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Article

Petrography of Ophiolitic Detritus from a Miocene Conglomerate Formation on Darnó Hill, SW Bükk Mts (N Hungary): A Unique Tool to Trace Covered Ophiolitic Sequences

by
Sándor Józsa
Department of Petrology and Geochemistry, Eötvös Loránd University, Pázmány P. sétány 1/C, H-1117 Budapest, Hungary
Minerals 2024, 14(10), 983; https://doi.org/10.3390/min14100983 (registering DOI)
Submission received: 22 July 2024 / Revised: 18 August 2024 / Accepted: 27 September 2024 / Published: 29 September 2024
(This article belongs to the Special Issue Submarine Volcanism, Related Hydrothermal Systems and Mineralizations)
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Abstract

:
Petrographic studies have been carried out on the Early Miocene Darnó Conglomerate Formation, which consists only of debris of ophiolitic mélange and is found today on Darnó Hill in SW Bükk, NE Hungary. The studied sediments are bounded by the Darnó line from Darnó Hill. The aim of this work was to show if it is possible to reconstruct the petrographic composition of the source area only from its debris. The rock types were determined in thin sections using a polarizing microscope, and a quantitative analysis of the different rock types was carried out using the grain counting method, the results of which were interpreted as volume ratios. The main rock types observed in the studied samples (textural varieties of basalt, dolerite/microgabbro, claystone, siltstone, and radiolarite) are similar to the rock types of the mélange assemblage of Darnó Hill. Based on the volume calculations of basaltic detrital grains with different textures characteristic for pillow basalts, it could be established that pillow basalts dominated the igneous rocks in the source area of the Darnó Conglomerate on Darnó Hill already in the Miocene. Thus, this work shows that the lithological composition of a source area can be precisely outlined by a detailed petrographic analysis of the debris eroded from the immediate vicinity.

1. Introduction

Geological research on clastic sedimentary rocks can have many purposes. These objectives can be in the areas of basic scientific research [1,2,3], industrial research [4], and environmental investigations [5]. Of course, the methods that are used in some areas and the results they obtained can also be useful for work in other areas. One of the main directions of scientific objectives is the provenance analysis of clastic sedimentary rocks. This trend now has very sophisticated, diversified, and modern methods (micromineralogy, geochemistry, geochronology, etc.) [6,7,8]. However, most of the above-mentioned methods can only be used to delineate a narrower or wider source area. In the case of coarse debris, the exact source rock or series of source rocks can be identified by gravel testing, but for debris particles in the mm size range, such as fine conglomerate or coarse sandstone, individual rock grain testing is not possible. This is where the fine-grained petrography of sandstone or fine conglomerate can come into play. I chose a sampling area where a well-known Mesozoic ophiolite mélange is located on the uplifted side of a distinct straight line of a tectonic zone, the Darnó Zone, and an Early Miocene shallow marine sedimentary assemblage appears on the depressed side. The present work provides the most complete micropetrographic description of the coarse sand grains of the Darnó Conglomerate, which has been compared with descriptions of similar rocks in the immediate source area found in the literature. The aim of this paper is to show that polarization microscopic petrography can be effectively used in source rock studies for siliciclastic rocks in the above grain size range. More specifically, one of my goals was to provide a detailed and complete petrographic description of the detrital grains of the Darnó Conglomerate, which has been lacking until now. Furthermore, I would like to define exactly what the main rock type was and its exact volcanological type (i.e., massive basalt, pillow basalt, or basalt–dolerite dykes) on Darnó Hill during the Miocene, and thus how much the geological composition of the surface of Darnó Hill has changed since the Early Miocene.

2. Geological Background

Our study area is Darnó Hill in SW Bükk, which lies on the southeastern border of the ALCAPA (Alpine–Carpathian–Pannonian) microplate, one of the two defining units of the Pannonian Basin [9]. The study area was once located at the western end of the Dinarides [10,11,12,13]. In the early Miocene, the ALCAPA microplate moved to its present position with a large-scale rightward shift of about 500 km along the Mid-Hungarian Shear Zone [14,15,16,17,18]. According to the most accepted view, the Szarvaskő–Darnó Complex, which is also known as one of the sources of the studied detrital assemblage [19], moved from the north onto the Bükk Paraautochton during the Early Cretaceous (Mónosbél–Szarvaskő nappe system) [20,21,22,23]. The presence of Silicic Nappe on the Bükk Paraautochton can be excluded [24]. The highlighted block of Bükkium is bordered on the west by the SSW-NNE-trending Darnó Shear Zone [25,26,27,28,29,30]. The object of this study is the Early Miocene Darnó Conglomerate, which lies on the Mesozoic basement along the Darnó Zone and is contemporaneous with the lower level of the westward-striking Pétervására Sandstone [19,31,32] (Figure 1). On the western slope of Darnó Hill, the studied assemblage, which can be examined on the surface, is a polymictic, poorly sorted conglomerate and coarse sandstone with gravel [33]. Among its debris, a large proportion of mafic igneous, silicic, and siliciclastic rock types occur [19,33,34], which show great similarity with the rock types (microgabbro, dolerite, pillow and massive basalt, hyaloclastite, shale, silt, radiolarite, slate, and limestone) that occur in the ophiolite mélange of the Darnó Hill, located in the vicinity immediately to the east [12,19,35,36,37].

3. Materials and Methods

The samples were taken from the excavation at the foot of Kis-hegy at Darnó Hill, just next to the Darnó line (47°57′10.4″ N 20°09′11.5″ E). The excavation revealed a series of reddish-brown, dark-brown siliciclastic rocks with clayey matrix and calcareous cement, poorly sorted, weakly layered, mostly sand- and gravel-sized grains [19,33,38]. After field sampling, normal (5 × 2.7 cm) and larger (max. 5 × 5 cm) covered thin sections were prepared (Figure 2) and examined using a Nikon OPTIPHOT2-POL OPTIPHOT polarizing petrographic microscope at the Department of Petrology and Geochemistry, Eötvös Loránd University, Budapest, Hungary. To determine the rock ratios, semiquantitative analyses of detrital grains were performed by estimation, and quantitative analyses were performed on two thin sections by counting all detrital grains larger than 300 µm. Since the sizes of the different rock grains considered in the grain counting method did not differ significantly from each other, the rock ratios obtained by grain counting were treated as volume ratios. The photomicrographs were taken with a Nikon DS Fi1 instrument and processed with the NIS Elements BR 3.2 program, also at the Department of Petrology and Geochemistry, Eötvös Loránd University, Budapest.

4. Results

4.1. General Petrographic Description

The collected samples came from the finer-grained part of the Darnó Conglomerate assemblage. They were coarse sandstone with calcite cement and calcareous shallow marine fossils. Their detrital grains disintegrated relatively easily. The samples were microscopically well-packed, polymictic, medium-sorted, mostly well-rounded detritus consisting mainly of mafic igneous and fine-grained siliciclastic sedimentary rocks. They also contained minor amounts of weakly rounded silica grains. A small amount of thick-shelled calcite fossils could also be observed. Coarse-grained sparite cement encrusted the detrital grains and filled the remaining space (Figure 3).

4.2. Description of the Detrital Rock Types

Much of the detrital material in the samples examined can be classified into mafic igneous and fine-grained sedimentary rock types. Igneous rock types are usually highly altered and often contain large amounts of secondary minerals (e.g., chlorite, limonite, hematite, sericite, and clay minerals). The original primary igneous minerals are difficult to identify, and in many cases, their former presence can only be inferred. This is aided by the texture of the rock, which is easily observed in almost all grains. Therefore, the rock types are grouped and described mainly on the basis of the rock texture. It is already known from previous studies [19,33] that most of the investigated debris may originate from the seafloor rock sequence discovered on Darnó Hill and its immediate surroundings. Therefore, for the detailed descriptions of the mafic igneous rock types, I have followed the fabric series [39], which was created on the basis of the cooling rate order of mafic rocks in the possible source rock sequence at Darnó Hill.

4.2.1. Magmatic–Metamagmatic Rocks

These types of rocks in the total debris made up about 60%. They were usually very well- or well-rounded, but there were also a few moderately rounded grains.
  • Hyalinic basalt
This type of rock was originally composed of basaltic glass. The studied samples were completely altered and homogeneous, consisting of very fine chlorite or, rarely, mosaic coarse quartz. Quartz can also occur in a very small amounts as a single crystal or as a set of fine- or coarse-grained mosaic crystals. In some cases, tiny needles of pumpellyite occurred with some quartz (Figure 4b), but compact aggregates of small laths of pumpellyite were also found as single debris grains. Some altered glass grains contained microcrystalline, scattered indeterminate mineral particles, or porphyric pseudomorphs after olivine crystals (Figure 4d). In some cases, the vitreous material had a striated, mottled, or spherical appearance. In cases of chloritization, their color was pale green. With crossed polars, they were weakly anisotropic with slightly wavy extinction and a dark gray interference color. (Figure 4c). One grain appeared as a complex fragment of hyaloclastite breccia (Figure 4a). They were usually angular and small (max. of 300 µm), and their abundance was about 1% of the igneous rock fragments.
  • Spherulitic basalt
Spherulites are spherical aggregates of dense, radially arranged, very thin albite needles. They occurred in the devitrified rock glass matrix described in point 1, scattered individually (Figure 5a,b) or in a close cluster (Figure 5c). In the center of the spherules, sometimes a thin single crystal of albite could be seen (Figure 5b). The spherules were often dark brown in color due to the fine-grained alteration by opaque minerals and titanite, but in the case of chloritization, they showed a light green homogeneous appearance. They showed circular, radial extinction between crossed polars. In some grains, cumulates of chlorite pseudomorphs were visible after euhedral olivine. Some spherules were not completely spherical, showing a broad bow-tie shape as a transition to a variolitic texture (Figure 5a,d) The average size of the spherules was 100–200 µm. The spherulitic basalt grains were slightly rounded, and their size was small, with a max. of 400 µm. They made up about 1%–2% of the total magmatic grains.
  • Variolitic basalt
An aggregate of very thin albite needles arranged in a sheaf, fan, or bow-tie shape is called variolite. In the samples studied, the appearance of the sheaves formed by the albite grains varied from a shape similar to a bird’s feather, consisting of almost invisible thin fibers, to an array of about 20 µm thick needles (Figure 6a,b,d). The direction of elongation of individual sheaves was random in most rock grains, but there were versions with sheafs elongated in one direction (Figure 6c). The length of individual sheafs exceeded 500 µm. Their color was black or dark brown due to opacitization, and they were weakly anisotropic with crossed polars. They rarely contained fully chloritized olivine aggregates and other altered mineral grains as a porphyry component (Figure 6f). In some cases, larger albite laths could also be seen as initiations of intersertal texture (Figure 6e). The variolitic rock grains were typically well-rounded, had an average diameter of 1–2 mm, and made up 30% of the total magmatic rock fragments.
  • Intersertal and intervariolitic basalt
The base of these rock grains is provided by a generally unoriented framework of thin, long albite laths. These albite laths are of variable thickness (20–40 µm) and length (about 0.5 mm), sometimes forming skeletal crystals (Figure 7b,c), and sometimes more widely spaced or closely associated (Figure 7d–f). Fine-grained alteration products appeared in the spaces between the albite laths, which were difficult to identify by light microscopy. In many cases, clusters of variolitic (Figure 7b), rarely well-identified pyroxene needles could also be seen. For these textures, I propose the name intervariolitic. Transitional textures with shorter albite laths at the core of variolites have also occurred (Figure 7a). These can contain porphyry minerals, like albite (Figure 7e) and also pseudomorphs after olivine. Vein and amygdale-filling pumpellyite were common, indicating anchimetamorphic processes (Figure 7f). These basaltic grains were usually well-rounded, 1–2 mm in size, and made up about 40% of the total number of magmatic rocks.
  • Intergranular basalt
In this rock type, the plagioclase laths also provide the framework, but they are slightly thicker (max. 0.08 mm) than in the previous texture type (Figure 8a,b). In the space between the laths, the primary igneous minerals—in this case, the augites—were usually unrecognizable due to their strong, complete chloritic, nontronitic, titanitic, calcitic, quartz, and limonitic–hematitic alterations, making this type difficult to distinguish from the previous intersertal and subsequent subophitic-ophitic types (Figure 8b). In other cases, the remnants of elongated interstitial limonitized augites could be seen (Figure 8c,d). These basaltic grain types usually formed smaller, moderately rounded fragments and accounted for 10% of the total magmatic grains.
  • Subophitic-ophitic basalt, dolerite, microgabbro
These are rock types similar in appearance to the intergranular type, but with coarser grains and even more intensive alteration. As a result, instead of augite, only green chlorite, dark gray fine-grained titanite and yellowish-red limonite appeared between albite laths (Figure 9a–c). Some of these grains appeared to form pseudomatrix, indicating that these grains were usually originally softer than the finer-grained basalts (Figure 9d). Their grains were smaller in size (less than 0.5 mm), and their proportion was about 5% of the magmatic rock fragments.
  • Serpentinite
Only a few grains of this rock type were found. These grains were well to moderately rounded and 0.5–1 mm in size. Most of them are serpentinites with a characteristic mesh structure of serpentine minerals covering the entire grain (Figure 10b–d). Uncertain relics of irregularly shaped, completely altered, poorly preserved former pyroxene grains can also be observed (Figure 10c). A fully chloritized/serpentinized composite grain has a subhedral granular texture and was originally composed of two types of minerals. Most likely, the smooth surface mineral was olivine and the rough surface mineral was pyroxene, so this grain was most likely a fragment of an ultramafic rock (Figure 10a).

4.2.2. Sedimentary–Metasedimentary Rocks

The proportion of sedimentary rock fragments in the total debris was about 40%. They were usually very well- or well-rounded, but angular grains were rare.
  • Siliciclastic rocks
Most sedimentary rocks, about 85%, were fine-grained siliciclastic rocks. They occurred mainly as claystone (Figure 11e,f), silty claystone (Figure 11a,b), siltstone (Figure 11g), and less commonly as fine-grained sandstone (Figure 11h). They often appeared as anchymetamorphic slate with oriented, schistose, slightly sheared fabric (Figure 11c,e,f). The main minerals, in addition to the clay mineral, were quartz, sericite, and muscovite, and secondary fine-grained calcite. They also contained a small amount of tiny, thin-shelled spherical fossils. These debris fragments were usually elongated, and well-rounded, less often moderately rounded or angular, and reached a large size (even several mm). Thin veins of albite-chlorite were present. Within a grain, even two versions of them appeared tightly intertwined (Figure 11e,f). Some grains showed a micromélange feature (Figure 11d). In one of these rock types, larger angular basalt with intergranular texture occurred in a highly opacitized fine-grained breccia matrix.
  • Radiolarite
The amount of radiolarite in the sedimentary rocks was about 15%. They were mostly angular, and consisted of fine-grained siliceous material and, in some cases, small secondary calcite grains. They also occurred in light red and light grey-white varieties and contained large amounts of radiolarians and smaller amounts of sponge spicules. Quartz veins were visible in some grains (Figure 12).
  • Calcareous grains
These rare component types can be divided into two main types. Grains of one type are usually homogeneous, fine-grained micritic limestones with a slightly oriented texture, sometimes containing recrystallized fossil shell fragments (Figure 13a). Different limestone grains made up less than one percent of the sedimentary rocks. Large, thick fragments of calcareous skeletons of shallow marine fossils were also present (Figure 13b).

4.3. Quantitative Analysis of Detrital Grains

Grain counting of the detrital components of sandstone samples from the Darnó Conglomerate was performed (see details in Section 3). Both main types of igneous (about 63%) and sedimentary (about 37%) grains were represented, including slightly metamorphosed types, (Table 1). All grains counted could have belonged to a rock series of a preexisted ophiolitic mélange. Hyalinic, spherulitic, variolitic, intersertal, and intergranular basalts are considered representatives of pillow basalts. They make up slightly more than 90% of all igneous rocks. See Section 5 for further discussion.

5. Discussion

The geology of Darnó Hill is well known [12,34,40,41,42,43]. Here, the Paleozoic–Mesozoic basement crops out on a 2–3 km-wide, 4–5 km-long NNE-SSW-striking area [34,41,44]. It is mainly represented mainly by mafic igneous rocks (diabase, hyaloclastite, pillow basalt, massive basalt, dolerite, microgabbro, and gabbro), sandstone, siltstone, slate, shale, siliceous shale, radiolarite, and limestone. These occur as fragments and matrixes of an ophiolite mélange [34,41,44,45].
The rock types of the debris of the Darnó Conglomerate can be interpreted as follows: Hyalinic basalt grains may represent the chilled glassy margin of submarine basalt bodies, such as pillow or massive basalt, and may also have composed the hyaloclastite breccias. Grains of spherulitic, intersertal, and intergranular textures follow each other toward the interior of these basalt bodies. Intergranular, subophitic, and ophitic grains may be associated with rock types to form subvolcanic and intrusive mafic bodies, such as dolerite and gabbro. The sedimentary rock fragments among the studied debris may partly represent the deep water sediments near the underwater extrusion of the basalt (radiolarite, claystone, shale, and siliceous shale), but some of them should have been formed in a shallower water environment (siltstone, sandstone, and limestone). The observed type of serpentinite may have been derived from tectonite, the upper mantle part of an ophiolite rock series.
Summarizing the above descriptions and interpretations, it can be concluded that the vast majority of the most clearly identifiable rock fragments of igneous origin from the Darnó Conglomerate samples (except for serpetinite grains) have a mafic appearance. Their texture and mineral composition are similar to the rock types described both on the surface and in boreholes on Darnó Hill [12,19,37,42,44,45]. The same is true for the siliciclastic detrital grains. In summary, there is a lack of rock types that are foreign to the rock series excavated on Darnó Hill. On this basis, it can be concluded that the debris of the investigated assemblage originate from the nearby Darnó Hill and its surroundings.
As a first step of evaluation, I tried to deduce, on a theoretical level, what kind of debris could have been left behind during the early Miocene erosion of the Darnó Hill mélange. Looking at the strata of the boreholes (Rm-131, -135, -136, Sirok-1) drilled in the nearby source area [12,35,41,42,45], or at the general geological map of Darnó Hill [43], the proportions of the main rock types appearing in the present structure of Darnó Hill can be estimated. Based on this rough estimate, it can be seen that the total rock material of the Darnó Hill ophiolite mélange known today is composed of about two-thirds igneous and one-third sedimentary rocks. It is also known that the igneous rocks of Darnó Hill are dominated by pillow basalts, while the sedimentary rocks are dominated by fine detrital rocks [12,35,41,42,45].
The sensitivity of the different rock types to weathering must also be taken into account when studying the erosion process. The main rock types of the two main rock groups on Darnó Hill, the fine detrital grains and the basalts, are physically less resistant to weathering, which is also evident from the well and moderately rounded nature of their debris grains. As a result, their proportions do not change significantly during transport. In addition, among the sedimentary rocks, silicic rocks, such as radiolarite, are among the most resistant, which can also be observed in field excavations, as indicated by the weakly rounded character of their debris grains (Figure 3b). These rocks must, therefore, represent a higher proportion of the debris than is observed in their source area. At the same time, igneous rocks also include highly malleable types, such as those originally rich in glass (e.g., hyalinic basalt) or olivine (e.g., olivine gabbro, peridotite). Based on all these considerations, if we assume that the Miocene geological composition of Darnó Hill was similar to the present composition, the igneous rocks in the debris of the studied assemblage should represent about 60%–70% and the sedimentary rocks, of course, 30%–40%. The proportions of the main rock types obtained by grain counting of the analyzed sandstones (igneous rocks = 62.6%, sedimentary rocks = 37.4%, Table 1) are in good agreement with these theoretical data. This overlap allows us to conclude that the mélange assemblage eroded from the eastern side of the Darnó line in the early Miocene had almost the same main rock composition as today.
On the basis of the literature [12,34,41,42,43,45,46] and my own field observations, it can be stated that on the surface of Darnó Hill today, among the different rock types of igneous origin, the basaltic variants (mainly pillow basalt, hyaloclastite) predominate; their proportion can be estimated to be well over 90%. They include hyalinic, spherulitic, variolitic, and intersertal basalts, as well as some intergranular basalts. Based on the grain counts of the thin sections, the proportion of these grains within the igneous rock types is 90.2% (Table 1). The good agreement between the two datasets above shows that pillow basalts also dominated the surface in the early Miocene.
It is also true that the ratio of pillow basalt-origin rock grains measured in the thin sections (90.2%) was slightly lower than the estimated ratio that appears on the surface today. This is probably because in the early Miocene, the estimated ratio of course-grained igneous rocks on the surface may have been slightly higher. These rock types may have been eroded from the mélange blocks representing the deeper members of the ophiolite assemblage (such as massive basalt, sheeted dyke, isotropic and layered gabbro, and upper mantle serpentinite). This concept is supported by the magmatite/metamagmatite grains (ophitic gabbro, metaperidotite, and serpentinite with mesh fabric) found in the thin sections examined. These rock types are the least resistant, so they must have been present at the surface of the former source area in a higher proportion than the 7.2% measured in the igneous rocks observed in the debris analyzed from there. In addition, some of them, including the serpentinite previously detected [19] and observed in greater abundance in this work, are not present on the source area today, leading to further interesting plate tectonic conclusions [19] and possibly providing opportunities in the future. To discuss this is not the purpose of this paper.
In the following, a simple calculation is presented to prove that pillow basalt was the dominant igneous rock in the source area of the investigated debris, just as it can be observed on Darnó Hill today. For this purpose, the grain count data (Table 1) were interpreted as volume ratios (Table 2). To simplify the calculation, the basalt pillows were treated as regular spheres, and the average basalt pillow diameter was calculated, which is about 70 cm on Darnó Hill [36,47].
In the first step, the relative proportions of basalt debris typically found within pillow basalts [48,49,50,51] were calculated and classified into five main fabric groups in the presented samples. Then, the data were arranged in the fabric sequence observed in the Darnó Hill basalt pillows from the outside to the inside [36,42], and their volume ratio was calculated. Finally, the possible thickness of the rock parts with different textures in a 70 cm-diameter basalt pillow was calculated (Table 2). According to this calculation, the outermost vitreous rim could have been 1 cm thick, the spherulitic zone 1.4 cm, the variolitic zone 4 cm, the intersertal zone 13.5 cm, and the innermost intergranular zone 11.5 cm thick (Table 2). These results are, not surprisingly, very close to the generally accepted fabric zonation presented here (Figure 14) on the basis of a sketch published by Marescotti and co-workers [50] as a typical basalt pillow (mean values: 1.2 cm, 1.7 cm, 4.7 cm, 16.3 cm, 11.1 cm, respectively) and observed on Darnó Hill [36,42].
Table 2. Results of volume and thickness calculations for detrital basalt types from the Darnó Conglomerate Formation with different textures, which are usually present in the inner pillow. In this calculation, a basalt pillow with a diameter of 70 cm was considered. Comparative data were estimated on the basis of the pillow section sketch in [50]. Legend: no. = number; Marescotti = [50].
Table 2. Results of volume and thickness calculations for detrital basalt types from the Darnó Conglomerate Formation with different textures, which are usually present in the inner pillow. In this calculation, a basalt pillow with a diameter of 70 cm was considered. Comparative data were estimated on the basis of the pillow section sketch in [50]. Legend: no. = number; Marescotti = [50].
Inner Pillow Rock TypesGrain No.Grain (Volume) PercentVolume in
d = 70 cm Pillow		Thickness in Pillow
This WorkMarescottiThis WorkMarescotti
Piece%cm3cmcmcmcm
Hyalinic basalt4410.719,2071.30.9–1.51.30.9–1.5
Spherulitic basalt5112.422,2581.61.4–2.01.61.4–2.0
Variolitic basalt15337.166,5966.44.0–5.46.44.0–5.4
Intersertal basalt11327.449,1848.215.5–17.125.725.7–29.1
Intergranular basalt5112.422,25817.510.2–12.0

6. Conclusions

This work provides a detailed polarization microscopic description of the detrital grains of sandstone samples from the Early Miocene Darnó Conglomerate. The lithological appearance of the detrital grains shows good agreement with different rock types of the ophiolite mélange known today in the probable source area located in the immediate vicinity, on Darnó Hill. This identification is also confirmed by the presence of basaltic grains containing very low and low-grade metamorphic minerals (pumpellyite and epidote), observed both in the studied debris and on Darnó Hill. Based on the quantitative evaluation, it can be concluded that both the sedimentary–igneous rock ratio and the ratio of different basalt types within the magmatic group show good agreement with the rock ratios that can be determined on Darnó Hill today. A theoretical basalt pillow zonation was derived from the quantitative ratio of rock debris with textures characteristic of pillow basalts. This zonation agrees well with the textural zonation of the pillow basalts found on Darnó Hill. These calculations show that pillow basalts dominated the surface igneous rocks of the source area also in the early Miocene. The sure presence of metalultramafic rocks (mainly serpentinites) in the investigated debris has been confirmed. These rock types are not known in the Darnó Hill area today, but were, therefore, present in the early Miocene on the upper, now eroded part of the ophiolite mélange sequence. This study shows that certain detailed lithological knowledge of the source area of a detrital assemblage, deposited after short-distance transport, can be effectively recognized on the basis of polarizing microscopic petrography.

Funding

This research received no external funding.

Data Availability Statement

All data used for calculations are presented within this article in tables and figures.

Acknowledgments

I would like to thank Gabriella Kiss and Tamás Sági for reviewing the manuscript and helping to publish this article. The Deepl Write application was a great help in improving the English language of the manuscript. The English level of the text of this article required for publication could only be achieved with the effective help of the MDPI Editors, for which I am grateful.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. (A) Map of the location of Darnó Hill in Northern Hungary, Central Europe. (B) Simplified geological map of Darnó Hill. The Darnó Conglomerate Formation is marked by a diagonal dense grid (modified from [19]).
Figure 1. (A) Map of the location of Darnó Hill in Northern Hungary, Central Europe. (B) Simplified geological map of Darnó Hill. The Darnó Conglomerate Formation is marked by a diagonal dense grid (modified from [19]).
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Figure 2. Macro photographs of two of the six thin sections under study, which were also used for grain counting. (The length of the glass was 5 cm).
Figure 2. Macro photographs of two of the six thin sections under study, which were also used for grain counting. (The length of the glass was 5 cm).
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Figure 3. General textural views of sandstone samples from the Darnó Conglomerate. (a) Well-rounded intersertal (ISB), spherulitic basalt (SPB), and claystone (CLS) grains and thick calcite cement (PPL, sample Kis-hegy-1); (b) angular radiolarite (RAD), well-rounded claystone (CLY), and moderately rounded variolitic basalt (VRB) (polymict sandstone, PPL, sample Kis-hegy-4); (c) equigranular grain supporting well-rounded polymictic sandstone with different magmatic originated rock fragments (polymict sandstone, PPL, sample Kis-hegy-5); (d) radiolarite (RAD) and intervariolitic basalt (IVB) in grain-supported sandstone (transmitted light-polarizing microscopic images, PPL, sample Kis-hegy-5).
Figure 3. General textural views of sandstone samples from the Darnó Conglomerate. (a) Well-rounded intersertal (ISB), spherulitic basalt (SPB), and claystone (CLS) grains and thick calcite cement (PPL, sample Kis-hegy-1); (b) angular radiolarite (RAD), well-rounded claystone (CLY), and moderately rounded variolitic basalt (VRB) (polymict sandstone, PPL, sample Kis-hegy-4); (c) equigranular grain supporting well-rounded polymictic sandstone with different magmatic originated rock fragments (polymict sandstone, PPL, sample Kis-hegy-5); (d) radiolarite (RAD) and intervariolitic basalt (IVB) in grain-supported sandstone (transmitted light-polarizing microscopic images, PPL, sample Kis-hegy-5).
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Figure 4. Rock grains from the Darnó Conglomerate, originally consisting mainly of basaltic glass. (a) Hyaloclastite breccia (HYB) and calcitic fossil (FOS) fragments (PPL and XPL, sample Kis-hegy-5); (b) glass shard from hyaloclastite filled with pumpellyite (rough surface, pale green), chlorite (smooth, pale green) and quartz (PPL, sample Kis-hegy-3); (c) chloritized angular basaltic glass fragment (PPL, sample Kis-hegy-6); (d) glomeroporphyres of chloritized euhedral olivine crystals in devitrificated spherulitic matrix (PPL, sample Kis-hegy-3). (Transmitted light-polarizing microscopic images).
Figure 4. Rock grains from the Darnó Conglomerate, originally consisting mainly of basaltic glass. (a) Hyaloclastite breccia (HYB) and calcitic fossil (FOS) fragments (PPL and XPL, sample Kis-hegy-5); (b) glass shard from hyaloclastite filled with pumpellyite (rough surface, pale green), chlorite (smooth, pale green) and quartz (PPL, sample Kis-hegy-3); (c) chloritized angular basaltic glass fragment (PPL, sample Kis-hegy-6); (d) glomeroporphyres of chloritized euhedral olivine crystals in devitrificated spherulitic matrix (PPL, sample Kis-hegy-3). (Transmitted light-polarizing microscopic images).
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Figure 5. Different types of spherulitic basalt grains from the Darnó Conglomerate. (a) Individual spherules in altered glassy matrix (PPL, sample Kis-hegy-2); (b) albite laths in the cores of elongated spherules (PPL, sample Kis-hegy-1); (c) tight-fitting spherules making up the basalt (PPL, sample Kis-hegy-5; (d) as a transition from spherulite, bow-tie variolite in fine-grained devitrificated basalt matrix (PPL, sample Kis-hegy 1). (Transmitted light-polarizing microscopic images).
Figure 5. Different types of spherulitic basalt grains from the Darnó Conglomerate. (a) Individual spherules in altered glassy matrix (PPL, sample Kis-hegy-2); (b) albite laths in the cores of elongated spherules (PPL, sample Kis-hegy-1); (c) tight-fitting spherules making up the basalt (PPL, sample Kis-hegy-5; (d) as a transition from spherulite, bow-tie variolite in fine-grained devitrificated basalt matrix (PPL, sample Kis-hegy 1). (Transmitted light-polarizing microscopic images).
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Figure 6. Different types of variolitic basalt grains from the Darnó Conglomerate. (a) Well-rounded grain composed of variolite and rimmed by coarse-grained calcite crust (PPL, Kis-hegy-6); (b) sharp contact between typical coarse-grained (CVB) and fine-grained (FVB) variolitic texture (PPL, sample Kis-hegy-1); (c) limonitized augite laths (ochre-colored) in the interstitial places of large albite variolite, (PPL, sample Kis-hegy-1); (d) well-developed variolites in moderately rounded basalt grain (PPL, sample Kis-hegy-1); (e) as an initiation of the intersertal skeleton structure, few larger albite laths appeared in the variolitic matrix (PPL, Kis-hegy-1); (f) euhedral olivines comprising glomeroporphyr aggregates in variolitic groundmass (variolitic glomeroporphyric basalt) (PPL, sample Kis-hegy-1). (Transmitted light-polarizing microscopic images).
Figure 6. Different types of variolitic basalt grains from the Darnó Conglomerate. (a) Well-rounded grain composed of variolite and rimmed by coarse-grained calcite crust (PPL, Kis-hegy-6); (b) sharp contact between typical coarse-grained (CVB) and fine-grained (FVB) variolitic texture (PPL, sample Kis-hegy-1); (c) limonitized augite laths (ochre-colored) in the interstitial places of large albite variolite, (PPL, sample Kis-hegy-1); (d) well-developed variolites in moderately rounded basalt grain (PPL, sample Kis-hegy-1); (e) as an initiation of the intersertal skeleton structure, few larger albite laths appeared in the variolitic matrix (PPL, Kis-hegy-1); (f) euhedral olivines comprising glomeroporphyr aggregates in variolitic groundmass (variolitic glomeroporphyric basalt) (PPL, sample Kis-hegy-1). (Transmitted light-polarizing microscopic images).
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Figure 7. Basalt grains with intersertal and intervariolitic textures. (a) Fine-grained secondary minerals between unordered albite laths (intersertal basalt, PPL, sample Kis-hegy 5); (b) variolites appeared between skeletal albite laths, forming the preface to the intersertal structure (intervariolitic basalt, PPL, sample Kis-hegy-1); (c) skeletal albites touched each other, forming a disordered skeletal structure (intersertal basalt, PPL, sample Kis-hegy-1); (d) loose skeletal structure of albite laths with porphyric plagioclase grains (porphyric intersertal basalt, PPL, sample Kis-hegy-1); (e) secondary minerals (chlorite—green, limonite—yellow) appeared throughout the skeleton, forming plagioclase laths. Large twinned tabular plagioclase phenocrysts also appeared (intersertal porphyric basalt, PPL, sample Kis-hegy-3); (f) pumpellyite (small green laths) and quartz-bearing amygdale in intersertal basalt (PPL, samples Kis-hegy-4). (Transmitted light-polarizing microscopic images).
Figure 7. Basalt grains with intersertal and intervariolitic textures. (a) Fine-grained secondary minerals between unordered albite laths (intersertal basalt, PPL, sample Kis-hegy 5); (b) variolites appeared between skeletal albite laths, forming the preface to the intersertal structure (intervariolitic basalt, PPL, sample Kis-hegy-1); (c) skeletal albites touched each other, forming a disordered skeletal structure (intersertal basalt, PPL, sample Kis-hegy-1); (d) loose skeletal structure of albite laths with porphyric plagioclase grains (porphyric intersertal basalt, PPL, sample Kis-hegy-1); (e) secondary minerals (chlorite—green, limonite—yellow) appeared throughout the skeleton, forming plagioclase laths. Large twinned tabular plagioclase phenocrysts also appeared (intersertal porphyric basalt, PPL, sample Kis-hegy-3); (f) pumpellyite (small green laths) and quartz-bearing amygdale in intersertal basalt (PPL, samples Kis-hegy-4). (Transmitted light-polarizing microscopic images).
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Figure 8. Strongly altered intergranular basalts with intersertal structure. (a) Between the framework-forming albite laths, strongly limonitized pyroxene grains appeared in a slightly rounded basalt grain; (b) the skeletal structure was formed by wider plagioclase laths; between them, secondary alteration products formed, dominantly chlorite (intersertal basalt, PPL, sample Kis-hegy-5); (c) intersertal structure formed by skeletal albite and limonitized clinopyroxene laths; well-rounded grain was crossed by calcite-filled fissures (intersertal-intergranular basalt, PPL, sample Kis-hegy-6); (d) unordered skeletal structure formed by less albite and more later-crystallized limonitized clinopyroxene (yellowish red) laths. Large white grain (upper left side) is an euhedral plagioclase phenocryst (intergranular basalt) (PPL, sample Kis-hegy-6). (Transmitted light-polarizing microscopic images).
Figure 8. Strongly altered intergranular basalts with intersertal structure. (a) Between the framework-forming albite laths, strongly limonitized pyroxene grains appeared in a slightly rounded basalt grain; (b) the skeletal structure was formed by wider plagioclase laths; between them, secondary alteration products formed, dominantly chlorite (intersertal basalt, PPL, sample Kis-hegy-5); (c) intersertal structure formed by skeletal albite and limonitized clinopyroxene laths; well-rounded grain was crossed by calcite-filled fissures (intersertal-intergranular basalt, PPL, sample Kis-hegy-6); (d) unordered skeletal structure formed by less albite and more later-crystallized limonitized clinopyroxene (yellowish red) laths. Large white grain (upper left side) is an euhedral plagioclase phenocryst (intergranular basalt) (PPL, sample Kis-hegy-6). (Transmitted light-polarizing microscopic images).
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Figure 9. Strongly altered coarser-grained mafic rock fragments. (a,b) Plagioclase framework with interstitial pyroxene, totally altered to chlorite (green), opaque mineral, and limonite (intergranular/subophitic basalt/microgabbro, PPL, samples (a) Kis-hegy-2 and (b) Kis-hegy-6); (c) between thick plagioclase grains, chlorite and leucoxene appeared in the place of pyroxene and ilmenite (ophitic or poikilitic microgabbro, PPL, sample Kis-hegy-1); (d) as a result of compaction, coarse-grained basaltic grain consisting of wider plagioclase laths and fine-grained groundmass appeared as a pseudomatrix of sandstone. A multi-celled foraminifera is visible in the right up corner (intersertal basalt) (PPL, sample Kis-hegy-5). (Transmitted light-polarizing microscopic images).
Figure 9. Strongly altered coarser-grained mafic rock fragments. (a,b) Plagioclase framework with interstitial pyroxene, totally altered to chlorite (green), opaque mineral, and limonite (intergranular/subophitic basalt/microgabbro, PPL, samples (a) Kis-hegy-2 and (b) Kis-hegy-6); (c) between thick plagioclase grains, chlorite and leucoxene appeared in the place of pyroxene and ilmenite (ophitic or poikilitic microgabbro, PPL, sample Kis-hegy-1); (d) as a result of compaction, coarse-grained basaltic grain consisting of wider plagioclase laths and fine-grained groundmass appeared as a pseudomatrix of sandstone. A multi-celled foraminifera is visible in the right up corner (intersertal basalt) (PPL, sample Kis-hegy-5). (Transmitted light-polarizing microscopic images).
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Figure 10. Totally serpentinized peridotites. (a) Large, chloritized orthopyroxene (left upper grain) in coarse-grained intrusive rock fragment (hypidiomorphic–granular, most probably gabbro/lherzolite, PPL, sample Kis-hegy-2); (b) well-developed mesh structure in serpentinite with serpentine vein (Kis-hegy-5); (c,d) less rounded serpentinite grain with mesh structure and calcite crust (PPL, XPL, sample Kis-hegy-3). (Transmitted light-polarizing microscopic images).
Figure 10. Totally serpentinized peridotites. (a) Large, chloritized orthopyroxene (left upper grain) in coarse-grained intrusive rock fragment (hypidiomorphic–granular, most probably gabbro/lherzolite, PPL, sample Kis-hegy-2); (b) well-developed mesh structure in serpentinite with serpentine vein (Kis-hegy-5); (c,d) less rounded serpentinite grain with mesh structure and calcite crust (PPL, XPL, sample Kis-hegy-3). (Transmitted light-polarizing microscopic images).
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Figure 11. Siliciclastite, metasiliciclastite, and mélange fragments. (a,b) Well-rounded silty claystone grains with different color (PPL, samples Kis-hegy-1); (c) well-rounded, elongated, schistose metaclaystone (slate) grain with oriented muscovite grains (thin elongated yellowish grains) (XPL, sample Kis-hegy-1); (d) composite grain with several fine-grained sedimentary rock fragment from mélange (PPL, sample Kis-hegy-6); (e,f) composite grain from mélange with sharp boundary between claystone (dark part) and slate (light part) embedded in it (PPL, XPL, sample Kis-hegy-3); (g) slightly angular siltstone grain (XPL, sample Kis-hegy-1); (h) elongated mica-rich, layered, coarse-grained siltstone grain (PPL, sample Kis-hegy-2). (Transmitted light-polarizing microscopic images).
Figure 11. Siliciclastite, metasiliciclastite, and mélange fragments. (a,b) Well-rounded silty claystone grains with different color (PPL, samples Kis-hegy-1); (c) well-rounded, elongated, schistose metaclaystone (slate) grain with oriented muscovite grains (thin elongated yellowish grains) (XPL, sample Kis-hegy-1); (d) composite grain with several fine-grained sedimentary rock fragment from mélange (PPL, sample Kis-hegy-6); (e,f) composite grain from mélange with sharp boundary between claystone (dark part) and slate (light part) embedded in it (PPL, XPL, sample Kis-hegy-3); (g) slightly angular siltstone grain (XPL, sample Kis-hegy-1); (h) elongated mica-rich, layered, coarse-grained siltstone grain (PPL, sample Kis-hegy-2). (Transmitted light-polarizing microscopic images).
Minerals 14 00983 g011
Minerals 14 00983 g012
Figure 12. Silicic rocks are mostly radiolarites. (a,b) Silicic rock fragment with spicula and radiolarian. (PPL, XPL, sample Kis-hegy-1); (c) disintegrated angular radiolarite in calcareous cement (XPL, sample Kis-hegy-1); (d) radiolaria-rich claystone grain (PPL, sample Kis-hegy-5). (Transmitted light-polarizing microscopic images).
Figure 12. Silicic rocks are mostly radiolarites. (a,b) Silicic rock fragment with spicula and radiolarian. (PPL, XPL, sample Kis-hegy-1); (c) disintegrated angular radiolarite in calcareous cement (XPL, sample Kis-hegy-1); (d) radiolaria-rich claystone grain (PPL, sample Kis-hegy-5). (Transmitted light-polarizing microscopic images).
Minerals 14 00983 g012
Minerals 14 00983 g013
Figure 13. (a) Well-rounded, micritic, probably Permian limestone grain with irregular sparite veins; (b) magmatic rock fragments were embedded in thick calcite shell remnants (PPL, sample Kis-hegy-1). (Transmitted light-polarizing microscopic images).
Figure 13. (a) Well-rounded, micritic, probably Permian limestone grain with irregular sparite veins; (b) magmatic rock fragments were embedded in thick calcite shell remnants (PPL, sample Kis-hegy-1). (Transmitted light-polarizing microscopic images).
Minerals 14 00983 g013
Minerals 14 00983 g014
Figure 14. Sketch of pillow section with textural zoning (based on Pacific Ridge Pillow Basalts), supplemented with average thickness values measured on the image and with reinterpretation of the textural order based on Darnó Hill observations (modified after [50]). Legend: 35 cm = radius of average basalt pillow on Darnó Hill. Textural zones with thicknesses: A = hyalinic (1.2 cm), B = spherulitic (1.7 cm), C = variolitic (4.7 cm), D = intersertal (16.3 cm), E = intergranular (11.1 cm) zones.
Figure 14. Sketch of pillow section with textural zoning (based on Pacific Ridge Pillow Basalts), supplemented with average thickness values measured on the image and with reinterpretation of the textural order based on Darnó Hill observations (modified after [50]). Legend: 35 cm = radius of average basalt pillow on Darnó Hill. Textural zones with thicknesses: A = hyalinic (1.2 cm), B = spherulitic (1.7 cm), C = variolitic (4.7 cm), D = intersertal (16.3 cm), E = intergranular (11.1 cm) zones.
Minerals 14 00983 g014
Table 1. Results of grain counting and percentage calculations of two thin sections of sandstone samples from the Darnó Conglomerate Formation. Legend: sediment. = sedimentary rocks, magm. = magmatic rocks). Comment: Among magmatic rocks, metamagmatics are listed too: quartzite, serpentinite, and metaperidotite).
Table 1. Results of grain counting and percentage calculations of two thin sections of sandstone samples from the Darnó Conglomerate Formation. Legend: sediment. = sedimentary rocks, magm. = magmatic rocks). Comment: Among magmatic rocks, metamagmatics are listed too: quartzite, serpentinite, and metaperidotite).
Rock TypeKis Hill-3Kis Hill-6SumWithin AllWithin MagmWithin MagmWithin Pillow
Grain No.%Grain No.%Grain No.%%%%
Magmatic rocksHyalinic basalt205.9246.17446.039.6 10.7
Spherulitic basalt308.8215.40516.9911.2 12.4
Variolitic basalt6318.59023.1415320.9633.5 37.1
Intersertal basalt5115.06215.9411315.4824.7 27.4
Intergranular basalt247.0276.94516.9911.2 12.4
Ophytic dolerite, microgabbro144.1174.37314.256.8
Quartz, quartzite51.571.80121.642.6
Serpentinite, metaperidotite0020,.5120.270.4
Sediment.Claystone9728.48722.3718425.21 67.4
siltstone, sandstone102.9123.08223.01 8.1
Radiolarite195.6307.71496.71 17.9
Limestone82.3102.57182.47 6.6
All grains341 389 730
Magmatic20760.725064.2745762.60
Sedimentary13439.313935.7327337.40
Pillow originated188 224 412
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MDPI and ACS Style

Józsa, S. Petrography of Ophiolitic Detritus from a Miocene Conglomerate Formation on Darnó Hill, SW Bükk Mts (N Hungary): A Unique Tool to Trace Covered Ophiolitic Sequences. Minerals 2024, 14, 983. https://doi.org/10.3390/min14100983

AMA Style

Józsa S. Petrography of Ophiolitic Detritus from a Miocene Conglomerate Formation on Darnó Hill, SW Bükk Mts (N Hungary): A Unique Tool to Trace Covered Ophiolitic Sequences. Minerals. 2024; 14(10):983. https://doi.org/10.3390/min14100983

Chicago/Turabian Style

Józsa, Sándor. 2024. "Petrography of Ophiolitic Detritus from a Miocene Conglomerate Formation on Darnó Hill, SW Bükk Mts (N Hungary): A Unique Tool to Trace Covered Ophiolitic Sequences" Minerals 14, no. 10: 983. https://doi.org/10.3390/min14100983

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