Peter Marchev

2.8-Ma ash-flow caldera at Chegem River in the northern Caucasus Mountains (Russia), contemporaneous granites, and associated ore deposits

Abstract Diverse latest Pliocene volcanic and plutonic rocks in the north-central Caucasus Mountains of southern Russia are newly interpreted as components of a large caldera system that erupted a compositionally zoned rhyolite-dacite ash-flow sheet at 2.83 ± 0.02 Ma (sanidine and biotite 40Ar/39Ar). Despite its location within a cratonic collision zone, the Chegem system is structurally and petrologically similar to typical calderas of continental-margin volcanic arcs. Erosional remnants of the outflow Chegem Tuff sheet extend at least 50 km north from the source caldera in the upper Chegem River. These outflow remnants were previously interpreted by others as erupted from several local vents, but petrologic similarities indicate a common origin and correlation with thick intracaldera Chegem Tuff. The 11 × 15 km caldera and associated intrusions are superbly exposed over a vertical range of 2,300 m in deep canyons above treeline (elev. to 3,800 m). Densely welded intracaldera Chegem Tuff, previously described by others as a rhyolite lava plateau, forms a single cooling unit, is > 2 km thick, and contains large slide blocks from the caldera walls. Caldera subsidence was accommodated along several concentric ring fractures. No prevolcanic floor is exposed within the central core of the caldera. The caldera-filling tuff is overlain by andesitic lavas and cut by a 2.84 ± 0.03-Ma porphyritic granodiorite intrusion that has a cooling age analytically indistinguishable from that of the tuffs. The Eldjurta Granite, a pluton exposed low in the next large canyon (Baksan River) 10 km to the northwest of the caldera, yields variable K-feldspar and biotite ages (2.8 to 1.0 Ma) through a 5-km vertical range in surface and drill-hole samples. These variable dates appear to record a prolonged complex cooling history within upper parts of another caldera-related pluton. Major W-Mo ore deposits at the Tirniauz mine are hosted in skarns and hornfels along the roof of the Eldjurta Granite, and associated aplitic phases have textural features of Climax-type molybdenite porphyries in the western USA. Similar 40Ar/39Ar ages, mineral chemistry, and bulk-rock compositions indicate that the Chegem Tuff, intracaldera intrusion, and Eldjurta Granite are all parts of a large magmatic system that broadly resembles the middle Tertiary Questa caldera system and associated Mo deposits in northern New Mexico, USA. Because of their young age and superb three-dimensional exposures, rocks of the Chegem-Tirniauz region offer exceptional opportunities for detailed study of caldera structures, compositional gradients in volcanic rocks relative to cogenetic granites, and the thermal and fluid-flow history of a large young upper-crustal magmatic system.

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.

40Ar/39Ar geochronology of magmatism and hydrothermal activity of the Madjarovo base-precious metal ore district, eastern Rhodopes, Bulgaria

Abstract The Madjarovo volcanic complex and ore district comprise alteration styles from potassium silicate, advanced argillic and sericite alteration to adularia-sericite alteration/mineralization with a close and unambiguous spatial relationship to specific magmatic events. New 40Ar/39Ar laser fusion and incremental heating experiments on nine sanidine, biotite, adularia, K-feldspar, and alunite samples constrain the ages and time span of lavas and tephras comprising the complex and their relationship to the hydrothermal activity. These results demonstrate that high-K calc-alkaline to shoshonitic volcanic activity began c.32.7 Ma and terminated c.500 ka later with the extrusion of quartz latite lavas at 32.2 Ma. The final stage of volcanism was accompanied by intrusion of compositionally similar monzonite stocks and trachytic dykes (c.32.2–32.1 Ma) and associated barren advanced argillic and sericite alteration (lithocap) and adularia-sericite base/precious metal vein mineralization. A probable thermal event at c.12–13 Ma disturbed the ages of alunite and sericite-bearing alteration at low stratigraphic levels. However, field relations combined with a plateau age of 32.1 ± 0.2 Ma from adularia in low-sulphidation veins that cross-cut lithocap indicate that hydrothermal activity, including base- and precious-metal vein deposition, was coeval with the youngest magmatic activity.

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.

Sedimentary-rock-hosted epithermal systems of the Tertiary Eastern Rhodopes, Bulgaria: new constraints from the Stremtsi gold prospect

Abstract Precious metal epithermal, sedimentary-rock-hosted prospects constitute a new class of ore deposits recently described in the Tertiary Eastern Rhodopes of southeastern Bulgaria. The Stremtsi prospect investigated in this contribution is located in a distal location with respect to the main cluster of sedimentary-rock-hosted Ada Tepe and Rosino gold prospects of the Eastern Rhodopes. The Stremtsi prospect is hosted by a Priabonian clastic sedimentary rock sequence, overlying metamorphic rocks of the Central Rhodopean dome. The eastern part of the Stremtsi prospect contains high gold grades, and is characterized by a strongly silicified zone, including adularia and silicified dolomite blades, diagnostic for boiling conditions during ore formation in such low-sulphidation epithermal systems. The western part of the Stremtsi prospect consists of a barite, sphalerite and galena mineralization, associated with silicification, and illite and carbonate alteration. Both parts are underlain by subvertical quartz-carbonate-pyrite veins. Primary and secondary fluid inclusions, respectively, in dolomite and barite yield homogenization temperatures ranging between 90 and 247 °C. The salinity of primary inclusions in dolomite falls between 1.9 and 5.1 wt% NaCl equivalent, whereas the one of secondary fluid inclusions in barite ranges between 0.0 and 3.1 wt% NaCl equivalent. The variable homogenization temperatures reflect post-entrapment re-equilibration of the fluid inclusions, whereas the salinities were preserved and the inclusions in dolomite are interpreted in terms of dilution of a saline fluid in the western part of the Stremtsi prospect. The sulphur isotope compositions of sulphides from Stremtsi range mainly between −4 and +4‰. They are not diagnostic and can be attributed to magmatic, metamorphic, and sedimentary sources. They overlap with the main compositional range of sulphides from other sedimentary-rock-hosted epithermal systems and reveal the existence of hydrothermal fluids with common characteristics during ore formation throughout the Eastern Rhodopes. In addition, at Stremtsi, negative δ34S values between −42.6‰ and −8.8‰ combined with framboidal pyrite and elevated δ34S values of +7.0‰ to +19.5‰ support locally derived sulphur generated, respectively, by bacterial and thermochemical sulphate reduction. Modelling of O, C, and Sr isotope data of dolomite support the above described ore-forming processes. A positive correlation between δ18O (+12.7‰ to +19.7‰ V-SMOW) and δ13C (−2.8‰ to +1.5‰ V-PDB) values for dolomite from the eastern, silicified and gold-enriched zone of the Stremtsi prospect is satisfactorily modelled by boiling between 140 and 180 °C of a deeply circulating fluid characterized by δ18O and δ13C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively, and radiogenic strontium leached from the metamorphic basement rocks or its clastic counterparts in the Priabonian host rocks. By contrast, negative correlations of δ18O values (+13.4‰ to +23.3‰ V-SMOW) with δ13C values (−0.6‰ to −3.9‰ V-PDB) and 87Sr/86Sr ratios of dolomite from the western, barite and base metal-rich zone are adequately modelled by a shallow, low temperature (70 °C), intra-formational fluid recharged by meteoric water, which interacted with organic matter, that is, coal layers, and carbonate rocks from the Priabonian host sequence, mixing with a deep, moderate temperature (190 °C), 87Sr-enriched fluid characterized by δ18O and δ13C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively. Disequilibrium conditions revealed by sulphur isotope thermometry of two galena-barite pairs yielding discrepant temperatures of 190 and 306 °C are consistent with fluid mixing. A plateau age of 37.57±0.31 Ma obtained by 40Ar/39Ar dating of adularia from Stremtsi is interpreted as a maximum age because of the saddle-shaped age spectrum. Combined with 40Ar/39Ar age data from previous studies, it reveals that the sedimentary-rock-hosted epithermal prospects constitute an independent, regional and older ore-forming hydrothermal system, distinct from the younger volcanic-rock-hosted epithermal deposits of the Bulgarian and Greek Eastern Rhodopes.

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.

Metasomatic reaction phenomena from entrainment to surface cooling: evidence from Mantle Peridotite Xenoliths from Bulgaria

We present a detailed study of the mineralogical and chemical modifications of peridotite xenoliths during magma transport and cooling at the Earth’s surface. The xenoliths are entrained in three small-volume (<1 km3), monogenetic, Miocene–Oligocene, basanite domes in the Moesian Platform, north Bulgaria, arranged along a NNE-directed right-lateral strike-slip fault. The domes show symmetrically decreasing volumes of erupted magma correlated with the size and quantity of the entrained xenoliths, according to their position along the fault. The xenoliths exhibit different degrees of mineralogical and chemical interaction with their host rocks, with the extent of interaction strongly depending on their position in the dome. Xenoliths from the fine-grained brecciated carapaces of the domes show very thin, fine-grained, reaction rims around orthopyroxene and spinel, and thin diffusion zones around olivine, limited mostly to the rims of the xenoliths. Clinopyroxene shows almost no visible reaction and is always strongly depleted in light rare earth elements (LREE) and Sr. Melt pockets in the xenoliths are small, composed mostly of fine-grained olivine and clinopyroxene. Modelling of Fe–Mg zoning in olivine suggests a very short residence time of a few days in the magma during transport and fast cooling at the Earth’s surface. In contrast, xenoliths from the interior of the domes, hosted in holocrystalline groundmass, are much more strongly affected by the host basanite magma. Their constituent minerals have wider reaction rims around orthopyroxene, sometimes leading to its entire consumption, and show transformation of spinel into chromite. Fe–Mg diffusion profiles in olivine are up to 400 μm wide and calculations indicate diffusion times up to 200 days, recording protracted cooling in the inner part of the dome. Melt pockets are much larger and coarser-grained, composed of minerals identical to the host groundmass. With few exceptions, clinopyroxene is sieve-textured and shows variable enrichment in LREE and Sr, ranging from several times higher than in the depleted xenoliths to complete equilibration with the host basanite. Strongly veined xenoliths show stronger chemical modifications, facilitated by infiltrated melt, which also progressively increase depending on the position of the xenoliths in the dome. The most enriched xenoliths from the core of the dome exhibit large inter- and intra-grain variations in Sr and LREE. Our study demonstrates that chemical and mineral modifications, although starting at the time of entrainment of the xenoliths at mantle depths, were completed mostly during their residence in the magma at the surface. The reaction phenomena are the result of post-entrainment partial melting, and reactions between xenolith minerals and infiltrated fluids and melts from the host basalts. The large inter- and intra-crystal chemical variations in a single xenolith suggest that reactions strongly depend on the access of fluids and melts (permeability) in different parts of the xenoliths. The results of this study allow us to introduce the term ‘host basalt metasomatism’ for those mantle xenoliths that have undergone chemical alteration at or near the surface during cooling of the host magma. Comparison with xenoliths stored in large-scale magmatic systems under La Palma (Canary Islands) shows that, although the products of interaction between xenoliths and host rocks are similar, there are considerable differences in the mechanism of entrainment, depth and longevity of the reactions between small-volume and large-scale magmatic systems.

Trace element and isotopic composition of mafic and ultramafic cumulate xenoliths in alkaline basalts from the Eastern Rhodopes, Bulgaria: Inferences on deep processes under the metamorphic core complexes

Oligocene within-plate alkaline basalts, intruded in the Eastern Rhodope metamorphic core complexes Kesebir and Biala Reka, carry ultramafic and mafic xenoliths, consisting of clinopyroxenites, websterites, orthopyroxenites, 2-pyroxene- and clinopyroxene gabbros. These xenoliths are interpreted as samples of layered intrusions formed at upper mantle-lower crustal depths. Calculated parental liquids from LA ICP-MS analyses of clinopyroxene in the ultramafic clinopyroxenites, orthopyroxenite and websterite indicate that they may have formed from melts similar in composition to their host basanitic magma. The clinopyroxene REE patterns of least magnesian websterites suggest amphibole and clinopyroxene fractionation. Trace element compositions of gabbros and high-Fe clinopyroxenites indicate that they are the most differentiated product from the same parent. Calculated parental liquids for some of them, however, show geochemical features typical for subduction-related rocks. In addition, Sr and Nd isotopic compositions of evolved websterites and gabbros are more radiogenic than those of the host basanites. This transition from within-plate to subduction-related signature can be related to fractionation of amphibole, accompanied by crustal contamination of the basanitic magma with more radiogenic lower crustal material.