neogene calc-alkaline magmatism in oas-gutai mts ...

NEOGENE CALC-ALKALINE MAGMATISM IN OAS-GUTAI MTS., EASTERN CARPATHIANS, ROMANIA. TIME AND SPACE EVOLUTION Marinel Kovacs1, Alexandrina Fülöp1, Zoltan Pécskay2 1.

North University Baia Mare, Romania; 2 Institute of Nuclear Research, Debrecen, Hungary

Abstract. Oas-Gutai Mts. had been built up during Neogene, as a consequence of subduction processes developed in the CarpathianPannonian Region. The felsic calc-alkaline volcanism (Badenian-Early Sarmatian) was explosive and besides the rhyolitic ignimbrites from Gutai Mts. (15.4 Ma), most of the released pyroclastics underwent submarine reworking. The intermediate calc-alkaline volcanism from Oas Mts. ((12.0-9.5 Ma) was mainly extrusive and submarine; dome-like structures are predominat, associated with both nonexplosive and explosive volcaniclastics. The intermediate calc-alkaline volcanism from Gutai Mts. (13.4-7.0 Ma) was very complex, with an associated intrusive activity which ceased with a mafic phase (CA basalts); thick piles of effusive volcano-tectonic fillings coexist with dome structures, effusive cones and intrusions. Lavas (from basalts to rhyolites), associated volcaniclastics of both explosive and non-explosive origin, primary and reworked, suggesting both submarine and subaerial evolution, are overlapped hiding the original volcanic centres and morphologies. The volcanism developed continuously, migrating parallel to the volcanic front of the subduction zone. It was contemporaneous with the volcanism from Tokay Mts., Vihorlat Mts. and Transcarpathia. Keywords: calc-alkaline, magmatism, geochronology, andesite, Neogene, dome

Introduction Oas-Gutai Mts. belong to the Eastern Carpathian Neogene-Quaternary volcanic chain (Fig. 1). A calcalkaline magmatism developed in the area during the Neogene, related to the complex processes involving the subduction of the European Plate beneath Alcapa and Tisza-Dacia/Tisia microplates and the hinterland extensional reply in the Pannonian Basin (Csontos, 1995, Seghedi et al, 1998). Two types of magmatism took place in Oas-Gutai Mts.: a felsic calc-alkaline “back-arc type”, consisting of rhyolitic volcaniclastics of explosive origin and an intermediate calc-alkaline “arc-type”, represented by typical calc-alkaline series of rocks with a final mafic, basaltic phase. A main intrusive magmatism is related to the intermediate volcanism. The intermediate calc-alkaline volcanism developed in the entire area of the Oas-Gutai Mts., as submarine and subaerial events. The magmatic rocks are often in connection with sedimentary deposits (Paleogene, Neogene and Quaternary). The succession in time of the volcanic rocks has been established based on the relationships with the sedimentary deposits biostratigrafically dated and on the spatial relationships of volcanic deposits. The radiometric datings on the most important types of rocks, volcanic and intrusive (more than 120 K-Ar determinations), correlated with the biostratigraphical data, enable the approach of the magmatic evolution in time and space in the area of Oas-Gutai Mts. Volcanological and petrological data The felsic calc-alkaline volcanism has been recorded in Oas and Gutai Mts. Magmatic explosive volcanism developed in both areas. Boreholes data from Oas Mts. show that a small scale felsic explosive volcanism is represented by volcaniclastics of pyroclastic origin. None

of the volcaniclastics, mostly interbedded with submarine mudstone, suggest primary deposits. The submarine reworking by mass flow transport is responsible for the

Fig. 1 Location of Oas-Gutai Mts. on the sketch of CarpathianPannonian Region. 1. Intracarpathian units belonging to the Alpine Orogen; 2. Carpathians flysch belt; 3. Neogene volcanic rocks; 4. Subduction.

median to distal facies of water-supported pyroclastic flows (Fülöp, Crihan, 2002). A large scale explosive volcanism developed in Gutai Mts. Magmatic explosions and caldera collapse released large amounts of welded ignimbrites (Fülöp, 2003). Ongoing volcanic activity, by minor explosive events from unknown sources, provided large amounts of pyroclastics emplaced locally, in submarine subbasins entrained by volcanic subsidence. Most of the volcaniclastics show mass flow or mass movement transport mechanism beneath water, by one ore more episodes of submarine reworking (Fülöp, 2004). The components are pyroclastic: pumice clasts/fiame, crystals of plagioclase, quartz, biotite and pyroxene (very scarce) glass shards, cognate pyroclasts as well as

accidental lithic clasts (sedimentary, metamorphic and igneous). Recent geochemical study on the ignimbrites from Gutai Mts. (Fülöp, Kovacs, 2003) has emphasized the rhyolitic composition, the calc-alkaline rhyolitic character of their parental magmas and geochemical signatures ressembling subduction zone-related volcanics. The intermediate calc-alkaline volcanism is well represented in both Oas and Gutai Mts. Oas Mts. show a well-defined extrusive phase with lava domes, dome-coulees and scarce effusive cones developed mostly subaqueously (Fig. 2). The subsequent co-genetic fragmentation processes had a major contribution in the geological record of the mountains: in situ and resedimented hyaloclastites and associated explosive volcaniclastics, mostly phreatomagmatic, are frequent deposits (Kovacs, Fülöp, 2002). The volcanic structures are less obvious in Gutai Mts. The original morphologies are not preserved, excepting the lava domes, dome-coulees and complexes of domes. Different sources were simultaneously active and the geometry of the structures is largely controlled by composition and rheology, volumes and setting. It is difficult to outline the eruptive centres. Most of the volcanic activity started subaqueously, along tectonovolcanic alignments responsible for the onset of volcanism and for the subsidence and entrapment of the volcanic piles. Therefore, lava flows associated with specific volcaniclastics and sedimentary deposits form thick successions filling volcano-tectonic depressions (Fig. 2). Effusive cones are scarce and the distinction between subaqueous and subaerial activity is difficult to approach. The co-genetic volcaniclastics show typical quench and phreatomagmatic fragmentation in subaqueous setting and

autobrecciation and magmatic explosive disruption in terrestrial setting. Such processes are related to both lava domes and lava flows. The petrography of the extrusive rocks belonging to the intermediate volcanism shows a large variety of petrotypes, from basalts to rhyolites. Pyroxene andesites/basaltic andesites are the predominant rocks. Many of the dacitic rocks have an exclusive pyroxene character (e.g. Oas Mts.) and they are the main rocks of the extrusive domes. The rhyolites are scarce, they form only three dome structures. Porphyritic textures of pyroxene andesites and microdiorites are predominant in the shallow level intravolcanic intrusions. The subvolcanic intrusions consist of a gabbroes-microgranodiorites series of rocks (pyroxene quartz diorites and quartz monzodiorites being predominant). The basaltic rocks belonging to the mafic phase are represented by aphyric basalts. Many mineralogical and petrographical peculiarities, suggesting mixing and mingling processes in the petrogenesis of the intermediate volcanics, have been outlined (Kovacs, 2002; Kovacs, Fülöp, 2003). The calc-alkaline predominant medium-K character and typical subduction zones geochemical signatures (LILE and LREE enrichments and HFSE depletions, negative Eu anomaly, negative correlation between 87/ Sr/86Sr and 143Nd/144Nd ratios) have been emphasized for intermediate, arc–type volcanism (Kovacs, 2002; Kovacs, Fülöp, 2003). Geochemical and isotopic data assert a significant crustal assimilation and a depleted MORB type mantle wedge, as the mantle source for Oas-Gutai arc-type magmas.

Fig. 2 Volcanological map of Oas-Gutai Mts.1.Effusive cones; 2. Extrusive domes; 3. Intrusions; 4. Volcano-tectonic depression effusive filling; 5. Lava flows (unknown centres); 6. Hyaloclastic deposits; 7. Ignimbrites, pyroclastic flow deposits; 8. Fine, primary and/or reworked tuffs; 9. Coarse pyroclastic deposits; 10. Coarse epiclastic deposits; 11. Medium and fine epiclastic deposits; 12. Quaternary; 13. Neogene sedimentary deposits; 14. Oligocen-Miocene sedimentary deposits; 15. Paleogene sedimentary deposits; 16. Faults; 17. Overthrusts.

Time-space distribution of the magmatism Oas Mts. The felsic calc-alkaline volcaniclastics buried in the north-central part of the mountains have been dated on biostratigraphical data, as Late Badenian-Early Middle Sarmatian (Fülöp, Crihan, 2002). The intermediate calc-alkaline volcanism had started during Sarmatian, when some acidic pyroclastic flows emplaced in the central part of the area (11.9 ±0.7 Ma, Kovacs et al, 1997a). The main magmatic activity had taken place during a short time interval, in Pannonian. The eldest volcanic products belong to an andesitic phase developed in the north-western part of Oas Mts., probably at the Sarmatian-Pannonian boundary (11.8±0.6 Ma). A first rhyolitic phase developed in the southernmost part of the area (an isolated dome complex of 11.0 Ma age) had been followed by a major paroxistic volcanic phase building up the great bulk of Oas Mts. volcanics. It consists of andesitic and dacitic volcanics belonging to different structures, spatially and temporarily connected and often overlapped. Andesitic lava flows and associated hyaloclastic breccias filled volcano-tectonic depressions and some effusive cones evolved during a very short time interval (10.9-10.5 Ma) in the central part of the area and in the eastern and northern part, along the RomanianUkrainian boundary. The main extrusive phase – extrusive domes, dome-coulees and complexes of domes with associated volcaniclastics consisting of dacites, quartz bearing andesites and hyalodacites/hyaloandesites- had taken place during Early Pannonian (10.9-9.5 Ma) all over the area. Many of these volcanic structures are domes surrounded by Pannonian and Quaternary sedimentary deposits, specially in the central-western part of the mountains.

The intrusive magmatism was contemporaneous with the paroxistic volcanic phase (10.8-9.6 Ma, Fig. 4) (Kovacs et al., 1997a). Gutai Mts. The felsic calc-alkaline volcanism had started 15.4 Ma ago in the south-western corner of the mountains and had taken place during Badenian and Early Sarmatian (Fülöp, 2003). Some other Badenian and Early Sarmatian acidic volcaniclastics developed in the south-eastern part of the area. The intermediate calc-alkaline magmatism had taken place during Sarmatian and Pannonian (13.4-7.0 Ma, fig. 4). An important distinct Sarmatian volcanic activity took place in Gutai Mts. It started in the south-eastern part of the area, by submarine andesitic lava flows and a dacitic extrusive dome (13.4-13.2 Ma, Edelstein et al, 1992, Pécskay et al., 1994) and continued with the main andesitic phase developed in the south-western part of Gutai Mts. (13.1-12.1 Ma) (Fig. 3).

Fig. 4 Geochronological evolution of the Neogene magmatism from Oas-Gutai Mts.; 1. volcaniclastics; 2. lava flows; 3. intrusions. Fig. 3 Simplified geochronological map of Oas-Gutai Mts. 1. Quaternary; 2. Neogene sedimentary deposits; 3. Paleogene sedimentary deposits; 4. Pannonian volcanics; 5. Sarmatian volcanics; 6. Badenian volcanics.

The main intermediate volcanic activity had taken place during Pannonian, when a very complex and continuous volcanism developed (Fig. 3). A first acidic phase emplacing dacitic to rhyolitic domes developed in the southern part of Gutai Mts. (11.6±0,5 Ma is the K-Ar age of the Danesti biotite dacitic/rhyolitic dome, Pécskay et al., 1994). It was followed by some andesitic phases developed everywhere in the mountains as the climax of

the volcanic activity (11.5-9.1 Ma). These andesitic phases evolved separately but they were contemporaneous. In the central-western area, the quartz bearing andesite lava flows, lava domes and volcaniclastics had been emplaced (11.5-10.5 Ma), whereas in the eastern and central part, pyroxene-amphibole lava flows and volcaniclastics filling volcano-tectonic depressions had developed during 10.99.9 Ma. The major paroxistic volcanic phase had taken place during 10.6-9.0 Ma in the northern part of the volcanic area, when the andesitic products of different effusive complexes overlapped. Some isolated biotite andesite and dacite domes represent the last events of these paroxistic phase (9.3-9.0 Ma, Pécskay et al., 1994, 1995). The intrusive magmatism from Gutai Mts. had taken place during 11.9-7.0 Ma (Fig. 4). The main intrusive magmatic activity, consisting of very different petrographic rock types, developed in the entire area, in connection with the Sarmatian and Pannonian sedimentary deposits and volcanics (11.9-9.6 Ma, Kovacs et al., 1997b). An outstanding intrusive complex (biotite dacites with a biotite andesites envelope) had been emplaced in the central southern part of Gutai Mts., during 8.5-8.0 Ma (Kovacs, 2002). A late intrusive phase, a mafic one, consisting of calc-alkaline high Al basalts, ceased the magmatic activity from Gutai Mts. (8.1-7.0 Ma, Edelstein et al., 1993) ( Fig. 4). Conclusions Two types of magmatism had developed in Oas-Gutai Mts. during the Middle Miocene: a felsic calc-alkaline “back-arc type” of explosive origin and an intermediate calc-alkaline “arc-type” of extrusive and intrusive origin. Typical subduction zones geochemical signatures have been outlined for the volcanic rocks of the both types of volcanism. There was a complex volcanic activity in the area of Oas-Gutai Mts.: acidic phases (dacites-rhyolites) and intermediate phases (basaltic andesites-andesites) developed successively, laterally overlapping their products. The intermediate arc-type magmatism had developed during a longer time interval (13.4-7.0 Ma) in Gutai Mts., comparative with Oas Mts. (12.0-9.5 Ma). The main magmatic activity in Oas-Gutai Mts. ceased around 9.0 Ma, the paroxistic phase developing during 11.0-9.0 Ma. The main intrusive magmatic activity was contemporaneous with the climax of the volcanic activity from Oas-Gutai Mts. and with the intrusive magmatism from Poiana Botizei and Tibles-Toroiaga-Rodna-Bargau subvolcanic units. In Gutai Mts., there is a time-space migration of the volcanism, parallel to the volcanic front of the subduction zone. The magmatism from Oas-Gutai Mts. was contemporaneous with the magmatism from Tokay Mts., Vihorlat Mts. and Transcarpathia but the paroxysm of volcanism was younger.

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