Nikolay Bonev

Gabbro, plagiogranite and associated dykes in the supra-subduction zone Evros Ophiolites, NE Greece

The incomplete Evros ophiolites in NE Greece form a NE-SW-oriented discontinuous belt in the Alpine orogen of the north Aegean. Field data, petrology and geochemistry are presented here for the intrusive section and associated mafic dykes of these ophiolites. Bodies of high-level isotropic gabbro and plagiogranite in the ophiolite suite are cross-cut by NE–SW-trending boninitic and tholeiitic–boninitic affinity dykes, respectively. The dykes fill tensile fractures or faults, which implies dyke emplacement in an extensional tectonic regime. The tholeiitic–transitional boninitic gabbro is REE- and HFS-depleted relative to N-MORB, indicating derivation from melting of a refractory mantle peridotite source. Associated boninitic dykes are slightly LREE-enriched, showing mineral and whole-rock geochemistry similar to the gabbro. The plagiogranite is a strongly REE-enriched high-silica trondhjemite, with textures and composition typical for an oceanic crust differentiate. Plagiogranite-hosted tholeiitic and transitional boninitic dykes are variably REE-enriched. Geochemical modelling indicates origin of the plagiogranite by up to 75 % fractional crystallization of basaltic magma similar to that producing the associated tholeiitic dykes. All mafic rocks have high LILE/HFSE ratios and negative Ta–Nb–Ti and Ce anomalies, typical for subduction zone-related settings. The mafic rocks show a similar trace-element character to the mafic lavas of an extrusive section in Bulgaria, suggesting they both form genetically related intrusive and extrusive suites of the Evros ophiolites. The field occurrence, the structural context, the petrology and geochemical signature of the studied magmatic assemblage provide evidence for its origin in a proto-arc (fore-arc) tectonic setting, thus tracing the early stages of the tectono-magmatic evolution of Jurassic arc-marginal basin system that has generated the supra-subduction type Evros ophiolites.

Record of a Palaeogene syn-collisional extension in the north Aegean region: evidence from the Kemer micaschists (NW Turkey)

In NW Turkey, the medium-grade Kemer micaschists of the Biga Peninsula record NE- directed extension related to ductile to brittle-ductile shearing during the Palaeogene period: a lower limit for their exhumation is given by the Late Maastrichtian age of the HP-LT metamorphism of a similar nearby area (Camlica micaschists); an upper limit is given by the Early Eocene intrusion age of the post-kinematic Karabiga granitoid, dated as 52.7 ± 1.9 Ma using the U-Pb LA-ICP-MS method on xenotime. Correlations with the northeasterly Rhodope region and integration into the geodynamic regional frame indicate that the Kemer micaschists experienced an extensional deformation connected to a collisional context in latest Cretaceous-early Tertiary times. The Kemer micaschists therefore represent a new area (the first in Turkey), which suffered synorogenic extension in the north Aegean domain at the very beginning of Tertiary times.

40Ar/39Ar geochronology constraints on the Middle Tertiary basement extensional exhumation, and its relation to ore-forming and magmatic processes in the Eastern Rhodope (Bulgaria)

The interaction of distinct geologic processes involved during late orogenic extensional exhumation history of the metamorphic units in the Eastern Rhodope is refined by new and reviewing 40Ar/39Ar geochronological and structural data. Minerals with different closure temperatures from metamorphic rocks investigated in this study are combined with those from magmatic and ore-forming hydrothermal rocks in two late stage metamorphic domes – the Kesebir-Kardamos and the Biala reka-Kehros domes. The 38-37 Ma muscovite and biotite cooling ages below 350°-300°C characterize basement metamorphic rocks that typified core of the Kesebir-Kardamos dome, constraining their exhumation at shallow crustal levels in the footwall of detachment. These ages are interpreted as reflecting last stage of ductile activity on shear zone below detachment, which continued to operate under low-temperature conditions within the semi-ductile to brittle field. They are close to and overlap with existing cooling ages in southern Bulgaria and northern Greece, indicating supportively that the basement rocks regionally cooled between 42-36 Ma below temperatures 350°-300°C. The spatial distribution of ages shows a southward gradual increase up structural section, suggesting an asymmetrical mode of extension, cooling and exhumation from south to the north at latitude of the Kesebir-Kardamos dome. The slightly younger 36.5-35 Ma crystallization ages of adularia in altered rocks from the ore deposits in the immediate hanging-wall of detachments are attributed to brittle deformation on high-angle normal faults, which further contributed to upper crustal extension, and thus constraining the time when alteration took place and deformation continued at brittle crustal levels. Silicic dykes yielded ages between 32-33 Ma, typically coinciding with the main phase of Palaeogene magmatic activity, which started in Eastern Rhodope region in Late Eocene (Priabonian) times. The 40Ar/39Ar plateau ages from the above distinct rock types span time interval lasting approximately ca. 6 Ma. Consequently, our geochronologic results consistently indicate that extensional tectonics and related exhumation and doming, epithermal mineralizations and volcanic activity are closely spaced in time. These new 40Ar/39Ar age results further contribute to temporal constraints on the timing of tectonic, relative to ore-forming and magmatic events, suggesting in addition that all above mentioned processes interfered during the late orogenic extensional collapse in the Eastern Rhodope region.

Geochemistry, tectonics, and crustal evolution of basement rocks in the Eastern Rhodope Massif, Bulgaria

Orthogneisses derived from granitoids with Variscan protolith ages dominate the lower unit of high‐grade metamorphic basement of the Eastern Rhodope Massif in south Bulgaria. We present whole‐rock geochemistry and Sr–Pb isotopic composition of these orthogneisses, which are compared with Pb isotopes of parametamorphic rocks, and hydrothermal ore deposits and associated rocks, to better constrain their composition, origin, and contribution to late Alpine hydrothermal processes. The igneous mineral assemblage is partly preserved, and the field textures and microstructures of the orthogneisses are consistent with a ductile, amphibolite‐grade tectono‐metamorphic overprint during Alpine time, when they were involved in the metamorphic nappe stack. Whole‐rock geochemistry revealed compositions of the orthogneisses largely unaffected by the amphibolite‐grade metamorphism, displaying a magmatic differentiation trend of the igneous protoliths. The protoliths are peraluminous medium‐K calc‐alkaline S‐type granitoids, whose tectono‐magmatic setting discrimination consistently indicates a continental volcanic arc origin. The orthogneisses present trace element and rare‐earth elements (REE) patterns based on which a group of high‐field strength elements‐depleted and REE fractionated orthogneisses and a group of LREE‐enriched orthogneisses can be distinguished. Both geochemical groups show compositions similar to the bulk and upper continental crust and its sedimentary counterparts. Crustal Pb isotope ratios (206Pb/204Pbi = 18.24–18.66) of the orthogneisses are comparable to the paragneisses (206Pb/204Pbi = 18.31–18.93) and uniform in both (207Pb/204Pbi = 15.64–15.72) and 208Pb/204Pbi ratios in the paragneisses (38.23–38.60) and the orthogneisses (38.32-38.56). The trace element data and 87Sr/86Sri isotopes of the orthogneisses (0.7050–0.7117) overlap those of the parametamorphic rocks (0.7039–0.7144), and confirm the supra‐crustal origin of the igneous precursors. A heterogeneous crustal source region is suggested in which melting and crustal contamination during magma genesis with subsequent fractional crystallization was involved in the petrogenesis. Comparative Pb isotope systematics suggests that a significant crustal Pb input to ore‐forming hydrothermal fluids was derived primarily from the metamorphic basement, implying that the brittlly deformed basement during crustal extension acted as an immediate environment for fluid leaching during late Alpine hydrothermal ore‐forming processes.

Geochemistry and tectonic significance of proto‐ophiolitic metamafic units from the Serbo‐Macedonian and western Rhodope massifs (Bulgaria‐Greece)

Dispersed Triassic (?) metamafic rocks occurring in the Serbo‐Macedonian and western Rhodope massifs of southern Bulgaria and northern Greece are important crustal components of early Mesozoic oceanic crust formation in the Tethyan realm. The Volvi and Therma metamafic bodies of the Serbo‐Macedonian Massif and metamafic rocks at Rila Mountain of the western Rhodope Massif consist mainly of gabbros and basalts that show a strong overprint of Alpine amphibolite‐facies metamorphism and deformation. Only clinopyroxenes in the Volvi body represent preserved primary phases. High‐Ti and low‐Ti groups have been identified geochemically within the metamafic rocks. The incompatible trace‐element and REE‐enriched high‐Ti group displays an E‐MORB signature consistent with a rift‐spreading centre tectonic setting; in contrast, the low‐Ti group exhibits geochemical features largely comparable to the high‐Ti group as well as HFSE and REE depleted compositions and an arc‐related signature. Comparison with Triassic rift‐related volcanic suites in adjacent tectonic zones show that these metamafic rocks are fully comparable in terms of their trace elements, REE ratios, and normalized patterns, suggesting that collectively they represent the remnants of a regionwide Triassic rifting event. Enriched mantle components, together with an arc component likely influenced by the late Palaeozoic‐early Mesozoic Palaeotethyan subduction, were involved in the petrogenesis of the metamafic rocks. Based on regional geology and geochronology, we interpret the metamafic rock suites as the products of intra‐continental rifting within the Serbo‐Macedonian/Rhodope continental margin. Thus, these proto‐ophiolitic mafic rock assemblages represent precursors of the Neotethyan early crust formation along the Eurasian plate boundary following the closure of Palaeotethys.

Peri-Gondwanan Ordovician crustal fragments in the high-grade basement of the Eastern Rhodope Massif, Bulgaria: evidence from U-Pb LA-ICP-MS zircon geochronology and geochemistry

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.

Nd–Sr–Pb isotopic composition and mantle sources of Triassic rift units in the Serbo-Macedonian and the western Rhodope massifs (Bulgaria–Greece)

We report on the field occurrence and isotopic compositions of metamafic rocks exposed in the Serbo-Macedonian (Volvi and Therma bodies) and western Rhodope (Rila Mountains) massifs of Bulgaria and Greece. These metamafic units consist of high- and low-Ti gabbroic and basaltic rocks, whose Nd–Sr–Pb isotopes are compatible with mantle-derived MORB and OIB components with a small amount of crustal material involved in their melt source. These isotopic features combined with the field observations are consistent with an intra-continental rift origin of the metamafic rocks protolith, and are comparable to those of the Triassic rift-related mafic rocks in the northern Aegean region.

Structural and 40Ar/39Ar age constraints on the Kulidjik nappe: A record of an early Alpine thrust tectonics in the northeastern Rhodope Massif, Bulgaria

In the northeastern Rhodope Massif, a unique juxtaposition of assumed high-grade basement allochthon onto a low-grade Mesozoic unit is exposed in the Kulidjik nappe. The thrusting is interpreted as post-Jurassic to pre-Late Eocene. The tectonostratigraphy comprises the following units: (i) an upper unit of high-grade basement of the eastern Rhodope consisting of various amphibolite facies rocks with metaophiolite lenses; (ii) an overlaying Mesozoic (including Jurassic radiolarians) low-grade unit; (iii) a structurally overlaying allochthon of the Kulidjik nappe; and (iv) unconformable Late Eocene sedimentary and Oligocene volcanic cover rocks. The basement unit contains a NNE-SSW oriented lineation rarely associated with top-to-the NNE ductile shearing. Greenschist facies retrogression of this unit is diagnostic of local thrusting of the overlying low-grade unit. The greenschists of the low-grade unit represent arc metavolcanic rocks overlain by phyllites. The structural elements in this unit have similar attitudes as the basement unit. Kinematic indicators in the low-grade unit demonstrate internal top-to-the NNE ductile shearing during greenschist facies conditions. The allochthon consists of klippen composed of muscovite-albite gneisses. The flat-lying thrust contact with ductile-brittle deformation contains mylonites and cataclasites. Previously reported local top-to-the SSW ductile shearing in the allochthon is probably a consequence of shear instabilities. Deformation-crystallization relationships revealed fabrics in the allochthon compatible with greenschist up to lower amphibolite facies conditions without retrogression and similar ENE-verging fold patterns in the low-grade unit and the allochthon at the thrust contact. Two muscovite samples from distinct klippen yielded inverse isochron 40Ar/39Ar ages of 156.07±1.30 Ma and 162.62±2.69 Ma, which reflect cooling below 350°C of the allochthon associated with its exhumation during emplacement after greenschist-lower amphibolite facies metamorphism. Structural data indicate NNE-directed tectonic transport of the low-grade footwall unit, which is consistent with emplacement in comparable metamorphic conditions, and the thrust direction of the allochthon in the hanging wall. The subduction-accretionary origin of the low-grade unit and the new age results allow us to link this tectono-metamorphic event to Late Jurassic thrust imbrication of arc units on the Rhodope basement, which is the first time that early Alpine thrust tectonics in the northeastern Rhodope Massif have been recognized.

Comment on Georgiev et al. "Structure and U–Pb zircon geochronology of an Alpine nappe stack telescoped by extensional detachment faulting (Kulidzhik area, Eastern Rhodopes, Bulgaria)

In this discussion, we evaluate the field, chemical, sedimentary, structural and metamorphic data related to the Kulidzhik area tectonic proposal.

From an ocean floor wrench zone origin to transpressional tectonic emplacement of the Sithonia ophiolite, eastern Vardar Suture Zone, northern Greece

In the Hellenides of northern Greece, the Sithonia back-arc ophiolite constitute an element of the Vardar suture zone against the Chortiatis island arc magmatic suite, the Melissochori Formation and the Serbo-Macedonian Massif further north at the Mesozoic continental margin of Eurasia. A granodiorite from the Chortiatis island arc magmatic suite crystallized at 160 Ma as derived from new U-Pb zircon geochronology and confirms the end of arc magmatic activity that started at around 173 Ma. Located southerly of the Chortiatis island arc magmatic suite, the Sithonia ophiolite had igneous life from 159 to 149 Ma, and the ophiolite interfinger with clastic-carbonate Kimmeridgian sediments. Magmatic structures (i.e., sheeted dykes) in the ophiolite witness for NE-trending rift axis, while the transform faults and fracture zones sketch NW–SE transcurrent transtension-like propagation of the rift-spreading center at Sithonia that is consistent with a dextral wrench corridor already proposed for the ophiolite origin in the eastern Vardar zone. The tectonic emplacement of the Sithonia ophiolite involved dextral ENE to SE strike-slip sense of shear and SW and NE reverse thrust sense of shear on mostly steep foliation S1, subhorizontal lineation L1 and associated variably inclined F1 fold axes. This structural grain and kinematics are shared by adjacent Chortiatis island arc magmatic suite and the Melissochori Formation. The coexistence of strike-parallel and thrust components of displacement along discrete dextral strike-slip shear zones and internal deformation of the mentioned units is interpreted to result from a bulk dextral transpressive deformation regime developed in greenschist-facies metamorphic conditions. The back-arc ocean floor previous structural architecture with faults and fracture zones where Kimmeridgian sediments deposited in troughs was used by discrete strike-slip shear zones in which these sediments involved, and the shear zones become the sites for strain partitioning of transpressional deformation. Available biostratigraphic and radiometric age constraints define latest Jurassic-earliest Cretaceous (Tithonian-Berriasian to early Valanginian) time frame for the Sithonia ophiolite northeastward tectonic emplacement accomodated by dextral transpression that led to the ophiolite accretion to the Chortiatis island arc magmatic suite and its trench-fill exposed in the Melissochori Formation and further north toward the Serbo-Macedonian margin of Eurasia.