437 research outputs found
Palaeotethys-related sediments of the Karaburun Peninsula, western Turkey: constraints on provenance and stratigraphy from detrital zircon geochronology
No full textDetrital zircon U-Pb geochronology of 15 Late Palaeozoic to Early Mesozoic siliciclastic sandstones from the Karaburun Peninsula in western Turkey determines maximum sedimentation ages, identifies possible source areas, and anchors the study area within the Palaeotethyan realm. Siliciclastic sandstones yielded ages from Triassic to Archean with major input from Palaeozoic to Neoproterozoic sources and very few Mesoproterozoic zircons. The youngest age groups set the new limit of the maximum depositional ages to Late Carboniferous-Early Permian for the Ku double dagger ukbah double dagger e and DikendagA +/- formations. Detrital zircons from Triassic sandstones are mainly Neoproterozoic and Palaeozoic in age. Zircons from the Scythian-Anisian Gerence Formation are predominantly Devonian and Carboniferous in age, while also Permian and Triassic zircon grains occur in the Carnian-Rhaetian Guvercinlik Formation. According to the zircon age populations and the data available from possible source regions, the Karaburun siliciclastic sediments, with the exception of two samples from the DikendagA +/- Formation, record sediment supply from units located at the southern margin of Eurasia during Late Palaeozoic and Early Mesozoic times. This interpretation is in agreement with palaeotectonic reconstructions for the closely related Greek islands of Chios and Inousses. The presence of Devonian accompanied by Carboniferous zircons in some of the Karaburun samples reveals similarities with Karakaya Complex sandstones of the Sakarya Zone in NW Turkey
Permian–Triassic magmatism in response to Palaeotethys subduction and pre-Late Triassic arrival of northeast Gondwana-derived continental fragments at the southern Eurasian margin: Detrital zircon evidence from Triassic sandstones of Central Iran
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Provenance of the Surveyor Fan and Precursor Sediments in the Gulf of Alaska—Implications of a Combined U-Pb, (U-Th)/He, Hf, and Rare Earth Element Study of Detrital Zircons
No full textEvolution of the Palaeotethys in the Eastern Mediterranean: a multi-method approach to unravel the age, provenance and tectonic setting of the Upper Palaeozoic Konya Complex and its Mesozoic cover sequence (south-central Turkey)
No full textSiliciclastic sediments from the Upper Palaeozoic Konya Complex and its Mesozoic cover were studied by a multi-method approach combining thin-section petrography, bulk-rock geochemistry, mineral chemistry of rutile, and U–Pb geochronology of detrital zircons. Provenance sensitive data of samples from the Upper Palaeozoic Halıcı Formation indicate sediment supply from mainly low- to medium-grade metamorphosed sedimentary rocks of felsic character, while the contribution from volcanic rocks was rare. The detrital zircon record of sediments from the Halıcı Formation documents sediment supply from different sources and excludes a similar provenance. Some samples show great similarities with Palaeozoic sandstones from the cover sequence of the Saharan Metacraton and the Arabian–Nubian Shield, while the other samples indicate a provenance that must be sought in units with a southern Eurasian affinity. The upper limit for sediment deposition in the Halıcı Formation is mostly constrained by Early Palaeozoic zircon populations; however, sediment accumulation in Pennsylvanian–Cisuralian time is more likely, contemporaneously with the Upper Palaeozoic succession on the Karaburun Peninsula (western Turkey). The provenance of sediments from the Upper Triassic Ardıçlı Formation remains enigmatic, but the source should be sought nonetheless in units close to the depositional site. In any case, detrital zircon age spectra and compositional data exclude recycling of underlying rock units (i.e. Halıcı Formation). Overall, our new provenance data reveal great similarities between the Konya Complex and comparable units (Chios, Karaburun) but also highlight distinct differences in terms of sediment composition and provenance.</p
Primitive high-Mg andesites from the Western Alps, Italy: Products of interaction of sediment diapir derived melts with mantle-wedge peridotite in a continental collision zone
No full textMineralogy, geochemistry and C-O isotope study of the iron oxide deposit at Grammatiko, NE Attica, Greece: ore genesis in an extensional setting
No full textThe Grammatiko Iron oxide deposit (NE Attica region, Greece) is one of the economically most significant iron deposits in the Attic-Cycladic metallogenic province which was extensively mined until 1912. Mineralization is hosted in marbles and developed in the form of replacement bodies. These bodies are structurally controlled and follow large subvertical fracture zones with a NNE-trending strike that are associated with the Late Miocene extensional tectonic regime in the region. The mineral paragenesis consists mainly of Fe-Mn-Ba minerals including hematite, Mn-goethite, pyrolusite, hollandite, barite as well as traces of sulfides (e.g. galena). Gangue minerals are mostly calcite, ankerite and minor quartz and muscovite. Supergene minerals include limonite and Fe-Mn and Zn- oxy-hydroxides. Bulk ore analyses show concentrations of Mn up to 3 wt% and high contents of Pb and Cu. Microscopic observations, textural relationships and detailed SEM/EDS analyses showed minor contents of sulfur (<1 wt %) in some coarse grains with tetrahedral morphologies possibly indicating hematite formation after primary pyrite. Carbon and oxygen isotope analyses of carbonates collected along a traverse extending from the ore-bearing zone to the unaltered/non-mineralized marble show δ13C values between −10.5 and −4.5‰ and δ18O values from 20.9 to 24‰. For the non-mineralized marble host rock, δ13C values range from 2.2 to 2.8‰ with δ18O values between 26 and 30.3‰, respectively. The isotopic composition records the variable degree of hydrothermal alteration of the marble host rock as well as the characteristics of the ore-forming fluid. Unmineralized marbles exhibit a marine signature whereas vein calcite in the mineralized parts reflects a contribution of isotopically light carbon, likely from oxidized organic matter. In summary, δ13C and δ18O values from the Grammatiko mineralized carbonates record a complex interaction between marine carbonate and surface (meteoric) fluid
On the Color and Genesis of Prase (Green Quartz) and Amethyst from the Island of Serifos, Cyclades, Greece
No full textThe color of quartz and other minerals can be either caused by defects in the crystal structure or by finely dispersed inclusions of other minerals within the crystals. In order to investigate the mineral chemistry and genesis of the famous prase (green quartz) and amethyst association from Serifos Island, Greece, we used electron microprobe analyses and oxygen isotope measurements of quartz. We show that the color of these green quartz crystals is caused by small and acicular amphibole inclusions. Our data also shows that there are two generations of amphibole inclusions within the green quartz crystals, which indicate that the fluid, from which both amphiboles and quartz have crystallized, must have had a change in its chemical composition during the crystallization process. The electron microprobe data also suggests that traces of iron may be responsible for the amethyst coloration. Both quartz varieties are characterized by isotopic compositions that suggest mixing of magmatic and meteoric/marine fluids. The contribution of meteoric fluid is more significant in the final stages and reflects amethyst precipitation under more oxidizing conditions
Biotite Chemistry from Porphyry-Style Mineralization in Western Thrace, Greece
No full textHydrothermally-altered volcano-sedimentary rocks of Eocene-Oligocene age in western Thrace, Greece, host several
porphyry-epithermal deposits. The Koryfes and Konos Hill at Sapes, Pagoni Rachi, and Papadokoryfi porphyry Mo-Cu-Au-
Re deposits are hosted in subvolcanic rocks of dioritic to granodioritic composition. Their central parts are dominated by
potassic and sodic-calcic/potassic alteration, characterized mainly by biotite, minor K-feldspar, albite, epidote,
hornblende-actinolite, chlorite, and calcite. EPMA data from both magmatic and hydrothermal biotite revealed the
following results: Hydrothermal biotite from the Sapes and Pagoni Rachi areas are relatively enriched in SiO2 and MgO
and thus are characterized as phlogopites, while the magmatic ones are more Fe-rich and fall close to the phlogopite-biotite transition. Fluorine, present in small amounts in the magmatic biotites (up to 1.86 wt. % F), is significantly
enriched in the hydrothermal biotites, with more F-rich compositions coming from the pervasively-altered samples from Koryfes, Sapes area (up to 5.69 wt. % F). In contrast, Ti content is much lower in the hydrothermal biotites, compared to their magmatic counterparts (values up to 4.63 and 8.68 wt. % TiO2, respectively). In the Papadokoryfi area, this trend is
weaker, as TiO2 depletion and SiO2-MgO enrichment is less significant as in the aforementioned areas and fluorine
content does not exceed 2.05 wt. %. Chlorine is present in minor amounts (up to 0.33 wt. % Cl) and cannot be used to
discriminate the two types of biotites. These mineral-chemical differentiations suggest enrichment of Si and Mg and
depletion of Ti under increasing oxygen fugacity conditions, which mark the transition from the late magmatic to the
hydrothermal stage. The magmatic-hydrothermal event is also characterized by a significant introduction of F, which is
reflected in biotites and enhanced by the presence of topaz and zunyite in the lithocaps overlying the cores of the system at Sapes, but also due to the presence of minor fluorite in the stockwork veins, in the Pagoni Rachi area
Rapid growth of phosphorus-rich olivine in mantle xenolith from Middle Atlas Mountains (Morocco, Africa)
No full textPhosphorus(P)-rich zones in olivine may reflect incorporation of P in excess of equilibrium partitioning during rapid growth (e.g. Milman-Barris et al. 2008). We investigated a mantle xenolith from Middle Atlas Mountains (Morocco) by optical microscopy and electron microprobe. It contains spinel-bearing lherzolite and orthopyroxenite layers, cross-cut by veins dominated by glass and secondary phases including P-rich olivines. The host lava, presumed to be alkali basalt (El Messbahi et al. 2015), is present on the margins of the hand sample but not included in our thin section. The studied melt veins (MV) generally contain Ol+Gl+Cpx+Pl+Spl_Ap. Olivines in the MV have (Fo72,1-83,4) with 0.02-0.3 wt.% P2O5; olivines with P2O5 >0.1 wt.% are Fo75,3-82,8. Some olivine grains are inclusion-free; others contain rounded glass inclusions or subhedral spinel or ilmenite inclusions. Olivines is generally found in contact with plagioclase and glass. Glass (5-15 vol%) has variable composition with P2O5 up to 1.52 wt.%, K2O 1.65-2.37 wt%, CaO 6.39-9.55 wt%, Na2O 0.78-6.70 wt% and SiO2 45.2-49.6 wt%. Where glass is in contact with matrix olivine, Fe-rich outer rims on olivine indicate mineral-melt reaction. In MgO variation diagrams, glass compositions display a coherent single trend for all oxides, with the exception of a discrete low-Na group. Clinopyroxene is present both as isolated subhedral to euhedral crystals within the MV and as replacive rims on matrix minerals. Very fine-grained dendritic clinopyroxene quench crystals up to 10 _m long are also present. Plagioclase occurs as prismatic, flow-oriented crystals parallel or sub-parallel to the layering. Spinel shows anhedral and euhedral shapes and occurs both as inclusions in olivine and as discrete grains associated with plagioclase and glass. Spinel in contact with glass shows a spongy outer rim and normal zonation towards Fe-rich rim compositions. Apatite is found mostly as very small crystals embedded in glass. High-resolution X-ray mapping of P in olivine reveals narrow P-rich bands parallel to crystal facets. P correlates negatively with Si4+, poorly with divalent cations (Mg+Fe+Ca), and positively with Al3+, suggesting a substitution 2IV Si4+ =IV P5++IV R3+. Furthermore, P is concentrated mainly at the rim of the olivine, in contact with surrounding glass. DP(OL/MELT) has a wide range (0.02 to 1.6), with the lowest numbers thought to represent equilibrium and higher numbers non-equilibrium partitioning via solute trapping during rapid growth (e.g. Watson et al. 2015). The imperfect correlation between P and Al in our data implies either diffusive relaxation of Al gradients or, judging by dynamic experiments (Grant & Kohn, 2013), cooling rates _1-10°C/h that generate disequilibrium P solute trapping but near-equilibrium Al incorporation. Early-crystallized olivine grew slowly enough to incorporate P by equilibrium partitioning, suggesting that no P-rich boundary layer developed despite slow diffusion of P in melts. Olivine rim crystallization, though was rapid enough to over-enrich P, by more than can be associated with concentration of P into a decreasing mass of residual melt (Shea et al. 2015). The apparent partition coefficient between olivine rims and adjacent melt suggests DP (OL/MELT) in the range 0.13-0.19
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