Download PETROGENESIS OF THE MIOCENE SILICIC PYROCLASTIC

Ph.D. Theses
PETROGENESIS OF THE MIOCENE SILICIC
PYROCLASTIC ROCKS OF BÜKKALJA:
INFERENCES FOR THE MAGMA RESERVOIR
PROCESSES
Réka Lukács
(Haranginé Lukács, Réka Zsuzsanna)
Earth Sciences PhD School
Geology and Geophysics Ph.D. Program
Prof. Szabolcs Harangi (DSc)
Supervisor
Prof. Gyula Gábris
SchoolLeader
Prof. Miklós Monostori
Program Leader
Eötvös Loránd University
Department of Petrology and Geochemisty
Budapest
2009
I. Introduction, aims of the study
Silicic (SiO2>65wt%) magmas evolve in shallow level magma reservoirs following
the segregation of their parental melts from the source regions. Eruption of the felsic residual
magmas, the erupted volume of the magma and the mechanism of the eruption are primarily
controlled by the processes occurred in this magmatic system. Thus, studies of these rocks
could contribute to our knowledge on the reason of large volume – sometimes supersized explosive eruption of silicic magmas and also to understand the growth and evolution of
silicic plutons, which add significant mass to the continental crust. Recently, there has been a
change of paradigm concerning the processes in the shallow silicic magma reservoirs
(Hildreth & Wilson, 2007; Bachmann & Bergantz, 2008 and references therein).
Silicic volcanism played a major role in the initial stages of the evolution of the
Pannonian Basin. This kind of volcanc activity occurred during a changing geodynamic
environment (Harangi and Lenkey, 2007), i.e. during the pre-extensional and syn-extensional
phases of the Pannonian Basin. The southern foreland of Bükkalja (Bükkalja) provides an
excellent area to study these silicic pyroclastic rocks. In spite of their fairly old ages (13-21
Ma; Márton és Pécskay, 1998), they are particularly fresh and preserved their original
compositions including also the less stable glass phases.
Among the so called Lower, Middle and Upper Tuff Horizons three representative
ignimbrite occurrences were selected (Szomolya Ignimbrite, Bogács Ignimbrite and Harsány
Ignimbrite belonging to the lithostratigraphic units of the Gyulakeszi Rhyolite Tuff
Formation, Tar Dacite Tuff Formation and Harsány Rhyolite Tuff Formation, respectively) to
carry out a detailed petrogenetic study in order to reveal the processes occurred in the shallow
magma reservoirs. This study involved an integrated textural and in-situ major and trace
element geochemical investigation of the juvenile glasses (i.e., glassy groundmass of the
pumices and lithic clasts, glass shards and silicate melt inclusions) and the mineral phases of
the ignimbrites combined with quantitative model calculations.
The Szomolya ignimbrite is located at the southern part of Bükkalja and represents a
well-preserved member of the Early Miocene Lower Tuff Horizon. One of the peculiarities of
this pyroclastic rock is the relative abundance of cogenetic lithic clasts, which could be
effectively used to have an insight into the deep magma reservoir processes. Furthermore, the
mineral phases of the lithic clasts contain abundant silicate melt inclusions. Their studies
provides a new perspective to reveal the genesis of the silicic magmas.
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The 17 Ma old Bogács Ignimbrite (Middle Tuff Horizon) consists of a dacitic to
rhyolitic pyroclastic sequence with a lower welded and an upper mixed scoria-pumice facies.
This formation cam be recognized throughout the Bükkalja volcanic field (Czuppon, 2003). It
shows an upward gradual change of volcanologic, petrographic and geochemical features.
The importance of magma mixing in its genesis was recognized by many researchers (eg.,
Póka et al., 1998; Czuppon et al., 2001; Czuppon, 2003). This study refined the petrogenetic
model of Czuppon (2003) using new in-situ major and trace element analyses of different
glasses, silicate melt inclusions and the mineral phases.
The 13.5 Ma old Harsány Ignimbrite is the youngest volcanic rock in the Bükkalja
volcanic field and belongs to the Upper Tuff Horizon. The peculiarity of this pyroclastic
formation is the occurrence of distinct rhyolitic pumices described previously by Lukács
(2002).
Petrogenetic studies of these rocks could provide valuable data and model for the
evolution of the silicic volcanic rocks in the Pannonian Basin. However, these results of these
case studies could contribute also to our knowledge on the silicic volcanic systems,
worldwide. Results of this study emphasize the open-system processes in the silicic magma
reservoirs, particularly the importance of ascent and intrusions of mafic magmas.
II. Methods
In this study, petrogenesis of the silicic rocks was revealed by combined application
of classic petrographic observations and in-situ major and trace element analytical data of
different juvenile glasses and mineral phases. Investigations of silicate melt inclusions
trapped in various minerals as well as interpretation of bulk rock compositional data were
also an important tool to describe the processes in the magma reservoirs. Based on these data
quantitative model calculations helped to reveal the closed and open system petrogenetic
processes in more detail.
The bulk rock analyses were performed by different analytical methods such as X-ray
fluorescent spectrometry (XRF), ICP-atomic emission spectrometry (ICP-AES), ICP-mass
spectrometry (ICP-MS) and instrumental neutron activation analysis (INAA). The XRF
analyses were done at the Department für Litosphärenforschung, Universität Wien, Austria
using a Philips PW2400 X-ray fluorescence spectrometer on fused glass discs (for major
elements) and pressed powder pellets (for trace elements). In addition, XRF data obtained by
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Sz. Harangi during the Royal Society Fellowship at the Royal Holloway College (RHUL),
Egham, U.K. were also used in this study.
Some of the trace elements of pumices and lithic clasts were determined by INAA at
the Nuclear Institute of the Technical University Budapest and by ICP-AES at the Royal
Holloway College (RHUL), Egham, U.K. Further bulk rocks analyses were carried out in the
AcmeLabs Ltd. (http://acmelab.com/), Vancouver, Canada by ICP-MS.
The major element composition of the glasses (glass shards, pumices and fiamme)
and the main mineral phases was determined using a CAMECA SX100 electron microprobe
at the Department für Litosphärenforschung, Universität Wien, Austria. In situ trace element
analysis of glasses, silicate melt inclusions and a couple of mineral phases was performed by
LA-ICP-MS using a 193 nm ArF excimer laser ablation system (MicroLas GeoLas 200Q) in
combination with a quadrupole ICP-MS (Micromass Platform ICP) at Utrecht University.
III. Results (theses)
1. Based on an integrated textural and geochemical investigations of various mineral phases
and glasses petrogenesis of three representative silicic ignimbrite occurrences from the so
called Lower, Middle and Upper Tuff Horizons of the Bükkalja volcanic field, were
reconstructed. Application of in situ geochemical investigations of glasses including the
silicate melt inclusions trapped in mineral phases combined with quantitative model
calculations was a novel research on silicic volcanic system. Several hundreds of mineral
and glass compositional data were produced. This data set can contribute to the general
geochemical characterisation of the volcanic formations of this area.
2. Studies of cognate lithic clasts in silicic ignimbrites could be crucial in order to reveal the
petrogenetic processes in the shallow magma reservoir. A detailed magma reservoir model
was produced based on the combined petrographic and geochemical investigation of the
host ignimbrite and the lithic clast of the Szomolya ignimbrite, Lower Tuff Horizon,
southern Bükkalja. In this model open-system processes played an important role including
in situ crystallization, repeated magma replenishments and crystal mush remobilization and
mixing of various crystal phases.
3. Mantle-derived mafic magmas often with fairly primitive composition (mg-number>0.6)
were shown to played a major role in the evolution of the silicic magma reservoir of the
Szomolya ignimbrite. Thus, it was the first firm indication that the Early Miocene silicic
magmas of the Bükkalja region were generated by mixing of mantle derived and crustal
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magmas and not by pure crustal anatexis as was proposed by many researchers. As a
working hypothesis, it is suggested that the Miocene silicic volcanism of Bükkalja could
belong to the intracontinental bimodal basalt-rhyolite style volcanic activities.
4. In the Bogács ignimbrite, which belongs to the Middle Tuff Horizon, several groups of
juvenile glasses were determined. They represent distinct melts variously interacted with
each other in the magma reservoir. Using in situ major and trace element data a quantitative
petrogenetic model was given for their genetic relationships. The previous petrogenetic
model proposed by Czuppon (2003) were refined and partly modified setting the complex
processes in a magma reservoir model where open system processes occur within an
extended crystal mush zone. Results of the silicate melt inclusions indicate that the mineral
phases particularly the orthopyroxenes in the less differentiated scoria clasts are entirely
xenocrysts, i.e. they were incorporated by the ascending dacitic melt from the mush zone.
5. In the Harsány ignimbrite (Upper Tuff Horizon) two distinct rhyolitic magmas were
recognized based on the bulk rock composition of the pumices. Results of quantitative
model calculations indicate that fractional crystallization could not result in the distinct
compositions, thus syn-eruptive mingling of rhyolitic melts coming from spatially separated
melt lenses was suggested.
6. Based on the in-situ (LA-ICP-MS) trace element data of a couple of mineral phases
(orthopyroxene, Na-rich plagioclase and zircon) and the accompanying glass phases,
mineral-melt distribution coefficients were determined. These values could be more precise
than the previous data (Mahood and Hildreth, 1983; Bacon and Druitt, 1988) calculated
from the composition of separated mineral and groundmass phases. Thus, they could be
more effectively used in the petrogenetic studied of silicic magmatic systems.
7. An important outcome of this study was that bulk rock composition of igneous rocks does
not necessarily indicate a magma, which played a role in the magma evolution. The
“andesitic” major element composition of lithic clasts of the Szomolya ignimbrite and the
juvenile clasts of the Bogács ignimbrite reflects complex mixing of crystals and melt
fractions with different origins rather than the existence of an andesitic magma. They can be
interpreted as consisting of mineral phases partly derived from mafic magmas and daciticrhyolitic residual melts. In this regard, origin of the pyroclastic rocks of the Tar Dacite Tuff
Formation should be also reinterpreted. They could not represent eruption of dacitic magma
as proposed by many researchers, but belong to a silicic magma system with a bimodal
mafic-silicic character.
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8. Silicate melt inclusions trapped in different mineral phases of ignimbrites and lithic clasts
could provide important information for the magma evolution. However, as this study
revealed, post-entrapment modification of their composition should be considered in the
genetic interpretation of the obtained geochemical data. Interpretation of the origin of the
host mineral, i.e. phenocrystic, antecrystic or xenocrystic is crucial. Results of quantitative
model calculations imply up to 30% post-entrapment crystallization in the plagioclase
hosted melt inclusions.
9. Results of quantitative petrogenetic model calculations for each silicic ignimbrites suggest
that the trace element content of silicic magmas is primarily controlled by the crystallization
of accessory minerals such as allanite and zircon even in very low – a few tens or hundreds
percent – amounts.
10. The petrogenetic models proposed in this research are consistent with the new views for
the silicic magma reservoirs put forward just recently by many authors (Hildreth, 2004;
Bachmann és Bergantz, 2004; 2008; Hildreth és Wilson, 2007) rather than the traditional
magma chamber models. Results of our studies could reveal some important processes
taking place in the silicic magma reservoirs. It is shown that evolution of silicic magmas is
quite complex involving repeated magma recharge, magma mixing and hybridization,
mixing of crystal mushes, in situ crystallization and most importantly ascent and intrusion
of mantle-derived mafic magmas.
IV. Conclusions
Detailed petrogenetic study of three silicic ignimbrites with different age and
compositional features could reveal the complex processes occurred in the shallow magma
reservoir. This implies that fairly different processes could take place even in a single
volcanic field. Recently, there is a change of paradigm in the understanding of silicic magma
systems. Interpretation of our combined petrographic and geochemical data is more
consistent with the new views on the silicic magma reservoirs and could help to reveal the
processes occurred in them in more detail. The wide variation of the magma reservoir
processes revealed in this study and the particularly fresh preservation of the silicic
pyroclastic rocks implies that the Bükkalja volcanic field could act as a natural laboratory in
such investigations.
There are a couple of important questions coming out from this research that should
be resolved during further studies. One of these is the possible interaction of silicic magmas
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from the Lower, Middle and Upper Tuff Horizons in the Bükkalja volcanic field, i.e. could a
new magma batch remobilize the older crystal mush and crystal sponge zones? A further
important question is the interpretation of chemical composition of igneous rock, particularly
whether andesitic composition does indeed reflect the existence of andesitic magmas in the
magma evolution? Resolving of such questions should involve integrated petrographic and in
situ geochemical investigations of mineral and galssy fractions of the igneous rocks.
V. References
Bachmann, O., Bergantz, G. (2008): The magma reservoirs that feed supereruptions. –
Elements, 4, pp. 17-21.
Bacon, C.R., Druitt, T.H. (1988): Compositional evolution of the zoned calc-alkaline magma
chamber of Mount Mazama, Crater Lake, Oregon. – Contributions to Mineralogy and
Petrology, 98, pp. 224-256.
Czuppon, G., Harangi, Sz., Ntaflos, T. Lukács, R., Szabó, Cs. Koller, F. (2001): Mixed
andesite-rhyolite ignimbrite from the Miocene Bükkalja Ignimbrite Volcanic Field,
Northern Hungary: evidence for magma mixing. – Mitteilungen der Österreichischen
Mineralogischen Gesellschaft, 146, pp. 61-63.
Czuppon, G. (2003): A bükkaljai Középsõ Ignimbrit Egység kőzetsorozatának eredete:
magmakeveredés egy rétegzett magmakamrában. – Diplomadolgozat. ELTE TTK
Kőzettani és Geokémiai Tanszék, Budapest, 114 pp.
Harangi, Sz., Lenkey, L. (2007): Genesis of the Neogene to Quaternary volcanism in the
Carpathian-Pannonian Region: role of subduction, extension and mantle plume. –
Geological Society of America Special Paper 418, 67-92.
Hildreth, W., Wilson, C.J.N. (2007): Compositional Zoning of the Bishop Tuff. – J.
Petrology, 48, pp. 951-999.
Lukács, R., Czuppon, Gy., Harangi, Sz., Szabó, Cs., Ntaflos, T., Koller, F (2002): Silicate
melt inclusions in ignimbrites, Bükkalja Volcanic Field, Northern Hungary – texture and
geochemistry. – Acta Geologica Hungarica, Vol. 45/4, 319-336.
Mahood, G., Hildreth, W. (1983): Large partition coefficients for trace elements in high-silica
rhyolites. – Geochimica et Cosmochimica Acta, 47, pp. 11-30.
Márton, E., Pécskay, Z. (1998): Complex evaluation of paleomagnetic and K/Ar isotope data
of the Miocene ignimbritic volcanics in the Bükk Foreland, Hungary. – Acta
Geologica Hungarica, 41, pp. 467-476.
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Póka, T., Zelenka, T., Szakács, A., Seghedi, I., Nagy, G., Simonits, A. (1998): Petrology and
geochemistry of the Miocene acidic explosive volcanism of the Bükk Foreland;
Pannonian Basin, Hungary. – Acta Geologica Hungarica, 41, pp. 437-466.
VI. Publications related to the Ph.D. dissertation
Publications in peer-reviewed journals:
1. Lukács, R., Czuppon, Gy., Harangi, Sz., Szabó, Cs., Ntaflos, T., Koller, F (2002): Silicate
melt inclusions in ignimbrites, Bükkalja Volcanic Field, Northern Hungary – texture and
geochemistry. –Acta Geologica Hungarica, Vol. 45/4, 319-336.
2. Lukács, R., Harangi, Sz., Ntaflos, T., Mason, P.R.D. (2005): Silicate melt inclusions in the
phenocrysts of the Szomolya Ignimbrite, Bükkalja Volcanic Field (Northern Hungary):
implications for the magma chamber processes. Chemical Geology, 223, 46-67. IF: 2.33
3. Harangi, Sz., Mason, P.R.D., Lukács, R. (2005): Correlation of silicic pyroclastic rocks in
the Northern Pannonian Basin, Eastern-Central Europe: a geochemical approach. Journal
of Volcanology and Geothermal Research, 143, 237-257. IF: 1.457
4. Lukács, R., Harangi, Sz., Ntaflos, T., Koller, F., Pécskay, Z. (2007): A Bükkalján
megjelenő felső riolittufaszint vizsgálati eredményei: a harsányi ignimbrit egység. –
Földtani Közlöny, 137/4, 487-514.
In press publications in peer-reviewed journals:
1. Lukács, R., Harangi, Sz., Mason, P.R.D., Ntaflos, T.: Bimodal pumice populations in the
13.5 Ma Harsány ignimbrite, Bükkalja Volcanic Field, Northern Hungary: syn-eruptive
mingling of distinct rhyolitic magma batches? – Central European Geology, 2009.
2. Harangi, Sz. és Lukács, R.: On the age of the Harsány ignimbrite, Bükkalja volcanic field,
Northern Hungary – a discussion. – Central European Geology, 2009.
3. Lukács, R., Harangi, Sz., Radócz, Gy., Kádár, M., Pécskay, Z., Ntaflos, T.: A
Nyékládháza-1, Miskolc-7 és Miskolc-8 sz. fúrások miocén vulkáni kőzetei és
párhuzamosításuk a Bükkalja vulkáni képződményeivel. – Földtani Közlöny, 2010.
Extended abstracts:
1. Lukács, R., Czuppon, Gy., Ntaflos, T., Harangi, Sz, Koller, F., Szabó, Cs. (2001): Silicate
Melt Inclusions in the mineral phases of the Bükkalja Ignimbrite Volcanic Field, Nothern
Hungary. F. Noronha, A. Doria, A. Guedes (eds): ECROFI XVI, Porto, Extended
abstracts, 275-277.
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2. Lukács, R., Harangi, Sz., Oláh, I. (2001): Correlation of Miocene pumice-bearing
pyroclastic deposits in the Pannonian Basin. Mitteilungen der Österreichischen
Mineralogischen Gesellschaft, 146, ÖMG MinPet 2001 Conference, Vienna, Austria,
180-182.
3. Czuppon, Gy., Harangi, Sz., Ntaflos, T., Lukács, R., Szabó, Cs., Koller, F: (2001): Mixed
andesite-ryolite ignimbrite from the Miocene Bükkalja Ignimbrite Volcanic Field,
Northern Hungary: evidence for magma mixing. – Mitteilungen der Österreichischen
Mineralogischen Gesellschaft, 146, ÖMG MinPet 2001 Conference, Vienna, Austria, 6163.
4. Lukács, R., Harangi, Sz. (2002): Petrogeneis of the Miocene silicic magmas in the
Pannonian Basin – a case study in the Eastern Bükkalja Volcanic Field, Northern
Hungary. – Geologica Carpathica, Vol.53, Special Issue, pp.13-14.
5. Lukács, R., Czuppon, Gy., Harangi, Sz., Szabó, Cs., Ntaflos, Th., Koller, F. (2002):
Silicate melt inclusions in ignimbrites of the Bükkalja Volcanic Field, Northern Hungary.
Geochemical and Geophysical Monitoring, Melt inclusions: methods, applications and
problems, Proceedings, pp. 149-152.
6. Harangi, Sz., Lukács, R., Czuppon, Gy., Szabó, Cs. (2002): Magma mixing in a
compositionally layered magma chamber: a silicate melt inclusion study. – WorkshopShort Course on Volcanic Systems, Geochemical and Geophysical Monitoring, Melt
inclusions: methods, applications and problems, Proceedings, pp. 101-106.
Selected abstracts:
1. Lukács, R., Czuppon, Gy., Ntaflos, T., Harangi, Sz, Koller, F., Szabó, Cs. (2000): Melt
inclusions in the Miocene silicic pyroclastic series of the Bükkalja Ignimbrite Volcanic
Field, Northern Hungary: preliminary results. – EUG XI, Abstract Volume(European
Union of Geosciences), Strasbourg, MS03, p. 443.
2. Lukács, R., Harangi, Sz. (2001): Geochemistry of Miocene pyroclastic deposits in the
Pannonian Basin: Correlation and genesis of silicic volcanic units in the Bükkalja
Ignimbrite Volcanic Field. – PANCARDI 2001 Abstract Volume, PO-8.
3. Czuppon, Gy., Harangi, Sz., Ntaflos, T., Lukács, R., Szabó, Cs., Koller, F: (2001): Scoria
and pumice-bearing ignimbrite from the Miocene Bükkalja Ignimbrite Volcanic Field,
Northern Hungary: evidences for magma mixing. - PANCARDI 2001 Abstract Volume,
PP-3.
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4. Lukács, R., Harangi, Sz. (2003): Mingling of rhyolitic melts in the Bükkalja Volcanic
Field, Northern Hungary – EGS-AGU-EUG Joint Assembly 2003, Nice, Abstract Volume.
5. Lukács, R., Libowitzky, E., Harangi, Sz., Ntaflos, T. (2003): Magma chamber processes
in the miocene silicic pyroclastic suites of the Bükkalja Volcanic Field (Northern
Hungary) revealed by silicate melt inclusions in lithic clasts. – Acta MineralogicaPetrographica abstract series, Vol. 2. p. 118.
6. Lukács, R. (2004): Magmakamra folyamatok vizsgálata szilikátolvadék-zárványok
segítségével. – XXXV. Ifjú Szakemberek Ankétja, Sárospatak, 2004., p.32-33.
7. Lukács R., Harangi, Sz., Ntaflos, T., Mason, P. (2004): Magma chamber processes
revealed by silicate melt inclusions hosted by phenocrysts of the Szomolya Ignimbrite,
Bükkalja Volcanic Field. – Supplement to Geochimica et Cosmochimica Acta, 5.4.PO4,
p.A651.
8. Harangi, Sz., Czuppon, Gy., Lukács, R., Ntaflos, T. (2004): Magma mixing in a
compositionally layered magma chamber: Inferences from silicate melt inclusions. –
Supplement to Geochimica et Cosmochimica Acta, 5.4.62, p.A645.
9. Lukács, R., Harangi, Sz., Gméling, K., Ntaflos, T., Czuppon, Gy. (2004): Petrogenesis of
the Miocene silicic volcanic rocks in the Bükkalja Volcanic Field, Northern Hungary. –
32nd IGC Florence 2004, Abstract Volume, 280-10 Oral, p.1240
10. Lukács R. (2006): Tephrostratigraphy in the Carpathian-Pannonian Basin: Mineral
chemical constraints . – Acta Mineralogica-Petrographica, Abstract Series, Vol. 5., p. 67.
Popular science articles:
1. Harangi Sz., Lukács R. (2008): Pusztító vulkánkitörésektől a borospincékig. Élet és
Tudomány, 2008/36, 1136-1138.
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