Field trip day-2

Bükk-Szarvaskõ area

(Tardos quarry, Tóbérc quarry, Új-határ valley, Szarvaskõ-pillow basalts)

29.11.2003

Introduction

On the second day of our field trip, we visited some interesting localities of the Szarvaskõ complex, which is situated in the southwestern part of the Bükk Mountains north of the city Eger.

Geology of the Szarvaskõ area

This igneous body consists mainly of a differentiated basalt-gabbro association with few ultramafic rocks and plagiogranites intruded into terrigenous sediments. These sediments are chiefly sandstones, shales, or siltstones.
This complex has always been a subject of debates and there still are some questions which haven't been answered for sure.
One of these important, but difficult questions was the age of the complex, which seems to be cleared up know. The early researchers suggested that the complex was formed during the Cretaceous period (Schréter,1943; Szentpétery, 1953) , and this conception has dominated for years. After proving the Triassic age of the igneous rocks in the Eastern Bükk area, most of the authors correlated the Szarvaskõ complex with these igneous rocks and they considered it to be also Triassic by age. The Triassic age has been confirmed many times (Földessy, 1975; Balla et al., 1980; )
The first article which suggested a Jurassic age was by Balogh et al. (1984), who related Jurassic radiolarians to the Szarvaskõ complex. This hypothesis was proved by some K-Ar age determinations, which gave an avarage age of 166 Ma, thus the complex is know considered to be undoubtedly Jurassic by age.

Pillow basalts at Szarvaskõ (left) and the best specialist of Szarvaskõ: Prof. Friedrich Koller

Another difficult, and until know not solved question is the origin, the petrogenesis of the complex. But to understand the possibilities, first of all let us have a look at the types of the igneous rocks. The most detailed study of the rocks was made by Szentpétery (1923, 1943, 1953), who has described a wide variety of differentiation products.
Within the volcanic rocks, most abundant are the pillow basalts. The plutonic section is mostly made up of gabbroic rocks with lots of differentiation products. They are also varying in grainsize, from fine-grained up to coarse-grained, and pegmatitic types. Interesting plutonic rocks are the so called "plagiogranites", leucocratic rocks consisting mostly of plagioclase and quartz, but it very often also contains garnet, which is almandine-rich. From the most abundant gabbros there are transition rocks to the ultramafics. These can be called pyroxenemelagabbros, gabbro-hornblendites, peridotit-hornblendites etc.
The most famous ultramafic rock in this area is the Ti-rich ore peridotite, which is better known as "wherlite" in the literature. This name was given after Wehrle, who analyzed this rock the first time. The most important ore-minerals are ilmenite, magnetite and titanomagnetite which give an avarage ore-content of 25 % to this rock.
To sum up all, at first sight we can see that this complex was a result of intruding magmas which went through differntiation processes. The dominance of pillow-basalts show us (in agreement with the surrounding deep-water sediments) that this intrusion must have taken place in marine environment.

Like we mentioned it before, the origin of this complex with its igneous rocks is a very embarrassing and difficult question, and despite of the newest analyzes on petrology and geochemistry, it remains a subject of guessing. On our fieldtrip we where accompanied by Prof. Friedrich Koller from Vienna, who has done lot of work on the Szarvaskõ area. One of his students, Aigner-Torres has done the latest geochemical examination on the complex (Aigner-Torres, 1996)
Among the conceptions we will just introduce the recently most supported idea, which has been confirmed also by Prof. Koller and his student.
According to this model the igneous rocks of Szarvaskõ are fragments of a dismembered ophiolitic sequence, thus remnants of Jurassic oceanic lithosphere. This model was put forward by E.Szádeczky- Kardoss (1974,1976), and it soon got the dominant model, wich most of the authors supports with more or less modifications (Balla et al., 1980; Onuoha, 1977). If we believe this model, there still remains a question whether if this ophiolit was located in an oceanic ridge or an other special environment?
Recent studies on the paleotectonic of the region and geochemical analyzes of the igneous rocks have shown that it is very likely that the ophiolite was situated in a back-arc basin, and not an oceanic ridge (Aigner-Torres, 1996).
Even on our fieldtrip Prof. Koller introduced us this model, reminding us also on the problems and unsolved questions concerning it.

Stop-1: Tardos quarry

This quarry is situated north of the village Szarvaskõ on the road to Mónosbél. It is built up of different types of gabbros, which have a sharp contact to each other. If we view the wall from distance, we can well separate two types by colour, one is light, the other is much more darker. The colours can be explained by the mineral-constitution: the darker one is clinopyroxene-rich, and the lighter one contains much more plagioclase. Between the gabbros there are also differences in grainsize; it varies from fine- up to coarse-grained, but the real pegmatits-like those of the Tóbérc quarry or the Újhatár valley- are missing.
In this quarry also the surrounding sediments can be seen.

Tardosi quarry, north from Szarvaskõ

The guider of the field trip: (right) Prof Friedrich Koller (University of Vienna), (middle) Dr Molnár Ferenc (ELTE University Budapest, academic sponsor of the SC, (left) Mr. Nigel Maund

Stop-2: Pillow basalts at the Kétvíz hill

At the crossing of the roads coming from Szarvaskõ to Mónosbél and Tarnalelesz, there are some nice outcrops of pillow basalts. Intresting in this locality is that here we don't find the common pillow structure of many pillows settled on each other. Here we can see separate drops of basalt within clayshale. The drops are really isolated from each other, and surrounded by the clay, which is extremely burnt.
We consider that these drops where formed on a steep edge of a pillow basalt field. When the lava tubes flow on the ground, reaching the edge of the failure they fall down, and sank into the unconsolidated clay, which was burnt by the hot drops.
It was very interesting for us to see this rare vulcanological phenomena.

Contact metamorphised claystone at Kétvíz Hill

Contact metamorphised claystone at Kétvíz Hill and separated pillow basalt drops at Kétvíz Hill

Stop-3. Pillow basalts at Várbérc Hill

If we walk out of the village Szarvaskõ a few hundred meters on the road to Mónosbél, we get to a narrow canyon where the hills on both sides break down with steep and rocky walls. These are built up of lots of "pillows" settled on each other. To tell the truth these forms not at all look like pillows, they much more resemble tubes. They got their elongated forms by moving, flowing on the ocean-bottom.
Between the pillows, at the triangular margin, the so called hyaloclastit can be seen. When the lava flows out on the ground the edge of each pillow gets glassy by the contact with water. Under cooling this edge breaks up into pieces and falls down to these triangular spaces, where it forms aquagene tuff, or hyaloclastit.

Pillow basalts at Szarvaskõ on the Várbérc Hill

Pillow basalts at Szarvaskõ on the Várbérc Hill

Stop-4: Tóbérc (Forgalmi) quarry

The best exposed are the plutonic rocks in this quarry, which is situated east to the village Szarvaskõ at the entrance of the Újhatár valley. Here most of the mafic plutonic differentiation products can be found.
The gabbros dominate, but they can be very different in mineralogical composition, and grainsize. Szentpétery (1923,1943,1953) has separated many differentiation products on the basis of the common rock-forming minerals. These minerals are the diallage-pyroxene, hypersthene, brown-amphibole, biotite etc., thus the given rocknames are diallageamphibole-gabbro, biotiteaugite-gabbro etc. The gabbros are mostly medium- and coarsegrained, but the pegamtitic texture -with minerals up to 3-4 cm- is very common to. These pegmatites are often near to the leucocratic rocks, collectivelly termed as plagiogranites. These rocks are more acidic, contain much plagioclase and quartz, and Szentpétery gave them also names like biotitequartz-diorite, diallagegabbro-diorite. At the contact of these rocks with the coarse-grained, or pegmatitic gabbro there often appear garnets in an 1-2 cm thick zone.
It is likely that these leucocratic rocks, and the pegmatites where formed by the contamination with the surrounding shales, and this process caused the increased fluid-content, and acidity.

Not only in this quarry, but in the hole igneous complex it can be established that the rocks underwent a low pressure- low temparature (pumpellyite-prehnite facies) metamorphism. This means that examining them with microscope lots of secondary minerals can be found. Instead of pyroxene and amphibole chlorite occurs, veins are filled with pumpellyite, prehnite.

Stop-5: Outcrop in the Újhatár valley

This valley, which ends at the Tóbérc quarry is built up by different types of gabbros, but there are only few outcrops, because the brook hasn't done big erosion, and there wasn't any quarrying in the valley at all. Despite of this fact there is a very exciting outcrop at the steep hill-side on the left shore of the brook, about 1 km inside the valley. There are some rocks of gabbro which contain the most beautiful pegmatites in the area. The pegmatites are mostly some decimetres in diameter, and have different shapes. They can be elongated sills, and tubes, or round, oval forms. In these pegmatites the amphiboles can reach a length of 8-10 centimetres, and there also can be cavities with grown-up crystals. The minaralogy of these pegmatites is under examination.

Epilogue

The financial background of our first Field Trip was estabilished by the Student Chapter Field Trip Grant dominated by the Committee of Student Chapters of the Society of Economic Geologists. We owe special thanks to the SEG for supporting our programs and activity at Eötvös Loránd University in the Carpatho-Panonian region. Our Academic sponsor is sincerely ackowledged for providing an excellent guide and understanding of the geology and mineral deposits of the Recsk and Szarvaskõ area. The executive also thank the members for their contributions and chearful attendance.