40Ar/39Ar and U-Pb geochronology of the Iran Tepe volcanic complex, Eastern Rhodopes

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Article

Reference

40Ar/39Ar and U-Pb geochronology of the Iran Tepe volcanic complex, Eastern Rhodopes

MARCHEV, Peter, et al.

Abstract

The Iran Tepe volcanic complex occurs in the south-eastern part of the Eastern Rhodope massif. The rocks are represented by calc-alkaline and high-K calc-alkaline basaltic andesite to dacite epiclastics, lava flows and dikes, which are crosscut by andesitic and latitic dikes and rhyolitic dykes from the Planinets dyke swarm. Stratigraphic data and existing K/Ar ages suggest that the Iran Tepe volcanic complex is Upper Eocene (35-39 Ma), and is one of the oldest volcanic structures in the Eastern Rhodopes. However, new 40Ar/39Ar laser fusion and incremental step-heating experiments on biotites and isotope dilution – thermal ionization mass spectrometry (ID-TIMS) U-Pb age data on single zircons from the bottom and top lava flows and dykes more precisely constrain the ages and time span of volcanic activity, and show that the volcanism is younger. Volcanic activity started with calc-alkaline andesites and dacites at the beginning of the Oligocene (~33.9 Ma) and culminated with the intrusion of latitic dykes at ~33.0 Ma. Rhyolites from the Planinets dyke swarm yield a similar age (32.8 Ma), but their genetic relationship [...]

MARCHEV, Peter, et al . 40Ar/39Ar and U-Pb geochronology of the Iran Tepe volcanic complex, Eastern Rhodopes. Geologica Balcanica , 2010, vol. 39, no. 3, p. 3-12

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http://archive-ouverte.unige.ch/unige:23835

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GEOLOGICA BALCANICA, 39. 3, Sofia, Dec. 2010, p. 3–12.

40

Ar/

³⁹

Ar and U-Pb geochronology of the Iran Tepe volcanic complex, Eastern Rhodopes

Peter Marchev

¹

, Peter Kibarov

¹

, Richard Spikings

²

, Maria Ovtcharova

²

, István Márton

²

, Robert Moritz

²

1 Geological Institute of the Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria (e-mail: [email protected])

2 Section des Sciences de la Terre et de l’Environnement, Université de Genève, Rue de Maraîchers 13, 1205 Genève, Switzerland

(Accepted for publication: November 2010)

Abstract. The Iran Tepe volcanic complex occurs in the south-eastern part of the Eastern Rhodope massif.

The rocks are represented by calc-alkaline and high-K calc-alkaline basaltic andesite to dacite epiclastics, lava flows and dikes, which are crosscut by andesitic and latitic dikes and rhyolitic dykes from the Planinets dyke swarm. Stratigraphic data and existing K/Ar ages suggest that the Iran Tepe volcanic complex is Upper Eocene (35-39 Ma), and is one of the oldest volcanic structures in the Eastern Rhodopes. However, new

40Ar/³⁹Ar laser fusion and incremental step-heating experiments on biotites and isotope dilution – thermal ionization mass spectrometry (ID-TIMS) U-Pb age data on single zircons from the bottom and top lava flows and dykes more precisely constrain the ages and time span of volcanic activity, and show that the volcanism is younger. Volcanic activity started with calc-alkaline andesites and dacites at the beginning of the Oligocene (~33.9 Ma) and culminated with the intrusion of latitic dykes at ~33.0 Ma. Rhyolites from the Planinets dyke swarm yield a similar age (32.8 Ma), but their genetic relationship with the more mafic Iran Tepe lavas re- mains unclear.

Marchev, P., Kibarov, P., Spikings, R., Ovtcharova, M., Márton, I., Moritz, R. 2010. ⁴⁰Ar/³⁹Ar and U-Pb geochronology of the Iran Tepe volcanic complex, Eastern Rhodopes. Geologica Balcanica 39(3), 3–12.

Keywords: Eastern Rhodopes, Oligocene volcanism, ⁴⁰Ar/³⁹Ar and U-Pb ages.

INTRODUCTION

Stratigraphic relationships at the Iran Tepe volcanic complex suggest that it marks the onset of Tertiary volcanic activity in the south-eastern part of the Rhodope Massif (Ivanov, 1960; Goranov et al., 1995).

Conventional K/Ar data of Iran Tepe volcanic rocks yield older ages than other paleo-volcanoes in the Eastern Rhodopes. However, the accuracy of the data is not estimated and it is likely that the K/Ar ages cannot be used to precisely constrain the timing of volcanic activity at Iran Tepe. For example, two K/Ar age determinations of lavas from the southern part of the complex yielded Priabonian ages of 36.5 and 35.0 Ma (Lilov et al., 1987), whereas Milovanov et al. (2003) obtained a K/Ar age of 39 Ma from lava located in the northern part of the complex. These age determinations suggest that the volcano was active for almost

3–4 Ma, which we consider to be unusually long and therefore unrealistic.

We report and discuss new and precise ⁴⁰Ar/³⁹Ar ages from lavas at the base (dacite) and top (andesite) of the volcanic sequence at Iran Tepe, and two late dykes (latitic and rhyolitic) in conjunction with high-precision U-Pb zircon data from the two lavas to determine the age and time span of volcanic activity. Additional data on the age of Iran Tepe and the nearby Ada Tepe ore deposit is published in a companion paper (Márton et al., 2010). The reported ⁴⁰Ar/³⁹Ar ages have similar precisions to ⁴⁰Ar/³⁹Ar ages acquired from other major volcanic areas in the Eastern Rhodopes (Singer and Marchev, 2000; Marchev and Singer, 2002; Moskovski et al., 2004; Georgiev and Marchev, 2005), which collectively provide a more accurate and precise geochronological framework for in- terpreting the timing and duration of volcanism in the Rhodope massif.

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GEOLOGICAL SETTING Regional setting

The Iran Tepe volcanic complex is located within the south-eastern part of the Eastern Rhodope Massif, which is a part of the large Macedonian–Rhodope–North- Aegean magmatic belt (Harkovska et al., 1989; Marchev et al., 2004). This ~500 km long and 130 to 180 km wide arcuate belt (Fig. 1) resulted from the Cretaceous collision of the Serbo-Macedonian and Rhodope Massifs with the Pelagonian microplate, a fragment of the African platform (Ricou, 1994). The collision caused large-scale intra-crustal deformation and thickening of the crust, which was followed by Paleocene-Eocene extension that drove a continuation of strong deformation, rock uplift and exhumation (Bonev et al., submitted). Extension was accompanied by the development of NNW–SSE-trending fault-bounded basins filled with continental clastic rocks and basic, intermediate and acid volcanic rocks (Ivanov, 1960, 1963; Harkovska et al., 1989; Yanev, 1998;

Marchev et al., 2004).

The tectonic pattern of the south-eastern Rhodope massif is dominated by two late-Alpine extensional metamorphic domes, the Kesebir-Kardamos and the Byala reka-Kechros domes, whose high-grade metamorphic rocks are uncomformably overlain by a low-grade Mesozoic unit, and Paleocene-Eocene sedimentary and volcanic units (Bonev, 2006).

District setting

The Iran Tepe volcanic complex is a north-south-trending eroded volcanic massif that is about 7 km wide and 14 km long. Recently, Milovanov et al. (2005) renamed the complex to Kalabach andesitic complex, however, we prefer the name Iran Tepe volcanic complex, which is closer to the original name Iran Tepe volcano, given by Ivanov (1960). The volcanic complex is located north of the Kessebir-Kardamos dome and west of the Biala Reka-Kechros dome in the south-eastern part of the Momchilgrad-Arda Depression (Ivanov, 1960; Boyanov and Goranov, 2001). New geological mapping of the Iran Tepe volcano at 1:25000 is summarized in Figure 1. The basement is represented by pre-Tertiary metamorphic rocks, which crop out south and east of the Iran Tepe volcanic complex. It is overlain by Paleocene (Krumovgrad group) and Upper Priabonian breccias-conglomerate, coal-bearing sandstone and marl-limestone formations (Goranov and Atanasov, 1992). Clasts of volcanic rocks in the uppermost part of the marl-limestone formation mark the onset of volcanism in the area. Volcanic rocks of the complex are covered by a regional tephro-stratigraphic marker, which is composed of rhyodacitic ash- fall and pumice-flow tuff of an unknown source, named the Reseda Tuff (Ivanov and Kopp 1969), dated at 32.44

±0.23 Ma (Marchev and Singer, 2002).

The Iran Tepe volcanic complex is dominated by lava flows and volcanoclastic rocks. The central vent has not been identified, although the large number of W–NW striking dykes and sills along the river Dyushun dere suggest that they might be the remnant of the volcanic

edifice (Fig. 1). These bodies are subparallel to a much larger structure, which is composed of felsic dykes and can be traced from Studen Kladenets to the west, to the village of Planinets in the east, named by Ivanov (1960) as the Planinets tensional zone of felsic rhyolites. In the Iran Tepe area, the acid dykes are 2–4 km displaced to the north from the intermediate dykes. The width of the intermediate dykes varies between less than a meter to several meters and they can be traced for less than 100 m. The lack of stratigraphic markers within the volcano makes stratification of the entire volcanic units difficult and the volcanic units (Fig. 1) are divided according to their spatial location, rather than their age. In general, stratigraphic observations reveal younging of the lavas from SE–E to W–NW. The stratigraphic relationships of dated lava flows and dykes, however, are clear.

The volcanic rocks from Iran Tepe form a suite of calc- alkaline and high-K basaltic andesites, andesites and dacites, with the youngest dyke yielding latitic compositions (Harkovska et al., 1989; Nedialkov and Pe-Piper, 1998;

Kibarov et al., 2007; Kibarov, 2008). The most notewor- thy chemical characteristics of the rocks (Kibarov, 2008) are the distinctive negative anomalies of Nb, Ta and Ti, which are typical for orogenic calc-alkaline magmas. The samples are also enriched in large-ion lithophile elements (LILE), U, Th and Pb, compared to heavy rare earth elements (HREE) and high field strength elements (HFSE).

The latitic dike is characterized by an overall enrichment of all trace elements and the absence of a negative P anomaly. The rhyolites from the Planinets dyke swarm are high-K and are enriched in Rb, Pb and Th.

GEOCHRONOLOGY

40Ar/³⁹Ar analytical technique

The ⁴⁰Ar/³⁹Ar isotopic analyses were performed at the University of Geneva, Switzerland. Mineral separates of biotite and sanidine were extracted from 100–250 mi- cron sieve fractions using standard magnetic, density, and handpicking methods. The samples were irradiated in the Oregon State University, CLICIT facility, and J values were calculated via the irradiation of Fish Canyon Tuff sanidines, which were separated by distances of <1 cm, throughout the columnar irradiation package. The ⁴⁰Ar/³⁹Ar incremental- heating experiments, consisted of 10–25 individual steps and were conducted on an Argus (GV Instruments), multi- collector mass spectrometer, equipped with four high-gain (10E12 Ohms) Faraday collectors for the analysis of ³⁹Ar,

38Ar, ³⁷Ar and ³⁶Ar, as well as a single Faraday collector (10E11 Ohms) for the analysis of ⁴⁰Ar. ⁴⁰Ar/³⁹Ar analyses follow the procedure outlined in Marshik et al. (2008).

U-Pb isotope analyses

The analytical procedures used for U-Pb dating on zircons analyzed in this study are identical to those described in Ovtcharova et al. (2006). The isotopic analyses were performed at the University of Geneva (Switzerland) on a Thermo Electron Triton mass spectrometer equipped with a linear MasCom electron multiplier, showing no

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Fig. 1. Geological map of the Iran Tepe volcanic complex showing sample locations and their preferred ages. Source: unpublished mapping by P. Marchev and P. Kibarov, 2007–2008

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Table 1 Summary of the zircon U/Pb age data from from bottom and top lava flows of the Iran Tepe volcanic complex a) calculated on the basis of radiogenic Pb208/Pb206 ratios, assuming concordancy b) corrected for fractionation and spike c) corrected for fractionation, spike, blank and common lead (Stacey & Kramers, 1975) d) corrected for initial Th disequilibrium, using an estimated Th/U ratio of 4 for the melt

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second-order non-linearity. Mass fractionation effects were corrected for 0.13 ± 0.03 per a.m.u. for Pb. U mass fractionation is calculated in real-time using a ²³³U–²³⁵U double spike. Both lead and uranium were loaded with 1 µl of silica gel-phosphoric acid mixture (Gerstenberger and Haase, 1997) on outgassed single Re-filaments, and isotope ratios were determined by sequential measure- ment of all ion beams on the electron multiplier. All uncertainties reported are at the 2 sigma level, following x/y/z systematic of Schoene et al. (2006). All data are reported in the Table 1 with internal errors only, marked as [x] – including counting statistics, uncertainties in cor- recting for mass discrimination, and the uncertainty in the common (blank) Pb composition. External or systematic errors [y] is comprising the uncertainty on Pb/U ratio of the mixed Pb-U tracer used in isotope dilution calcula- tions (0.05%, Condon et al. in prep) and [z] – uncertainties in the decay constants of ²³⁵U and ²³⁸U. Intercept ages are given with decay constant uncertainties. Data reduction and age calculation were applied using the algorithms of Ludwig (1980); the concordia plot was generated using the Isoplot/Ex v.3 program of Ludwig (2005).

Cathodoluminescence (CL) images

Cathodoluminescence (CL) images on individual zircon grains were obtained on a Tescan CamScan 2300MV

Scanning Electron Microscope (SEM) at the Institute of Geology and Palaeontology of the University of Lausanne.

Samples and results

The results of individual incremental heating experiments of the ⁴⁰Ar/³⁹Ar experiments are summarized in Table 2 and the plateau ages (expressed at the ± 2σ level) are il- lustrated in Figures 2 and 3. The location of the samples and their ages are shown on the geological map (Fig. 1).

A brief description, the significance of the data and the preferred ages for each sample is provided below.

Dacite PK-9 is from one of the oldest lava flows (Chemersko dacite), directly overlies the basement limestones and is composed of 63.5 wt.% SiO2 and 2.3 wt.%

K2O. The rock is fresh and hosts phenocrysts of plagioclase, orthopyroxene, clinopyroxene, amphibole and biotite. Five of the thirteen ⁴⁰Ar/³⁹Ar heating steps, which represent ~87% of the ³⁹Ar released from the target biotite grains, yield a plateau age of 33.88 ± 0.26 Ma.

Andesite PK20 is from the uppermost Rutlina lava flow and is composed of 57.1 wt.% SiO2 and 2.9 wt.%

K2O. The sample contains phenocrysts of plagioclase, clinopyroxene, orthopyroxene rimmed by clinopyroxene, amphibole, biotite and titanomagnetite, plus zircon and apatite. Six steps on the biotite ⁴⁰Ar/³⁹Ar age spectrum,

Fig. 2. ⁴⁰Ar/³⁹Ar incremental step-heating age spectra derived by CO2–IR laser for biotites from the bottom dacite (PK9) and the andesite top lava flow (PK20) from the Iran Tepe volcanic complex.

Table 2

Summary of the 40Ar/39Ar age data for the Iran Tepe volcanic complex

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which represent ~ 75% of the total 39Ar released, define a plateau age of 33.37 ± 0.29 Ma.

Sample PK24 is a latitic dyke (SiO2 = 60.3 wt.% and K2O = 4.2 wt.%) composed of plagioclase, biotite, amphibole, clinopyroxene and rare orthopyroxene and tita- nium magnetite. A fifteen step⁴⁰Ar/³⁹Ar incremental heating experiment with biotite yielded a three step plateau age of 32.97 ± 0.23 Ma, obtained from ~54% percent of the total ³⁹Ar released.

Sample PK38 is a rhyolitic dyke from the Planinets dyke swarm with 72.9 wt.% SiO2 and 5.8 wt.% K2O. The sample contains phenocrysts of quartz and sanidine, sub- ordinate plagioclase and biotite and rare amphibole and magnetite hosted in a perlitic groundmass. Thirteen of fifteen ⁴⁰Ar/³⁹Ar heating steps of sanidine, which represent 99% of the total ³⁹Ar gas-released, yielded a plateau age of 32.80 ± 0.28 Ma.

The argon isotopic data for each of the samples men- tioned above yielded a non-radiogenic Ar reservoir that is indistinguishable from an atmospheric composition, and inverse isochron ages that are indistinguishable from the plateau ages. However, the applicability of the inverse isochrons is questionable because >90% of ⁴⁰Ar released is radiogenic, precluding a reliable extrapolation of the data towards the non-radiogenic axis.

Zircons extracted from samples PK-9 and PK-20 yield a significant variety of morphologies. We analyzed only zircons with long prismatic to euhedral habits, length of up to 100 µm and slightly pink in color. When imaged by cathodoluminescence, all zircons display oscillatory and sector zoning typical of an igneous origin (Hoskin and Schaltegger, 2003). Most zircons contain old inherited cores and are full of inclusions. Nevertheless, some long-prismatic zircons with simple oscillatory zoning and without inheritance were observed in sample PK-9, and similar were analyzed by TIMS.

Sample PK-9 yielded a single concordant point at 33.24 Ma ± 0.03/0.05/0.07 Ma, while the remaining 5

analyses were discordant because of the combined effects of inheritance, lead loss and overgrowth (Fig. 4a).

Analysis PK9/4 is touching as well the concordia line at a date of 34.02 ± 0.07 Ma and could be interpreted as dating the onset of crystallization. We, however, do not favor such interpretation because crystallization over 0.77 Ma (between PK9/2 and PK9/4) is highly unlikely for a single pulse of volcanic activity. Moreover, a recent study of magmatic rocks from Adamello intrusion in Northern Italy (Schaltegger et al., 2009) demonstrates that the youngest zircons are usually reflecting the emplacement age. We rule out the possibility of lead loss for PK9/2, since the annealing – leaching technique applied here is proved to eliminate such problem efficiently in most cas- es (Mattinson et al., 2007). If we calculate a regression line with the concordant zircon (PK-9/2) and the points PK-9/3, PK9/4 and PK-9/5, the lower and upper intercept ages are 32.7 ± 1.6 and 307.7 ± 5.1 Ma (MSWD = 1.6), respectively. The upper intercept reflects a Variscan lead component, which is well documented within the entire Rhodope (Peytcheva et al., 1995). Since the lower intercept is indistinguishable from the ²⁰⁶Pb/²³⁸U age of 33.24

± 0.03/0.05/0.07 Ma of the concordant point PK-9/2, we consider this age to be the best estimate for the crystallization age of the dacite. Analyses PK-9/1 and PK-9/6 define a separate discordia line with a lower intercept of 19.6 ±7.5 Ma and an upper intercept of 231.1 ± 3.4 Ma, which reflects probably a combination effect of different sources of old lead component and overgrowths.

All six analyzed zircons from sample PK-20 show similar effects of inheritance (mainly Variscan) and lead loss; they do not belong to a single population but represent a mixture of inherited xenocrysts, overgrowths and lead loss (Fig. 4b). There is only one young zircon among the analyzed ones – PK20/4, which is discordant with a ²⁰⁶Pb/²³⁸U date of 29.88 +/- 0.03/0.05/0.07.

This analysis represents a fragment part of a zircon grain with high U concentration (1012 ppm, Table 1), caus- Fig. 3. ⁴⁰Ar/³⁹Ar incremental step-heating age spectra derived by CO2–IR laser for biotite from latite dyke PK24 from the Iran Tepe volcanic complex and the rhyolite dyke PK38 from the Planinets dyke swarm.

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ing a higher degree of radiation damage and therefore the annealing-leaching treatment was inefficient. If we nevertheless consider this date as meaningful, the time span between the dacite PK9 and andesite PK/20 will be 3.36 +/- 0.06/0.10/0.14My, which is neither supported by field relationships, nor by the internally consistent Ar-Ar dating of the same samples (the age difference between PK9 and PK20 is 0.51 +/- 0.54 My). Therefore we cannot give a reliable U-Pb age for andesite sample PK20.

DISCUSSION AND IMPLICATIONS

Minerals extracted from the Iran Tepe volcanic complex yield ⁴⁰Ar/³⁹Ar plateau ages and U-Pb ages that are consistent with their stratigraphic relationships. Furthermore,

we also consider the ages to be accurate, and to closely approximate the timing of their crystallization. Our precise ⁴⁰Ar/³⁹Ar data, which has been acquired from rocks that span the stratigraphic range of intermediate volcanic rocks of the Iran Tepe complex, clearly indicate that volcanism began at ~ 33. 9 Ma and ended at ~ 33.0 Ma.

Despite the fact that the age of the rhyolitic dyke from the Planinets dyke swarm (32.80 ± 0.28 Ma) lies within the uncertainty of the late latite, their genetic relationship to the intermediate rocks is not clear. The rhyolites are separated from the less evolved rocks at Iran Tepe volcanics by a 10 wt. % SiO2 compositional gap and yield a much larger spatial distribution. For example, a rhyolitic dyke of the same dyke swarm, located 20 km to the east of the Iran Tepe volcanic complex, yields an indistinguishable sanidine ⁴⁰Ar/³⁹Ar plateau age of 32.88 ± 0.23 Ma Fig. 4. (a) Concordia plot showing the results

of single-zircon analyses from dacite sample (PK-9) and (b) andesite (PK-20) from Iran Tepe. Individual analyses are shown as 2σ error ellipses. Cl image is demonstrating the internal structure of the zircons with inherited core and oscillatory rim.

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(Marchev et al., 2004). These geochemical and temporal characteristics suggest that the rhyolites may represent a separate magmatic system. Additional geochemical and mineralogical research is required to demonstrate the relationship between the intermediate volcanic rocks and the rhyolites.

The ⁴⁰Ar/³⁹Ar age for the onset of volcanism is approximately half a million years older than that obtained from the only concordant U-Pb point of 33.24 ± 0.03 Ma from sample PK9. No obvious reason for the discrepancy can be reached from the data acquired by both methods.

However, given that there is no a priori reason to suggest that the U-Pb zircon age is inaccurate, we must consider the possibility that the ⁴⁰Ar/³⁹Ar age of PK9 is too old.

The inverse isochron of sample PK9 yields an imprecise initial atmospheric ⁴⁰Ar/³⁶Ar composition of 205±85, suggesting that we cannot confidently state that excess

40Ar is present. However, clustering of the points on the inverse isochron opens the possibility that the inverse isochron is not valid, precluding the identification of excess

40Ar (e.g. Sherlock and Kelley, 1999). Consequently, we suggest that the most accurate age for the oldest volcanic strata at Iran Tepe is 33.24 ± 0.03 Ma. Nevertheless, the concordance between our new, and pre-existing ⁴⁰Ar/³⁹Ar ages, coupled with ⁴⁰Ar/³⁹Ar ages that corroborate the stratigraphic relationships between the samples suggests that the remaining ⁴⁰Ar/³⁹Ar ages are probably accurate, and are at least internally consistent. The upper discor-

dia intercept U-Pb zircon age of 307.8 ± 1.1 Ma of those samples may reflect the assimilation of zircon grains from Variscan basement lithologies. Magmatic protoliths with similar ages are known from many granitoid bodies from the Eastern Rhodope massif (e.g. Peytcheva and Quadt, 1995).

An evaluation of the errors shows that the overall time span of intermediate volcanism (samples PK9 and PK24) at Iran Tepe was between 400 k.y and 1.4 Ma. If we in- clude the rhyolitic activity, the time span enlarges up to 1.1 Ma. Published K/Ar ages of the Iran Tepe volcanic rocks indicate a much wider period of activity, ranging between 35 and 39 Ma. The newly obtained age determinations are somewhat closer to the K/Ar ages (36.5–35 Ma) of Lilov et al. (1987), and are notably younger from the age of 39 Ma determined by Georgiev et al. (2003). Our new precise age data confirm that Iran Tepe is the oldest volcanic complex in the south-eastern part of the Eastern Rhodopes. However, they do not confirm the previously accepted Priabonian age of volcanism. Volcanic activity in the region started at the Priabonian/Rupelian boundary, after the Priabonian marine transgression in the Eastern Rhodopes (Boyanov and Goranov, 2001), and was simul- taneous with the deposition of uppermost shallow marine limestones. The deposition of organogenic limestones (mostly coral reefs) at the periphery of the volcano accompanied the entire volcanic activity. The ages of 33.37

± 0.29 and 32.97 ± 0.23 Ma from the stratigraphically

Fig. 5. Summary of weighted mean or plateau ages from ⁴⁰Ar/³⁹Ar laser-fusion and incremental-heating age determinations of the major volcanic areas in the Eastern Rhodopes; uncertainties ± 2‫. Shown are for comparison K-Ar ages of Iran Tepe from Lilov et al. (1987).

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youngest lava flow, and the feeding latitic dyke corroborate an age of 32.44 ±0.23 Ma, obtained by Marchev and Singer (2002) for the Reseda Tuff of Ivanov and Kopp (1969), which covers the Iran Tepe complex. This age is indistinguishable within the error from that of the rhyolites (32.80 ± 0.28) from the Planinets dyke swarm, which intrudes the Rezeda Tuff.

The duration of the volcanic activity of the Iran Tepe of approximately 1 Ma is similar to that obtained for a series of volcanic structures in the Eastern Rhodopes (Fig. 5), which were recently dated: e.g. Madjarovo and Zvezdel (Marchev et al., 2004; Georgiev and Marchev, 2005), and is slightly shorter than that of the largest volcanic structure at Borovitsa (Singer and Marchev, 2000). These age intervals fall well within the lifespan of well-documented composite volcanoes in Japan, the Cascades and the Southern Andes, which were active for 80 ka to 900 ka (e.g., Wohletz and Heiken, 1992;

Singer et al., 1997).

CONCLUSIONS

The new precise ⁴⁰Ar/³⁹Ar and U-Pb age data for the Iran Tepe volcanic complex do not confirm an Upper Eocene (Bartonian-Priabonian) age, obtained by previous K/Ar age determinations. Our new data show that the entire volcanic activity occurred in the Lower Oligocene time, in the interval between 33.9Ma and 33.0 Ma.

The time span of volcanic activity (~ 1 Ma) is similar to the duration of other volcanic centers in the Eastern Rhodopes, and well-documented volcanoes in Japan, the Andes and the Cascades.

The age of the rhyolites from the Planinets dyke swarm is close to that of the late Iran Tepe latites. However, their spatial distribution is far beyond the boundary of the Iran Tepe volcano, rendering their genetic relationship with the more mafic Iran Tepe volcano unclear.

The occurrence of inherited zircons of Variscan age in lavas at Iran Tepe is an evidence for considerable crustal contamination during transport of the magmas through the metamorphic basement.

Acknowledments

Supported by the University of Geneva through the SCOPES program of the Swiss National Science Foundation for cooperation with Eastern European countries (grant IB 7320-111046/1). Partial support for field work was provided from the Bulgarian National Fund for Scientific Research project no. VU -NZ- 02/06. PK gratefully acknowledges support from a SE European Geoscience Foundation Scholarship during this study. We thank Svetoslav Georgiev and Alexandra Harkovska for providing the constructive reviews on the paper. The authors are grateful to Balkan Mineral and Mining and Danko Jelev for the logistic support during the field work.

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