Corundum mineralization in the marbles from the vicinity of Mishevsko village, the Eastern Rhodopes, Bulgaria - preliminary data

Ruby and sapphire occur as corundum mineralization in marbles near the village of Mishevsko, Eastern Rhodopes, Bulgaria. Corundum samples were investigated using UV-Vis, Raman and luminescence spectroscopy. UV-Vis spectra show characteristic absorption bands of Cr³⁺ at ~405 and ~550 nm, while sharp R1–R2 emission lines near 693 nm confirm Cr3+ as the principal activator of red coloration. Variations in the UV-blue region (350–450 nm) are attributed to charge-transfer transitions involving traces of Fe, V and Ti, influencing hue saturation.

Graphitization degrees and temperatures of the regional metamorphism in the Central and Eastern Rhododpes have been determined by the values of the structural parameter d002 (Å) of graphite in graphite-bearing marbles and shists, from Vacha and Madan-Erma Reka areas (Madan lithotectonic unit), Ardino–Nedelino area (Startsevo lithotectonic unit), and Chernichevo–Boturche area (Byala Reka lithotectonic unit). Equations of different authors, formulated between 1951 and 2021, were used. They are integrated in one system, allowing a conversion of the results and deriving new quantitative correlations between the temperature of metamorphism, the structural parameter d002 (Å) and the degree of crystallinity order of semi-graphite and graphite. The comparison of the data allows the creation of a new scheme, GD(0–30), based on the values of the parameter d002 (Å) of the natural carbon matter. The validity range of this geothermometer is large, from 84 °C to 804 °C (from zeolite to granulite facies of the regional metamorphism). The temperature peak of metamorphism in the studied areas is 660 °C (i.e., the upper limit of the amphibolite facies), and the lowest temperature is 468 °C, which is characteristic for the lower part of the greenschist facies P-T field. The temperature range and graphitization degrees of the regressive metamorphism in the Central and Eastern Rhodopes are: 660 °C (GD(0–30) = 24, well-crystallized graphite); 588 °C (GD(0–30) = 21, graphite); 564 °C (GD(0–30) = 20, graphite); 516 °C (GD(0–30) = 18, graphite); 492 °C (GD(0–30) = 17, graphite); and 468 °C (GD(0–30) = 16, graphite). The calculated temperatures of metamorphism in the Central and Eastern Rhodopes, by equations of all authors, correspond to the conditions of metamorphism established by other methods without using of the structural parameter d002 (Å) of the graphite. The GD(0–30) system for determination of graphitization degree of the natural carbon is simple, informative and functional. It is more convenient than earlier schemes showing correlation: metamorphic temperature − d002 (Å) − graphitization degree. In addition to the influence of the temperature, the values of the degrees of graphitization also include the effect of all secondary factors on the graphitization processes.

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 remains unclear.

Two Mn and Fe nodule bearing early Oligocene volcano-sedimentary sequences located to the north and north-west of Perperek village in the Eastern Rhodopes were studied. The main feature of the sequences is a manifold rhythmic alternation of medium to coarse grained sandstones and granule conglomerates intercalated by thin clayey layers. Two types of nodules: (i) oval, ochreous and red, Fe-bearing; (ii) spherical, black, Mn-bearing were studied. Mn nodules are composed of quartz, cristobalite, plagioclase, and holandite. Typical are high contents of Mn, Ba, Pb, V, Co, Mo, W, Tl, and REE. Fe nodules contain chamosite and show high content of Si and Al and P, V, Co, Cu, REE. Geochemical data support diagenetic origin of the studied Mn and Fe nodules.

The geotectonic setting for plumasite-type corundum occurrences is understudied, even though it is of importance for the understanding of trace-element patterns used for fingerprinting of ruby and sapphire. Mineral reactions related to metasomatism caused by pegmatite intrusion into ultramafic rock result in a characteristic trace element signature in corundum and thereby control its colour. The Nattivit area, Isertoq Terrane, South-East Greenland, provides a natural laboratory to investigate these mineral reactions and corundum trace element patterns given the excellent exposure of a typical plumasite-type occurrence where pegmatites intruded ultramafic rocks of different composition, namely lherzolite and dunite. The pegmatite dykes are 10 to 50 cm wide in the ultramafic rocks, whereas the adjacent alteration zone in the ultramafic rock reaches widths between 10 and 30 cm. Metasomatism resulted in desilication of the pegmatites and a decrease in Ca, Mg, K, Mn, Al and Fe away from the centre of the pegmatite dyke. Chromium, Ni, Mg, Fe, Sc, Co, V, Zn, Ti and Mn in the metasomatic reaction zones are predominantly derived from the ultramafic rock. We identify three zones with different mineral assemblages. In the lherzolite, tschermakite and biotite are formed in the centre of the reaction zone, which is followed by anorthite-rich plagioclase, hercynite, dolomite and ultimately pink corundum that occurs in the most heavily reacted part of the metasomatic reaction zone. The metasomatic reaction zones in the ultramafic rock include an intense reactionzone at direct contact to the pegmatite dyke with biotite and actinolite, and two alteration zones further away from the pegmatite dyke with enstatite, actinolite, anthophyllite, phlogopite, dolomite, sulphide, apatite and chrome-spinel. In the dunite, no biotite formed and hence, corundum contains more Fe, Mg and Ti. The plumasite-type corundum from Nattivit contains more Fe than Cr, which is typical of pink corundum hosted in mafic-ultramafic rocks. The corundum-forming reaction is dated from the pegmatite vein to 1843±4 Ma (U-Pb zircon age), which is coinciding with convergence of the Rae and North Atlantic cratons resulting in the Nagssugtoqidian Orogen. Only syn-tectonic, corundum-normative, peraluminous, calc-alkaline pegmatites of granitic to granodioritic composition that intrude into ultramafic rocks in the upper plate formed corundum in this area. The pegmatites are classified as muscovite class granitic pegmatites and intruded at upper amphibolite facies conditions. These pegmatites possible originated from partial melting of mafic granulite or a subducted oceanic plate. The formation of granitic pegmatites and related corundum mineralization in the upper plate of a collisional orogen described here is comparable to other corundum occurrences, e.g. Polar Urals, and thus is regarded as a typical geotectonic setting for plumasite-type corundum.

Formation of corundum and associated mineral zones in the hybrid ultramafic-pegmatite association of the Neoproterozoic Hafafit core complex, South-Eastern Desert, Egypt

The transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by 40Ar/39Ar laserprobe mica analyses. Preservation of pre-Alpine (~ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time. © 2000 Editions scientifiques et médicales Elsevier SAS

Middle-Late Alpine thermotectonic evolution of the southern Rhodope Massif, Greece

This paper presents a geological interpretation of the deep seismic profile Ivaylovgrad–Ardino, which was published in 1996. Four plates are distinguished according to their seismic features that build up the Eastern Rhodope Terrane. They have a total thickness of 22–24 km and layered structures with sub-oceanic character. The lower two plates (1, 2) are westward obducted on the Rila-Western Rhodope Terrane, forming one obduction complex. Plate 3 is probably also a part of the obduction complex. Plate 4 is thrust southward on all plates. A Kobilino Crypto-dome, comprising Plates 1 and 2 and covered discordantly by the third one, is found at the eastern part of the profile at 7–7.5 km depth. The two lower plates do not appear on the surface, but some ultrabasic to basic bodies are supposed to be parts of them, rising as tectonic slices. The Plate 3 is revealed on the surface at the eastern part of the seismic profile (east of Avren Fault), and is built up mainly by the Pre-Rhodopean Supergroup metamorphic rocks, respectively from the seismically indistinguishable Upper and Lower Allochthon. The Plate 4 appears mainly at the western end of the profile (between Mishevsko Village and Ardino Town), and is built up by the metamorphic rocks of the Rhodopean Supergroup from the Startsevo Lithotectonic Unit (or Middle Allochthon). Five fault and shear zone systems of different ages are distinguishable in all plates. Transparent areas in the plates are interpreted as magma chambers of the Paleogene volcanoes, others as Upper Cretaceous and Paleogene (?) plutons. Because these magmatic chambers are located in the sub-oceanic crust, both intermediate and acid Paleogene volcanics have isotopic characteristics similar to those of the metasomatized mantle.

Te relevance of the work is due to the need to improve the efciency of appraisal work on new and unconventional types of gold mineralization within the known ore-placer regions and clusters. Te purpose of the work is to show the results of the study of the spatial and genetic relationship between endogenous and exogenous gold content using the example of the Pervomaisk-Zverevsky ore-placer cluster (OPC). Research methodology. Mineralogical and geochemical sampling of ore bodies outcrops, chemical weathering crusts; schlicho-mineralogical studies; study of native gold on a digital basis, taking into account the shape, size and composition of particles; mapping of parameters of gold particles in the section of the crust, the upper horizon of the Mesozoic eluvium. Results. Te factors that determine the localization of gold content within the OPC are substantiated. Among the endogenous factors: structural-tectonic, magmatic, metallogenic specialization of individual blocks. In the volume of the Verkhotursky granodiorite massif (an object of prospecting), a linear substockwork with a low level of mineralization of the gold-polysulfde-quartz type was revealed. When considering exogenous factors, the confnement of ore-bearing chemical weathering crusts to smoothed relief areas, comparable with Mesozoic denudation leveling surfaces and containing dispersed gold mineralization in the basement, was established. Te profle of the weathering crust in the Verkhotursk area is complex, without a distinct vertical hypergene zonality. Tese crusts are considered as “superimposed”, fnally formed under the infuence of later low-temperature argillization processes. Te most signifcant concentrations of gold are established in the upper horizon of the crust - the zone of the Mesozoic eluvium. Based on the mineralogical and geochemical study of metal particles, a model of its accumulation, sufusion-gravity, is proposed. Under certain conditions, a gold-enriched crustal horizon can have an independent applied value. In past years, gold-bearing areal accumulations (poddernikovye placers) were successfully mined. When mapping free gold, a wide range of sample fuctuations (from low- to high-grade) was established, indicating in favor of the multi-stage processes of ore formation in the OPC area. Conclusions. Many years of experience in the geological-geomorphological, mineralogical-geochemical study of the OPC of the Ural region has been tested by the practice of forecasting and prospecting. Te approaches outlined, taking into account the close spatial relationship between endogenous and exogenous gold content, are recommended for implementation and further improvement.

Due to its meteorological, geological and geomorphological settings, Brazilian’s Atlantic coast have suffered severe landslide phenomena, particularly along the cost of São Paulo, Rio de Janeiro, and Santa Catarina states. Studies have shown that these processes are complex and triggered by several factors. However, within the study area, many landslide locations are placed on gneiss migmatitic terrain with pronounced geological discontinuities. The objective of this paper is to assess landslide occurrences in Serra do Mar (Caraguatatuba, SP) and investigate, by means of kinematic analysis, how they correlate with foliation and fractures present on the rocks. Kinematic analysis is used to investigate which discontinuities are more favorable to landslides occurrence and their relationship with slope directions. Structural information and kinematic data models are used to evaluate cause–effect relationship of landslide processes. Results show that dip slopes, with NE–SW direction, have greatest potential for the occurrence of planar landslides. On the other hand, anti-dip and cross-dip slopes have a greater potential to develop wedge landslides. Rock weathering and fluid flux along foliation planes can intensify mass movement. Such effect is also seen in residual soil, since they are able to preserve relict structures of the rock. This study contributes to a better understanding of how geological structures, preserved in tropical residual soils, influence shallow landslides.

Paleoproterozoic structural evolution of the southern segment of the Jiao-Liao-Ji Belt, North China Craton

Timing and nature of the partial melting processes during the exhumation of the garnet–bearing biotite gneiss in the southern Altyn Tagh HP/UHP belt, Western China

Petrofabric, P-wave anisotropy and seismic reflectivity of high-grade tectonites

The study is focused on geological mapping, petrography and metamorphism of Jhyallaphat–Barpak–Bhachchek area, a part of Gorkha District, Central Nepal using a base of 1:25000 scale covering an area of 139.80 sq. km. The rocks of the study area can be broadly divided into two tectonic zones; the Lesser Himalaya consisting of fie lithological units, and the Higher Himalaya consisting of Formation I of the Tibetan Slab. Three metamorphic zones can be distinguished in the study area; biotite zone, garnet zone and kyanite zone. The biotite zone of the mineral assemblage in pelitic rocks consists of biotite+muscovite+chlorite+quartz, in psammitic rocks comprises of biotite+muscovite+chlorite+feldspar+quartz and in carbonate rocks comprises of biotite+muscovite+calcite/dolomite+feldspar+quartz, respectively. These mineral assemblages show that the area belongs to the greenschist facies. The mineral assemblage of the garnet zone in pelitic rocks constitutes garnet+biotite+muscovite+chlorite+quartz, and in psammitic rocks constitutes of garnet+biotite+muscovite+feldspar+quartz. The minerals assemblages found within the biotite and garnet zones represent the well-known inverted metamorphism in the Lesser Himalaya. Mineral assemblage of the kyanite zones constitutes of kyanite+garnet+biotite+muscovite+feldspar+quartz. The mineral assemblages of the both garnet and kyanite zones show that the area belongs to the epidote amphibolite facies.
 The bedding and foliation planes are almost parallel, showing that isograds also cut across the foliation. Therefore, the main metamorphic event should have followed development of foliation in the area. The rocks of the area show at least two metamorphic events: syntectonic prograde and post-tectonic retrograde. Syn-tectonic prograde metamorphism (M1), which has grown during a single phase of deformation and most frequently encountered garnet prophyroblast. Metamorphic deformation is represented by the presence of metamorphic foliation, stretching lineation, and S-C fabric. Post-tectonic retrograde metamorphism (M2), which is followed by retrograde mineral formation changing its P-T condition from high to low grade minerals, such as the formation of the biotite and chlorite minerals around the rims of the garnet porphyroblasts.