Abstract
The NE Tokaj Mountains at Pálháza in NE Hungary are made up of a complex association of Miocene rhyolitic shallow intrusions, cryptodomes and endogenous lava domes emplaced into and onto soft, wet pelitic sediment in a shallow submarine environment. The intrusive–extrusive complex shows a range of interaction textures with the host muddy sediment, ranging from blocky peperites, formed on a 0.1 m-scale, through to irregular contacts closely resembling globular mega-peperites, on a >10 m-scale. The over 200 m-thick igneous succession is interpreted to result from the pulsatory growth of shallow cryptodomes through muddy saturated host sediment. The intrusions eventually breached the sedimentary cover to build up thick in situ hyaloclastite piles in the shallow subaqueous environment. The coherent rhyolitic cryptodome facies is surrounded by intrusive hyaloclastite in the contact zone to the pelitic host sediment. In the upper level of the complex, rhyolitic dome rock is capped and surrounded by hyaloclastite formed due to quench fragmentation upon contact of the lava surface with sea water.
Lava domes in subaerial settings are common, especially in association with arc-related volcanoes with andesite–rhyolite composition (Anderson et al. 1995; Matthews et al. 1997; Sparks et al. 1998; Nakada et al. 1999; Zobin et al. 2002; Carn et al. 2004). Shallow subaqueous dome complexes (Smellie et al. 1998) of low volume (0.01 km3) are commonly associated with mafic volcanic fields. Direct observation of subaqueous dome formation and associated processes are commonly restricted to submersible investigations and based on photo-interpretation of small areas of view (S. M. White et al. 2000). Within the geological record, however, subaqueous volcanic successions and associated lava domes are common from a range of tectonic settings (Cas et al. 1990; Gimeno 1994; DeRita et al. 2001). Lava domes, cryptodomes and lava flows easily burrow and apparently rapidly expand into soft sediments in subaqueous settings (Kano 1989; Hanson & Hargrove 1999; Gifkins et al. 2002), forming various types of peperite and associated hyaloclastite units.
Peperite is used here as a genetic term applied to a rock formed essentially in situ by disintegration of magma intruding and mingling with unconsolidated or poorly consolidated, typically wet, sediment (J. D. L. White et al. 2000; Skilling et al. 2002). These rocks are common in the root systems of small-volume mafic phreatomagmatic volcanic complexes, especially in diatremes that are intruded by dykes and sills (Lorenz et al. 2002; Martin & Németh 2004a). In such settings, peperite forms upon disintegration of the magma and mixing with the host pyroclastic debris (Lorenz et al. 2002; Martin et al. 2005; Martin & Németh 2007). In larger-volume systems of mafic–more evolved magmas, especially in subaqueous conditions, groups of sills and dykes can develop with associated extensive peperite, as well as associated hyaloclastite units (Busby-Spera & White 1987; Hanson & Wilson 1993; McPhie 1993; Goto & McPhie 1996; Coira & Perez 2002). This is also the case for nested cryptodomes, and domes (Stewart & McPhie 2003). In this paper our focus is on the latter of these situations.
Here we use hyaloclastite to describe those rock types formed by non-explosive quenching and mechanical disintegration of hot melt upon contact with water (Rittmann 1962; Honnorez & Kirst 1975; Dimroth & Yamagishi 1987).
The term cryptodome is used here to describe coherent bulbous magmatic bodies emplaced at shallow levels into host sediment, although never breaching the sedimentary cover (Goto & McPhie 1998; Stewart & McPhie 2003). Cryptodomes in subaqueous settings are surrounded by intrusive hyaloclastites (McPhie et al. 1993; Stewart & McPhie 2003) formed by the quench fragmentation of the margin of the emplaced magmatic body. Intrusive hyaloclastite is also known as peperitic hyaloclastite (McPhie et al. 1993). Hence, intrusive hyaloclastite and peperite are very closely related terms. Some intrusive hyaloclastite bodies could also be termed peperite, but only if there is evidence of host sediments injected into the intrusive body.
By contrast, while endogenous lava domes are partially grown in soft sediment in the sea/lake floor (McPhie et al. 1993; Goto & Tsuchiya 2004), at the upper margins lava breaches the sedimentary cover and, upon contact with the sea/lake water, quench fragmentation of the lava surface forms in situ hyaloclastite. These hyaloclastite successions may laterally interconnect with redeposited hyaloclastite units formed by volcaniclastic deposits carried away from the in situ hyaloclastite piles by currents (Goto & Tsuchiya 2004). Extrusions in very shallow subaqueous environments may generate explosive eruptions and form tephra mounds and cones that overlie domes (Cas et al. 1990; Cas 1992). Distinguishing geological exposures of subaqueous cryptodomes from endogeneous lava domes may be challenging, not least owing to the potential that a cryptodome may expand enough to breach the host sediment. Documented here is a situation where cryptodome and dome structures overlap with a highly intricate architecture that contains a variety of rock textures.
Volcanic systems associated with the Miocene Carpathian Volcanic Arc (Lexa & Konecny 1974) in central Europe were commonly formed in shallow subaqueous settings, and produced various coherent and fragmented volcanic facies, interbedded with marine sediments (Gyarmati 1961; Mátyás 1974, 1987; Széky-Fux & Maury 1978; Pécskay & Molnár 2002). In spite of the general vegetation cover over central Europe, good exposures of the proximal facies of lava dome and cryptodome successions are common owing to intensive quarrying into the coherent magmatic bodies. Here we present a field-based textural study of one of the lava dome and cryptodome complexes of one of the largest rhyolitic perlite quarry in Europe, located in Pálháza within the Tokaj Mountains (Fig. 1). In spite of a long history of geological mapping in this region, few textural descriptions were available to provide clear evidence of the subaqueous emplacement of the volcanic successions. Moreover, many of the previous studies applied a non-standard range of terminology and definitions of hyaloclastite and peperite with no clear textural demonstration of the origins of the described units (Ilkey-Perlaki 1961). Here we demonstrate that the described volcanic rocks are part of a subaqueous shallow-marine endogenous lava dome and cryptodome complex, and we compare their formation to similar units described elsewhere – for example, from Ponza, Italy (Scutter et al. 1998; DeRita et al. 2001).
Overview geological map of the Tokaj Mountains and the Carpathian–Pannonian geological environment (inset).
The quarry sites studied allow detailed textural analysis of peperitic contact surfaces between intruding silicic magma and host sediment, as well as a concurrently growing hyaloclastite pile. In this respect, this work contributes to our understanding of eruption mechanisms, growth process and interaction characteristics of subaqueous, low-volume siliceous volcanism.
Geological setting
Miocene calc-alkaline andesitic, rhyo-dacitic and rhyolitic volcanic rocks form an erosional remnant of a subaqueous–subaerial volcanic complex in the central part of the subduction-related Carpathian volcanic arc, the Tokaj Mountains (Fig. 1). Volcanic activity in the Tokaj Mountains took place between 15.2 and 9.4 Ma, based on K/Ar age determination (Pécskay et al. 1989, 1995). The rhyolitic rocks of the Tokaj Mountains consist of coherent lava–intrusive bodies, as well as pyroclastic successions and their reworked counterparts. Pálháza is located in the NE of the Tokaj Mountains, and is a complex subvolcanic body of rhyolite cryptodomes, domes and intrusions, many of them with perlitic texture (Perlaki & Szoor 1973).
The intermediate–felsic calc-alkaline volcanic rocks of Badenian (Miocene – 18.5–16 Ma) to Pannonian (Mio/Pliocene – 12.6–2.4 Ma) age of the Tokaj Mountains were deposited in an approximately 100 km-long and 25–30 km-wide, N–S-trending graben-like structure in NE Hungary (Pantó 1968; Gyarmati 1975). The basement units consist of various Precambrian and Palaeozoic metamorphic rocks, which are exposed along the northeastern boundary of the graben (Fig. 1). The older (Badenian) volcanic stage began with ignimbrite and rhyolite tuff eruptions along the Szamos Fault Line (Fig. 1). The subsidence of the basement resulted in marine transgression, and the accumulation of thick andesitic and dacitic lava flows took place under submarine conditions in the axis of the graben. During the last stage of Badenian volcanism some areas were uplifted, and andesitic–dacitic subvolcanic bodies intruded the earlier volcanic and sedimentary units. The Badenian rocks crop out only in the northeastern part of the Tokaj Mountains; they are covered by the Sarmatian–Pannonian volcanic and sedimentary accumulations in other parts of the mountains.
The initial stages of the Sarmatian–Pannonian volcanic cycle of the Tokaj Mountains started in subaqueous–subaerial conditions, corresponding to a terrestrial environment dissected by lagoons and bays. This initial stage is characterized by accumulations of rhyolite tuff, often intercalated with shallow-marine clay and marl. In the central and northern part of the Tokaj Mountains initial felsic pyroclastic deposition was followed by formation of subaerial andesitic stratovolcanic structures (Telkibánya and Regéc; Fig. 1). Synchronously with andesitic volcanism, rhyolite dome-flow complexes were formed in the Mád, area as well as SE of Telkibánya (Fig. 1). The area of study near Pálháza village is located in this northern rhyolite field of the Tokaj Mountains (Fig. 1). In the late stages of Sarmatian-Pannonian volcanism, andesitic dykes and flows were emplaced in the axial part of the mountains. Dacite was extruded along the marginal zones of the graben structure. The youngest products of volcanism are olivine basalt accumulations known from drilling along the eastern boundary of the graben (Fig. 1).
In the vicinity of Pálháza, the quarry on the northern slope of the Gyöngykö hill exposes a perlitic part of a rhyolite intrusive and extrusive dome-flow complex, which penetrated the lower Sarmatian shallow-marine clay and marl unit (Fig. 2).
(a) Overview of the hills around the Pálháza Quarry from the north. The cross-section shows the main volcanic units in the same orientation as the photograph above. (b) Columnar jointed coherent rhyolitic body in the middle section of the exposed quarry wall.
Geomorphology
The Tokaj Mountains are a subdued mountainous (250–950 m above sea level, asl) region that is either forested or used for various forms of agriculture and industry. Large aggregate quarries of coherent magmatic bodies, such as the study site at Pálháza (Fig. 2a), provide an excellent opportunity to study the proximal facies of a dome–cryptodome complex. The highest points of the mountains are generally formed by intermediate (mostly andesitic) coherent magmatic bodies, lava flow and/or dyke complexes intruded into pumiceous pyroclastic, as well as reworked volcaniclastic successions deposited in near-sea-level subaerial and/or shallow-marine conditions (Gyarmati 1961; Ilkey-Perlaki 1961; Pantó 1966a, b; Székyné Fux et al. 1981). All the units commonly show strong epithermal mineralization (Molnár et al. 1999; Bajnóczi et al. 2000; Pécskay & Molnár 2002). Large intrusive bodies form relatively steep hills built up by proximal lava units and associated near-vent pyroclastic successions. The stratigraphy of local, dome-related pyroclastic units is not studied in detail; however, they can be traced in many areas. At Pálháza, coherent rhyolitic formations are emplaced onto and/or intruded into shallow-marine pelitic sediments. The immediate pre-volcanic mudstones and sandstones are semi-consolidated today, and can be traced about a third of the way uphill from the base of the present quarry of Pálháza. The exposed coherent magmatic units are about 100 m thick, lensoid in three-dimensional (3D) architecture and completely surrounded by monomictic coarse-grained volcaniclastic rocks. The uppermost volcanic unit forms a 500 m-across broadly conical hill with flat terraced slopes where finer grained volcaniclastic beds crop out in a concentric distribution. Similar facies architecture and morphological features are common elsewhere in the Tokaj Mountains, hence Pálháza provides a representative site to understand the volcanic processes of Miocene volcanism in this part of the Carpathian Arc.
Petrochemistry and age of rhyolitic rocks in the Tokaj Mountains
In general, the well-differentiated Sarmatian–Pannonian volcanic series show a medium–high calc-alkaline character, with up to 4 wt% potassium in the rhyolites (Fig. 3a). This feature is similar to other intermediate–acidic volcanic units of the Western Carpathian area, and reflects the highly contaminated character of the subduction-related syn- to post-collisional melts generated in the thickened crust of the Carpathian collisional and back-arc zone. The high modification of the melts is also reflected by Sr-, Nd- and Pb-isotope data (Downes et al. 1995).
(a) Geochemical data through the Tokaj Mountains show a great diversity of volcanic rock types from basalt to rhyolite. (b) K–Ar ages of different rock types of the Tokaj Mountains show a prolonged volcanism in the region.
Boron, trace-element (Cl, Sc, V, Co, Nd, Sm, Gd) and major oxide concentrations of two perlite and 12 rhyolite volcanic rock samples from Pálháza Quarry, and surrounding areas in the Tokaj Mountains, were determined by prompt gamma activation analysis (PGAA) at the Budapest Research Reactor (Hungary). The perlitic samples were collected from two coherent rhyolitic units of the Pálháza Quarry.
PGAA involves the detection of prompt γ-rays that originate in the (n, γ)-reactions during neutron irradiation of samples (Révay & Belgya 2004). Analytical conditions are outlined by others (Belgya et al. 1997; Molnár et al. 1997; Kasztovszky et al. 2004; Révay & Belgya 2004). Precision of boron (B) analyses by PGAA is 1–1.5% for B, as checked by measurements of geological reference materials (Gméling et al. 2005).
The rhyolitic samples from the Tokaj Mountains contain between 70 and 77.5 wt% SiO2 (Fig. 3a). No significant difference between perlites and the fresh rhyolites can be detected in most other major- and trace-element concentrations. Only H2O and Cl contents of the perlites are higher than the rhyolites (H2O=3.9–4.7 v. >1.4 wt%; and Cl=676–740 v. >460 µg g−1).
The B content of the examined whole-rock samples lies between 20 and 68 µg g−1, falling in the typical range for subduction-related volcanics (Leeman 1996). The B contents of the two examined perlites from the Pálháza quarry are indistinguishable within the analytical error (32.8 and 32.2 µg g−1). The darker perlite shows slightly higher H2O and Cl content than the light one.
Two representative whole-rock samples taken from the Pálháza perlite quarry were measured by conventional K–Ar dating (Table 1). The K–Ar ages are also concordant with the biostratigraphic data (Pantó 1966b, 1968; Székyné Fux et al. 1981). On the basis of the field observations and the analytical data, the studied perlitic cryptodome–endogeneous lava dome complex was formed slightly after the Lower Sarmatian sedimentation. However, as the studied rhyolitic bodies are encapsulated in those sediments, this indicates possible coeval sedimentation and intrusion. Other perlitic rhyolite samples were dated from adjacent areas in the Carpathians (Avas–Oas, Kárpátalja–Transcarpathia and Slansky Vrchy). Generally these perlites are slightly younger than the Pálháza perlite (Table 1).
K–Ar ages of samples from the Pálháza Quarry coherent rhyolites
Hence, in summary, two distinct age groups can be detected within the coherent rhyolitic units of the Tokaj Mountains (clustering around 13 and 11 Ma; Fig. 3b). The older rocks have a relatively lower B content (<28 µg g−1), although, as seen above, the two Pálháza perlites contain slightly higher B. The younger rhyolites show a much wider range in boron (27–68 µg g−1).
Coherent rhyolitic bodies at Pálháza
Description
A large quarry in the NE side of the Tokaj Mountains exposes in three dimensions a more than 200 m-thick succession of complex rhyolitic cryptodomes and endogeneous lava domes, partially intruded into wet, unconsolidated marine sediments (Figs 1 and 2a). A 150 m-thick vertical section through the sequence is exposed, allowing distinction of fine facies changes. The coherent rhyolite bodies at the base of exposure are partially emplaced into marine pelitic sediments with contacts of highly irregular geometry. Up-section, coherent magmatic bodies are more distinctive and laterally pass into fragmented volcanic rocks.
The individual cryptodomes, especially in the medial and higher levels of the exposures, are radially jointed, up to 50 m wide and 25 m thick, and lenzoidal in cross-section (Fig. 2b). Columnar jointing on a 0.1–0.7 m scale is well developed and slightly radial in the inner core of the bodies, with platy fracture zones occurring perpendicular to the main joints. The joint-bounded prisms are four- to seven-sided, although there appears to be no systematic relationship of this to distribution within the individual structures.
The margins of coherent bodies are glassy and strongly perlitic in texture, but often show a sub-metre carapace of fractured rock that can still be recognized despite the strong perlitic overprinting of the original structure. Joints are also separated by millimetre- to centimetre-wide strongly perlitic zones. In addition, perlitic zones form a patchwork pattern, occurring through the coherent bodies. Remaining unaltered areas are represented by fresh, darker-coloured obsidian in the larger coherent bodies. The better-preserved coherent rhyolites are typically flow banded, with alternating centimetre-wide black and grey zones. Vesicular textures occur only rarely and are exclusively located in the outer margins of the individual coherent igneous bodies. The coherent magmatic rocks are very glassy with porphyritic and aphanitic textures.
False vitriclastic textures also occur within many central parts of the coherent bodies, represented by 0.1 m-size relicts of original perlitic black glassy zones. Along the dome margins, thin darker bands of variably devitrified obsidian perlite are found, which show flow banding and false vitriclastic textures. Similar textured rocks occur commonly as clasts in the fragmented volcaniclastic units that encapsulate the coherent perlitic magmatic bodies in the medial and upper level of the exposed quarry sections (Fig. 4a). There are no clast-supported, massive, monomict breccias that may be interpreted as autoclastic carapaces of cryptodomes associated with the coherent rhyolitic bodies. However, the margins of the coherent magmatic bodies grade outward into jigsaw fit, hyaloclastite breccia in the upper half of the exposed sections. This breccia is rich in glass shards with ash-grade matrix-supported texture. Domains, 0.1–1 m wide and up to a few metres long, of hyaloclastite are also present within flow-banded coherent rhyolite.
(a) Flow-banded coherent rhyolitic block (Fbc) near to a coherent rhyolite intrusive body. (b) Jigsaw-fit flow-banded rhyolite (J-f b) near a large coherent rhyolite intrusion (Cb). Note the metre-sized muddy sediment (Ms) in the flow-banded jigsaw-fit rhyolite-clast-rich zone.
Interpretation
Coherent magmatic bodies described from Pálháza are interpreted to be small-volume silicic intrusive and extrusive units. They have irregular shapes in cross-section and common coarse-grained quench-fragmented volcaniclastic deposits (hyaloclastite) along their margins. This indicates emplacement of these silicic magmas into water in the upper section, and probably water-rich, unconsolidated soft sediment in the basal section (Scutter et al. 1998; Cas et al. 2000). The lensoid and steep convex architecture of their upper surfaces suggest emplacement as domes. The irregular-shaped contacts between igneous bodies with marine sediments in the lower levels of the sequence indicate shallow burrowing into the host sediment. Such processes occur in soft and especially water-saturated sediments, and can be explained by the higher density of invading lava compared to unconsolidated water-saturated pelitic marine sediments (Beresford & Cas 2001), but are also known when lava invades water-saturated non-marine sediments (Rawlings et al. 1999). Hence, these lower coherent igneous bodies are interpreted to be silicic cryptodomes.
In the upper levels of the sequence, the coherent silicic magmatic bodies are interpreted, by contrast, to be predominantly domes extruded onto a free surface. The similar crystal content, banding texture, and vesicle distribution of hyaloclastite clasts and the adjoining rhyolite in the upper sections imply also co-genetic origin (e.g. the hyaloclastites were not formed earlier, before rhyolite invaded). The generally crystal-poor glassy texture and commonly perlitic state indicates sudden cooling of the emplacing igneous body. The underlying marine sediments record a shallow-marine (from a few tens to a few hundreds of metres in depth) depositional environment (Pantó 1968; Gyarmati 1975). Hence, these domes were emplaced subaqueously.
The glassy coherent silicic bodies have a curving and intersecting network of cracks that surround intact obsidian-like glassy zones. This is typical for perlite (McPhie et al. 1993) and these cracks are inferred to develop as a response to a volume increase as glass is hydrated (McPhie et al. 1993; Davis & McPhie 1996; Zahringer et al. 2001). Similar perlitic rocks are well known in the Tokaj Mountains, however (Perlaki & Szoor 1973).
The banded texture of many coherent magmatic bodies is related to flow movement. In the strongly banded glassy rhyolite, perlitic fractures form a rectilinear texture, comprising cracks that are parallel and oblique to the banding. These banded perlite textures are more common as fragments within the volcaniclastic breccia than in the coherent igneous bodies. This suggests flow banding was more pronounced along the margins of the intrusions. These margins concurrently went through a process of autoclastic fragmentation, leading to the formation of various types of hyaloclastites.
Radial jointing patterns are commonly associated with cooling of pillow lavas including mega pillows, especially in mafic compositions such as basanite (Goto & McPhie 2004). Similar radial joints have also been reported from the top parts of feeder-dykes, cryptodomes and endogeneous lava domes emplaced in subaqueous environments (Yamagishi 1991a; Goto & McPhie 1998; Stewart & McPhie 2003). Joints and interiors of subaqueously emplaced coherent igneous bodies are distinct from subaerial ones, with the former exhibiting greater evidence for quench fragmentation along their margins mimicking tortoise-shell radial joint patterns in glassy rims (Yamagishi 1991b). While such textures are well documented in mafic compositions, silicic counterparts probably form similar jointing patterns. The examples presented here from Pálháza demonstrate a similar pattern, although the joints are wider and less pronounced than documented from more mafic systems.
The interconnecting coherent igneous bodies forming radially jointed silicic rock units at Pálháza are interpreted to be remnants of closely packed, small-volume silicic cryptodomes. These were part of a nested intrusive complex that began as cryptodomes burrowed into soft muddy marine sediments and then evolved to an endogeneous lava dome complex as the magmas emerged into the base of a shallow sea.
Fragmented volcaniclastic units
Description
Fragmented volcanic rocks form a complex architecture enclosing most of the coherent rhyolitic rock units in the quarry of Pálháza. The volcaniclastic successions are fairly uniform, and predominantly massive in texture. Near the dome–cryptodome bodies a coarse-grained, monomictic, matrix-poor volcanic breccia is common (Fig. 4b). The breccias are finer grained with increasing distance from the contact with the coherent rhyolitic units. Clasts in both the matrix-poor and -rich breccias range between 1 cm and up to 2 m. The fragments are generally angular, and show glassy textures with common flow-banding enhanced by alternation of perlitic and obsidian bands up to 2 cm thick (Fig. 4a). Flow-banded fragments are evenly banded (Fig. 4a), and vesicularity of all clasts is generally low. Clast rims are glassy, and in these zones millimetre- to sub-millimetre-sized vesicles form trains parallel with the bands. Clasts show a minor decrease in size outwards from the margins of coherent igneous bodies. Jigsaw-fit architecture of larger clasts is prominent close to the coherent igneous bodies and also forms irregular-shaped zones within the clastic rock units (Fig. 4b). At around 100 m from the coherent bodies, the volcaniclastics are weakly stratified with bed-aligned clasts and occasional grey ash-rich interbeds (Fig. 5a). This volcaniclastic succession shows a moderate fining upwards and away from the uppermost coherent igneous bodies. In the finer grained units, bedding is more strongly developed and plane-parallel. There is no indication of cross-bedding, antidune structures or impact sags; however, inverse grading is identified. The fine-grained volcaniclastic successions form a thin cover over the intrusive complex (Fig. 5b), and can be traced up to 200 m away from the centre of the Pálháza Hill. No interbeds of marine sediment have been identified in the volcaniclastic succession. The matrix content of the volcaniclastic sediments is up to 50 vol.%. In addition, there is an increasing abundance of non-volcanic lithic clasts (1–100 mm in diameter) towards the contacts between magmatic bodies and the host sediment.
(a) Weakly stratified hyaloclastite (Sh) and mud-rich hyaloclastite (Mr h) succession in the middle–upper section of the exposed quarry wall. (b) Overview of a large coherent rhyolite body (Cmb) surrounded by massive hyaloclastite (Mh) in its lower part, and capped by weakly stratified hyaloclastite higher up (Sh). The section is topped by bedded, finer grained hyaloclastite (Bh).
The volcaniclastics have a monolithological composition, with moderately perlitic textured volcanic lithic fragments hosted in a fine perlitic and hydrothermally altered glassy-ash matrix. The glass has variable vesicularity with commonly elongated vesicles. Occasional muddy aggregates occur, derived from the underlying marine sedimentary units, but there are no traces of fossil or organic material in the volcaniclastic succession. The uppermost part of the volcaniclastic succession contains large, plastically deformed, slightly thermally altered mud clasts up to 10 m in diameter (Fig. 6a). In the lower levels of the sequence, boudinage-like sedimentary clast-trains up to 5 m in length are common (Fig. 6a). The deformed sedimentary clasts are both compressed and sheared (Fig. 6a) to form highly irregular-shaped lensoid or flame-like mega structures (Fig. 6b). There is no apparent thermal and/or hydrothermal alteration zoning around the large coherent igneous bodies.
(a) Large muddy sediment clast (M) with irregular, plastically deformed shape in a hyaloclastite unit in the middle section of the exposed quarry wall. (b) Squeezed-up mud (arrow) intruding into a massive hyaloclastite body near to a coherent rhyolite intrusion (C).
Interpretation
The massive and monolithological volcaniclastic breccias are interpreted to be hyaloclastite associated with the emplacement of the silicic igneous units at Pálháza. Hyaloclastite is commonly referred to as a clastic, fragmented, volcanic sediment, formed by non-explosive fracturing and disintegration of quenched coherent igneous bodies (Rittmann 1962, 1973; Silvestri 1963; Pichler 1965). The fragmentation initially affects the outer rim of the coherent magmatic body as it is quenched, before fracturing gradually penetrates into the newly emplaced melt. This process produces large volumes of glassy fragments of a wide size range, generating a poorly sorted deposit with a matrix-supported massive texture. These characteristics are similar to the majority of fragmented volcaniclastic units at Pálháza. The monolithological fine–medium sand-sized glassy matrix fragments in the volcaniclastic sediment units at Pálháza support their origin as hyaloclastite rather than autobrecciated dome margins. For example, the matrix content (up to 50 vol.%) is much higher than typical autobreccias. There is an increasing abundance of non-volcanic lithic clasts (1–100 mm in diameter) towards the contacts between magmatic bodies and the host sediment, indicating that the volcancilastic rocks are not only fragmented magma. The increasingly jigsaw-fit character of the breccias near the coherent magma bodies also implies that there was no significant remobilization of clasts through gravity flows. Many workers suggest that the grain size of the hyaloclastite and the degree of the fragmentation can be related to the efficiency of the magma–water interaction (Busby-Spera & White 1987; Hanson 1991; Davis & McPhie 1996). The massive texture of the majority of the fragmented volcaniclastic rocks at Pálháza suggest that they were originally deposited in situ in a proximal position. The uppermost bedded, commonly inverse-graded, monolithological volcaniclastic sands are interpreted to be redistributed from a nearby hyaloclastite and emplaced by grain flows. These widely dispersed volcaniclastic units that cover most of the proximal hyaloclastite probably represent a mass-wasting process that redistributed hyaloclastites from nearby lava domes during and following their formation. In the lower levels of the sequence, boudinage-like sedimentary clast-trains indicate that soft sediments were squeezed into the volcaniclastic succession and strongly deformed.
Peperitic contact zones of dykes, sills, cryptodomes and lava domes
Description
The up to 50 m-long and 25 m-thick coherent rhyolite domes are surrounded by up to 2 m-wide breccia zones that overlie yellow mudstone (Fig. 7). The breccia zones consist of angular rhyolitic fragments, many of which are jigsaw fit near the coherent rhyolite body, as described earlier (Fig. 8a). Further from the coherent body, the breccias grade into matrix-supported units, with small domains of jigsaw-fit texture remaining (Fig. 8b). The contact of the magmatic bodies to the underlying sediments is highly irregular and undulates over tens of metres (Fig. 9a).
Contact between coherent rhyolite body (Cb) and underlying marine muddy sediments (M). The contact zone is characterized by a closely packed peperite (Cpp) zone near to the rhyolite. Close to the sediment, peperite is more dispersed (Dp), and the matrix of the rock is mud-rich.
(a) Closely packed peperite next to a rhyolitic body. (b) A well-developed contact zone shows characteristic facies changes from the coherent rhyolite to the sediment (M) through closely packed (Cpp) and dispersed peperite (Dp) zones. The dispersed peperite zone near the sediment is rich in large, deformed sediment fragements (circle). From the basal muddy sediments small decimetre-scale protrusions intruded into the dispersed peperitic zone (arrow).
(a) Three-dimensional relationship between muddy host sediment (M) and hyaloclastite units. Massive hyaloclastite (Mh) is in irregular contact with the sediment. Massive hyaloclastites are in contact with weakly stratified (Sh) and bedded hyaloclastite (Bh) units. (b) Decimetre-sized muddy sediment (M) chunks are common in massive hyaloclastites. A hammer for scale is in the circle.
In the basal level of the exposed quarry sections the proportion of marine sediments in the breccias increases dramatically with distance from the coherent rhyolite body (Fig. 9b). Highly variable thickness (0.01–0.5 m) muddy sedimentary dykes penetrate deep into the jigsaw-fit breccia zones (Fig. 10a). These homogenized mudstone dykes commonly form sheared dissected fragments near the coherent igneous bodies that are connected to each other through centimetre-thick zones in a boudinage-like fashion (Fig. 10a). Perlitic textured angular–fluidal-shaped ash-lapilli fragments of rhyolite form small clusters within the muddy dyke sediments (Fig. 10b). The rhyolitic fragments are commonly aligned and hosted in an homogenous mud that also has muscovite-grain trains and textures associated with shearing (Fig. 10c). Larger (0.1 m-scale) groups of ash–lapilli-sized rhyolite fragments are linked by homogenized lighter-coloured mud (Fig. 10c and d).
(a) Centimetre- to decimetre-scale intrusions (arrow) of mud (M) into dispersed peperite zones. (b) Mud-rich (white zones) and flow-banded rhyolite-rich (Fb) zones in dispersed peperite. (c) Centimetre- to decimetre-scale texture of a closely packed peperite (Cp) invaded by fluidized mud (Fm) and having intact mud clasts (M). (d) Rhyolite obsidian (Ro) concentration zones hosted in fluidized mud.
Within about 1 m of the coherent igneous body, ash–lapilli-sized angular volcanic fragments are hosted in a fine-mud-dominated, glass-shard-rich matrix to form a ‘halo’ around the coherent igneous bodies. In the higher levels of the quarry sequence, stratigraphically above the pre-eruptive sedimentary units, similar halo-like zones occur in hyaloclastite breccia surrounding coherent magma bodies. In these zones siliciclastic muds are rare or absent. In both cases the halo-like contact zone exposes volcanic fragments with jigsaw-like pattern, and/or rotated clasts surrounded by a fine-grained strongly homogenized, lighter coloured muddy matrix (‘halo in the halo’ texture). The number of dispersed juvenile fragments in the host muddy matrix increases over a few tens of centimetres from the coherent magmatic bodies.
The halo-like region passes over a sharp contact to an irregular and transitional matrix-rich zone that grades into to a volcaniclastic breccia. Patches of homogenized muddy glassy matrix-rich zones are occasionally present and may exhibit slight stratification. In the uppermost levels of the sequence, lobe-like zones (up to 1 m wide) of this type of breccia are distinguishable from the volcaniclastic host rocks. Star-like textures of angular coherent magmatic bodies in the homogenized and fluidized sediment zones commonly form zones that are a few decimetres wide.
In general, a transition from a jigsaw-fit juvenile clast-rich breccia zone through to a juvenile clast-rotated breccia to the host siliciclastic rock units is apparent (Fig. 7). An undulating sub-metre thickness zone of homogenized, often plastically deformed, mud with few occasional angular rhyolite clasts forms the most sediment-rich zone of the contact zones, commonly up to 10 m away from the contact with the magma body (Fig. 10).
Interpretation
The lowermost part of the hyaloclastite succession is connected to silicic coherent igneous bodies that are in intrusive contact with the pre-volcanic/syn-volcanic marine sediments. Along these contacts, peperitic margins are well developed, and this allows classification of the surrounding hyaloclastititc succession as intrusive or peperitic hyaloclastites following the terminology of McPhie et al. (1993). The dominance of jigsaw-fit texture and polyhedral clasts with glassy margins in the contact zone between the underlying mud as well as the hyaloclastite suggest that quench fragmentation of the rhyolite was important in the formation of the blocky peperite (Squire & McPhie 2002). The limited existence of fluidal (globular) peperitic texture (cf. blocky texture) between the mud and rhyolite could be interpreted as a progressive cooling of the magma during intrusion, and also from the breakdown of fluidization when the limited supply of fine glassy matrix in the host hyaloclastite was exhausted. This is similar to the processes well described in mafic intrusions into coarse-grained sediments (Busby-Spera & White 1987; Doyle 2000; Squire & McPhie 2002). Increasing magma viscosity as temperature decreased, combined with pulsatory intrusion, appear to have been important in the production of the blocky peperite. Moreover, the silicic composition of the melt results in an originally lower intrusion temperature in comparison to mafic melts, and the subaqueous environment probably enhanced the cooling of the magma, leading to the formation of blocky peperite and in situ hyaloclastite.
The complex network of coherent magmatic bodies encapsulated in jigsaw-fit, massive hyaloclastites shows evidence of both quench fragmentation and intrusive hyaloclastite formation processes (McPhie et al. 1993). The coarse-grained hyaloclastite partially surrounding the upper-level rhyolitic domes are interpreted to be formed upon intrusion of new silicic melt into the previously fragmented, but still water-saturated and unconsolidated, hyaloclastite in a similar way to those recognized at the Island of Ponza in Italy (DeRita et al. 2001). This could have prevented the direct contact of newly intruded melt with sea water, leading to less-efficient quench fragmentation and, therefore, coarser grained hyaloclastitic contact zones, similar to many dyke–hyaloclastite contacts in Ponza (DeRita et al. 2001). Intrusive hyaloclastite formed peperitic margins along the magma–host sediment interface, where heated pore fluids also generated strong fluidization of the sediment, as has been suggested elsewhere (Kokelaar 1982). Fluidization, in general, can strongly modify the original texture of the host mud, partially boiling it (Kokelaar 1982). Such fluidization is widely recognized in dykes of many different compositions when intruding soft, water-saturated sediments, either volcaniclastic (Kano 2002) or siliciclastic (Hanson & Hargrove 1999). Fluidization is thought to be the main driving force to carry fragmented coherent magmatic fragments deep inside the intruded host sediment, partially destroying the host sediment's original structure (Kokelaar 1982; Busby-Spera & White 1987; Brooks 1995; Doyle 2000; Wohletz 2002). Fluidization is also inferred to be responsible for the formation of halo-like homogenized sediment zones around igneous bodies, as recorded along the margins of a basanitic lava lake emplaced into wet tephra (Martin & Németh 2004b). The heated fluidised sediment functioned as a pathway to carry detached fragmented glassy fragments from the coherent intrusions deep into the host sediment, in a process commonly documented along sill margins (Curtis & Riley 2003). Star-like textures of angular coherent magmatic bodies in the homogenized and fluidized sediment zones document mild, suppressed explosions. This process is commonly inferred to generate angular-clast-dominated, blocky peperite domains in fluidized host sediment (Dadd & Van Wagoner 2002; Doyle 2000; Martin & Németh 2004b, 2007). The textural variations from the closely packed peperite with jigsaw-fit texture, through to the clast-rotated-peperite zones and then to more dispersed peperitic facies at Pálháza are similar to facies zones described elsewhere (Doyle 2000).
At greater distances from the coherent magmatic bodies, ‘hyaloclastite clast in matrix texture’ can be recognized due to variable fragmentation of the emplacing melt (McPhie et al. 1993). Such textures closely resemble individual clastic fragments in the hyaloclastite and suggest a more-interconnected network of matrix rich zones of hyaloclastite.
The type and scale of muddy-sediment clastic dykes described above have also been reported in association with sill emplacement, in western Dronning Maud Land, East Antarctica (Curtis & Riley 2003). Such clastic dykes, as well as diverse peperitic textures, may form in association with quench fragmentation along the margins of a cryptodome where fluidization-induced mixing of the fragmented debris and host sediment may occur (Kokelaar 1982). Clastic dyke induction could probably be facilitated by syn-eruptive seismicity. Sedimentary clastic dykes in the lowermost coherent magmatic bodies of Pálháza, as well as in the hyaloclastite piles, are inferred to have developed in association with increasing weight of the growing dome complex in the sea floor. This is a common process engendering clastic dykes documented along contacts of muddy sediments and more competent subsequent sedimentary successions. Such sedimentary dykes may reach hundreds of metres in length (Davies 2003; Davies & Stewart 2005). Sudden reorganization of the hyaloclastite pile as it is penetrated by hot magma was probably common in the steepening cryptodome and dome complex. Failure of the growing edifice on the sea floor was possibly enhanced by rapid loading of unconsolidated muddy sediments (Moretti et al. 2001) or volcano seismicity that could have engendered other similar soft-sediment deformation features (Mohindra & Bagati 1996; Moretti 2000; Massari et al. 2001; Menzies & Taylor 2003; Kotlia & Rawat 2004; Horvath et al. 2005).
Discussion
In subaerial conditions lava domes are commonly surrounded by a thin autobrecciated carapace (Kaneko et al. 2002; Riggs & Carrasco-Nunez 2004). In subaqueous environments this carapace may, instead, be hyaloclastite owing to intensive quench fragmentation and burrowing into soft, water-saturated sediments (Stewart & McPhie 2003).
At Pálháza we interpret the complex facies assemblages as representing the growth of a vertical sequence of cryptodomes passing upwards into submarine lava dome facies, with associated hyaloclastites (Fig. 11). This is similar to the interpretation of dyke and hyaloclastite complexes forming the Island of Ponza in Italy (Scutter et al. 1998; DeRita et al. 2001). An alternative, although more complex, interpretation could be the formation of extensive subaqueous viscous acidic lava flows that were intruded later by dykes, and local domes of the same composition. The morphological, textural and 3D relationships at Pálháza appear to document the evolution of many closely spaced submarine dome and cryptodome complexes each up to 100 m across (Fig. 11). The small coherent magmatic bodies are distributed in a random fashion within a relatively uniform, massive hyaloclastite breccia, which implies that emplacement of the magma took place in a pulsating mode, and therefore the volcanism was over a prolonged period (months–years).
Emplacement model of the rhyolite cryptodome, lava dome and hyaloclastite complex in Pálháza. (a) In the initial stage rhyolite intruded into soft marine sediments, developing complex intrusive hyaloclastite zones around the intrusion as well as peperitic zones with mud intrusion into the coherent rhyolite body, in addition to the newly formed massive hyaloclastites. (b) During growth of the complex, new intrusions breached the top of the complex and lava domes emplaced in the sea floor, forming hyaloclastites. Immediate remobilization of hyaloclastite formed stratified and minor bedded hyaloclastite over the complex. 1, fluidized mud intrusions; 2, intrusive hyaloclastite and peperite; 3, flow banded cryptodome margin; 4, radially jointed core of cryptodome; 5, radially jointed core of new lava domes; 6, flow banded margin of new lava domes; 7, in-situ hyaloclastite; 8, redeposited hyaloclastite.
The particulate nature of the in situ hyaloclastite breccia may have enhanced the remobilization of the freshly formed clastic carapace around the growing lava dome through submarine volcanic debris avalanches or debris flows. There is no alternative source for these quench-fragmented in situ hyaloclastites, such as large or extensive rhyolitic lava flows nearby. Therefore, we conclude that the cryptodome–dome system built up a steep-sided pile, partially burrowed into saturated and soft pelitic marine sediments, facilitating various degrees of hyaloclastite fragmentation when magma encountered sea water and water-saturated sediments (Fig. 11). Such processes falls into the original definition of hyaloclastite (Pichler 1965). DeRita et al. (2001) pointed out in their study of the hyaloclastite succession in Ponza, Italy, that hyaloclastite carapaces may form over intruding/extruding silicic magmas and accumulate around silicic domes by mass-wasting from the outer margins of the fragmenting coherent magmatic body. In the case of small-volume multiple intrusions and extrusions in a restricted area, a complex, intercalated, steep-sided, hyaloclastitic succession could accumulate with relatively homogeneous coarse-grained, proximal hyaloclastite facies. Newly intruding silicic magmas may then come in contact with both the water-saturated marine sediments and the earlier-formed and variably saturated hyaloclastite units nearby. Such a scenario is the likely cause for accumulation of the thick, steeply sloped piles of hyaloclastite and small-volume coherent magmatic bodies identified at Pálháza.
In Ponza, Italy, a wide range of hyaloclastite types have been identified (Scutter et al. 1998; DeRita et al. 2001), in contrast to the more uniform facies architecture of Pálháza. The facies variety in Ponza has been explained by different degrees of quench fragmentation (Scutter et al. 1998); however, the relatively uniform textures at Pálháza suggest a similar degree of quench fragmentation throughout the evolution of the dome–cryptodome. This can be explained by the inferred proximal position of the preserved rocks, as well as the relatively homogeneous composition of the emplaced magmas and stable subaqueous conditions of the eruptive environment. The general limited existence of bedded hyaloclastite units could be a result of intensive erosion (i.e. only the core zone of the dome–cryptodome complex is preserved).
The geometrical scale of the rhyolitic intrusions, their texture, and the contact features with host siliciclastic and hyaloclastite sediments at Pálháza are broadly similar to those documented from mafic intrusive/extrusive complexes in fluvio-lacustrine basins such as the western Pannonian Basin (Németh & Martin 2007). The curviplanar architecture of coherent magmatic bodies recognized at Pálháza is also similar to those documented from many mafic intrusions from the Mio/Pliocene phreatomagmatic volcanic fields of the western Pannonian Basin (Martin & Németh 2004a; Németh & Martin 2007). This is also true for the scale and texture of rosette-like columnar jointing of mafic intrusions and their associated fractured and peperitic margins (Rawlings 1993; Schmincke et al. 1997; Lyle 2000; Martin 2002).
The complex geometry and contact relationships between host sediment and coherent igneous bodies at Pálháza indicate a multiple injection and pulsatory magma supply. A single magma inflation would produce a larger and more regular, simple structure such as at Rebun Island, Hokkaido (Goto & McPhie 1998) or Milos Island, Greece (Stewart & McPhie 2003). Submarine dacite lava domes in NE Japan have a similar size to those at Pálháza (90–180 m in diameter, 55 m high: Goto & Tsuchiya 2004). The internal architecture of the Japanese example is simpler with individual radially jointed cores surrounded by hyaloclastite. The hyaloclastite is partially invaded by lava lobes up to 10 m wide (Goto & Tsuchiya 2004). Also in this Japanese example, the lava dome concordantly covers the immediate pre-volcanic mudstone, which is undeformed and has not been intruded. Hence, no peperite occurs at the contacts (Goto & Tsuchiya 2004), in contrast to Pálháza where the lowermost coherent intrusions invaded and partially encapsulated a marine mud, forming peperite along the contact. This textural difference also indicates that at Pálháza the initial magma emplacement led to cryptodome formation, which subsequently broke through the sedimentary cover to form a lava dome complex with an associated hyaloclastite pile around the network of feeder zones.
Conclusion
The identification of peperitic/intrusive hyaloclastite margins within the Pálháza rhyolite complex attest to its predominantly shallow-subsurface intrusive origin. Rhyolitic magma initially intruded unconsolidated, soft and wet, fine-grained sediments forming a cryptodome complex (Fig. 11). The low-volume, but sustained and pulsatory, magma supply led to the unsteady growth of a network of several cryptodomes. Some of these eventually broke through the sedimentary cover to form a network of small-volume endogenous lava domes. Quench fragmentation of the surfaces of these domes led to development of a capping and surrounding hyaloclastite pile.
Most of the hyaloclastite at Pálháza is in situ and only a small volume of re-sedimented hyaloclastite has been identified in the uppermost part of the more than 200 m-thick succession. During the growth of the nested cryptodome and dome complex, minor destabilization events transported freshly deposited hyaloclastite short distances away from the intrusive/extrusive centres (Fig. 11).
The identified facies relationships and geometry of coherent and fragmented volcanic units suggest a largely high-level intrusive emplacement of cryptodomes with associated formation of in situ hyaloclastite (Fig. 11). During the ongoing effusion more melt broke through the sediment cover and poured out to the sea floor, building up a growing hyaloclastite pile dominated by quench-fragmented and flow-banded angular rhyolite clasts hosted in a glassy matrix. Subsequently and/or parallel with this process, newly erupted rhyolitic melt intruded into the hyaloclastite pile causing local disturbances. The peperite formation beside the hyaloclastite and rhyolite attests to the large pore-water content and weak, unconsolidated nature of the hyaloclastite pile upon intrusion, which, in turn, suggests a relatively continuous, pulsating emplacement mechanism over a short period.
Acknowledgments
This work has been partly supported by the following research grants OTKA F43346 (K. Németh), OTKA T (Z. Pécskay and F. Molnár), NZ FRST Post-doctoral Fellowship (K. Németh) (MAUX0405) and FRST-PGST grant MAUX0401 (S. J. Cronin). The authors thank D. Brown (University of Glasgow), C. Breitkreuz (TU-Freiberg) and K. Thomson (University of Birmingham) for constructive reviews and editorial work that improved this manuscript significantly.
- © The Geological Society of London 2008
References
Hyaloclastites, peperites and soft-sediment deformation textures of a shallow subaqueous Miocene rhyolitic dome-cryptodome complex, Pálháza, Hungary
