The origin of volcanic rock fragments in Upper Pliocene Grad Member of the Mura Formation , North-Eastern Slovenia

Fresh-water, coarse-grained and detritus-dominated Mura Formation in NorthEastern Slovenia includes pyroclastic and volcaniclastic deposits originating from Upper Pliocene volcanic activity of basaltic geochemical character. Although localized in occurrence at the hamlet Grad, these pyroclastic and volcaniclastic sediments form a distinctive depositional unit, for which the term “Grad Member” is proposed and introduced in this paper. In the Grad area no lavas or cinder cones are preserved, and the origin of volcaniclastic fragments still uncertain. For this reason, chemical composition of basaltic rock fragments from the Grad Member volcaniclastics has been studied and compared with basaltic rocks from the neighboring locations at Klöch, Kindsberg, Dölling and Neuhaus. The Grad Member pyroclastic and volcaniclastic deposits seem to be fed from the same source which is different from the occurrences in Austria. That supports the idea about the existence of a local volcanic centre in the present Grad area. The old volcanic edifices were possibly destroyed by the late-stage hydrovolcanic eruptions, and pyroclastic and volcaniclastic deposits subjected to constant reworking by fluvial currents in a dynamic sedimentary environment of alluvial fan and braided river systems.


Introduction
Fresh-water, coarse-grained and detritus-dominated Mura Formation in North-Eastern Slovenia includes pyroclastic and volcaniclastic deposits originating from Upper Pliocene volcanic activity of basaltic geochemical character. Although localized in occurrence at the hamlet Grad, these pyroclastic and volcaniclastic sediments form a distinctive depositional unit, for which the term "Grad Member" is proposed and introduced in this paper.
Upper Pliocene Grad Member consists of pyroclastic, syn-eruptive resedimented volcaniclastic and mixed volcaniclastic-fluvial deposits. Their formation is closely related to continental alkali basaltic volcanism which was active about 3 million years ago in the area of the present medieval castle and the surrounding hamlet Grad in Gori~ko, North-Eastern Slovenia. The volcanism forms a part of a broader volcanic province encompassing South Styrian Basin (Pöschl, 1991;Winkler, 1927;Poulditis, 1981;Pouldi t is & Scharbert, 1986) and Little Hungarian Plain (Martin & Németh, 2004), and developed as a consequence of postcollisional extension of the south-western realm of the Pannonian Basin.
At the present, no lavas are preserved in the Grad area, but only their fragmented remains (Plate 1 - Fig. 1) in volcaniclastic debris flow deposits. For this reason, doubts have been posed again recently about the existence of local volcanic centre in the Close proximity -about 10 km -of large lava flows on the crest of the South Burgenland Swell and in the neighboring Styrian Basin in Austria -at Stradner Kogel, Klöch, Kindsberg, Dölling and Neuhaus, supported a possibility that some of them might have been an additional source of volcaniclastic debris at Grad. The present contribution deals with detailed chemical composition of potential rock occurrences in Austria, and lava fragments in the Grad Member volcaniclastic rocks in order to characterize the source and consider possible paleotransport directions for volcaniclastic rock fragments of the Grad Member.

Geological setting outline
North-easternmost Slovenian territory ( Fig. 1) is a hilly country that forms a part of the Mura Basin -the south-easternmost extending of the Pannonian Basin. The Mura Basin is filled with clastic, and to minor extent carbonate sediments, that range in age from Neogene to Quaternary. The Mura Basin consists of two depressions -northerly positioned southwest -northeast trending Radgona depression, and nearly west-east trending Ljutomer depression. They are separated by the Murska Sobota Swell (K i s ovar, 1979). The Radgona depression is separated from the neighboring Styrian Basin in the north by the South Burgenland Swell (Toll m a n n, 1986; O b e r h a u s e r, 1980). The basement of the Mura Basin mainly consists of Paleozoic metamorphic and clastic sedimentary formations; only in the deepest parts of depressions, Mesozoic carbonates are preserved. Tertiary sediments were deposited in a marine environment during Karpatian, Badenian and Sarmatian stage (R i j a v e c et al., 1985). Except for Badenian, they are developed as clastics -clays, marls, silts and sands.
During Pannonian, brackish conditions prevailed. Lower Pannonian sediments are silts and marls characterised by the occurrence of ostracods. They are overlain by the beds with Paradacna abichi molluscans, termed the »Abichi beds« (P l e n i~a r, 1968). Overlying Lower Pliocene -Pontian sediemnts are mainly limnic, and consist of quartz sands, sandy silts and clayey silts. These deposits are regarded as »the freshwater equivalent of the Rhomboidea beds« (Plen i~a r, 1968). Middle Pliocene deposits overlie discordantly Pontian beds and are developed as sands and gravels. Fluvial sedimentation that started with Middle Pliocene persisted during Upper Pliocene and Quaternary.
Sarmatian sediments are united in the Murska Sobota Formation, Pannonian and Lower Pontian in the Lendava Formation, and Upper Pontian and Quaternary in the Mura Formation ([imon, 1966).
During Pliocene, volcanic activity of basaltic composition occurred in the Styrian Basin and in the northern margins of the Radgona depression. On the crest of the South Burgenland Swell large lava flows occur (Fig. 2). Towards the north, maars, tuff rings and tuff cones are more common in occurrence. Strongly differentiated and alkalies-rich varieties include nephelinites, basanites, nepheline basanites, thrachybasalts and alkali basalts. Peridotite and/or lherzolite xenoliths are common and indicate the origin of magmas and their rapid ascend towards the surface (Embey-Isztin & Kurat, 1996). Volcanic activity at Grad occurred in an active continental sedimentary environment characterized by alluvial fan and braided river systems (Kralj, 2000 b). Rapid deposition of coarse-grain dominated detritus was closely related to the rise of the South Burgenland Swell and the subsidence of the Radgona depression. During Early Pliocene, a system of alluvial fans formed along the south-eastern slopes of the South Burgenland Swell, and towards the south, east and south-east it continued as a system of braided rivers, although the main transport direction was from north-west to south-east. The present maximum thickness of clastic deposits in the Radgona depression amounts to about 2000 m. Lavas and pyroclastic deposits occurring in such active depositional environment had little preservation potential and rapidly underwent redistribution by fluvial currents. Magmas ascending towards the surface reached water-bearing strata and consequently, and with the time, the style of eruptions became essentially influenced by hydrovolcanic processes. Their violent explosions additionally contributed to destruction of primary volcanic edifices and lavas (Plate 1 - Figure 1) which already had little preservation potential in such dynamic fluvial sedimentary environment (Kralj, 1995;2000a, b).

Chemical composition of alkali basaltic rocks
Chemical composition of alkali basaltic rocks in Styria and Burgenland is extensively treated by P o u l d i t i s (1981) and P o u l ditis & S c h a r b e rt (1986). Their absolute age was determined by B a l o g h et al. (1994). At Stradner Kogel, the most differentiated varieties -nephelinites occur in the form of lava massive. At Klöch and Kindsberg, lavas of nephelinite basanite composition outcrop. At Neuhaus, alkali basalts occur. They penetrated soft sands, probably soaked with water, and consequently, they underwent extensive autobrecciation (Plate 1 - Fig. 2). In such form, they could be easily eroded and transported by water currents.
In order to minimize analytical errors related to procedures in different laboratories, rock samples from potential locations in Austria were analyzed in the same laboratory and under the same analytical conditions as the samples from Grad. The analyses were performed in X-RAL Activation Services Inc. in Ann Arbor, Michigan and Don Mills, Ontario, and it encompasses determination of 73 elements by combined wet chemical method, atomic absorption spectroscopy, and inductively coupled plasma source and mass spectroscopy.
The rocks from Klöch, Kindsberg, Dölling and Neuhaus were determined as potential sources with respect to the general pa-leotransport direction. Stradner Kogel was eliminated in the first place since the composition of nephelinite lavas is too declined from the composition of lava clasts from of the Grad Member volcaniclastics as evidenced from preliminary petrographic studies. Chemical composition of the studied rocks is shown in Table 1 and Table 2.
General overview of chemical analyses indicates that among major oxides silica and K 2 O do not differ significantly in the studied samples. The rocks from Austria have higher abundance of TiO 2 , CaO, MgO and Na 2 O, and are lower in P 2 O 5 . Rock fragments from the Grad Member volcaniclastics tend to be enriched in almost all trace elements -Rb, Be, Sr, Ba, Ag, Zn, Th, U, Zr, Hf, Ta, W, Y, REEs, As and Sb, and depleted in Cu, V, Ni and Sc. The magma(s) producing rock fragments from the Grad Member volcaniclastics seem to be more differentiated than those from Austria. Depleted CaO, MgO, TiO 2 , Cu, V, Ni and Sc might be related to the removal of pyroxenes from the melt by crystal fractionation.

Discussion
The studied basaltic rock samples occupy mainly the fields of trachybasalt and basalt (Fig. 3) in the Na 2 O + K 2 O vs. SiO 2 diagram after LeBas et al. (1986). Only one sample from Klöch falls in the field of tephrite and basanite. Based on the content of alkali oxides and silica, the samples from Austria and the Grad Member volcaniclastics do not vary significantly, although the samples from the Grad Member tend to be more rich in silica at a given Na 2 O + K 2 O content. Similar trend can be observed in the Al 2 O 3 vs. SiO 2 variation diagram (Fig. 4). In the diagram MgO vs. SiO 2 , the samples from Klöch, Kindsberg and Dölling are clearly separate from the others. One of the samples from Neuhaus is positioned close to the Grad Member population, while the other shows extremely low abundance of MgO, possibly owing to alteration processes. In the diagram TiO 2 vs. SiO 2 the samples from Austria clearly separate from the Grad Member rock fragments. This trend is even more obvious in the diagrams of SiO 2 vs. Zr, Sc vs. Zr and TiO 2 vs. Zr (Fig. 5); herein, a further distinction between the samples from Klöch, Kindsberg and Dölling, and Neuhaus can be seen.

Conclusions
Geochemical characteristics of basaltic rocks from Klöch, Kindsberg, Dölling, and Neuhaus and basaltic rock fragments from the Grad Member volcaniclastics based on the abundance of major oxides and trace elements have shown the following: • Chemical composition of the Grad Member rock fragments and basaltic rocks from potential source locations in Austria is different. It is not likely that the lava fragments from the Grad Member are eroded and redistributed detritus from the occurrences in Austria. • Rock fragments from the Grad Member volcaniclastics could have two sources (volcanoes), or one, from which the magmas underwent differentiation during volcanic activity. • Late-stage hydrovolcanic explosions produced proclastic surges, large lahars and volcanic debris flows that destroyed the former volcanic edifice(s) including lava flows and cinder cones. In spite of numerous works (Hinterl echner- Ravnik & Mi{i~, 1986;Kralj, 1995;2000a, b;Lugovi} & Kralj, 2006), the study of the Grad Member is far from completed and will continue in the future.