shallow sill and dyke complex in western hungary as a possible feeding system of phreatomagmatic...

al Research 159 (2007) 138–152www.elsevier.com/locate/jvolgeores

Journal of Volcanology and Geotherm

Shallow sill and dyke complex in western Hungary as a possiblefeeding system of phreatomagmatic volcanoes

in “soft-rock” environment

Károly Németh a,b,⁎, Ulrike Martin c

a Geological Institute of Hungary, Department of Mapping, Stefánia út 14., Budapest H-1143, Hungaryb Massey University, INR, Volcanic Risk Solutions, PO Box 11 222, Palmerston North, New Zealandc Würzburg University, Physicalisch vulkanologisch Laboratory, Pleicherwald 1, Würzburg, Germany

Received 5 November 2004; accepted 11 June 2006Available online 12 September 2006

Abstract

Neogene alkaline basaltic rocks in the western Pannonian Basin are eroded remnants of maars, tuff rings, tuff cones, scoriacones and lava fields. The erosion level of these volcanoes is deep enough to expose diatreme zones associated with thephreatomagmatic volcanoes. The erosion level is deeper yet in the west, exposing shallow dyke and sill swarms related to formerintra-plate volcanoes. The basanitic sills are irregular in shape and their lateral extent is highly variable. Individual sills reach athickness of a few tens of metres and they commonly form dome-like structures with rosette-like radial columnar joint patterns. Thelargest sill system identified in this region is traceable over kilometres, and forms a characteristic ridge running north-east to south-west. Elevation differences in the position of the basanitic sills within an otherwise undisturbed “layer cake-like” siliciclasticsuccession indicate emplacement of the basanite magma at multiple levels over kilometre-scale distances. The margins of sills inthe system are irregular at a dm-to-mm-scale. Undulating contacts of the sills together with gentle thermal alteration in the hostsediment over cm-to-dm distances indicate the soft, but not necessarily wet state of the host deposits at the time sills were intruded.Parts of the sill complex show a complicated relationship with the host sediment in form of peperitic zones and irregularly shaped,disrupted, peperite textures. This is interpreted to reflect inhomogenities in water content and rheology of the siliciclastic depositsduring intrusion. The current summit of the longest continuous ridge preserves a small diatreme that seems to cut through anotherwise disk-like sill indicating of relationship between sill emplacement and phreatomagmatic explosive eruptions.© 2006 Elsevier B.V. All rights reserved.

Keywords: maar; diatreme; monogenetic; erosion; sill; dyke; dome; basalt; peperite

1. Introduction

Small-volume mafic volcanoes (both effusive andexplosive) are commonly considered as a monogenetic

⁎ Corresponding author. Present address: Massey University, INR,Volcanic Risk Solutions, POBox 11 222, PalmerstonNorth, NewZealand.

E-mail addresses: [email protected] (K. Németh),[email protected] (U. Martin).

0377-0273/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jvolgeores.2006.06.014

and result of a single batch of magma rise causing aneruption generally in a short time (Walker, 2000). Thisassumption is based on the fact that mafic volcanic fieldsare composed of volcanic edifices generally smaller than0.05 km3 (Wood, 1980), they are observed to be shortlived (Thordarson and Self, 1993; Vespermann andSchmincke, 2000) and commonly host dm-size nodulesderived from the mantle region (Sinclair et al., 1978;D'Orazio et al., 2000) believed to require high speed of

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139K. Németh, U. Martin / Journal of Volcanology and Geothermal Research 159 (2007) 138–152

flow in themagmatic feeding system to successfully reachthe surface (Sparks et al., 1977; Spera, 1984). Theseobservations are generally suggestive of a simple pipe-like conduit for a monogenetic volcano. Magma propa-gation models appointed two major controlling para-meters of magma movement toward the surface, themagmatic pressure and the gravity controlled neutralbuoyancy (Rubin and Pollard, 1987;Walker, 1989, 1999).Individual small-volume mafic volcanoes of continentalmonogenetic volcanic fields are generally presumed toerupt single magma batches during brief eruptions.Nevertheless, in many places it has been identified thatsystematic compositional differences among products ofindividual volcanoes exist (Houghton and Schmincke,1986) and may indicate development of temporal storageplaces ofmelt en-route to the surface (Németh et al., 2003).Density controlled rise of mafic melt modelled to be de-pendent on the general stress field of the lithosphere andmay have difficulties to reach the surface in case whenstress fields are unfavourable, such as transtensional and/ortranspressional situation (Canon-Tapia and Walker,2004). Mafic magma movement in near surface condi-tions is likely different for those in deep level. The freesurface effect on the density control especially if magmaencounters thick low density near-surface successionsmay force the melt to pond as sill and dyke complexinstead of erupt to the surface (Connor et al., 2000;Canon-Tapia and Walker, 2004). Such conditions arelikely to occur in thick fluvio-lacustrine sediment filledbasins such as the Pannonian Basin was during the Mio/Pliocene mafic volcanism. In such environment the po-tential eruption style (effusive versus explosive and/ormagmatic explosive versus phreatomagmatic) is stronglycontrolled by the host rock environment what the maficmelt encounters (Connor et al., 2000). In the case of “soft-rock environment”, as a term often used for loose andwater saturated country rocks (Lorenz, 2002) that thefeeding dykes intrude, is commonly water saturated andmay enhance phreatomagmatism (Lorenz, 1985).

Magma emplacement into wet, unconsolidated sedi-ments is likely to result in a highly interactive magma-sediment-fluid system (Krynauw et al., 1994). Suchsystems in case of acidic and large volume melt emplace-ment are potentially important for the establishment ofhydrothermal activity (Kokelaar, 1982) and associatedmineralization. Injection of magma into wet, poorlyconsolidated sediment may also result in mixing andformation of peperite (White et al., 2000; Skilling et al.,2002) both blocky and/or globular in texture (Busby-Speraand White, 1987; Doyle, 2000). Although the peperitestudies recently have identified different types of peperiteinvolving diverse combinations of magma and sediment

mixing, there are only few studies on the peperiteformation in intrusive complexes associatedwith subaerialintracontinental phreatomagmatic volcanoes (Martin andNémeth, 2000; Lorenz et al., 2002; Martin and Németh,2004; Hooten and Ort, 2002). Careful description andinterpretation of textures in outcrop to thin-section scaleexamples are important for achieving a better understand-ing of peperite formation and the emplacement andmagmatic feeding history of small-volume phreatomag-matic volcanoes. Parts of a Pliocene basanite dyke and sillcomplex in the Bakony–BalatonHighlandVolcanic Field,Western Hungary (Fig. 1), display remarkable wellexposed shallow sub-surface mafic dyke and sill com-plexes associated with exposed diatreme remnants offormer phreatomagmatic volcanoes. Such intrusive com-plexes are common with diverse peperite textures.Because both coherent and peperitic domains of thedyke and sill complex in Western Hungary are wellexposed due to intensive quarrying and the 3D setting ofintrusions are well-defined, some controls on emplace-ment mechanism of mafic melts in intracontinental settingcan be inferred. The more or less continuous outcropavailability and the detailed mapping of the intrusivecomplexes have reviled some genetic relationship betweenphreatomagmatic volcanoes and such intrusive systems asa potential feeding network of this type of volcanism. Thispaper describes and interprets themorphology and internaltextures of an extensive dyke and sill complex that isclearly associated with phreatomagmatic volcanoes, andconsiders implications for the control on the formation ofpeperite in this setting as well the emplacement mecha-nism of melt in wet, “soft-rock” environment.

2. Geological setting

Pliocene volcanic rocks crop out in the western part ofthe Bakony–Balaton Highland Volcanic Field and forman elevated mesa that reaches an elevation about 300 m(200 m above the surrounding base level (Fig. 1). Thiselevation is equal to the top level of the Triassic limestoneand dolomite blocks (Csillag et al., 1995) just fewkilometres south of the volcanic rocks (Fig. 1). Thevolcanic rocks in this area form a north-west to south-easttrending zone, but the ridge alignments are different fromthis direction (Fig. 1). The erosional remnants in thecentral part of the area form a very characteristic north-east to south-west ridge (Figs. 1 and 2). Volcanic rocksformmesa-like hills to the west (Kovácsi-hegy) and to thesouth-east (Szebike) of this ridge (Figs. 1 and 2). The pre-volcanic rocks are Neogene siliciclastic siltstones,sandstones, silt and sand with approximately a hundredmetres thickness (Budai and Cummings, 1987; Jámbor,

140 K. Németh, U. Martin / Journal of Volcanology and Geothermal Research 159 (2007) 138–152

1989). The southern margin of the area is a fault boundedagainst Triassic limestone with a dolomite ridge thatreaches an elevation of over 400 m. Lava capped mesas(e.g. Tátika — 413 m, Uzsa lava plateau — 340 m) aremore or less at the same elevation. The volcanic rocks aresituated in a graben bounded by faults, that is the north-westward continuation of the Tapolca Basin (Fig. 1).

Pyroclastic rocks crop out in significant volumes onlyat the Uzsa locality (Fig. 1). Small outcrops of pyroclasticrocks have been identified high up on Tátika (at about380 m), which is the southwestern edge of a 5 km long akilometre wide ridge formed by intrusive rocks (Figs. 1and 2). The basanite ridge of the Tátika — Sümegprága(Sarvaly) is a chain of irregularly shaped, commonlyrosette-like columnar jointed compositional homogeneousbasanites, e.g. basanite dyke at Tátika characterised bymajor element composition of SiO2 — 44.94, FeOtotal —10.15, MgO — 10.07, Na2O — 3.90, K2O — 2.55. The

Fig. 1. Simplified geological map of the dyke and sill complexes of the WesteBlack rectangular in the inset map shows the area of the main map.

coherent basanite bodies are in intrusive contact with thehost Neogene siliciclastic rock units. In contrast, theKovácsi-hegy is a tabular basanite flowwithmultiple flowunits characterised by vertically oriented columnar joints.A basanite ridge formed by dissected bud-like basaniteintrusions forms a north-west to south-east ridge just southof Uzsa (Kő-orra).

The ages of the lava flows are among the youngest ofthe Pliocene intraplate volcanic rocks in the westernPannonianBasin and range between 3.4 to 2.7MabyK/Arwhole rock and isochron age determination methods(Balogh et al., 1986; Balogh et al., 2001). A recent mea-surement of intrusive rocks from Sümegprága (Figs. 1and 2) applying high resolution 40Ar/39Ar incrementalheating ages, however, gave an age of 4.15±0.05Ma over100% of the gas release and slightly lower isochron ages(3.95±0.16 Ma) (Wijbrans et al., 2004). The agedifferences among different methods highlight the need

rn Pannonian Pliocene volcanic field just north of the Keszthely Mtns.

Fig. 2. 3D oblique view of a digital terrain model of area where thedyke and sill complex are located. The Tátika— Sümegprága ridge isabout 6 km long. Note the NE–SW running ridge formed by basaniteterminating into the rounded flat hill of Tátika.


for a detailed age determination survey in this region.However, the identified age range is somehow similar tothat of the volcanic rocks of the Tapolca Basin (Baloghet al., 1986; Borsy et al., 1986). This similarity also high-lights the genetic relationship between these volcanicrocks and those of the Tapolca Basin.

In this paper we describe in detail the longest intrusivecomplex forms the Tátika to Sümegprága (SW to NE)ridge and shows the relationship with the host rock thatwas unconsolidated and moderately water saturatedduring the intrusion.

3. Sümegprága

Sümegprága is the northernmost part of a ∼5 kmlong ridge exposing volcanic rocks and trending fromnorth-east to south-west (Figs. 1 and 2). The ridge atSümegprága reaches an elevation of about 260 m.Coherent basanite appear at elevations around 220 mand above. At this site the basanite has been quarried inthe past, exposing the three dimensional architecture ofthe coherent lava facies. The coherent basanite bodyforms a small, flat hill that has been opened up duringthe quarrying (Fig. 3). The basanite is in intrusivecontact with the host Neogene siliciclastic units. Thereare tabular and rosette-like sill and dyke complexes thatare in sharp but irregular contact with the host sediment(Fig. 4A).

The contact of the basanite dyke and sill complex atSümegprága with the host sediment is wavy but sharp,and ismostly non-brecciated. Along this contact, the dykeand sills have a dark grey, glassy quenched rim 0.5–2 cmthick that contains. There is no apparent flow banding inthe dyke/sill body. There is neither characteristic orientedtexture of the disturbed host sediment. The contact zonesof the dyke and sill complex are purely thermally altered(Fig. 4B). There is no evidence that hornsfel developed inthe contact zones. The sand and silt slightly stronglycemented by a combination of calcite and quartz near thecontact zone as a sign of hydrothermal alteration atthe contact zone where fluids were free to move along theintrusive margins (Fig. 4B). Small protrusions fromthe master sills commonly form irregularly shaped dm-to-m thick dykes with chilled margins (Fig. 4C). Peperiticmargins are rare at Sümegprága, and only exposed insmall, dm-scale zones where slightly thermally alteredsand/silt is in contact with the coherent intrusive bodies(Fig. 4D). The textural characteristics of the host sedimentare the same as for other Neogene siliciclastic unitsforming the immediate pre-volcanic successions else-where in the region. Small outcrops around Sümegprágashow intrusive contacts between coherent lava bodies and

host sediments, indicating that the ridge is consist of a silland dyke complex. At Sümegprága a complex jointingpattern is characteristic for the intrusive bodies formingrosette-like joints in the centre of the exposed complex(Fig. 4E). In the coherent part of the basanite units, rapidchanges in joint pattern orientation indicate complexcooling history and potential rejuvenation intrusions intothe complex (Fig. 4F ).

4. Bazsi to Tátika

To the south-west of the Sümegprága another dyke andsill complex is exposed in a small quarry (Figs. 1 and 2).The sills are very irregular in shape and have chilledmargins. The contact of the basanite dyke and sillcomplex at Sümegprága with the host sediment is wavy.InBazsi however the contact is fluidally shaped inmost ofthe exposures, and it is mostly non-brecciated (Fig. 5A).Along the contacts, the dyke and sill have a dark grey,glassy quenched rim up to 2 cm thick. The glassy rim isricher in vesicles than the coherent sill and dyke body, but


the total vesicularity of the coherent bodies are low. Theglassy quenchedmargin of the intrusive bodies consists ofporphyritic basanite. Along the rim, the brown, moder-ately palagonitized glassy rim comprises euhedral pheno-crysts of olivine set in a groundmass palagonitised glass.There is no characteristic flow banding along the marginneither in the inner parts of the intrusive bodies. The hostsediment within 50 cm of the contact zone is yellowishbrown, homogenised and non-stratified, whereas beyondthis zone, it is creamy and is well-bedded. Angular tofluidally shaped basanite clasts l–10 cm across are dis-persed in the yellowish brown host sediment that sur-rounds the intrusive bodies and form globular peperite.The clasts have quenched glassy rims.

In other places, irregular margins and globularpeperites are more well developed (Fig. 5B). In thesezones, the lava showed fluidal behavior and blobs whichare mixed with the host sediment (Fig. 5B). Becausethere is no obvious evidence of high temperaturealteration of the silt/sand along the coherent magmaticbodies (e.g. hornfels) we can only state that at least somehydrothermal effect on the host sediment took placealong the intrusive bodies. The Bazsi outcrops about 50to 100 m above the location of Sümegprága (Fig. 3). Incomparison to the Sümegprága locality, in the centerpart of the ridge, irregular sill and dyke contact zoneswith the host sediments are more common, and usuallywell developed and wider (decimetre to metre scale).

In the upper quarry of Bazsi, metre-scale finger-likeintrusions into the host sediment can be observed, andterminate into an irregularly shaped zone mixture of

Fig. 3. Airphotos of the dyke and sill complex. A) surrounding of the Tátikintrudined into Pliocene fluvio-lacustrine siliciclastic units. B) the northern tegenerated by basanite dyke buds. Lines represent 1 km length.

fragmented volcanic rocks and plastically deformed silt(Fig. 5C). The contact of the basanite dyke and sillcomplex with the host sediment at the upper level of Bazsiis characterized by common blocky peperite texture(Busby-Spera andWhite, 1987;Doyle, 2000), comprisingangularly-shaped basanite clasts 1 to 50 cm acrossseparated by homogenised host sediment (Fig. 5C). Inmany places, along the exposure of fragmented basaniteand host sediment mixtures, the basanite clasts show amoderately developed jigsaw-fit texture similar to dykemargins reported in many sites where mafic dyke swarmintruded into a siliciclastic sediment filing basins (Caset al. 2000; Van Wagoner et al., 2001; Dadd and VanWagoner, 2002; Van Wagoner et al., 2002). Occasionallysome clasts have been rotated and dispersed up to a metreinto the host sediments. Larger (dm-to-m size) angularclasts seem to float in the host sediment relatively far fromthe main intrusive body. Most basanite clasts have a well-developedmm-to-cm thick glassy quenched rim.Basaniteclasts are poorly vesicular. Occasional vesicles however,may reach cm-sizes and are ellipsoidal. Vesicles arecommonly filled with homogenised fine host sediment.The basanite clasts do not show preferred fracturing ori-entation nor a concentric radial joint. The original textureof the host sand and silt cannot be recognized through thesub-horizontal bedding between such irregular fragmen-ted zones are often preserved in a dm-scale. The basaniticclasts are generally finely crystalline or tachylitic in tex-ture, but have up to cm-thick palagonite rims (Fig. 5D).The interior of the basanite dyke and sill is dense withvariously oriented joints. Joints are continuous in long

a diatreme. Note the quarry of Bazsi where a complex dyke and sillrmination of the dyke and sill complex. Note the irregular topography


(tens of metres) distances. The joint orientations at Bazsivary largely. Radially jointed, rosette-kind jointing patternform a central section of the exposed sills. Between thenumerous curviplanar joints no host sediment filling havebeen recognized, however toward the intrusive body andhost sediment interface small patches of host sedimentalong the joints are more prominent. In the upper contactzones of the Bazsi intrusive complex part of the contact

Fig. 4. A) The sharp but slightly undulating (at metre scale) upper contacB) Undulating vertical contact of a basanite dyke bud at Sümegprága. Note thethe dyke margin. The units on the marker are in cm. C) Meter scale dyke budsD) Peperitic contact of a dyke bud in a cm scale at Sümegprága. Units on thcomplex of Sümegprága. Note the changable but sharp upper contact of the cocenter of the Sümegprága dyke and sill complex attest a complex cooling hist10 m high.

with the sediments expose blocky peperitic domains in acm- to dm-scale in the otherwise planar and sharp contacts(Fig. 5E). In this zones the angular fragments of basaniteshows contact parallel oriented texture due to fluidisationalong the intrusive margins (Fig. 5E). In other places thecontact is wider (dm- to m-scale) and the peperite is moreglobular in texture (Fig. 5F ). Such complex coexistenceof different peperite textures suggests rapid changes in

t of the Sümegprága sill. The length of the outcrop is about 50 m.colour changes of the host sediment due to hydrothermal activity alonginitiating from the master sill of Sümegprága. Trees are about 3 m high.e marker are cm. E) Complex joint pattern of the master dyke and sillmplex. The trees are about 4 m tall. F ) Complex jointing pattern in theory of the system upon intrusion to the host sediment. The wall is about

Fig. 4 (continued ).


water content as well as compaction of the near surfacehost sediments.

5. Tátika diatreme

The south-westernmost hill of the ridge terminates ina circular shaped plateau-like coherent basanite regionforming the plateau of the hill Tátika (Figs. 1–3). Thequarry of Bazsi exposes basanitic sills up to the level of300 m, and is covered by the same sand as it is exposed

in the quarry itself. The plateau at Tátika reaches 350 m.This plateau is cut through by few basanite dyke budsthat are characterized by rosette-like columnar jointing.In the basal zone pyroclastic rocks are exposed.

Pyroclastic rocks, collected from poor metre-scale sizeof outcrops, are rich in fine sand, silt, quartz grains, andmud chunks (mm-to-cm size), derived from rock typescharacteristic of the Neogene pre-volcanic rock sediments(Fig. 6A). Mud chunks are evenly distributed in theexposed lapilli tuff units and form fluidal shape lapilli size


fragments. They doesn't appear in a fine grained, fluidisedhost zone in the lapilli tuff, they rather form individualpyroclasts with an own explosive fragmentation history.Their texture differ from fluidised mud-zones have beenidentified in the near vicinity of dykes and sills peperiticzones from Bazsi or Sümegprága. They are interpreted tobe disrupted mud fragments travelled in one piece in theeruption cloud similarly to reports from mud chunks from

Fig. 5. A) Sharp but undulating upper contact of the Bazsi dyke and sill compbetween individual sill tongues in the upper level of the complex. The wall ibasanite dyke buds at Bazsi. Note the partially preserved bedding in the cmundulating vertical contact of a master dyke at Bazsi. In the upper level oterminating in fragmented zones. The outcrop is about 6 m wide. D) Fragfragmented basanite. The view is about 3 m wide. E) Sharp contact along a sithe margin. The peperitic domains are highly oriented with angular basanite cbud at Bazsi.

deposits associated with the Hopi Butte maar/diatremevolcanoes (White, 1991). The pyroclastic rocks of Tátikacontain sideromelane glass shards that are blocky in shapeand moderately vesicular (Fig. 6B), typical products ofphreatomagmatic fragmentation (Heiken and Wohletz,1986; Wohletz, 1986; Heiken and Woheltz, 1991). Theglass shards are tephritic in composition, similar to glassshards from the Neogene phreatomagmatic lapilli tuff and

lex. Note the undeformed bedding characteristics of the host sediments about 15 m high. B) Globular peperite along the steep contact of the-scale in the peperitic domain. The pen is 12 cm long. C) Sharp, butf the intrusive complex at Bazsi, small (metre scale) sills commonlymented zone of a small sill at Bazsi. Note the angular shape of thell margin at Bazsi with peperitic domains in irregular distribution alonglasts. F ) Small globular basanite buds in dm-scale initiated from a dyke

Fig. 5 (continued ).


tuff units elsewhere in the western Pannonian Basin(Martin and Németh, 2004). The chemical compositiondifference between the volcanic glass shards and the silland dyke complex infers a potential differentiation process,that is recorded in many other places in western Hungary(Németh et al., 2003) but not fully understood yet.

The poorly exposed pyroclastic rocks form a semi-circular collar-like region with dip direction toward thecentre of the circular hill top of Tátika. The dip direction isdefined by poorly developed stratification in the other-wise massive lapilli tuff. The exposed lapilli tuff is poorlysorted, and occasional fine interbeds can be identified inlarger exposure walls. Such interbeds, however, pinch outquickly, and changing in their bed thickness over short(dm-scale) distances. In the western side of the Tátika, ageneral increase in scoriaceous lapilli is characteristic. Inspite of the limited number of outcrops to define theposition of the identified pyroclastic rocks, there is noevidence of large lateral extent of the pyrcolastic rocks.Their areal extension is confined to the central, top part ofthe Tatika. There is also no indication of preservedpyroclastic rocks associated with the nearby hills alongthe strike of the dyke and sill complex. Tátika seems to bethe only location in the region beside 10-km radius wherepyroclastic rocks have been mapped. The steep beddingcharacteristics of the preserved pyroclastic rocks, theirunsorted texture and their general well-confined, circulardistribution with no lateral connection with other similarpyroclastic rock units suggest that the pyroclastic rocks ofTátika represent a vent filling succession cutting throughthe syn-eruptive stratigraphy and preserved in a pipe-likevolcanic conduit. The evidences of sudden chilling ofpyroclasts in these pyroclastic rocks, as well as the largevolume of sand, silt, quartz andmuscovite in the matrix ofthese lapilli tuffs indicates magma/water interactiondriven phreatomagmatic eruptions as a principal frag-mentation mechanism. In the phreatomagmatic fragmen-tation history with the 3D stratigraphy relationship ofthese pyroclastic rocks to the surrounding rock unitssuggest, that Tátika is an erosion remnant of a diatreme,and best to interpret it as a sub-surface architecture of aphreatomagmatic volcano, a shallow maar or tuff ring.

6. Sill and dyke complexes and their relationship tophreatomagmatic volcanoes

The 3D relationships of the basanitic rocks with thehost rock indicate that a ridge from Sümegprága throughBazsi to Tátika a sill and dyke complex is exposed.The location of the Bazsi quarry is about 2 km to theSümegprága quarry (Figs. 1–3). Elongation of the basanitebody along the length of the ridge aswell as the continuousoutcrops of tens of metres-scale wide radially jointedbasanite between these two locations indicate that Bazsi ispart of the same major sill and dyke system that runs fromnorth-east to south-west. Similar dyke and sill complexesin poorly exposed settings have been mapped out to thesouth-west supporting the interrelationship between these

Fig. 6. A) Phreatomagmatic lapilli tuff from the Tátika diatreme that is rich in sand and silt fragments (cm to dm scale) derived from the Pliocenefluvio-lacustrine siliciclastic successions that the sill and dyke complexes intruded. The pen is about 10 cm long. B) Microphotograph of aphreatomagmatic lapilli tuff from the Tátika diatreme. Note the chilled, blocky, weakly vesicular, but microlite-rich texture of the moderatelypalagonitized sideromelane glass shards. The view is about 4 mm across. C) An overview from the southwest of Tátika diatreme. Note the flat circularbasement of the diatreme formed by a basanite sill that is punctured by the elevated feature of the diatreme.


Fig. 7. An emplacement model of the dyke and sill complexes in thewestern Pannonian Pliocene mafic volcanic region. 1) dyke and sillcomplexes intruding into the thick host sediments of the Pliocenefluvio-lacustrine siliciclastic units. The deeper contact of thecomplexes are sharp due to the lower water content and possiblemore consolidated state of the host rocks. Reaching the upper watersaturated and less consolidated zones of the host rocks by theintrusions lead to the formation of peperitic margin of the intrusivebodies. Upon repeated intrusions, dykes may feed sills that are laterallyburrow under the soft sediment. 2) In areas where the host sedimentshad elevated water content the lateral burrowing intrusions mayinteract explosively with the water saturated sediments and formbroad, bowl shape maars with shallow diatremes (a). Alternatively thediatreme formed due to a new intrusion below the topmost sill complex(b). The preserved position of the diatreme on the end of a chain ofdyke and sill complex however suggest close genetic relationshipbetween the topmost dyke and sill complex and the diatreme and rathersupport the model “a”. In other words there is no reason to penetrateand disrupt a sill body by a new intrusion when it could have reachedthe surface much easier just hundred metres off where no sill wouldhave blocked its way to the surface.


zones. South-west of Bazsi, other major ridges representrosette-like columnar jointed bud-like basanite bodies.These rosette-like features preserved as small irregularitiesin morphology in the otherwise 1–2 km wide zone ofplateau-like coherent body.

The general spatial relationship between coherentbasanite units of Bazsi and Tátika, and the Neogenesiliciclastic succession indicate that the basanite is inintrusive contact with the sedimentary rocks, and is partof an elongated sill and dyke system in this region. Theonly location where pyroclastic rocks are exposed isTátika (Fig. 6C), indicating that explosive eruption tookplace in that site. The textural characteristics of thepyroclastic rocks indicate that the explosive eruptions ofTátika were driven predominantly by magma/waterinteraction. The semi-circular inward dip direction ofthese pyroclastic rocks indicates angular unconformitybetween the circular basanite plateau and the pyroclasticrock units. The present plateau elevation of the Tátikabasanite platform is more or less equal to 300 m today.In such elevation just NE of Tátika sand and silt unitsform a cap over the NW–SE trending cohrent basanitebody that is connected to the Tátika basanite platform.In this 3D relationships it is a reasonable reconstructionthat the Tátika pyroclastic rocks form a pipe-like feature(diatreme) cut through the Tátika basanite platform.

However, the Tátika pyroclastic succession also seemsto be cut by smaller basanitic dykes, which suggests afinal dyke intrusion after the phreatomagmatic eruption ofTátika. Such intrusions are interpreted to be the feedingchannels of the last stage of the eruptions andmay emittedlava flows that have already been eroded. In summary it isinferred that dyke and sill complex geometry is com-plicated in the studied area and the Tátika diatreme hasclose relationship with the identified dyke and sillcomplex (Fig. 7). Due to the limited exposures there aretwo equal model to demonstrate the spatial relationshipbetween the Tátika diatreme and the dyke and sill com-plex; a) the lateral burrowing sill complex interact withwater saturated portions of the host sand and silt and forma shallow diatreme, or b) the sill pre-date the formation ofthe diatreme, which produced by a new intrusion belowthe sill (Fig. 7). The preserved position of the diatreme onthe end of a chain of dyke and sill complex, howeversuggests close genetic relationship between the dyke andsill complex and the diatreme. It needs more energy todisrupt a near surface sill by a new intrusion especiallywhen it could have reached the surface much easier justhundred metres off where no sill would have blocked itsway to the surface. In either way it is clear that in a shortdistance (10 km-scale), complex intrusive processes andassociated phreatomagmatic explosive eruptions took

place, indicating the importance of the hydrogeology ofthe topmost few hundred metres of the country rocks tocontrol mafic volcanism.

The erosion level at these locations is deep. The syn-volcanic paleo-surface is estimated to be presently at theelevation above the present top of Tátika (413 m),considering that

1. There has been no uplift nor tectonic dissection of thevolcanic rock-capped ridges between Tátika andSümegprága,

2. Tátika pyroclastic rocks represent the top, rather thanlower parts, of a diatreme (Fig. 6C). In this estimate the


present position of the top of the Tátika should repre-sent a level below the syn-volcanic paleo-surface usinggeneral considerations for the 3D architecture of adiatreme (Lorenz, 1986; White, 1991). It is hence aconservative estimate that the pre-volcanic Neogenesiliciclastic units were still forming a thick pile ofsediment at the time of eruption. The sill and dykecomplex hence inferred to be intruded at depths of atleast 50–100mbelow the paleo-surface.However, thisdepth is likely an underestimate.

7. Discussion

More than other types of volcanic activity, maar-diatreme volcanism is strongly affected by the nearsurface geological setting (Kienle et al., 1980; Lorenz,1986; Luhr and Arandagomez, 1997; Németh et al.,2001). Maar volcanoes have been classified into twomajor types according to the ground water source withmagma interact (Lorenz, 2003). The first one are maarsdeveloped over a “hard-rock” environment where anascending magma interacts with water in cemented, oftenfracture controlled aquifer-bearing rocks. The second typemaars are those that develop over “soft-rock” country rockenvironment, where the ascending magma enters wetunconsolidated sediment. Shallow intrusive dykes, whichare entered water saturated sediments, may form peperiticmargins as a first stage of a Fuel–Coolant Interaction(FCI) (Hooten and Ort, 2002) but not necessarily aphreatomagmatic volcano. However a genetic relation-ship and a gradual transition between shallow sub-surfaceintrusive processes in “soft-rock” environment and theirpotential evolution to develop phreatomagmatic volca-noes are expected.

This paper reports on the identification of dyke and sillcomplexes in association with a volcanic field producedmanymafic phreatomagmatic volcanoes that are preservedcommonly as diatremes cutting through a “soft-rock”country rock environment. The studied dyke and sillcomplexe gradually show more pronounced peperitetextures along their contact with the host fluvio-lacustrinesediments and associated breccia zones comprisingangular to lobate variably vesiculated basalt clasts in amud and silt matrix. The peperite formed as a result ofemplacement of the basanite sheet as a shallow sill anddyke complex based on the 3D field relationships thatburrowed into and migrated laterally within an unconsol-idated moderately water-saturated fluvio-lacustrine sedi-ment pile. The studied dyke and sill complex shows acomplex architecture with lava buds tens of metres indiameter, rapidly changing orientation of columnar joints;rapid changes (in tens of metres distances) of dyke–sill–

dyke geometry and developments of laterally large(hundreds metres across) lobe-like tens of metres thicksill units. These geometrical relationships closely resemblethose complex 3D geometries that have been reportedfrom the North Rockall Trough (Atlantic margin, North-west Europe) dolerite sill and dyke complexes on the basisof 3D seismic sectioning (Thomson and Hutton, 2004).The elongated architecture of the studied sill and dykecomplex suggests fissure-controlled melt propagation inthe feeding system. The flowmovement such as generallysuggested styles as compensation surface mirroring(Bradley, 1965), magmatic overshoot and magma backflow (Francis, 1982), ring dyke model (Chevallier andWoodford, 1999) or centrally sourced laccolithmodel withperipheral dyking (Thomson and Hutton, 2004) is cannotbe established at the present stage of research. for the dykeand sill complex in western Hungary. However, the totalvolume of the western Hungarian sill and dyke complex issmaller than the examples from the North Atlantichowever its geometrical complexity is comparable tothose sill and dyke complexes reported from the NorthAtlantic margin or those generally considered sub-surfacefeeding systems of composite volcanoes (Thomson andHutton, 2004). In this way the relationship betweendiatreme(s) as a root zone of mafic small-volumephreatomagmatic volcanoes and such shallow but com-plex and reasonable large intrusive complexes suggeststhat such volcanic fields may have been far more complexfeeding systems than it is considered before. Such feedingsystems are especially expected to exist in associationwithphreatomagmatic volcanic fields in well-drained basinswith thick basin filling “soft sediments”.

The estimated emplacement depth of the sill and dykecomplex in western Hungary is at least 100 m below thesyn-volcanic paleosurface in the north-easternmost partof the complex. However this estimate should be taken asa minimum value because the reference point used toestablish the emplacement depth is the presentlypreserved top level of a diatreme. The exposed diatremefacies consists of pyroclastic rocks are characteristics fordiatremes that are developed in “soft-rock” environment.Such diatremes in general do not form a deep pipe-likearchitecture and therefore the present exposure veryunlikely to represent large depth in comparison to thepaleosurface. Here we suggest, that the preserveddiatreme was about 100 m below a broad bowl-shaped,shallow maar/tuff ring, that developed in the alluvialplain filled with soft and water saturated fine grainedsilisciclastic sediments in the Pliocene. Basins that arefilled with thick units of such sediments are excellentplaces to develop broad, shallow bowl shape maars andtuff rings that are underlain by shallow diatremes. Such


broad maars are expected to develop in association withlarge igneous provinces where the large volume ofuprising mafic melt encounter thick water-saturatedporous media aquifers (White andMcClintock, 2001). Insuch systems shallow burrow of uprising melt isexpected and in many places have been described inancient settings (Rawlings et al., 1999; Dadd and VanWagoner, 2002; Van Wagoner et al., 2002) as well assome sort of lateral migration of vents along a verticalmafic feeding system that may fully develop as aphreatomagmatic volcano in sites where conditions arefavourable (White and McClintock, 2001). The newshallow sill and dyke complex interpretation of thestudied coherent mafic bodies has significant implica-tions on the perceived sedimentary architecture andMio/Pliocene mafic melt emplacement model for the westernPannonian Basin. We suggest that large volume “lavaflows” in the region may be attributed to emplacement asshallow sills that have propagated through a regionallyextensive sediment pile similarly as it has been suggestedfrom other places such as Karoo (Rawlings et al., 1999).Such potential emplacement mechanism of coherentmafic bodies with uncertain stratigraphy position inwestern Hungary should be carefully re-examined in thenear future.

8. Conclusion

The volcanic rocks mapped north of the KeszthelyMts. have long been a object of geological research in theregion (Lóczy, 1913). Researchers noted the general lackof pyroclastic rocks in these locations in comparison toother Neogene alkaline basaltic regions in the westernPannonian Basin. The ridge between Sümegprága andTátika is clearly a shallow subsurface sill and dyke com-plex. This system, at least in one place, has been cutthrough by a diatreme (near Tátika). However, the pyro-clastic rocks are invaded by thin dykes. This implies thatthe emplacement of the majority of sills and dykes pre-dates formation of the phreatomagmatic volcanoes.

The intrusive origin of the majority of the basaniticrocks north of the Keszthely Mts. suggests thatsignificant erosion took place since their emplacementaround 3 Ma. A conservative estimate would reconstructthe level of the paleo-surface at the level of the presenttop of the Tátika, where pyroclastic rocks crop out.However, the Tátika is the remnant of an exhumeddiatreme, and that implies that its present top sectionrepresents a level below the syn-volcanic paleo-surface(Fig. 7). An almost continuous and well exposed sill anddyke complex is traceable from the Tátika diatreme andlocated about 100 to 250 m below the estimated syn-

volcanic paleo-surface. The present day high altitude ofthe intrusive rocks at this region in comparison to othereffusive coherent lava rock locations in the westernPannonian Basin in a more or less similar elevationsuggests some sort of differential base level changesthrough the Neogene which process needs further study.

The recognition that a majority of the coherent igneousrocks exposed north of the Keszthely Mts. originated asintrusive sill and dyke complexes also highlights thecomplexity of magma emplacement and feeding systemsfor small-volume intracontinental alkaline basaltic volca-nic systems (Fig. 7). There are growing examplesworldwide that demonstrate that maar/diatreme volca-nism is often associated with complex effusive (extensivelava flows) and intrusive (dykes, sills and laccoliths)processes (Van Wagoner et al., 2001; Van Wagoner et al.,2002; Lorenz and Haneke, 2004). Both recent andpreceding seismic studies have identified severalmound-like, high velocity zones within Neogene strata afew tens to a hundredmetres below the surface in the LakeBalaton basin (Cserny and Corrada, 1989; Sacchi et al.,1999; Sacchi and Horváth, 2002). These structures arebest interpreted to represent sill and dyke systems thatnever made it to the syn-volcanic surface, and which arenot yet exhumed like the Sümegprága–Tátika sill anddyke complex.

Acknowledgements

This research was a subject of Károly Németh'sMagyary Zoltán Post doctoral Fellowship. The authorsare grateful for funds such as from the Hungarian ScienceFoundation OTKA F 043346, DFG, DAAD GermanHungarian Academic Exchange Program 2002/2003 andTÉT CONICET, Hungarian–Argentine Bilateral Cooper-ation that helped to carry out this research or contributedto better understanding of dyke and sill emplacements involcanic fields, many thanks to these organisations.

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