploads/2012/07/24-Seghedi-Gataia...2012/07/24  · lamproites compositions. The rock is characterized

Abstracts

Atypical Ultrapotassic Rock occurrence in Romania Gătaia Lamproite

Ioan Seghedl, Theodor Ntafloi, Zoltan Pecskal JInstitute ofGeodynamics, 19-21, str. Jean-Luis Calderon, 020032 Bucharest, Romania 2Dept. ofGeological Sciences, University of Vienna-Geozentrum, Althanstr. 14, 1090 Vienna, Austria 3 Institute of Nuclear Research ofthe HungarianAcademy ofSciences, p.D. Box 51, Bem ter 18/c, H-4001 Debrecen, Hungary

Introduction

73

Major and trace element, SrN d isotope data have been recently presented for post-collisionai ultrapotas;ic lamproite from Gătaia (SWRomania), with KJ Ar age at 1.3 Ma (Seghedi et al., 2008). The lamproite occurrence, as a small shield vo1cano, is situated ca. 5 km south of Gătaia (Banat) and was considered up to now as an alkali basaltic occurrence. The Şumiga hill (198m a.s.l.) is a lava cone, constituted by a sequence of vesicular lava and by some intercalations of falI-out scoria deposits. The lamproite sample belongs from a ca. 1 Om thick lava flow pierced by an exploration drilling in the peak area ofthe vo1cano. The lamproite magma gets into relatively undeformed flat-lying Miocene sedimentary rocks at the margin ofthe Pannonian Basin along an important NE-SW faults system. The lava cone overlies an older Pleistocene terrace deposits and crystalline basement that experienced intense lithospheric deformation aud orogeny during Cretaceous. The Gătaia lamproite represent a short lived magmatic episode, which was generated in similar time interval with other contemporaneous vo1canic activity, but at 50-150 km toward N-NE, along the South Transylvanian fault system (Lucareţ alkali basalts, Uroiu shoshonites), but also with another ultrapotassic occurrences in the Pannonian Basin (Bar) (Harangi et al., 1995; Seghedi et al., 2004). The rock is fresh and has a slightly porphyritic texture with phenocrysts ofhigh-Mg olivine aud microphenocrysts of euhedralleucite, in a glassy matrix, which contain microcrysts of olivine, arma1colite, apatite, sanidine, low Al-diopside, fluor-bearing titanium phlogopite, fluor-bearing amphibole and as accessory chrome spinels (mostly inc1uded in olivines) and rare titauomagnetite (Fig. 1 ). Ba-sulphate aggregates fill shlall vesic1es. Rare Al-phlogopite, surrounded by secondary spinels, is enc10sed by leucite aggregates, which suggest formation at high pressure in an earlier event.

Fig. 1. Back-scattered electron image showing the texture ofthe Gataia lamproite and the main mineral components: olivine (ol); leucite (le); phlogopite (Phl) ; apatite (ap); armalcolite (am); clinopyroxene (cpx); spinels (sp) and glass.

74 Geodiversi

The major and trace element geochemistry attests that the rock is a typicallamproite, cl0se to Leucite Hills and Gaussberg lamproites compositions. The rock is characterized by high Mg# (75 .7), high abundances ofNi (478 ppm) and Cr (539 ppm), as well as high primary K20 contents (8.67 wt %) and K201Na20 values (6.84). lts place in the lamproite clan is given by reiativeiy low contents of AI203 (9.34 wt %) and CaO (3.43 wt %) in combination with high abundances ofRb (205 ppm), Ba (2750 ppm), Sr (868 ppm) Zr (1243 ppm), La (134 ppm) and Ce (253 ppm). Slight depletions ofNb relative to Ba and La and high ratios of Bal La suggest a metasomatically enriched litbospheric mantie source.

GeochemicalAssumtions Higher Rb/Sr and to higher RblBa for Gătaia lamproite, as compared with the most basalts, including MORB and OIB,

reveal an enrichment of Rb relative to Ba and Sr, demonstrating that partial melting of a similar source to that seen in other lamproites occurrences, which reflect a style of an enrichment closer to anorogenic lamproites, then orogenic ones, which show an increasing trend of Rb/Sr ratio (e.g. SE Spain) (Fig. 2). According to this diagram the source of our rock is closer to phlogopite-bearing peridotites.

10 ~'-TT,"TIO-'-'OTnnr--'"Tn~

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Fig. 2. Ba/Sr vs. Rb/Sr ratios in igneous rocks, including MORE, PM (primitive mantie) and OIB (Sun &McDonough, 1989), gamet peridotites (Gar Per; Erlank et al. , 1987) result in a trendwith narrow limits of 0·03 and 0·1 ofRb/Ba ratio; In contrast, the ultrapotassic lavas from phlogopite peridotite xenoliths (Erlank et al., 1987), from SE Spain (Venturelli et al. , 1984) and from W Australian lamproites (Mitchell & Bergman 1991), Gataia lamproite and Bar leucitite are characterized by an enrichment ofRb relative to Ba and Sr.

10 100 e.ISr

Melting of phlogopite veins in a harzburgitic substrate, may explain the fact that the Gătaia lamproite have very low CaO/ AI203 (0.36), low CaO (3.43 wt %) and Sc contents (14 ppm). As well, liquidus and near-liquidus studies on various lamproite compositions (e.g. Edgar & Vukadinovic, 1992) suggest that lamproitic magma, as the Gătaia, may have been resulted by partial melting of a phlogopite-bearing mantie source. Using as well, Lai Yb vs. Yb that can distinguish mantie melting processes, it is suggested that Gataia lamproite was generated in at low degree of partial melting of a phlogopite - gamet Iherzolite source. The Sr-Nd isotopic composition (0.70571; 0.51257) is quite primitive as compared with other lamproites in Mediterranean area (Prelevic et al. , 2007), however closer to Bar leucitite, attributed, as well, to a phlogopite-bearing mantie source (Harangi et al. , 1995), that agree with a lithosphere-metasomatic enrichment, though different from orogenic lamproites.

ConcIusions We conci ude that the source of Gătaia lamproite, as it is generally envisioned, was probably a Iherzolitic-harzburgitic

lithospheric mantie (e.g. Mitchell & Bergman 1991, Foley, 1992), which has subsequently been metasomatized in an ancient event, as a necessary requirement due to its high content in incompatible trace elements as Ba, Sr, Rb and Zr. However, even the Gătaia lamproite is situated at the westem edge ofthe Carpathian unit, a typical subduction related environment, our data are not conclusive that subduction-related H20-rich fluid have been involved in metasomatic processes. The large presence of fluor-bearing minerals suggests, as well, that the source was F -rich. Gătaia lamproite had probably a limited available source volume for melting in direct relationship with the ambient thermal regime in a typical post-collisional tectonic setting, during the Late Neogene to Quatemary tectonic evolution, that indicate surf ace uplift and eros ion, marking the collapse oftheAlpine orogen.

References

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