Testing the validity of detrital zircon chemistry as a provenance indicator

Zircon grains from the Nile Delta beach sediments were mineralogically investigated for their provenance perspective. Electron microprobe backscattered electron images, and wavelength dispersive spectroscopy line scans showed well-developed oscillatory and sector zoning in the majority of the studied zircon grains. In addition, some grains showed inclusions represented mainly by apatite. The zoning in zircon reflects compositional variations of Zr, Si, Hf, P, Y, and rare earth elements (REE). Based on the electron probe microanalyzer data, the calculated formula of the studied detrital zircon is ^A(Zr_3.82Hf_0.05Th_0.003Ca_0.10Fe_0.008REE_0.033Tl_0.03Y_0.001)∑_4.04 ^T(Si_3.90P_0.08Al_0.011V_0.003Sc_0.004)∑_4.00O₄. Th, Y, P, Sc, and Fe are substituted into zircon either by simple or coupled substitution mechanisms. Additional zircon grains separated from Abu Swayel granites and metamorphic gneisses, south Eastern Desert of Egypt were similarly investigated for comparison. Although evidence based on morphological characteristics, zoning, apatite inclusions, and mineral chemistry supports a derivation of zircon from granitic rocks including the hydrothermally altered varieties, the possibility that the zircon grains may have more than one source cannot be ruled out due to the large area of the Nile watershed.     

Zr-in-rutile thermometry in blueschists from Sifnos, Greece

Zr-in-rutile thermometry on samples of blueschist from Sifnos, Greece, yields temperatures that reflect progressive crystallization of rutile from ca. 445 to 505°C with an analytical precision of + 18/−27 and ± 10°C using the electron microprobe and ± 1.5–3.5°C using the ion microprobe. Individual grains are generally homogeneous within analytical uncertainty. Different grains within a single sample record temperature differences as large 55°, although in most samples the range of temperatures is on the order of 25°. In several samples, Zr-in-rutile temperatures from grains within garnet are lower than temperatures from matrix grains, reflecting growth of rutile with increasing temperature of metamorphism. Although the specific rutile-producing reactions have not been identified, it is inferred that rutile grows from either continuous reaction involving the breakdown of lower grade phases (possibly ilmenite), or from pseudomorph reactions involving the breakdown of relic igneous precursors at blueschist-facies conditions. No systematic variation in rutile temperatures was observed across the blueschist belt of northern Sifnos, consistent with the belt having behaved as a coherent block during subduction.     

Linking juvenile fish and their habitats: An example from Narragansett Bay, Rhode Island

We used two methods and existing field survey data to link juvenile fish and their habitats. The first method used seine survey data collected monthly from July to October 1988–1996 at fixed stations in Narragansett Bay, Rhode Island. Thirteen fish species making up 1% or more of the catch were analyzed by principal components analysis for two time periods: July–August and September–October. The stations were then plotted by their principal component scores to identify station groupings and habitat types. The second method used environmental data collected in July and August 1996 at the established survey stations in a principal components analysis. The stations and 13 most abundant species were plotted by principal components scores resulting from the environmental data. For the environmental data, the first two principal components explained 59% of the variance. The first principal component described the amount of energy shaping the habitat and was positively correlated with salinity, dissolved oxygen, current flow, and slope, and negatively correlated with silt. The second component was positively correlated with depth and silt, and negatively correlated with dissolved oxygen. The environmental data grouped the stations according to their distance from the ocean and three habitat types emerged. The uppermost station was a silty barren having low salinities and dissolved oxygen. Three other stations grouped together as low energy, protected habitats with sandy substrates. Lower bay stations had higher salinities, higher dissolved oxygen, higher flow rates, greater slopes, and larger size substrates, mostly cobble and gravel. Results from the fish data grouped the stations similarly. Combining results from both datasets revealed the uppermost station had the highest catches, most species, and greatest number of winter flounder (Pseudopleuronectes americanus) juveniles. Plots of winter flounder catches with principal component scores from the environmental data indicated the winter flounder distribution in the bay has shrunk from baywide to mostly the upper estuary near their primary spawning grounds. Results illustrate the value of coupling historic fish survey data with environmental measurements for identifying previously undervalued habitats important to fish.     

Pliocene clays from Aegina (Greece): Reference material for chemical provenance studies on bronze age pottery from the Island

Normally, the use of clays as reference materials in chemical provenance studies of ancient ceramics is complicated due to the original clay paste processing. The primary mixing and/or refining of raw materials during pottery production makes a straightforward comparison of archaeological ceramics with extant geological materials difficult if not impossible in many cases. However, in the case of Pliocene clays from Aegina (Greece), which were examined chemically and mineralogically and compared with Bronze Age pottery produced on the island, a successful exception can be presented. The chemical composition of a large group of Aeginetan pottery resembles the chemical composition of clays from a deposit in close vicinity to the main Bronze Age settlement of the island. Clays from specific outcrops exhibit considerable chemical and mineralogical homogeneity, and the suitability of those clays for pottery production apparently made substantial clay paste processing unnecessary. © 2004 Wiley Periodicals, Inc.     

Mobility of metamorphic fluids inferred from infrared microspectroscopy on natural fluid inclusions: the example of Tinos Island,Greece

OH structure of metamorphic fluids has been studied by high temperature infrared (IR) microspectroscopy on natural fluid inclusions contained in quartz veins, over the temperature range 25–370 °C. Blueschist-facies veins from Tinos island core complex (Cyclades, Greece) display H₂O–NaCl–CaCl₂–CO₂ inclusions whereas greenschist-facies veins contain H₂O–NaCl ± CO₂ inclusions. From 25 to 370 °C, peak positions of OH stretching IR absorption bands increase quasi-linearly with slopes of 0.25 and 0.50 cm^–1 °C^–1 for inclusions trapped under blueschist and greenschist conditions, respectively. Extrapolation to 400 °C yield peak positions of 3,475 cm^–1 for blueschist inclusions and 3,585 cm^–1 for greenschist inclusions. Because the smaller wave number indicates the shorter hydrogen-bond distance between water molecules, fluids involved in the greenschist event have a loose structure compared with blueschist fluids. We suggest that these properties might correspond to a low wetting angle of fluids. This would explain the high mobility of aqueous fluids suggested by structural observation and stable isotope analysis.Editorial responsibility: J. Hoefs     

Vorticity analysis in calcite tectonites: An example from the Attico-Cycladic massif (Attica,Greece)

Although calcite tectonites are widespread in nature their use to quantify flow vorticity is limited. We use new (micro-)structural, petrofabric and vorticity data to analyse the kinematics of flow in outcrop-scale calcite mylonite zones. These zones are genetically related to a crustal-scale NE-directed ductile thrust (Basal Thrust) that emplaced the Blueschist over the Basal unit during the exhumation of the Attico-Cycladic Massif. Calcite microstructures reveal that the last stage of deformation occurred at temperatures 200–300 °C achieved by mild heating, which is possibly related with the reburial of the Basal Thrust's footwall. Vorticity analyses were based on the degree of asymmetry of calcite c-axis fabrics as well as on the assumption that the orientation of the long axes of calcite neoblasts within an oblique foliation delineates the direction of instantaneous stretching axis. Both methodological approaches provide consistent estimates with a simple shear component between 55% and 82% (W_n = 0.76–0.96). The use of the stress axis (σ₁) orientation recorded by twin-c-axis-pairs to quantify vorticity generally gives significantly lower simple shear component. Comparison of our vorticity estimates with previous estimates inferred from quartz fabrics and rigid porphyroclasts reveals that exhumation-related deformation in the nappe pile was steady state.     

Reconstructing deep‐time histories from integrated thermochronology: An example from southern Baffin Island,Canada

We present a multi‐chronometric approach for reconstructing deep‐time thermal histories using southern Baffin Island as a case study. This continuous thermal history begins with the Palaeoproterozoic Trans‐Hudson Orogeny and is derived from inverse and forward models that integrate thermochronometers spanning some 500°C: new apatite U–Pb ages and K‐feldspar ⁴⁰Ar/³⁹Ar multi‐diffusion domain data, published (U–Th)/He zircon ages and new multi‐kinetic fission‐track results. Integration of data from a wider temperature range reduces ambiguities in thermal‐history modelling and permits us to constrain the timing of geological processes including, extended post‐orogenic cooling, enhanced later Proterozoic cooling, and then episodic burial and exhumation in the Palaeozoic–Mesozoic.     

Crystal chemistry and reaction relations of piemontites and thulites from highly oxidized low grade metamorphic rocks at Vitali,Andros Island,Greece

Piemontite- and thulite-bearing assemblages from highly oxidized metapelitic and metacalcareous schists associated with braunite quartzites at Vitali, Andros island, Greece, were chemically investigated. The Mn-rich metasediments are intercalated in a series of metapelitic quartzose schists, marbles, and basic metavolcanites which were affected by a regional metamorphism of the highP/T type (T=400–500° C,P>9 kb) and a later Barrovian-type greenschist metamorphism (T=400–500° C,P～-5–6 kb). Texturally and chemically two generations of piemontite (I and II) can be distinguished which may show complex compositional zoning. Piemontite I coexisted at highP/T conditions with braunite, manganian phengite (alurgite), Mn³⁺-Mn²⁺-bearing Na-pyroxene (violan), carbonate, quartz, hollandite, and hematite. Zoned grains generally exhibit a decreasing Mn³⁺ and an increasing Fe³⁺ and Al content towards the rim. Chemical compositions of piemontite I range from 2.0 to 32.1 mole % Mn³⁺, 0 to 25.6 mole % Fe³⁺, and 60.2 to 81.2 mole % Al. Up to 12.5 mole % Ca on the A(2) site can be substituted by Sr. Piemontites formed in contact or close to braunite (±hematite) attained maximum (Mn³⁺+Fe³⁺)Al_?1 substitution corrresponding to about 33 mole % Mn³⁺+Fe³⁺ in lowiron compositions and up to about 39 mole % Mn³⁺+ Fe³⁺ at intermediate Fe³⁺/(Fe³⁺+Mn³⁺) ratios. Piemontite II which discontinuously overgrows piemontite I or occurs as separate grains may have been formed by greenschist facies decomposition of manganian Na-pyroxenes according to the reaction: (1) $$\begin{gathered} {\text{Mn}}^{{\text{3 + }}} - Mn^{2 + } - bearing omphacite/chloromelanite \hfill \\ + CO_2 + H_2 O + HCl \pm hermatite \hfill \\ = piemontite + tremolite + albite + chlorite \hfill \\ + calcite + quartz + NaCl \pm O_2 . \hfill \\ \end{gathered} $$ Thulites crystallized in coexistence with Al-rich piemontite II. All thulites analysed are low-Fe³⁺ manganian orthozoisites with Mn^tot～-Mn³⁺ substituting for Al on the M(3) site. Their compositions range from 2.9 to 7.2 mole % Mn³⁺, 0 to 1.2 mole % Fe³⁺, and 91.8 to 96.7 mole % Al. Piemontites II in thulite-bearing assemblages range from 5.8 to 15.9 mole % Mn³⁺, 0 to 3.7 mole % Fe³⁺, and 83.7 to 93.6 mole % Al. By contrast, piemontites II in thulite-free assemblages are similarly enriched in Mn³⁺ + Fe³⁺ — and partially in Sr²⁺ — as core compositions of piemontite I (21.1 to 29.6 mole % Mn³⁺, 2.0 to 16.5 mole % Fe³⁺, 60.6 to 68.4 mole % Al, 0 to 29.4 mole % Sr in the A(2) site). The analytical data presented in this paper document for the first time a continuous low-Fe³⁺ piemontite solid solution series from 5.8 to 32.1 mole % Mn³⁺. Aluminous piemontite II is enriched by about 3 mole % Mn³⁺+Fe³⁺ relative to coexisting thulite in Fe³⁺-poor samples and by about 6 mole % Mn³⁺+Fe³⁺ in more Fe³⁺-rich samples. Mineral pairs from different samples form a continuous compositional loop. Compositional shift of mineral pairs is attributed to the effect of a variable fluid composition at constantP _fluid andT on the continuous reaction: (2) $$\begin{gathered} piemontite + CO_2 \hfill \\ = thulite + calcite + quartz \hfill \\ + Mn^{2 + } Ca_{ - 1} [calcite] + H{_2} O + O{_2} \hfill \\ \end{gathered} $$ Further evidence for a variable \(x_{H_2 O} \) and/or \(f_{O_2 } \) possibly resulting from fluid infiltration and local buffering during the greenschist metamorphism is derived from the local decomposition of piemontite, braunite, and rutile to form spessartine, calcite, titanite, and hematite by the reactions: (3) $$\begin{gathered} piemontite + braunite + CO_2 \hfill \\ = sperssartine + calcite + quartz \pm hermatite \hfill \\ + H{_2} O + O{_2} \hfill \\ \end{gathered} $$ and more rarely: (4) $$\begin{gathered} piemontite + quartz + rutile + braunite \hfill \\ = spessartine + titanite + hematite + H{_2} O + O{_2} . \hfill \\ \end{gathered} $$      

Rutile occurrence and trace element behavior in medium-grade metasedimentary rocks: example from the Erzgebirge, Germany

Metamorphic textures in medium-grade (~500–550°C) metasedimentary rocks from the Erzgebirge give evidence of prograde rutile crystallization from ilmenite. Newly-crystallized grains occur as rutile-rich polycrystalline aggregates that pseudomorph the shape of the ilmenites. In-situ trace element data (EMP and SIMS) show that rutiles from the higher-grade samples record large scatter in Nb content and have Nb/Ti ratios higher than coexisting ilmenite. This behavior can be predicted using prograde rutile crystallization from ilmenite and indicates that rutiles are reequilibrating their chemistry with remaining ilmenites. On the contrary, rutiles from the lowest grade samples (~480°C) have Nb/Ti ratios that are similar to the ones in ilmenite. Hence, rutiles from these samples did not equilibrate their chemistry with remaining ilmenites. Our data suggest that temperature may be one of the main factors determining whether or not the elements are able to diffuse between the phases and, therefore, reequilibrate. Newly-crystallized rutiles yield temperatures (from ~500 to 630°C, Zr-in-rutile thermometry) that are in agreement with the metamorphic conditions previously determined for the studied rocks. In quartzites from the medium-grade domain (~530°C), inherited detrital rutile grains are detected. They are identified by their distinct chemical composition (high Zr and Nb contents) and textures (single grains surrounded by fine grained ilmenites). Preliminary calculation, based on grain size distribution of rutile in medium-grade metapelites and quartzites that occur in the studied area, show that rutiles derived from quartzites can be anticipated to dominate the detrital rutile population, even if quartzites are a minor component of the exposure.     

Application of the revised universal soil loss equation model on landslide prevention. An example from N. Euboea (Evia) Island, Greece

This study aims to evaluate the application of the Revised Universal Soil Loss Equation (RUSLE) on predicting of sites susceptible to slope failures caused by the soil erosion. It was carried out at the northern part of Euboea Island presenting one of the highest frequencies of landslide occurrence in Greece. All required datasets for the application of the RUSLE model were engaged to compile a potential soil erosion map of the study area. In addition, historical slope failure data were used to evaluate the produced map. Moreover, the frequency ratio statistical analysis was applied for the verification of the soil erosion map. The results showed a satisfactory agreement between the soil erosion intensity zones and distribution of landslides events. Therefore, the RUSLE model can be applied for the localization of sites susceptible to landslides that were prepared or triggered by the soil erosion. Finally, taking under consideration the contribution of erosion mechanisms over the landslides manifestation, a combination of measures were proposed for preventing and supporting these catastrophic phenomena.     

Neoproterozoic glaciation in the mid-oceanic realm: An example from hemi-pelagic mudstones on Llanddwyn Island, Anglesey, UK

We report the first occurrence of ice-rafted dropstones in mid-oceanic sediments belonging to an ocean plate stratigraphy within a Neoproterozoic accretionary complex on Llanddwyn Island, Wales, UK. Dropstones of sandstone, chert, and basalt occur as matrix-supported exotic clasts in a 1 m-thick, hemi-pelagic mafic mudstone; the largest clast is 20 × 25 cm across. These dropstones occur specifically in hemi-pelagic mafic mudstone that is located at the structural top of ocean plate stratigraphy that records a ridge-trench transition; they are supplementary to dropstones associated with extensive tillites reported in shallow marine sequences of continental shelf facies and in back-arc basins.     

High-resolution seismic reflection profiles for the reconstruction of postglacial transgressive shorelines: An example from Greece

In 1982 we carried out a shallow-penetration, high-resolution seismic profiling study on the shelf of the southern Argolid, Peloponnesos, Greece, to identify and map the shores of the late Pleistocene and Holocene, and to establish their coastal environments. Portable equipment and a local boat were used. The lowest glacial shore occurs at ?115 to ?118 m, within the range of global values. The subsequent rise across the distinct old land surface left behind many shore features (scaps, beaches and beach ridges, channels and lagoons) now buried under a few meters of post-transgressive deposits. These features cluster at a small number of depths below present sea level, suggesting that the rise of the sea, usually too fast to leave an imprint, was episodically interrupted by brief stillstands or even temporary reversals of climatic or tectonic origin. The clusters can be roughly dated with reference to a global sea-level-rise curve; after 6000 yr B.P. sparse archaeological data establish a local curve. The seismic profiling technique, convenient and not costly, holds promise for the identification of postglacial shores elsewhere prior to sampling for dating. It has wide application for environmental reconstructions of vanished coastal zones as a basis for prehistoric resource assessments. These applications are illustrated with examples from this study.     

Sediment geochemistry as a provenance indicator: Unravelling the cryptic signatures of polycyclic sources,climate change,tectonism and volcanism

Interpretation of bulk‐sediment geochemistry is one of several approaches for determining sediment provenance. This study investigates the value added by bulk‐sediment geochemical analysis in interpreting provenance in a passive margin clastic basin, the Upper Jurassic–Lower Cretaceous deltaic sediments of the Scotian Basin. Provenance studies in this basin are challenging because source tectonic terranes are parallel to the basin margin and polycyclic sediment sources are abundant. More than 400 samples of mudstone and sandstone representing the geographical and stratigraphic range of interest were analysed for 57 elements. Diagenetic processes added calcium to many samples and removed potassium in rocks buried below 3 km, thus impacting principal component analysis and published weathering indices. However, multiple geochemical approaches to assessing the degree of weathering showed climatically controlled changes in weathering in the Tithonian and Barremian, and changes in supply from major tectonic events, such as the top‐Aptian uplift in the Labrador rift. Covariance of elements in heavy minerals demonstrates the varying magnitude of polycyclic supply and stratigraphic changes in sources. Geochemical analyses revealed a previously unsuspected Tithonian alkali volcanic sediment source, characterized by high niobium and tantalum. The lack of highly contrasting sources means that geochemistry alone is inadequate to determine sediment provenance. Published discrimination diagrams are of limited value. Statistical analysis of geochemical data is strongly influenced by diagenetic processes, episodic volcanic inputs and polycyclic concentration of resistant heavy minerals in sandstones. Single indicator elements for particular sources are generally lacking. Nevertheless, careful consideration of geochemical variability on a case by case basis, integrated with detrital mineral studies, provides new insights into palaeoclimate, sediment provenance and, hence, regional tectonics. Although there is no simple template for such analysis, this study demonstrates an approach that can be used for other basins.     

An example of high-pressure low-temperature metamorphic rocks from an island-arc: the Paikon Series (Innermost Hellenides, Greece)

Abstract The Paikon Series is considered to be a volcanic arc sequence with a mainly neritic sedimentary sequence and bimodal tholeiitic volcanism of early Mesozoic age. The metamorphic assemblages are syn- to post-kinematic with respect to a pre-Tithonian tectonic phase and range from the lawsonite-chlorite-albite facies through transitional Na-amphibole-greenschist facies to the chlorite sub-zone of the greenschist facies. The metamorphic imprint of the Paikon Series corresponds to a temperature range from less than 330° C to ± 450° C under a total pressure from 3 kbar to 6–7 kbar. The overprinting of these facies on an earlier blueschist assemblage, related either to a subduction zone or to a tectonic overpressure caused by thrusting, is suspected.     

Rutile geochemistry and thermometry of eclogites and associated garnet-mica schists in the Biga Peninsula,NW Turkey

In northwest Turkey, high-pressure metamorphic rocks occur as exotic blocks within the Çetmi mélange located on the south of the Biga Peninsula. Rutile chemistry and rutile thermometry obtained from the eclogite and associated garnet-mica schist in the Çetmi mélange indicate significant trace element behaviour of subducted oceanic crust and source-rock lithology of detrital rutiles. Cr and Nb contents in detrital rutile from garnet-mica schist vary from 355 to 1026 μg/g and 323 and 3319 μg/g, respectively. According to the Cr-Nb discrimination diagram, the results show that 85% of the detrital rutiles derived from metapelitic and 15% from metamafic rocks. Temperatures calculated for detrital rutiles and rutiles in eclogite range from 540 °C to 624 °C with an average of 586 °C and 611 °C to 659 °C with an average of 630 °C at P = 2.3 GPa, respectively. The calculated formation temperatures suggest that detrital rutiles are derived from amphibolite- and eclogite-facies metamorphic rocks. Amphibolite-facies rocks of the Kazdağ Massif could be the primary source rocks for the rutiles in the garnet-mica schist from the Çetmi mélange. Nb/Ta ratios of metapelitic and metamafic rutiles fall between 7–24 and 11–25, respectively. Nb/Ta characteristics in detrital rutiles may reflect a change in source-rock lithology. However, Nb/Ta ratios of rutiles in eclogite vary from 9 to 22. The rutile grains from eclogites are dominated by subchondritic Nb/Ta ratios. It can be noted that subchondritic Nb/Ta may record rutile growth from local sinks of aqueous fluids from metamorphic dehydration.     

Petrographic and geochemical characteristics of Paleogene turbidite deposits in the southern Aegean (Karpathos Island,SE Greece): Implications for provenance and tectonic setting

The provenance and depositional setting of Paleogene turbidite sediments from the southern Aegean are investigated using petrography and whole-rock geochemistry. Petrography indicates that Karpathos Island turbidites are consisting of compositionally immature sandstones (graywackes–litharenites) derived from igneous (plutonic–volcanic), sedimentary, low-grade metamorphic and ophiolitic sources. The studied sediments probably reflect a mixing from an eroded magmatic arc and from quartzose, recycled sources. Major and trace element data are compatible with an acidic to mixed felsic/basic source along with input of ultramafic detritus and recycling of older sedimentary components. Geochemical data also reveal that the sediments have undergone a minor degree of weathering and no significant sediment recycling. Chondrite-normalized REE plots show a light REE enrichment (LaN/YbN ca. 7) and absence of significant negative Eu anomalies, indicating provenance from young undifferentiated arc material with contribution from an old upper continental crust source. Turbidite sedimentation probably took place in a continental island arc depositional setting as a result of subduction of a branch of Neotethys beneath a continental fragment of the Anatolide domain in Early Tertiary times. The relation of Karpathos turbidites with the Pindos foreland basin (Gavrovo and Ionian Zones of western Greece) remains problematic.