Detrital heavy mineral and bulk rock geochemistry and a review of sandstone petrographic data have been used to investigate the post-collisional effusive magmatism that followed the closure of the ...Vardar Ocean and the generation of volcanic sediments in a complex and compositionally variable volcanic region. Available petrographic data gives evidence of contributions from three key source areas corresponding to the three main tectonic units: the structurally lower Gneiss–Migmatite Complex (Byala Reka-Kechros and Kesebir-Kardamos domes) and the upper Variegated (Kimi) Complex, both fringed by the low-to-medium-grade Mesozoic rocks of the Circum-Rhodope Belt. Besides the deposition of siliciclastic material, volcanic contributions from both, intermediate and acid products represent an important source of sediment in the area. Despite dominant intermediate to acid volcanic products, volcanic lithic fragments in sandstones (microlithic, lathwork and brown vitric textures) indicate main inputs from intermediate and basic-intermediate products generating questions on the interpretation of volcanic detritus in reconstruction of provenance. Detrital amphibole and pyroxene chemistry is used to characterise the supply of volcanic material as well as the dispersal mechanisms and understand the role played by each of the volcanic centers present in the area in the infill of the north-western Thrace basin. Amphibole chemistry reveals high compositional heterogeneity according to both compositional variability of the numerous volcanic centres active at the time of deposition and presence of metamorphic amphibole. 4Al and AlT apfu values indicate that most of the amphiboles from the NERZ are not of volcanic origin and their presence can only be attributed to the numerous amphibolite facies metamorphic rocks abundantly documented in the area. Detrital amphibole compositions from the ZKVS indicate major contributions from the Iran Tepe and Zvedzel volcanoes. Analysed detrital clinopyroxenes from the NERZ are mostly diopside–augite, with no hedenbergite or Fe–augite detected. The 4Al/6Al ratio is comparable with compositions of volcanic pyroxenes from the Momchilgrad–Arda (ZKVS) region and products from the pre-caldera phase of the Borovitza.
Field, geochemical, and geochronologic data of high-grade basement metamafic and evolved rocks are used to identify the nature and timing of pre-Alpine crustal growth of the Rhodope Massif. These ...rocks occur intrusive into clastic-carbonate metasedimentary succession. Petrography and mineral chemistry show compositions consistent with Alpine amphibolite-facies metamorphism that obliterated the original igneous textures of the protoliths. Bulk-rock geochemistry identifies low-Ti tholeiitic to calc-alkaline gabbroic-basaltic and plagiogranite precursors, with MORB-IAT supra-subduction zone signature and trace elements comparable to modern back-arc basalts. The U-Pb zircon dating revealed a mean age of 455 Ma for the magmatic crystallization of the protoliths that contain inherited Cambrian (528-534 Ma) zircons. Carboniferous, Jurassic, and Eocene metamorphic events overprinted the Ordovician protoliths. The radiometric results of the metamorphic rocks demonstrate that Ordovician oceanic crust was involved in the build-up of the Rhodope high-grade basement. Dating of Eocene-Oligocene volcanic rocks overlying or cross-cutting the metamorphic rocks supplied Neoproterozoic, Ordovician and Permo-Carboniferous xenocrystic zircons that were sampled en route to the surface from the basement. The volcanic rocks thus confirm sub-regionally present Neoproterozoic and Paleozoic igneous and metamorphic basement. We interpret the origin of the Middle-Late Ordovician oceanic magmatism in a back-arc rift-spreading center propagating along peri-Gondwanan Cadomian basement terrane related to the Rheic Ocean widening. The results highlight the presence of elements of Cadomian northern Gondwana margin in the high-grade basement and record of Rheic Ocean evolution. The eastern Rhodope Massif high-grade basement compared to adjacent terranes with Neoproterozoic and Cambro-Ordovician evolution shares analogous tectono-magmatic record providing a linkage among basement terranes incorporated in the Alpine belt of the north Aegean region.
The Elatsite–Chelopech ore field in the northern part of the Panagyurishte district in Central Bulgaria comprises numerous spatially associated porphyry copper and epithermal gold deposits and ...prospects. In addition to the mineralization and alteration features, trace elements, lead and sulfur isotope signatures of sulfide minerals from porphyry copper, base metal and gold-base metal deposits/prospects have been studied. LA-ICP-MS analyses of pyrite, arsenopyrite and sulfosalt minerals validate them as major carriers for Au, Ag, Sb, Se and Co. Pyrite from the three types of mineralization has specific geochemical characteristics. Pyrite from the porphyry copper deposits/prospects has generally lower total trace element content compared to pyrite from the epithermal prospects, except for Se, Co and Ni. Pyrite from the base metal and gold-base metal veins is enriched in As, Au, Ag, Sb and Pb. In pyrite from the base metal deposits, Co and Ni have contents comparable to the pyrite from the porphyry copper deposits, while pyrite from the gold-base metal veins shows lower Co and Ni. Arsenopyrite from these deposits shows similar features. Similarly, sphalerite from the gold-base metal veins also has lower Co content compared to sphalerite from the base metal veins but higher In and Cu contents. In addition to the close spatial relationships between the Elatsite and Gorna Kamenitsa porphyry Cu deposits and Negarstitsa-West and Dolna Kamenitsa base metal prospects, as well as similarities in the mineralization and alteration styles, the lead isotopic (206Pb/204Pb = 18.61–18.68, 207Pb/204Pb = 15.64–15.65 for porphyry and 206Pb/204Pb = 18.55–18.67, 207Pb/204Pb = 15.64–15.68 for base metal) and sulfur isotopic (δ34S values of −3 to +1‰ for porphyry and δ34S values of −1.7 to +3.5‰ for base metal) signatures of sulfides support the idea of a genetic link between these two types of deposits. The porphyry and base-metal mineralization result from a common major ore-forming event during the Late Cretaceous, corresponding to deep/higher-temperature and shallower/distal/lower-temperature environments, respectively. In particular, more radiogenic lead (206Pb/204Pb = 18.41–18.47, 207Pb/204Pb = 15.67–15.76) and slightly different sulfur isotopic compositions (δ34S values of +3.5 to +10.6‰) of sulfides from the distal gold-base metal veins of Kordunsko Dere, Svishti Plaz and Shipkite might be a consequence of the interaction of the ore-forming fluids with an external older crustal and isotopically positive S source. Alternatively, a different fluid source/event for the formation of these gold-base metal veins may be suggested.
Basaltic magmatism occurred only rarely within the extensive Eocene–Oligocene volcanic activity in the Eastern Rhodope Mts., SE Bulgaria. The earliest mafic volcanism started at ca. 34 Ma with K-rich ...trachybasalts strongly enriched in large ion lithophile elements (LILE), particularly Ba, Sr, Pb, Th, and light rare earth elements (REE) relative to the high field strength elements (HFSE). They have high
87Sr/
86Sr ratios (0.70688–0.70756), low
144Nd/
144Nd (0.51252–0.51243), and very high
207Pb/
204Pb (15.74–15.76) and
208Pb/
204Pb (39.07–39.14) at low
206Pb/
204Pb (18.72–18.73) ratios, reflecting high degrees of crustal contamination. Shoshonitic basalts and absarokites and calc-alkaline and high-K calc-alkaline magmas, which erupted between 33 and 31 Ma, have decreasing Sr isotope initial ratios from west (0.70825) to east (0.70647) at approximately constant
143Nd/
144Nd isotopic compositions (0.51252–0.51243) and slightly decreasing
207Pb/
204Pb (15.66–15.72) and
208Pb/
204Pb (38.80–38.96) and increasing
206Pb/
204Pb (18.73–18.90) in comparison with the trachybasalts. All these rocks are characterized by negative Nb–Ti and Eu anomalies. They resulted from different degrees of partial melting of enriched asthenosphere, and the magmas were later contaminated by the Rhodopian crust. The end of the magmatic activity (28–26 Ma) was marked by emplacement of alkaline dykes, spatially associated with metamorphic core complexes. They are characterized by low
87Sr/
86Sr (0.70323–0.70338), high
144Nd/
144Nd (0.51290–0.51289), and
206Pb/
204Pb (18.91–19.02) at lower
207Pb/
204Pb (15.52–15.64) and
208Pb/
204Pb (38.59–38.87) ratios, consistent with an origin from a source similar to OIB-like European Asthenospheric reservoir contaminated by depleted mantle lithosphere.
The Eastern Rhodope Eo-Oligocene mafic magmatism formed as part of the prolonged extensional tectonics of the whole Rhodope region in Late Cretaceous–Paleogene time, similar to those in the U.S. Cordillera and Menderes Massif (Turkey). Initiation of extension is constrained by the formation of metamorphic core complexes, low-angle detachment faults, and supradetachment Maastrichtian–Paleocene sedimentary basins, intimately associated with 70–42 Ma granitoids and metamorphism which record mantle perturbation. The Eo-Oligocene stage started with block faulting, sedimentary basin formation, and extensive acid-intermediate and basic volcanism over the entire Eastern Rhodope area. The order of emplacement of the basalts from high-Ba trachybasalts through shoshonites, calc-alkaline and high-K calc-alkaline basalts, and finally to purely asthenospheric-derived alkaline basalts, with progressively decreasing amount of crustal component, reflects upwelling asthenospheric mantle. Most of the models proposed in the literature to explain extension and magmagenesis in the Rhodopes and the Mediterranean region cannot be applied directly. Critical evaluation of these models suggest that some form of convective removal of the lithosphere and mantle diapirism provide the most satisfactory explanation for the Paleogene structural, metamorphic, and magmatic evolution of the Rhodopes.
This paper presents and discusses the water and gas geochemistry of a large number of thermal springs occurring along the N-S trending Strymon Valley, from its source, near Sofia (Bulgaria), to the ...Aegean Sea (Greece). In Bulgaria springs have markedly alkaline pH, relatively low Total Dissolved Solids and prevalent Na-HCO3 to Na-Cl(SO4) in composition while the associated gas phase is mostly N2-dominated. When moving to the Greek sector, the thermal springs, Ca(Mg)-HCO3 to Na-HCO3, become less alkaline and more saline whereas the associated gas phase is CO2-dominated. The abrupt geochemical change in the Greek sector is caused by a variation in the thickness and nature of the sediments filling the Strymon Valley, the latter being characterized by a relevant amount of Neogene marine material. Such changes occur south of an important E-W lineament named Middle Mesta, south of which marble formations extensively crop out and are likely occurring below the sedimentary succession. The presence of these carbonate sequences embedded in the Neogene sediments is explaining the CO2-rich gases associated to the Greek springs. Water isotopes indicate a meteoric origin for the studied waters. From a geothermometric point of view, solute (previous studies) and gas (this work) geothermometers suggest that no high enthalpy systems occur in the Bulgaria and northern sector of Greece with estimated temperatures <120 °C. Consequently, these thermal springs can be regarded as tectonically-derived along the many fault systems that border the Strymon Valley. The convective circuits are thus originated from rainfall in the crystalline massifs that border the valley, i.e. the Serbo-Macedonian to the west and the Rhodope to the east.
•Water and gas discharges from the Strymon Valley (Bulgaria-Greece) were investigated.•Chemical and isotopic differences between the Bulgarian and Greek samples occur.•No high-enthalpy systems were recognized.•Fluid discharges are mostly driven by tectonic lineaments.•A conceptual model of fluid circulation is proposed.
The Oligocene Krumovgrad alkaline basalts and lamprophyre (KABL) intruded into the Kesebir-Kardamos and Byala Reka metamorphic domes carry various mantle and crustal xenoliths. This study presents ...the results from petrological and geochronological investigations of one granitic xenolith entrained in an alkaline (camptonite) dyke intruded into the Byala Reka dome, close to the village of Gorni Yurutsi (SE Bulgaria). The studied xenolith is granite formed at an estimated depth of 6–9 km. During the transport to the surface, it records some host-melt interaction with the penetrating melt, reflected mainly in the reaction with the biotite. In situ U/Pb dating of four zircons and one monazite from the xenolith yield different and slightly discordant to nearly concordant ages. One zircon reveals Ediacaran (606±19 Ma) 206Pb/238U age of the core and Permian (277±16 Ma) of the rim (all error uncertainties are 2σ). Two zircons at three analytical points overlap in their error uncertainties at 198±10 Ma and 207±8 Ma, and one zircon rim yields a concordant Jurassic age of 157±10 Ma. The age of this zircon should reflect the age of the crystallization of the granitic xenolith, whereas the older zircons are inherited and are typical for the metamorphic and magmatic rocks in the Rhodope Massif. The monazite crystal yields two almost concordant Chattian ages of 28.8±1.3 Ma and 27.6±1.8 Ma. They coincide with previous K/Ar dating of the alkaline basalts, thus confirming their Oligocene (Chattian) age.
In the Thrace region of NE Greece, the crystallization and post-solidification cooling below 600°C of the intrusive suite of the Circum-Rhodope Belt arc-related Evros ophiolite span from 176.4±0.93 ...to 163.5±3.85Ma. In the underlying Rhodope high-grade metamorphic basement, (ultra-) high-pressure metamafic rocks are cross-cut by orthogneisses with granitoid protoliths and crystallization ages between 160±0.69 and 154±1.5Ma. This new U–Pb LA–ICP-MS zircon geochronological data provides evidence that the timing of the evolution of the Evros ophiolite coincides, within analytical error, with granitoid magmatism in the underlying high-grade basement. One of the dated metagranitoid rocks cross-cuts amphibolitized eclogite, indicating that the high-pressure conditions have a maximum age of 160Ma postdating one of the proposed ages of ultra-high pressure conditions in the Rhodope Massif. The currently earliest recognised Rhodope high/ultra-high pressure metamorphic event is therefore related to the subduction setting which formed the Evros ophiolite. Based on these new temporal constraints and the regional tectonic–geochronologic framework, we discuss and update the geodynamic context connected to the Jurassic subduction–collisional setting at the Rhodope continental margin of Eurasia.
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•Evros ophiolite has magmatic life between 176-164Ma south to the Rhodope margin.•The timing of (UHP) HP event coincides with the Evros ophiolite magmatic history.•Basement granitoid magmatism overlaps the ages of HP event and Evros ophiolite.
A prominent subduction-related magmatic arc hosting significant mineralization formed in SE Europe during the Late Cretaceous. Previous studies on major magmatic centers and ore deposits suggested ...that this belt formed through southwards retreat of a subducting Neotethys oceanic slab. However, the timing and the petrologic characteristics of magmatic products from the less mineralized eastern portions of this belt remain largely unknown. The complete lack of radiometric ages and limited geochemical characterization of this magmatism adds considerable uncertainty on existing large-scale geodynamic reconstructions focused on the Late Cretaceous magmatism in SE Europe. Here, we address this question by studying little known Cretaceous lavas and sub-volcanic bodies from the Eastern Balkan, Bulgaria. Major and trace element contents of these volumetrically limited rocks show a clear subduction signature. The most primitive rocks are high-Al basalts, which further differentiated into andesites and dacites via fractional crystallization in relatively small magma chambers. The mineral chemistry and assemblages constrain magmatic conditions prior to crystallization to pressures of 3–7 kb, temperatures of 900–1020 °C, water contents of ~4–7 wt% and high oxygen fugacities. Phenocryst features like reverse zonation of clinopyroxene and amphibole and sieve and patchy textures of plagioclase suggest magma mixing processes. Initial εHf values of Cretaceous zircons (+2 to −2) and inherited, mainly Variscan and older zircons (+3 to −11) provide clear evidence for assimilation of crustal lithologies by mantle-derived Late Cretaceous magmas. Amphibole phenocrysts from an andesite and a dacite give 40Ar/39Ar plateau ages of 94.67 ± 0.40 Ma and 94.56 ± 0.40 Ma, respectively. These dates are the oldest recorded in the entire Late Cretaceous magmatic belt and constrain the onset of the subduction magmatism to the later parts of the Cenomanian stage. Regional correlations based on these results reveal that processes of slab retreat were active also in the eastern part of the magmatic arc. Further, these results outline a clear temporal along-arc trend of progressively younger initiation of the arc magmatism from east to west.
•We discus the petrology and geochronology of East Balkan magmatism, Bulgaria.•Two overlapping precise Ar/Ar amphibole ages date the magmatism at ~94.5 Ma.•East Balkan volcanism marks the initiation of a major subduction zone in SE Europe.•The Ar/Ar ages are consistent with a southward slab retreat with time.•The ages outline an E-W, along-arc trend for initiation of volcanism with time.
Generally all orogenic ultrapotassic rocks are formed after melting of metasomatized sub-continental lithospheric mantle via subducted crustal mica-bearing lithologies. Here we present another ...possible model, based on the study of the small Stomanovo ultrapotassic monzonite porphyry intrusion in the Central Rhodope Massif, Bulgaria. The monzonite dated at 30.50 ± 0.46 Ma is intruded into the voluminous Oligocene (31.63 ± 0.40 Ma) Bratsigovo–Dospat ignimbrite. The monzonite hosts both normally and reversely zoned clinopyroxene phenocrysts. The normally zoned clinopyroxene is characterized by gradually diminishing core-to-rim Mg no. (89–74), whereas the reversely zoned clinopyroxene has green Fe-rich cores (Mg no. 71–55) mantled by normally zoned clinopyroxene (Mg no. 87–74). Neither the core of the normally zoned clinopyroxene nor the Fe-rich green cores are in equilibrium with the host monzonite. This ultrapotassic monzonite shows more radiogenic Sr isotopes ((87Sr/86Sr)i = 0.71066) and ϵNd(t) = −7.8 to −8.0 that are distinct from the host ignimbrites with (87Sr/86Sr)i = 0.70917–0.70927 and ϵNd(t) = −4.6 to −6.5. The Sr–Nd isotopic data and the presence of copious zircon xenocrysts from the underlying metamorphic basement suggest extensive crustal assimilation. Our observations indicate that the Stomanovo ultrapotassic monzonite formed after extensive lower or middle crustal fractional crystallization from an evolved magma producing cumulates. The process was followed by hybridization with primitive mantle-derived magma and subsequent continuous crustal contamination. We suggest that instead of inheriting their high K2O and large-ion lithophile element enrichments from slab-derived/metasomatic fluids, the Stomanovo ultrapotassic monzonite may owe some of its unusually high alkalinity to the assimilation of potassium-rich phases from the Rhodope Massif basement rocks.
