Rhodope Metamorphic Core Complex Research Articles

Thrace Basin—An Oligocene Clastic Basin Formed During the Exhumation of the Rhodope Complex

AbstractSome orogenic sedimentary basins are difficult to assign to a particular category. An example is the hydrocarbon‐bearing Thrace Basin in the northern Aegean. It has more than 9‐km‐thick Cenozoic clastic sediment, and is spatially associated with the Rhodope metamorphic core complex in the west, and with the Tethyan subduction‐accretion complexes in the south, and is cut by the North Anatolian Fault and its precursors. It has been interpreted variously as an intramontane, a forearc, or an orogenic collapse basin. Here, we provide new geochronological and biostratigraphic data to constrain the tectonic evolution of the Thrace Basin. The new data indicate that as an individual depocenter the Thrace Basin has a short age span (late Eocene—Oligocene, 36–28 Ma) and more than 90% of the basin fill consists of early Oligocene (34–28 Ma) siliciclastic turbidites, deposited at rates of 1.0 km/my. Paleocurrents and new detrital zircon U‐Pb ages show that the Rhodope Complex was the main sediment source. The exhumation of the northern Rhodope Complex (36–28 Ma) was coeval with the main subsidence in the Thrace Basin (34–28 Ma), and involved clockwise crustal rotation in the northern Aegean and possibly crustal flow from underneath the Thrace Basin. Crustal rotation is indicated by the paleomagnetic data, regional stretching lineations in the Rhodope Complex, and the triangular shape of the Thrace Basin. The rotating crustal block must have been bounded in the south by a sinistral fault zone; the location of which corresponds largely with the present day North Anatolian Fault.

Slip preference analysis of faulting driven by strike-slip Andersonian stress regimes: an alternative explanation of the Rhodope metamorphic core complex (northern Greece)

Slip preference analysis (SPA) is accomplished by examining fault activation driven by different Andersonian strike-slip stress regimes. Strike-slip regimes favour the activation of strike-slip faults striking as high as 45° from the trend of the σ 1 axis, thus favouring the predominance of R shears over P shears in a strike-slip shear zone. Reverse and normal faults cannot be activated simultaneously by a common strike-slip stress regime, and their activations depend on the stress ratio. The SPA provides new constraints on stress inversion methods that take into account only the misfit angle minimization criteria. Insights from the SPA give rise to an alternative explanation of the origin of the Tertiary Rhodope metamorphic core complex exposed in the inner part of the Hellenic orogen, which attributes the activation of the associated extensional shear zones and detachments to an Andersonian, transpression–strike-slip (TRP–SS) to strike-slip (SS) stress regime, and not to an extensional one as widely assumed. The variation from TRP–SS to SS is critical for the activation of the extensional structures and is attributed to the local and at-depth increase of the vertical crustal stresses owing to the ascent and emplacement of large plutonic bodies.

Genesis and tectonic implications of podiform chromitites in the metamorphosed ultramafic massif of Dobromirtsi (Bulgaria)

Podiform chromitites occur in the meta-dunite horizon of the Dobromirtsi ultramafic massif, in the Central Rhodope metamorphic core complex (southern Bulgaria). Although these were deformed and metamorphosed together with their host rocks several chromite cores from massive chromitite samples still preserve compositions unaffected by metamorphism – at least in terms of major elements. In situ laser ablation ICP-MS analyses of these chromite cores show a bimodality in the composition of chromite between high-Cr and high-Al {Cr#=[Cr/(Cr+Al)], atomic ratio=0.74–0.55} with Mg# between 0.69 and 0.60 [Mg#=Mg/(Mg+Fe2+), atomic ratio]. These compositions of chromite and the whole rock enrichment of Os–Ir–Ru relative to Pt–Pd in the chromitites resemble that reported for podiform chromitites in ophiolites elsewhere.Magmatic platinum-group minerals (PGM), including laurite (RuS2), Os–Ir alloys and sulfarsenides (irarsite, IrAsS), exhibit a wide range of 187Os/188Os (from 0.1097 to 0.1272) whereas 187Re/188Os is nearly zero. The distribution of the Os model ages calculated for these PGM ranges between 0.13 and 2.6Ga, with age peaks at 0.25, 0.4, 0.7, 2.1 and 2.6Ga. Two xenocrystic zircons were identified in the chromitites and yield concordant U–Pb age of 2257±80Ma (2σ) and 1952±82Ma (2σ). The oldest zircon exhibits a depleted-mantle Hf model age (TDM) of 2.77Ga and a “crustal” Hf model age (TCDM) of 3.1Ga, whereas the youngest has a TDM of 2.21Ga and a TCDM of 2.35Ga.We interpret the coexistence of both types of chromitites in a horizon of meta-dunites as the result of their precipitation from fractionating island arc tholeiite melts. The first reactions produced dunite; later the batches of magma were isolated from reaction with the peridotite wall-rocks and progressively fractionated in a network of dunite channels developed in the upper mantle beneath the back-arc spreading centre. The Os and Hf model ages suggest that the ultramafic protolith of the Dobromirtsi ophiolite was derived from an original Archean mantle at least 3Ga old, which underlay a continental crust and was reworked in the Paleo-proterozoic and possibly even younger times.

Genesis of alpinotype fissure minerals from Thasos Island, Northern Greece - Mineralogy, mineral chemistry and crystallizing environment.

Alpinotype fissure-minerals in Thasos Island are hosted in gneisses, amphibolites, Mn-rich schists and calc-silicate layers, and marbles of the Carboniferous-Permian Pangeon Unit, which represents the lower tectonostratigraphic unit of the southern Rhodope metamorphic core complex. Alpinotype fissures crosscut metamorphic fabrics and are closely related to the exhumation processes of the core complex during the Oligocene-Miocene. Most mineralized fissures occur close to a major detachment fault, which separates gneisses from marbles and amphibolites. The mineralogy of the alpinotype fissures is closely related to the host rocks: amphibolite-hosted fissures include adularia, albite, quartz, titanite, apatite, actinolite, chlorite, calcite, hematite and rutile. Fissures in para- and orthogneisses- and in metapegmatites are characterized by smoky and clear quartz, adularia, muscovite and hematite. Fissures within spessartite-piemontite schists contain quartz, chlorite, spessartite, hematite, rutile, albite, epidote and traces of zircon. Finally fissures in calc-silicate layers include Mn-grossular, quartz and Mn-clinozoisite. Hydrothermal alteration halos surrounding the fissures may suggest leaching of the wall rocks as a potential mechanism for mineral deposition. Scepter quartz crystals consist of a lower Tessinhabit crystal and several generations of upper prismatic quartz crystals, suggesting several stages of crystallization and changing P-T-x conditions with time. Chlorite geothermometry indicates temperatures of formation in the range between 286 and 366 °C. Tessin habit quartz was deposited from CO2-bearing fluids, probably at the transition from a compressional to an extensional tectonic regime and was later dissolved by meteoric water dominated fluids resulting in the formation of quartz scepters. Oxidizing conditions are indicated by the widespread occurrence of hematite in the mineralization. The studied area represents a unique mineralogical geotope. Its geological-mineralogical heritage should be protected through establishment of a mineralogical-petrological geopark that will also promote sustainable development of the area.

Metamorphism disturbs the Re-Os signatures of platinum-group minerals in ophiolite chromitites

The Os-isotope compositions of platinum-group minerals (PGMs) in ophiolite chromitites are commonly regarded as resistant to fluid-related processes, and have been used to track the evolution of Earth’s convecting mantle. However, we have found significant differences in 187Os/188Os between primary and secondary PGMs from metamorphosed ophiolite chromitites of the Dobromirtsi Ultramafic Massif, in the Central Rhodope Metamorphic Core Complex of southeastern Bulgaria. Primary (magmatic) PGMs hosted in unaltered chromite cores have 187Os/188Os from 0.1231 to 0.1270, and 187Re/188Os ≤ 0.002. T MA and T RD model ages, calculated relative to the Enstatite Chondrite Reservoir, cluster around three main peaks: ca. 0.3, 0.4, and 0.6 Ga. Secondary PGMs, produced by alteration of magmatic PGMs, have a wider range of variation (187Os/188Os = 0.1124–0.1398, 187Re/188Os ≤ 0.024); these grains yield T MA and T RD model ages from –1.7 Ga up to 2.2 Ga. The larger range in 187Os/188Os in the secondary PGMs is interpreted as due to reactions between the primary PGMs and infiltrating metamorphic-hydrothermal fluids with a range of Os-isotope compositions. This redistribution of Os in PGMs during metamorphism has significant implications for the interpretation of both whole-rock and in situ Os-isotope data in mantle-derived rocks.

Is the Palea Kavala Bi–Te–Pb–Sb ± Au district, northeastern Greece, an intrusion-related system?

Intrusion-related gold systems are generally characterized by a Au–Bi–Te ± Sn–W metal assemblage genetically linked to the emplacement of granitoids. The Palea Kavala ore system, Greece, consists of ~ 150 minor Fe–Mn (Pb ± Zn ± Ag), Fe–Mn–Au, Fe–As–Au, Fe–Cu–Au, and Bi–Te–Au occurrences that occur primarily in quartz–calcite–sulfide veins (hypogene mineralization), or as supergene bodies, in overlapping zones centered on the ~ 21–22 Ma granodioritic Kavala pluton, which intrudes metamorphic rocks of the Paleozoic Rhodope metamorphic core complex. The pluton consists mostly of granodiorite with lesser amounts of diorite, tonalite and monzodiorite, which was emplaced along the regional E–W trending Kavala–Komotini fault. The recently discovered, ~ 4 km long, E–W trending so-called Kavala vein is a sheeted quartz vein system of Bi–Te–Pb–Sb ± Au mineralization that crosscuts the Kavala pluton and the schists and gneisses of the Rhodope Massif. The Kavala vein system is comprised of quartz with lesser amounts of K-feldspar, plagioclase and muscovite. Quartz–sericite–pyrite alteration is pervasive but minor kaolinite is also present. Pyrite (~ 5% of vein volume) contains inclusions of tetradymite (some gold-bearing), bismuthinite, and cosalite. Sulfur isotope values (n = 27) of pyrite from the Kavala and Chalkero veins, as well as pyrite and galena from Garizo Hill Fe–Mn–Pb vein range from − 1.9 to 1.0‰ (with one outlier of − 4.6‰) and suggest a magmatic sulfur source. Homogenization temperatures (T h) of type I (two-phase aqueous liquid–vapor) and type II (three-phase, H 2O–CO 2-rich) fluid inclusions that homogenize into the liquid phase in quartz from the Kavala and Chalkero veins range from 216.0° to 420.0 °C (n = 216) and 255.7° to 414.0 °C (n = 112), respectively. The T h of type III (two-phase aqueous liquid–vapor), which homogenize into the vapor phase, ranges from 210.4° to 323.4 °C (n = 28). The salinities of type I and type II inclusions range from 15.9 to 22.6 wt.% NaCl equiv. and 5.5 to 11.2 wt.% NaCl equiv., respectively. Eutectic temperatures of − 58.5° to − 44.3 °C for type I inclusions suggest the presence of appreciable CaCl 2 in addition to NaCl. Clathrate melting temperatures for type II inclusions of ~−56.7 °C indicate that CO 2 is the major component of the gaseous phase, however up to ~ 6% CH 4 is present in some inclusions. The presence of a zoned metallogenetic district centered on Bi–Te–Pb–Sb ± Au mineralization within the Kavala pluton, the presence of both magnetite and ilmenite in the Kavala pluton, and the two high-temperature, high-salinity, immiscible carbonic and aqueous fluids associated with the Kavala and Chalkero veins are consistent with them being part of an intrusion-related gold system that formed along the ilmenite–magnetite buffer.

Geochemistry, petrogenesis and tectonic setting of the Samothraki mafic suite, NE Greece: Trace-element, isotopic and zircon age constraints

The Samothraki mafic suite in the north-eastern Aegean Sea, Greece, is an ‘in situ’ magmatic complex comprising gabbros, sparse dykes and basalt flows and pillows cut by late dolerite dykes. We have determined the age of the complex by SHRIMP zircon geochronology of a gabbro as 159.9 ± 4.5 Ma (i.e. Oxfordian; early Late Jurassic), which precludes any correlation with the so-called Lesvos ophiolite further south (253.1 ± 5.6 Ma; Latest Permian). Six distinct, hitherto unrecognised, geochemical groups have been identified among the basalts and dolerites of the Samothraki mafic suite on the basis of trace-element and Nd–Sr isotopic characteristics. All groups show the presence of an enriched component in their source region, be that a long-term enriched mantle reservoir or an enriched melt. They also invariably show evidence of the addition of subduction-related hydrous fluids. Some groups testify to recent depletion events of long-term enriched mantle, some to the involvement of strongly depleted, recently refertilised mantle, and others to significant addition of melts either from subducted sediments or continental crust. None of the groups bear resemblance to mid-ocean ridge or back-arc basin basalts thus suggesting that the Samothraki mafic suite cannot represent mature back-arc basin crust. In view of recent advances in back-arc basin geology and the data collected here, the Samothraki mafic suite could represent a slab-edge segment of an evolved back-arc basin near the trench, a situation that satisfies the inflow of fertile mantle, the strong signature of sediment melts and the rare occurrence of boninite-like melts. Alternatively, it could represent the embryonic stages of back-arc basin formation either by a propagating rift tip into continental crust or by oblique subduction causing back-arc rifting of continental crust and creation of nascent transtensional basins. Restoration of block configuration in NE Greece before extensional collapse of the Hellenic hinterland and exhumation of the Rhodope Metamorphic Core Complex (mid-Eocene to mid-Miocene) results in a continuous ophiolite belt from Guevgueli to Samothraki, thus assigning the latter to the Innermost Hellenic Ophiolite Belt. We propose that the Samothraki mafic suite originated via rift propagation of the Sithonia ophiolite spreading ridge into the Chortiatis calc-alkaline arc (and its basement) which was partly assimilated. Our model does not favour southward intra-oceanic subduction of the Meliata–Maliac Ocean under the Vardar Ocean whereby Samothraki is located in the back-arc spreading centre. The autochthonous rift-related origin, the enriched geochemical characteristics and the lack of common features with known ophiolite types suggests calling the igneous complex of Samothraki Island a “mafic suite” rather than an “ophiolite”.

Late Cenozoic extension of the Alpine collisional orogen, northeastern Greece: Origin of the north Aegean basin

The Rhodope metamorphic core complex, exposed beneath the Strymon Valley detachment in northeastern Greece, comprises a platform carbonate sequence >5000 m thick intruded by Tertiary calc-alkaline plutons. The final thickening of the north Aegean Alpine collisional orogen in Paleogene time and its Neogene-Quaternary extensional dismemberment in the backarc of the Hellenic subduction zone produced structures within and above the Rhodope core complex that are related here to four successive deformations. In early–middle Eocene time (D1), the Falakron marble series was subducted northeastward beneath the Serbo-Macedonian–West Thracian gneiss complex, a heterogeneous ophiolite-bearing high-grade metamorphic terrane that was accreted to southeastern Europe in Cretaceous time. The orogen began to extend on a northeast-southwest axis in earliest Miocene time, evidenced in the Rhodope core complex by the emplacement of the Symvolon granodiorite ca. 21 Ma within a northwest-trending midcrustal coaxial rupture of the Falakron slab (D2). The Strymon Valley detachment system succeeded the D2 Symvolon rupture and related structures, facilitating unroofing of the core complex and a transition from ductile to brittle deformation ca. 16–3.5 Ma (D3). The Serbo-Macedonian gneiss complex, the island of Thasos, and a supradetachment basin were translated relatively southwestward as much as 80 km in the D3 hanging wall. Balanced reconstructions of D2 and D3 displacements predicate two new hypotheses concerning the Cenozoic tectonic evolution of northeastern Greece. First, the southwest-vergent Thasos detachment and northeast-vergent low-angle normal faults in the Olympos region may both have rooted in the coaxial Symvolon rupture zone to form a bivergent early Miocene extensional system. Second, the Olympos and Falakron carbonate platforms may be correlative remnants of the subducted eastern margin of the Apulian microcontinent, implying that the “Vardar zone,” the putative Alpine suture, is a rootless ophiolite belt, and that segments of the Alpine suture are actually exposed over a zone as wide as 200 km, including the Rhodope province. Cumulative D2 and D3 stretching of greater than or equal to 100% created the north Aegean basin. The North Anatolian fault, which accommodates the westward escape of Anatolia from the Pontide suture, propagated into the north Aegean region in late Pliocene time. Its offshore continuation, the North Aegean Trough, transfers dextral strike-slip displacement into extension to the north, principally expressed in northern Greece by the Thermaikos, Strymon, and Drama half grabens (D4).

Pre-Priabonian Palaeogene formations in southwestern Bulgaria and northern Greece: stratigraphy and tectonic implications

The Paril Formation (South Pirin and Slavyanka Mountains, southwestern Bulgaria) and the Prodromos Formation (Orvilos and Menikion Mountains, northern Greece) consist of breccia and olistostrome built up predominantly of marble fragments from the Precambrian Dobrostan Marble Formation (Bulgaria) and its equivalent Bos-Dag Marble Formation (Greece). The breccia and olistostrome are interbedded with thin layers of calcarenites (with occasional marble pebbles), siltstones, sandstones and limestones. The Paril and Prodromos formations unconformably cover the Precambrian marbles, and are themselves covered unconformably by Miocene and Pliocene sediments (Nevrokop Formation). The rocks of the Paril Formation are intruded by the Palaeogene (Late Eocene–Early Oligocene) Teshovo granitoid pluton, and are deformed and preserved in the two limbs of a Palaeogene anticline cored by the Teshovo pluton (Teshovo anticline). The Palaeocene–Middle Eocene age of the formations is based on these contact relations, and on occasional finds of Tertiary pollen, as well as on correlations with similar formations of the Laki (Kroumovgrad) Group throughout the Rhodope region.The presence of Palaeogene sediments within the pre-Palaeogene Pirin–Pangaion structural zone invalidates the concept of a ‘Rhodope metamorphic core complex’ that supposedly has undergone Palaeogene amphibolite-facies regional metamorphism, and afterwards has been exhumed by rapid crustal extension in Late Oligocene–Miocene times along a regional detachment surface. Other Palaeogene formations of pre-Priabonian (Middle Eocene and/or Bartonian) or earliest Priabonian age occur at the base of the Palaeogene sections in the Mesta graben complex (Dobrinishka Formation) and the Padesh basin (Souhostrel and Komatinitsa formations). The deposition of coarse continental sediments grading into marine formations (Laki or Kroumovgrad Group) in the Rhodope region at the beginning of the Palaeogene Period marks the first intense fragmentation of the mid- to late Cretaceous orogen, in particular, of the thickened body of the Morava-Rhodope structural zone situated to the south of the Srednogorie zone. The Srednogorie zone itself was folded and uplifted in Late Cretaceous time, thus dividing Palaeocene–Middle Eocene flysch of the Louda Kamchiya trough to the north, from the newly formed East Rhodope–West Thrace depression to the south.

U‐Pb and 40Ar/39Ar geochronology of the Symvolon granodiorite: Implications for the thermal and structural evolution of the Rhodope metamorphic core complex, northeastern Greece

North Aegean continental lithosphere was thickened by southwest vergent thrusting and continental subduction within the Alpine collisional orogen but has subsequently been greatly extended on a northeast‐southwest axis in the back arc of the Hellenic subduction zone. Crosscutting relationships with two granodiorite bodies emplaced at ∼31–33 Ma, the Xanthi and eastern Vrondou plutons, constrain a pre‐mid‐Oligocene origin of Alpine convergent structures in northeastern Greece. Post‐Alpine thinning of the north Aegean nappe pile began in earliest Miocene time and has been accommodated by a succession of distinct structural systems. The earliest of these, the “Symvolon shear zone”, appears to represent a midcrustal, coaxial rupture of the Falakron marble series, a carbonate platform >5000 m thick that was subducted northeastward beneath high‐grade rocks of the Rhodope metamorphic province in late Alpine time. Zircon and titanite U‐Pb dates and hornblende 40Ar/39Ar dates obtained in this study constrain the intrusion and incipient mylonitization of the Symvolon or “Kavala” granodiorite within the Symvolon shear zone at ∼21–22 Ma. Following its emplacement, the Symvolon body resided at temperatures between 300°C and 500°C for 5–7 m.y., during which coaxial deformation may have continued within a widening Symvolon rupture. The Strymon Valley detachment, a regionally south‐west dipping low‐angle normal fault, succeeded the Symvolon shear zone in middle Miocene time. Southwestward displacement of the Serbo‐Macedonian gneiss complex by as much as 80 km in the hanging wall of this detachment facilitated the unroofing of the Rhodope metamorphic core complex, including the Falakron marble series and several Tertiary plutons, in its footwall. Biotite and K‐feldspar 40Ar/39Ar dates from 11.1 ± 0.2 Ma to 15.5 ± 0.3 Ma yielded by Symvolon granodiorite samples document the cooling of the Rhodope core complex below 150°C–300°C during its southwestward‐progressive exhumation in the footwall of the Strymon Valley detachment.

Late Cenozoic extension in northeastern Greece: Strymon valley detachment system and Rhodope metamorphic core complex: Comment and Reply

Late Cenozoic extension in northeastern Greece: Strymon valley detachment system and Rhodope metamorphic core complex: Comment and Reply

Late Cenozoic extension in northeastern Greece: Strymon Valley detachment system and Rhodope metamorphic core complex

The Strymon Valley detachment system accommodated at least 25 km of extension at the northern margin of the Aegean extensional province from middle Miocene through early Pliocene time. The primary detachment surface is exposed as a gently southwest-dipping low-angle normal fault—similar to those widely known in core complex settings of the U.S. Basin and Range province—for more than 150 km along strike, from the Aegean Sea coast in northeastern Greece northward into the Struma Valley of southern Bulgaria. Hanging-wall rocks in the Strymon Valley detachment system moved in a S53°W direction relative to the footwall, resulting ultimately in the tectonic unroofing of the Rhodope metamorphic core complex. Several segments of the Strymon Valley detachment have previously been interpreted as thrust faults formed during a Miocene compressional event. Our results negate the principal evidence for this event and imply instead that extensional deformation has characterized the northern Aegean region since at least middle Miocene time. Displacement ceased on the Strymon Valley detachment with the late Pliocene development of the Strymon and Drama basins. We propose that those basins, together with the other principal basins of the northern Aegean region, are subsiding above an active northeast-dipping, extensional detachment zone that forms a unified kinematic system with the western offshore continuation of the dextral North Anatolian strike-slip fault, the North Aegean trough. If this hypothesis is correct, then the North Aegean trough, the western strands of the North Anatolian fault, and the principal modern depocenters of the northern Aegean region may all have originated no earlier than late Pliocene time.