Dacitic glassy lava flow from Trlično at Rogatec, Eastern Sloveniad

Glassy lava from Trli~no is dacitic in composition and developed by incomplete mixing of two magmas of similar composition but different degree of groundmass crystallinity. By laminar lava flow banded texture developed consisting of bands with different magma types. Abundance of major and trace elements is within the variation range for dacites. Comparison with the Smrekovec andesites in Northern Slovenia indicates that the rocks do not originate from the same volcanic complex, displaced by tectonic activity.


Introduction
Tertiary volcanic rocks outcropping in the Roga{ka Slatina and Rogatec area form a part of a widespread volcanic complex which extend discontinuously from the Smrekovec Mts. towards the southeast along the Donat transpressive zone (Fig.1). The age of volcanism is not solved yet, although recent tectonostratigraphic and biostratigraphic studies (J e l e n e t a l . 2001) indicate the existence of two Tertiary volcanic sequences: the lower, and the upper (Upper Oligocene -Egerian) volcanic sequence. Both sequences have entirely submarine character as evi-denced from nannoplankton and plankton foraminifera fauna found in the underlying interstratified, and overlying fine-grained clastic sediments. Lavas and high-level intrusive bodies in the Smrekovec Mts., and in the Roga{ka Slatina and Rogatec areas, both seem to belong to the lower volcanic sequence.
The present contribution is focused on petrographic characteristics and chemical composition of a glassy lava flow which outcrops in the easternmost part of the Rogatec area. The influence of depositional environment to the rock structure and a brief comparison with the Smrekovec volcanics is discussed, too.

Brief geological setting
The Roga{ka Slatina and Rogatec area forms a part of the southwesternmost part of the widespread system of Pannonian basins. The area is tectonically disturbed by several faults. The most prominent are the Šo{tanj and Labot faults which join into the Donat fault near Roga{ka Slatina. Along the Donat fault, pre-Teritary carbonate basement outcrops. At the Rudnica Mt., south of Roga{ka Slatina, Mesozoic volcanic rocks, mainly spilites, occur. Tertiary sediments are characterised by dominating clastic development, in Rupelian and Oligocene mainly as silts, and in Miocene as interstratified silts, sands and conglomerates. Rupelian to Egerian volcanism produced submarine lavas, pyroclastic deposits, autoclastic and resedimented volcaniclastic deposits. Based on foraminifera fauna occurring in the undelaying and overlying clastic sediments indicate that the environment was marine in the life span of volcanic activity. Basic geological map 1: 100.000, Sheet Rogatec, has been elaborated by A n i~i } & J u r i { a (1989).

Petrography and chemical composition
Lava flow from Trli~no is a plagioclaseaugite-magnetite vitrophyre which consists of glassy groundmass, phenocrysts, microphenocrysts and microlites. The rock is relatively fresh, except for marginal, autobrecciated parts which may grade into peperites. In a hand specimen, the rock is black; plagioclase phenocrysts are up to 1,5 mm sized and amount up to 10 % of the bulk rock. Under the microscope, in most of the plagioclase phenocrysts, lamellar twinning and zonation is seen. The composition of plagioclases ranges from andesine to labradorite. Augite phenocrysts are less common than plagioclases, and they are frequently associated with irregularly shaped magnetite. Sometimes, they contain inclusions of plagioclase microphenocrysts. Up to 0,5 mm sized olivine microphenocrysts are rarely fresh. They are replaced by brownish-green filosilicate minerals. Plagioclase microlites are not very abundant, but still they indicate flow texture.
The bulk of the glass contains very small greenish crystallites with elongated prismatic habitus and very high index of refraction. Most probably, they belong to augite. In the crystal surface, a few micrometer sized irregular inclusions or exsolutions of opaque minerals -very possibly magnetite -occur, and impart dark color to the rock.
The lava flow is actually a mixture of three magmas of similar composition, but slightly diverse degree of groundmass crystallinity, i.e. the size of (augite) crystallites and plagioclase microlites. The smallest crystallites attain some µm, the intermediate sized microlites some ten µm, and the largest some 100 µm. Magmas with different degree of crystallinity seem to start mixing during the lava flow. In the zone of laminar flow, the bands with different degree of groundmass crystallinity developed, but in more turbulent zone of the lava flow, more irregular lenses formed. Locally, sediment inclusions occur. The included sediment is finegrained and strongly altered.
There are cracks in the rock, related to the hydration processes, and they are infilled with brownish-green filosilicates. In the same thin section, the cracks may form angular and subangular or rounded perlitic parts. Locally, lava margins grade into peperite, the mixture of still flowing lava and the enclosing sediment. Peperite is strongly altered into filosilicates, mainly montmorillonite, quartz and analcime. Chemical composition of the rock is slightly variable owing to the mixing processes (Table 1). The lava from Trli~no is dacitic in composition according to classification based on SiO 2 vs. Na 2 O + K 2 O (after L e B a s e t a l . 1986), and the Zr/TiO 2 vs. SiO 2 contents (Fig.  2, after W i n c h e s t e r & F l o y d 1977). On the SiO 2 /K 2 O diagram (after P e c c e r i l l o & T a y l o r 1976) the samples belong to calcalkali dacites. In comparison with the data for dacites (after E w a r t 1979), the samples from Trli~no are very low in magnesium and phosphorous with respect to the silica content.
Trace element abundance is in the variation span for dacites (E w a r t 1979). Among incompatible elements, Li and Rb are relatively low. K/Rb ratios amount to 148 and 158 respectively, and they are lower than in the Smrekovec andesites (K r a l j 1996). Abundance of compatible trace elements Cu and Ni is low. The samples are very rich in Nb and rare earth elements with respect to the avearge dacite composition after E w a r t (1979). In comparison with the Smrekovec andesites and with respect to a relatively higher silica content, the analysed samples seem to be rich in scandium, chromium, cobalt, zinc, arsenic, rubidium, yttrium and barium and very low in zirconium. Rare earth element abundance is higher than in the Smrekovec volcanics (Fig. 3), what can be expected from a higher silica content, but the diagram of the ratios Ce/Y vs. Ce (Fig.  4) two different lines for the Smrekovec and Rogatec volcanic rocks.

Conclusions
Glassy lava from Trli~no at Rogatec is banded and shows flow texture. It seems to form by mixing of two magmas with similar composition but diverse degree of groundmass cristallinity. Chemical composition is within the variation range for dacites, although magnesium and phosphorous are relatively low. Among trace elements, lithium, rubidium, copper, nickel and vanadium are low, and rare earth elements and niobium are high. Comparison with the chemical composition of Smrekovec andesites suggests that the rocks probably do not belong to the same volcanic complex, displaced by tectonic activity.