Monazite and xenotime U–Th–Pb geochronology by ion microprobe: dating highly fractionated granites at Xihuashan tungsten mine, SE China

Zircon, monazite, and xenotime have proven to be valuable chronometers for various geological processes due to their commonly high-U–Th and low common Pb contents. However, zircons that have crystallized in highly fractionated granites often have such high-U contents that radiation damage can lead to scattered U–Pb ages when measured with secondary ion mass spectrometry (SIMS). In this study, monazite and xenotime were separated from a number of highly fractionated granites at the Xihuashan tungsten mine, Southeast China, for alternative dating methods by SIMS. For monazite analysis, obvious excess ²⁰⁴Pb signal (mainly from interference of ²³²Th¹⁴⁴Nd¹⁶O₂ ⁺⁺) was observed in high-Th (>2 wt%) monazite, which hinders ²⁰⁴Pb-based common Pb corrections. A ²⁰⁷Pb-based common Pb correction method was used instead. By employing power law relationships between Pb⁺/U⁺ versus UO₂ ⁺/U⁺, Pb⁺/Th⁺ versus ThO₂ ⁺/Th⁺ and suitable exponentials, monazites with ThO₂ contents in the range of ~3–19 % do not exhibit this matrix effect. Independent SIMS U–Pb ages and Th–Pb ages of three phases of Xihuashan granite samples were consistent with each other and yielded dates of 158.7 ± 0.7, 158.0 ± 0.7, and 156.9 ± 0.7 Ma, respectively. Xenotime does show marked matrix effects due to variations of U, Th, and Y [or total rare earth element (REE), referred as ΣREE hereafter] contents. Suitable correction factors require end-member standards with extremely high or low U, Th, and Y (or ΣREE) contents. No excess ²⁰⁴Pb was observed, indicating that the ²⁰⁴Pb-based common Pb correction method is feasible. Independent ²⁰⁷Pb/²⁰⁶Pb ages can be obtained, although multi-collector mode is necessary to improve precision. The main difficulties with dating xenotime are when high-Th (U) mineral inclusions are ablated. We can identify when this occurs, however, by comparing the measured UO₂ ⁺/U⁺ and ThO₂ ⁺/Th⁺ with those in xenotime standards. Three xenotime samples from the first phase of Xihuashan granite yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb date of 159.5 ± 4.4 Ma (MSWD = 1.0) and a ²⁰⁶Pb/²³⁸U date of 159.4 ± 0.9 Ma (MSWD = 1.6), which are consistent with monazite U–Pb and Th–Pb ages from the same granites. This study demonstrates that monazite and xenotime are better SIMS chronometers for highly fractionated granites than zircon, which can yield doubtful ages due to high-U contents.     

Two layer model of lithospheric compression and uplift/exhumation in an intracratonic setting: an example from the Cooper–Eromanga Basins,Australia

Seismic reflection profiles indicate the compressive nature of the structural style associated with the major uplift events in the Cooper–Eromanga Basins. Inversion geometries and reactivated features attest to a period of compression during Late Triassic–Early Jurassic times. In the Eromanga Basin, compressional structural styles associated with Late Cretaceous–Tertiary are apparent. Many of the Late Cretaceous–Tertiary structures coincide with exhumation highs in Late Cretaceous–Tertiary times. The two-layer lithospheric compression model is considered as the most complete explanation of both the uplift of areas subject to compression and crustal thickening, and of the regional uplift of areas not subject to any apparent Late Cretaceous–Tertiary compression. In the model, compression and thickening in the lower lithosphere is decoupled and laterally displaced from that in the upper crust. Thickening of the mantle lithosphere without thickening of the overlying crust can account for the initial subsidence then uplift of not inverted platform areas. The opening of the Tasman Sea and the Coral Seas can lead to stress transmission in the interior of the continent. These stresses are likely to generate uplift but cannot explain the distribution of uplift in areas not subject to compression.     

K–Ca ages of authigenic sediments: examples from Paleozoic glauconite and applications to low-temperature thermochronometry

K–Ca ages of Cambrian glauconites from the Llano uplift, central Texas, were determined in order to re-evaluate the ability of the K–Ca system to constrain the timing of deposition of sedimentary packages. All of the K–Ca ages presented here were found to be younger than their stratigraphic ages. In addition to being too young, the K–Ca ages are also highly variable, ranging in age from Silurian to Permian. The oldest subset of glauconite ages are in agreement with previously published Rb–Sr ages from the same outcrop and provide further evidence for there having been a post-depositional thermal or recrystallization event that reset both the Rb–Sr and K–Ca systems. The range of younger glauconite K–Ca ages is similar to the distribution of available apatite fission track ages for the Llano basement. K–Ca ages are interpreted as thermochronologic data reflecting partial retention of Ca in thermally fluctuating basin conditions. Estimates of the closure temperature of Ca in glauconite are found to be 60–90°C for cooling rates of ~0.5–1°C/My. The K–Ca system is potentially useful as a low-temperature thermochronometer with closure temperatures <100°C for glauconite.     

Weakening associated with the diaspore–corundum dehydration reaction in metabauxites: an example from Naxos (Greece)

Metabauxite lenses embedded in marble on Naxos consist of diasporites below the 420°C isograd, and dehydrate into corundum-rich rocks with increasing grades of metamorphism. While the diasporites are essentially undeformed, the corundum-rich rocks are strongly deformed, even though both diasporites and corundum-rich rocks are much stronger than the surrounding intensely deformed marbles. The observed structures can be explained as an effect of high fluid pressures during the prograde diaspore–corundum dehydration reaction, which causes dramatic temporary weakening of the metabauxites (to a strength comparable to that of the surrounding deforming marbles). Deformation of the metabauxite is thus largely restricted to the time span the phase transformation occurred, allowing the dehydrating bauxite mass to deform together with the surrounding marbles.     

Early subduction–exhumation and late channel flow of the Greater Himalayan Sequence: implications from the Yadong section in the eastern Himalaya

Based on metamorphic studies of the Yadong high-pressure (HP) granulite and multiple thermochronological investigations of granitoids from both upper and lower parts, the Yadong section in the eastern Himalaya constrains the Cenozoic tectonic evolution of the Greater Himalayan Sequence (GHS). The Yadong HP granulite, located at the top of the GHS, underwent a peak-stage HP granulite facies metamorphism and two stages of retrograde metamorphism. Granulite and hornblende facies retrograde metamorphism took place at 48.5 and 31.8 Ma, respectively, marking the time of exhumation of the subducted Indian slab to lower and middle crustal levels. Subsequently, an average young zircon U–Pb age obtained from the Yadong HP granulite indicated that this unit was captured by its surroundings in a partially molten condition at 16.9 Ma. In addition, three granitoids from both the lower and the upper parts of the GHS yielded biotite ⁴⁰Ar/³⁹Ar ages of 11.0, 11.3, and 11.5 million years. These consistent ages suggest that the GHS along the Yadong section was laterally extruded and synchronously cooled to ～300°C at ～11.3 Ma. Furthermore, the granitic gneisses yield apatite fission track ages of ～7 million years, documenting the cooling of the GHS to ～110°C. A two-stage model describes the Cenozoic tectonic evolution of the GHS: (1) the Indian slab had subducted under Tibet before ～55 Ma, and was exhumed to the lower crust (50-40 km) at 48.5 Ma, and to the middle crust (22-15 km) at 31.8 Ma; and (2) the partial melting occurred at middle crustal levels during the period 31.8 to 16.9 Ma, causing channel flow. In the late stage, the GHS was laterally extruded by ductile mid-crustal flow during the period 16.9 to ～7 Ma, characterized by a fast cooling rate of ～2 mm per year.     

Geological, mineralogical and geochemical characteristics of the Radzimowice Au–As–Cu deposit from the Kaczawa Mountains (Western Sudetes, Poland): an example of the transition of porphyry and epithermal style

The sheeted quartz–sulfide veins of the Radzimowice Au–As–Cu deposit in the Kaczawa Mountains are related to Upper Carboniferous post-collisional potassic magmatism of the composite Zelezniak porphyry intrusion. Multiple intrusive activity ranges from early calc-alkaline to sub-alkaline and alkaline rocks and is followed by multiple hydrothermal events. Early crustally derived dacitic magma has low mg# (<63) and very low concentrations of mantle-compatible trace elements, high large-ion lithophile elements (LILE), moderate light rare-earth elements (LREE), and low high-field-strength elements (HFSE). Later phases of more alkaline rocks have higher mg# (60–70), and LILE, LREE, and HFSE characteristics that indicate mafic magma contributions in a felsic magma chamber. The last episode of the magmatic evolution is represented by lamprophyre dikes which pre-date ore mineralization and are spatially related to quartz–sulfide–carbonate veins. The dikes consist of kersantite and spessartite of calc-alkaline affinity with K₂O/Na₂O ratios of 1.1–1.9, mg# of 77–79, and high abundances of mantle-compatible trace elements such as Cr, Ni, and V. They have high LILE, low LREE, and low HFSE contents suggesting a subduction-related post-collisional arc-setting. The mineralization started with arsenopyrite that was strongly brecciated and overprinted by multiple quartz–carbonate phases associated with base-metal sulfides and Au–Ag–Bi–Te–Pb±S minerals. The sulfur isotope composition of sulfides ranges from –1.1 to 2.8 ³⁴S and suggests a magmatic source. At least two generations of gold deposition are recognized: (1) early refractory, and (2) subsequent non-refractory gold mineralization of epithermal style. Co-rich arsenopyrite with refractory gold and pyrite are the most abundant minerals of the early stage of sulfide precipitation. Early arsenopyrite formed at 535–345°C along the arsenopyrite–pyrrhotite–loellingite buffer and late arsenopyrite crystallized below 370°C along the arsenopyrite–pyrite buffer. Non-refractory gold associated with base-metal sulfides and with Bi–Te–Ag–Pb–S mineral assemblages has an average fineness of about 685, and is represented by electrum of two generations, and minor maldonite (Au₂Bi). Fluid inclusions from various quartz generations co-genetic with base-metal sulfides and associated with carbonates, tellurides and non-refractory gold indicate fluids with moderate salinity (9–15 wt% NaCl equiv.) and a temperature and pressure drop from 350 to 190°C and 1.2 to 0.8 kbar, respectively. According to the result of the sulfur isotope fractionation geothermometer the temperature of base-metal crystallization was in the range from 322 to 289°C. Preliminary results of oxygen isotope studies of quartz from veins indicate a gradual increase in the proportion of meteoric water in the epithermal stage. The gold to silver ratio in ore samples with >3 ppm Au is about 1:5 (geometric mean). Hydrothermal alteration started with sericitization, pyritization, and kaolinitization in vein selvages followed by alkaline hydrothermal alteration of propylitic character (illitization and chloritization), albitization and carbonatization. The mineralization of the Radzimowice deposit is considered as related to alkaline magmatism and is characterized by the superposition of low-sulfidation epithermal mineralization on higher-temperature and deeper-seated mesothermal/porphyry style.Editorial handling: B. Lehmann     

Dating of Columbite-tantalite and Monazite from Pegmatites of the Kawadgaon–Challanpara Area,Bastar Craton,Central India

The results of geochronological studies on columbite-tantalite and monazite from the rare metal pegmatites of the Kawadgaon–Challanpara area in Bastar craton, central India are presented. Columbite-tantalite yielded U-Pb concordia upper intercept age of 1978±16 Ma (MSWD = 0.18). Radiogenic ²⁰⁷Pb*/²⁰⁶Pb* (T_7/6) ages on 4 out of 5 columbite-tantalite vary in a narrow range of 1903 to 2077 Ma and are similar to U-Pb age, whereas, one sample shows younger ²⁰⁷Pb*/²⁰⁶Pb*(T_7/6) age of 1728 Ma. Younger Pb-Pb age of 1744 ± 250 Ma (MSWD = 150) has also been indicated by these columbite-tantalite samples. Four out of five monazite samples define Pb-Pb errorchron age of 2050±370 Ma (MSWD = 165) and radiogenic ²⁰⁷Pb*/²⁰⁶Pb* (T_7/6) ages on 3 out of 5 monazites show a narrow range of 1983 to 2083 Ma. Other two samples show younger ²⁰⁷Pb*/²⁰⁶Pb*(T_7/6) ages as 1254 Ma and 1592Ma. Both monazite and columbite-tantalite indicate disturbance in Pb and U isotopic systematics as revealed by high MSWD. However, selected samples from both monazite and columbite-tantalite indicate age of their formation as c. 2000 Ma. Younger ages, i.e., 1254 to 1744 Ma are indicative of later geological disturbances. Reported age of c. 2000 Ma is comparable to Rb-Sr date of pegmatitic muscovite (1850-2330 Ma) from this area and is younger to intrusive granites of c. 2500 Ma. By analogy, therefore, it may be inferred that the age of the rare element mineralization may be ~2000 Ma old, and linked with younger granitic activity that spanned over the period from 2300 to 2100 Ma in the Bastar craton.     

On the miscibility gap in monazite–xenotime systems

The regular solid solution model has been applied to solid solubility in the monazite–xenotime systems and is verified against the available experimental data for LaPO₄–YPO₄ and CePO₄–YPO₄ systems. The model is then used to predict the miscibility gaps in a number of other monazite–xenotime systems. The implications for prospective two-phase monazite–xenotime fiber coatings for applications in ceramic matrix composites (CMCs) are discussed.

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Landslide susceptibility mapping by frequency ratio and logistic regression methods: an example from Niksar–Resadiye (Tokat,Turkey)

Turkey confronts loss of life and large economic losses due to natural disasters caused by its morphologic structure, geographical placement, and climate characteristics. The Kuzulu (Koyulhisar) landslide, which caused loss of life and property on 17th March 2005, occurred in an area near the country’s most important active fault, the North Anatolian Fault Zone. To mitigate and prevent landslide damages, prediction of landslide susceptibility areas based on probabilistic methods has a great importance. The purpose of this study was to produce a landslide susceptibility map by the logistic regression and frequency ratio methodologies for a 733-km² area near the North Anatolian Fault Zone from the southeast of Niksar to Resadiye in Tokat province. Conditioning parameters, such as elevation, slope gradient, slope aspect, distance to streams, roads, and faults, drainage density, and fault density, were used in the analysis. Before susceptibility analysis, the landslides observed in the area were separated into two groups for use in analysis and verification, respectively. The susceptibility maps produced had five different susceptibility classes such as very low, low, moderate, high, and very high. To test the performance of the susceptibility maps, area under curve (AUC) approach was used. For the logistic regression method, the AUC value was 0.708; while for the frequency rate method, this value was 0.744. According to these AUC values, it could be concluded that the two landslide susceptibility maps obtained were successful.     

Karst rivers’ particularity: an example from Dinaric karst (Croatia/Bosnia and Herzegovina)

The very complex system of sinking, losing and underground transboundary Karst rivers, lakes and aquifers in the central part of the deep and bare Dinaric karst in Croatia and Bosnia and Herzegovina is analysed. The groundwater and surface water are hydraulically connected through numerous karst forms which facilitate the exchange of water between the surface and subsurface. A complex underground conduit system is an inherent characteristic karst system analysed. Groundwater and surface water exchange with both adjacent and distant aquifers through underground routes or inflows from surface streams and artificial reservoirs. Because of a complex surface and underground karst features, which strongly influenced its hydrological and hydrogeological regime, the main open stream flow, with a longitude of about 106 km, undergoes eight name changes. In this paper, it is noted as “the eight-name river”. In fact, it represents one river with losing, sinking and underground stream sections. Different surface and underground karst forms play crucial roles in the way the water flowing over the surface and on the underground sections of its catchment. The analysed area is full of varied and often spectacular surface landforms, including for example the Blue and Red Lakes and the Kravice Waterfall. The analyses made in the paper show the existence of a decreasing trend of mean annual discharges on the eight-name river, which can cause numerous problems in the regional water resource management of this transboundary river and catchment.     

Carbon isotope and REE characteristics of the Paleocene–Eocene shallow marine Subathu formation from the NW Himalaya (India) and their paleo-environmental implications

Carbon isotope (δ¹³C) and rare earth element (REE) concentrations in representative samples of the shallow marine Subathu Formation, explored from the Neelkanth and Dogadda sections of Northwestern Himalaya (India) were determined to infer the palaeo-environmental condition during the late Paleocene and middle Eocene. δ¹³C values show variation of ～5.0‰ with maximum excursion (?27.34‰) in calcareous sandstone at the basal part and minimum (?22‰) in red shale towards the terminal end. Total REE concentration varies (due to lithology) from 27.23?ppm to 564.35?ppm with an average of 187.60?ppm. The chondrite and PAAS normalized patterns exhibit positive Ce anomaly (0.95–4.45), enriched LREE, and depleted HREE, medium Y/Ho ratio (～30–45) along with positive correlation between Y/Dy and Y/Ho ratio. In addition, calcite veins present in some shale samples indicate redox sensitive trace elements. The overall REE abundance and distribution suggests highly oxygenated environment under the shallow marine regressive phase of deposition. The depositional setting, biostratigraphical constrained age along with δ¹³C values and lower TOC suggested an intense warm period, that might be coeval with the Paleocene-Eocene Thermal Maxima event (PETM).     

Quantifying the role of sandy–silty sediments in generating slope failures during earthquakes: example from the Algerian margin

The Algerian margin is a seismically active region, where during the last century, several large magnitude earthquakes took place. This study combines geotechnical and sedimentological data with numerical modelling to quantitatively assess the present-day slope stability of the Algerian margin. Geotechnical laboratory tests, such as cyclic triaxial tests, oedometric tests and vane shear tests were carried out on sediment cores collected on the study area. The liquefaction potential of a sediment column located about 30 km from the Boumerdès earthquake epicentre of 21st May 2003 was evaluated theoretically for an earthquake of M _w  = 6.8. We show that thin sand and silt beds such as those described on recovered sediment cores are the main cause of sediment deformation and liquefaction during earthquakes. Numerical calculations showed that the slope failure may occur during an earthquake characterised by a PGA in excess of 0.1g, and also that, under a PGA of 0.2g liquefaction could be triggered in shallow silty–sandy deposits. Moreover, comparison of the predicted slope failure with failure geometries inferred from seafloor morphology showed that earthquakes and subsequent mass movements could explain the present-day morphology of the study area.     

Permian copper–polymetallic mineralization in the southern Great Xing’an Range,Northeast China: the Daolundaba copper–polymetallic deposit example

The Daolundaba Cu–polymetallic deposit is a newly discovered Cu–W–Sn deposit on the western slopes of the southern Great Xing’an Range, and its mineralization was related to an early Permian coarse-grained biotite granite. However, there is little information on the age of formation of the deposit. In this article, we present the results of our investigation into the age of the Daolundaba Cu–polymetallic deposit, which involved the selection of chalcopyrite and pyrrhotite samples for Rb–Sr isochron dating. A Rb–Sr isochron defined by the chalcopyrite samples yielded a Rb–Sr isochron age of 290.0 ± 11 Ma (MSWD = 1.2) with an initial Sr isotopic composition (I_Sr) of 0.71446. The pyrrhotite samples yielded a Rb–Sr isochron age of 283.0 ± 2.6 Ma (MSWD = 1.16) with an initial Sr isotopic composition (I_Sr) of 0.71447. The Rb–Sr isochron age determined from the chalcopyrite and pyrrhotite is 282.7 ± 1.7 Ma (MSWD = 1.13). These results indicate that the Daolundaba Cu–polymetallic deposit formed during the early Permian (282.7–290.0 Ma). The Rb and Sr contents of the chalcopyrite and pyrrhotite range from ~0.1325 to ~3.6810 ppm and from ~0.1219 to ~9.5740 ppm, respectively, and the initial Sr isotope ratios (I_Sr) range from 0.71047 to 0.71869, with an average of 0.714723. These isotopic characteristics indicate the ore-forming minerals of the Daolundaba Cu–polymetallic deposit originated mainly from the crust, but with small amounts of mantle material involved. The copper was derived from the associated magma whereas the W and Sn was derived from the surrounding strata. The Permian mineralization of the Xing’an–Mongolia region occurred in an active continental margin setting during subduction of the Palaeo-Asian oceanic plate beneath the Siberian Plate.