Structural evolution of the Mokrsko-West,Mokrsko-East and Čelina gold deposits,Bohemian Massif,Czech Republic: Role of fluid overpressure     

《Ore Geology Reviews》2016

The studied Mokrsko-West (90–100 t Au), Mokrsko-East (30 t Au) and Čelina (11 t Au) deposits represent three spatially and genetically interrelated deposits of supposed affiliation to the intrusion-related gold deposit type. The deposits differ in their dominant host rocks, which are represented by ca 354 Ma old biotite tonalite (Mokrsko-West) and Neoproterozoic volcanic and volcanosedimentary rocks (Mokrsko-East, Čelina). Another difference lies in the style of veining — densely spaced networks of 0.1–5 mm thin veins (Q₂) within the tonalite, compared to thick (usually 5–20 cm; Q_1–2) and widely spaced veins within the Neoproterozoic rocks.Five generations of quartz veins, referred to as Q₀ through Q₄ were distinguished: Q₀ veins are the oldest and ore-barren, Q₁ veins mark the onset of the Au-ore formation, Q₂ veins its culmination and Q₃ veins its fading. Late quartz gangue (Q₄) is associated with uneconomic Ag–Pb–Zn vein-type ores hosted by calcite–barite–(quartz) veins.Quartz vein thickness (~ 0.3 to ~ 300 mm), spacing (~ 3 mm to ~ 500 mm), distribution, and related extensional strain (ca. 3–25%) evolve systematically across the studied ore district, reflecting both the major host rock and other tectonic factors. Detailed study of vein dimension parameters (thickness, length, width, aspect ratios) allowed estimation of the probable depth of the fluid source reservoir (~ 2 km or ~ 4 km) below the present surface. The depth to the fluid source seems to increase through time, being the shallowest for the Q₀ veins and the deepest for the Q₂ veins. Two independent methods of estimating fluid overpressure are discussed in the paper. Fluid overpressure during vein formation decreases from the Q₀ through the Q₂ veins, from 10 to 4 MPa or from 26 to 10 MPa, depending on the assumed tensile strength of the tonalite (5.5 and 15 MPa, respectively).The origin of joints and veins is discussed in terms of the stress orientation and crack-seal and crack-jump mechanisms. Field relationships unambiguously indicate that the veins hosted by Neoproterozoic rocks originated by reopening of the pre-existing extension joints (J₁) due to fluid overpressure. The origin of the densely-spaced thin veins (Q₂) hosted by the tonalite at the Mokrsko-West deposit is, however, less certain. It is probable that the tonalite was already affected by microfracturing analogous to the J₁ joints prior to the formation of quartz veins.The formation of the Q_1–2 veins at the Mokrsko-East deposit was constrained by the Re–Os dating of molybdenite to 342.9 ± 1.4 Ma. The ore-bearing hydrothermal system is thus ca 12 Ma younger than the tonalite that hosts the Mokrsko-West deposit. A similar ca 15–2 Ma difference between the age of the host-intrusion and the age of the hydrothermal event was encountered in several other gold deposits in the vicinity of the Central Bohemian Plutonic Complex. Two hypotheses to explain this are discussed in the paper.  相似文献   

Relation between mantle shear zone deformation and metasomatism in spinel peridotite xenoliths of Jeju Island (South Korea): Evidence from olivine CPO and trace elements     

《Journal of Geodynamics》2011,51(5):424-440

This study presents geochemical and fabric analysis of coarse-grained, porphyroclastic and mylonitic spinel peridotite xenoliths derived from the shallow subcontinental lithospheric mantle of Jeju Island (South Korea). Fabric analysis of olivines in the studied peridotites shows activation of the high temperature (0 1 0)[1 0 0] slip system; however, in the porphyroclastic and mylonitic peridotites, activation of (0 k l)[1 0 0] slip system results in a weaker fabric. Formation of porphyroclastic and mylonitic fabrics are thought to form in a shear-dominated environment. The results of the trace element analysis reveal that the smaller the grain size and weaker the fabric, the more enriched in LREE and HFSE are the peridotites, which indicates a strong relationship between metasomatic agents and mantle shear zones.  相似文献   

Development of habitat models for Nephtys species (Polychaeta: Nephtyidae) in the German Bight (North Sea)     

《Journal of Sea Research》2009,61(4):276-291

The aim of the study was the development of habitat models for Nephtys species (Polychaeta: Nephtyidae). The investigation area was the German Bight, the southeastern part of the North Sea. Models were developed based on field data collected between 2000 and 2006. In addition, data on environmental variables were retrieved from long-term monitoring data sets and from the sediment map by Figge [Figge, K., 1981. Nordsee. Sedimentverteilung in der Deutschen Bucht. Map No. 2900. Publisher: Deutsches Hydrographisches Institut, Hamburg]. The statistical modelling technique used was multivariate adaptive regression splines (MARS). Models were fitted individually for each species. Evaluation of predictive discrimination and predictive accuracy of the developed models was by calculation of the area under the receiver operating curve (AUC) or sensitivity and specificity, respectively. Habitat models with best predictive fit were selected for the presentation of habitat suitability maps.Six Nepthys species were found: Nephtys assimilis, N. caeca, N. cirrosa, N. hombergii, N. incisa and N. longosetosa. N. hombergii was most common whereas N. incisa and N. longosetosa were rare. Habitat preferences varied considerably among the species. For all investigated Nephtys species except N. longosetosa a habitat model could be developed based on four predictor variables. The habitat models with best predictive fit were those for N. cirrosa and N. hombergii. The N. caeca habitat model was of limited predictive accuracy and only accept predictive discrimination. The number of predictors as well as the relative importance of the respective predictors in the model varied among the different species. Direct comparison of most suitable habitats for the different species based on modelling revealed that in the mostly sandy regions parallel to the German coast in water depths up to 20 m an overlap between N. caeca, N. hombergii and N. cirrosa exists. In the deeper central German Bight with mostly fine sands with increased mud contents N. hombergii, N. assimilis and, at least partially and rare in numbers, N. incisa co-occur. It can be concluded that important sediment characteristics like grain size median and mud content as well as water depth and mean salinity are useful parameters to describe the habitat requirements of most Nepthys species in the German Bight. However, additional variables need to be incorporated into such analyses.  相似文献   

Distribution of porphyry deposits in the Eurasian continent and their corresponding tectonic settings     

《Journal of Asian Earth Sciences》2014

In the Eurasian continent there are three huge metallogenic belts of Cu and Mo porphyry deposits, comprising the Paleozoic Central Asian Ore Belt in the north, the Tethyan Eurasian Ore Belt of Jurassic to Cenozoic age in the southwest, and the East Margin Ore Belt of the Eurasian Continent of Jurassic to Cretaceous age in the east. The latter is considered to be part of the vast Circum-Pacific ore belt. Some of the main features of the spatial–temporal distribution of Cu and Mo porphyry systems and related geodynamic processes of the three metallogenic belts are described. In particular, the key role of post-subduction – related porphyry ore systems is emphasized, comprising collisional and post-collisional Cu–Mo porphyry deposits during the geological history of the Eurasian continent. The recurrent feature of these ore systems and related felsic rocks is their derivation from partial melting of stagnant or residual oceanic slabs, and mixing with a variable amount of crustal material during magma ascent to shallower levels.  相似文献   

Pacific superplume-related oceanic basalts hosted by accretionary complexes of Central Asia,Russian Far East and Japan     

《Gondwana Research》2010,17(3-4):587-608

Plume-related oceanic magmatism form oceanic islands, seamounts and plateaus (hereafter “seamounts” or “paleoseamounts”), which are important features in geological history. The accretion of oceanic seamounts to active continental margins significantly contributed to the formation of the continental crust. This paper reviews occurrences of Late Neoproterozoic–Mesozoic seamounts of the Paleo-Asian and Paleo-Pacific oceans, which are hosted by accretionary complexes (ACs) of Russian Altai, East Kazakhstan, Mongolia, Russian Far East and Japan. The paleoseamounts commonly consist of Ti–LREE–Nb-enriched plume-related basalts (OIB-type or intraplate basalts) capped with massive limestone and associated with other units of oceanic plate stratigraphy (OPS): oceanic floor basalts (MORB), pelagic chert, epiclastic slope facies, etc. The paper presents available geochemical data on the plume-related basalts including the first geochemical data on the Middle Paleozoic OIB-type basalts of the Paleo-Asian Ocean hosted by the Ulaanbaatar AC of Mongolia. An emphasis is made for the structural setting of OPS units, specific geochemical features of intraplate basalts, problems of their identification, and distinguishing from magmatic units of a different origin such as MORB, island-arc and back-arc basalts. Finally, we propose a continuous, though periodical, evolution of the Pacific superplume-related magmatism, which can be more reliably proved by studying Middle Paleozoic OPS units hosted by ACs of Mongolia and Tien Shan, and discuss prospects of future studies.  相似文献   

Relative sea level changes and regional tectonic evolution of seven coastal areas in NW Greece since the mid-Holocene     

《Quaternary Science Reviews》2007,26(7-8):894-919

This study presents relative sea level (RSL) curves for seven coastal areas in Akarnania and the northwestern Peloponnese (NW Greece) since the mid-Holocene. RSL fluctuations are deduced from 48 ¹⁴C-AMS dated sedimentological sea level markers from 27 vibracores drilled in near-coast geological archives as well as from six geoarchaeological sea level indicators of known ages. Seven palaeo sea level curves including uncertainty bands are reconstructed for a coastal zone spanning a distance of 150 km. Considerable intra-regional differences in sea level evolution exist. These differences are mainly due to tectonic reasons. In general, RSL in northwestern Greece has never been higher than today. Rates of local sea level rise were highest until 5500–5000 cal BC (up to 12.3 m/ka) and lowest during 4000–500 cal BC (0.2–1.4 m/ka). During the past 2500 or so years, RSL has accelerated anew (0.7–2.7 m/ka). Calculating differences between local mean sea level curves provides quantitative information on intra-regional differences of tectonic activity. The coastal plains of Palairos and Elis show signs of uplift, whereas the Mytikas and Boukka plains are strongly subsiding. Compared to other areas of the eastern Mediterranean, northwestern Greece has been subject to significant net long-term subsidence. Regional tectonic events (RTEs) were detected for the time around 4000, 2500, 500 and 250 cal BC as well as around 250 and 1250 cal AD. RTEs are characterized by changes of uplift/subsidence rates or by the redirection of local tectonic movements. The question if some of the RTEs were of a supra-regional nature is still open. From a geodynamic point of view, the results presented show that Akarnania's southwestern fringe is being downwarped while the tectonic block as a whole is moving towards the southwest. Strongest subsidence rates are observed for central Akarnania. At Akarnania's fringes, subsidence is reduced by the influence of strong uplift of adjacent areas such as around Preveza and the northern Peloponnese.  相似文献   

U–Pb (zircon) and geochemical constraints on the age,origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu–Au district,southern Mongolia     

Alan J. Wainwright  Richard M. Tosdal  Joseph L. Wooden  Frank K. Mazdab  Richard M. Friedman 《Gondwana Research》2011,19(3):764-787

Uranium–Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu–Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (ε_Nd(t) range from + 3.1 to + 7.5 and ²⁰⁶Pb/²⁰⁴Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have < 1.5% TiO₂) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu–Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium–Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu–Au deposits are ~ 372 Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu–Au mineralization are ~ 366 Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu–Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu–Au deposits from younger magmatic suites in the district.  相似文献   

Permian basaltic rocks in the Tarim basin,NW China: Implications for plume–lithosphere interaction     

《Gondwana Research》2011,19(4):596-610

There are large areas of Permian basaltic rocks in the Tarim basin (PBRT) in northwestern China. Precise Ar–Ar dating of these rocks revealed an eruption age span of 262 to 285 Ma. Most of the PBRT is composed of alkaline basaltic rocks with high TiO₂ (2.43%–4.59%, weight percent), high Fe₂O₃ + FeO (12.63%–17.83%) and P₂O₅ (0.32%–1.38%) contents. Trace elements of these rocks have affinities with oceanic island basalts (OIB), as shown in chondrite normalized rare earth elements (REE) diagrams and primitive mantle normalized incompatible elements diagrams. The rocks show complex Sr–Nd isotopic character based on which they can be subdivided into two distinct groups: group 1 has relatively small initial (t = 280 Ma)⁸⁷Sr/⁸⁶Sr ratio (∼ 0.7048) and positive εNd(t) (3.42–4.66) values. Group 2 has relatively large initial ⁸⁷Sr/⁸⁶Sr ratio (0.7060–0.7083) and negative εNd(t) (from − 2.79 to − 2.16) values. Lead isotopes are even more complex with variations of (²⁰⁶Pb/²⁰⁴Pb)t, (²⁰⁷Pb/²⁰⁴Pb)t and (²⁰⁸Pb/²⁰⁴Pb)t ranging from 17.9265 to 18.5778, 15.4789 to 15.6067 and 37.2922 to 38.1437, respectively. Moreover, these two groups have different trace elements ratios such as Nb/La, Ba/Nb, Zr/Nb, Nb/Ta and Zr/Hf, implying different magmatic processes. Based on the geochemistry of basaltic rocks and an evaluation of the tectonics, deformation, and the compositions of crust and lithospheric mantle in Tarim, we conclude that these basaltic rocks resulted from plume–lithosphere interaction. Permian mantle plume caused an upwelling of the Tarim lithosphere leading to melting of the asthenospheric mantle by decompression. The magma ascended rapidly to the base of lower crust, where different degrees of assimilation of OIB-like materials and fractionation occurred. Group 1 rocks formed where the upwelling is most pronounced and the assimilation was negligible. In other places, different degrees of assimilation and fractionation account for the geochemical traits of group 2.  相似文献   

Taphonomy of a South African cave: geological and hydrological influences on the GD 1 fossil assemblage at Gondolin,a Plio-Pleistocene paleocave system in the Northwest Province,South Africa     

《Quaternary Science Reviews》2007,26(19-21):2526-2543

Occasional excavation of in situ and ex situ deposits at the formerly mined Gondolin paleocave system has yielded large and diverse samples of Plio-Pleistocene faunas, including isolated hominin and non-hominin primate remains. In 2003, new excavations into naturally decalcified, in situ sediments near the GD 1 datum point near the northwest corner of the cave system were undertaken. This paper describes the recovered faunal remains, taphonomy of the assemblage, and the geological and paleomagnetic context of the GD 1 deposits. The deposits represent a series of inter-stratified speleothem, in-washed sediments and talus deposits we suggest date to a time period prior to, and just after, the Olduvai normal-polarity event at around 1.7–1.8 Ma. Surface sediments and clasts were introduced into the cave by rain water runoff entering a vertically-oriented entrance that had formed along a rift in the area of GD 1. The faunal assemblage consists primarily of fragmentary diaphyseal fragments and isolated teeth. Taxonomically, the small collection of specifically identifiable bovid and equid fossils is generally consistent with remains previously excavated from in situ deposits in the Gondolin paleocave system (GD 2) and dated to around 1.8 Ma; however, the depositional histories of these two assemblages from Gondolin are remarkably different. The preservation and relative proportions of recovered skeletal elements at GD 1 is consistent with these materials having been initially accumulated outside the karstic system near the vertical cave entrance, and then later hydrologically sorted and deposited inside the cave. The sporadic to continuous water flow into the northwest corner of the cave system during the Pleistocene gradually decalcified the excavated fossilbearing breccias and further modified the composition and spatial distribution of the fossil assemblage by introducing potentially younger deposits and skeletal materials. This study highlights the variation in taphonomic processes that can occur within a single cave system, and the complex pre- and postdepositional geological and hydrological processes that can influence the taphonomic history of South African Plio-Pleistocene karstic fossil assemblages.  相似文献   

Late Cenozoic development of the Strouma and Mesta fluviolacustrine systems,SW Bulgaria and northern Greece     

《Quaternary Science Reviews》2007,26(22-24):2783-2800

The Strouma and Mesta are two of the largest rivers that drain across SW Bulgaria and northern Greece into the northern Aegean Sea. Their modern valleys, flanked by Quaternary river terraces, are incised into a diverse landscape, which records the region's complex tectonic history. A network of lacustrine basins existed in the region in the Late Oligocene to earliest Miocene, but was disrupted by thrusting and folding related to Early Miocene transpression. This deformation was followed by a period of erosion, covering most of the Early and Middle Miocene, which probably marked the initiation of the Strouma and Mesta fluviolacustrine system, with geometries unrelated to the older systems. The first clear evidence of these river systems dates from the Middle Miocene (late Badenian to Sarmatian). The systems evolved in the Late Miocene (Maeotian to early Pontian), when lakes existed, characterized by diatomaceous algae and by occasional burial of abundant plant fragments and coal formation. Areas in the south, south of the Kerkini fault, were intermittently submerged beneath the Aegean Sea at this time. Intense localized uplift of horst blocks in late Pontian and Pliocene, associated with crustal extension, resulted in deposition of thick alluvial fans, with tilting of sedimentary successions in adjacent grabens evident by the end of the Pliocene. The highest horsts (Osogovo, Rila, Pirin, and Belasitsa) experienced additional uplift in the Pleistocene, in part as a result of regional uplift and in part through continued normal faulting. Pleistocene climate change also resulted in influxes of glacial and fluvioglacial systems. The present form of the Strouma and Mesta fluviolacustrine systems is thus the result of interplay between crustal extension, regional uplift, and global climate change.  相似文献