40Ar/39Ar thermochronology along a western Québec transect of the Grenville Province, Canada

Ar/Ar thermochronology on 24 hornblendes, 3 biotites, 2 muscovites and 2 K-feldspars, collected along a 400 km-long NW-SE geotraverse through the Grenville Province in western Québec, is employed to provide time constraints on the intermediate and low temperature stages of cooling of part of the Grenville orogen. In the Grenville Front zone, the c. 1000 Ma time of exhumation previously established from thermobarometric and isotopic studies, is supported by the hornblende age data presented here. From 60 km to 160 km SE of the Front, reworked Archaean migmatites of the parautochthonous Réservoir Dozois terrane (RDT; 1004 Ma-old metamorphic monazites) contain hornblendes with 972– 950 Ma cooling ages. Assuming metamorphic geotherms between 25 and 30 °C km?1, calculated cooling and unroofing rates are about 6 °C Ma?1 and 0.33 km Ma?1 in the P–T range 725 °C–800 MPa and 450 °C–400 MPa. Hornblendes from monocyclic rocks of the Mont-Laurier and Morin terranes (MLT and MT; monazite ages c. 1165 Ma) give ages of about 1040 and 1010 Ma, respectively. Calculation of cooling-unroofing rates from peak metamorphic conditions in this area is hampered by thermal perturbations associated with the still poorly dated Grenville collision which took place approximately between 1060 and 1020 Ma. Cooling ages of c. 900 Ma for muscovite and biotite and 860–810 Ma for K-feldspar, show that cooling rates decreased to around 1.5 °C Ma?1 under retrograde greenschist facies conditions in the MLT. On a time vs. distance diagram, the hornblende data define several distinct age ranges, suggesting that each terrane had a characteristic thermal history. Thus, cooling was diachronous and probably non-homogeneous throughout this segment of the Grenville orogen. The time-lag between the cooling history of the parautochthon (972–950 Ma) and the allochthons (1040–1010 Ma) is compatible with an earlier (pre-1040 Ma) peak of metamorphism in the allochthons. The Réservoir Cabonga allochthon was transported toward the NNW from its probable root zone in the MLT during the 1060–1020 Ma Grenvillian collision as a partially cooled slab. The remobilization of the Archaean parautochthon is attributed to this collision. In the Grenville Front zone, slightly older cooling ages and cooling rates initially faster than in the remaining part of the parautochthon are probably as a result of rapid (tectonic?) exhumation shortly after collision. The minor delay (20–30 Ma) in unroofing of the MT compared to the adjacent MLT is most likely related to post-1040 Ma extensional displacement along the Labelle shear zone. In terranes like those described above where metamorphism is diachronous, determination of cooling rates and the history of exhumation may be meaningless without a firm control on the regional structure. However, identification of contrasting cooling histories contributes to unravelling the independent movement of terranes.     

Depositional magnetization of Carboniferous Limestones from the Craven Basin of northern England

Carboniferous Limestones in the Craven Basin of northern England carry a stable natural remanent magnetization (NRM) the intensity of which is facies dependent. Dark argillaceous limestones are most strongly magnetized and pure, pale coloured limestones most weakly magnetized. Partial thermal and alternating field demagnetization suggest that magnetite is the principal carrier of the remanence although some haematite is present in the limestone. The presence of magnetite is confirmed by the low temperature transition, isothermal remanent magnetization (IRM build-up curves) and microprobe analysis. Partial demagnetization of IRM and ARM suggest that the magnetite is relatively coarse grained and in the multidomain state. There are no indications of pseudo-single domain behaviour but magnetite of this type cannot be excluded as a possible remanence carrier. A grain size estimate of 10–20 μm based on coercive force and remanent coercive force is compatible with the theoretical consideration of grain size. The limestones show a weak but marked magnetic susceptibility anisotropy. This anisotropy defines a depositional fabric which indicates that the magnetization is a depositional remanent magnetization (DRM). A DRM was acquired by each specimen before compaction and cementation and was preserved because of the reducing conditions which prevailed in the early diagenetic environments of the limestones.     

A 70 Ma record of suprasolidus conditions in the large,hot, long‐duration Grenville Orogen

Petrochronology of magmatic monazite and apatite from a single paragneiss leucosome derived by in situ partial melting documents the thermal evolution of the Allochthonous Belt of the central Grenville Province. Monazite records suprasolidus metamorphism from ca. 1080 to 1020 Ma under high temperature up to 850°C. Apatite from the same leucosome yields an age of 960 Ma, consistent with cooling of this crustal segment down to subsolidus conditions of ca. 450°C. A pegmatitic granite dyke, with lobate contacts, previously dated at ca. 1005 Ma (Turlin et al., 2017 ) is interpreted to intrude the paragneisses at a temperature of ca. 650°C close to the wet‐solidus. These data document slow cooling at a rate of 2–6°C/Ma for the middle crust of this part of the Grenville hinterland marked by protracted suprasolidus conditions for at least 70 Ma. It supports the definition of the Grenville Orogen as a large, hot, long‐duration orogen.     

Mineral-chemical and isotopic studies of Namaqualand granulites,South Africa: A grenville analogue

The northwestern part of South Africa and southern South-West Africa/Namibia is amongst the most extensive granulite terranes in Africa. This work reports the results of electron microprobe studies of minerals from two-pyroxene, cordieriteorthopyroxene (-gedrite) (-sapphirine) and garnet and/or cordierite parageneses from Namaqualand, in the N.W. Cape Province of South Africa. Determined PT conditions of prograde metamorphism based on thermodynamic calculations are 800°–900° C and ca. 6–7 Kb; and it is argued that rocks of unusual composition, notably cordierite-orthopyroxene rocks, are restites after the extraction of granitic liquid from former argillites. This interpretation is consistent with previously published data on similar rocks, and with McCarthy's (1976) suggestion of extensive partial melting in the quartzofeldspathic rocks in the area. U-Pb isotopic studies of some 50 zircon fractions have been carried out and confirm an age of 1,200 m.y. for the high-grade regional metamorphism; but certain zircon populations record inherited ages greater than 1,700 m.y. Garnet-sillimanite rocks that contain retrograde kyanite reflect PT conditions of 550°–650° C and ca. 7–8 Kb; and constituent biotite has yielded a K-Ar age of ca. 950 m.y. These data, the regional stratigraphy and structure, and the mineralisation are compared with data from the Grenville Province of Canada. Notable similarities are the possible basement-cover relationships, and the calendar of tectonothermal events, while differences include the important stratiform base-metal mineralisation in the supracrustal sequence in Namaqualand, and the Cu-mineralisation in hypersthenebearing intrusives, emplaced some 1,100 m.y. ago, that are areally, and believed to be genetically, related to the granulite facies metamorphic regime.     

Reliability of Relative Paleointensity Recorded in Chinese Loess–Paleosol Sediments

A detailed paleomagnetic and rock-magnetic investigation spanning loess L7 to paleosol S8 has been carried out at the Baoji and Xifeng sections. Results of anisotropy of magnetic susceptibility confirm that the studied loess–paleosol sediments retain primary sedimentary fabrics. Stepwise thermal demagnetization shows that two well-defined magnetization components can be isolated from both loess and paleosol specimens. A low-temperature component, isolated between 100°C and 200°C, is consistent with the present geomagnetic field direction, and a high-temperature component, isolated between 200–300°C and 620–680°C, includes clearly normal and reversed polarities. Isothermal remanent magnetization and thermomagnetic analyses indicate that characteristic remanent magnetization is mainly carried by magnetite and hematite. The Day plot, together with the stratigraphic variations of rock-magnetic parameters, shows that the uniformity of magnetic mineralogy and grain size fulfills the criteria for relative paleointensity (RPI) studies. RPI records have been constructed using natural remanent magnetization (NRM) intensity after thermal demagnetization at 300°C normalized by low-frequency magnetic susceptibility (NRM300/χ). The results show that the RPI record from the Baoji section, where pedogenesis is quite weak, is compatible with the stacked PISO-1500 paleointensity record, suggesting that it might reflect the paleointensity variation of the geomagnetic field. The RPI record from the Xifeng section, where pedogenesis is rather strong, indicates a clear dissimilarity with the stacked PISO-1500 paleointensity record, implying that it does not reflect the paleointensity variation of the geomagnetic field. Our new results show that the NRM300/χ from the strongly pedogenetic paleosols does not completely eliminate the pedogenetic (climatic) influence, so it might be unsuitable for a reliable paleointensity study.     

Magnetic properties of the Chelyabinsk meteorite: Preliminary results

This paper presents the distribution of magnetic susceptibility, χ₀, in fragments of the Chelyabinsk ordinary chondrite (LL5, S4, W0, fall of February 15, 2013) from the collection of the Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, and results obtained by standard magnetic techniques for the meteorite material, including thermomagnetic analysis, measurements of natural remanent magnetization (NRM) and saturation isothermal remanent magnetization (SIRM), as well as the spectra of their alternating field demagnetization at amplitudes up to 170 mT, measurements of hysteresis loops and back-field remanence demagnetization curves at temperatures from 10 K to 700°C etc. The mean logχ₀ values for the light-colored (main) lithology of the meteorite material and impact-melt breccia from our collection are 4.54 ± 0.10 (n = 66) and 4.65 ± 0.09 (n = 38) (×10^?9 m³/kg), respectively. According to international magnetic classification of meteorites, Chelyabinsk falls within the range of LL5 chondrites. The mean metal content was estimated from the saturation magnetization, M _s, of the light- and dark-colored lithologies as 3.7 and 4.1 wt %, respectively. Hence, the dark lithology is richer in metal. The metal grains are multidomain at room temperature and show low coercive force, B _c (<2 mT) and remanent coercive force, B _cr (15–23 mT). The thermomagnetic analyses of the samples showed that the magnetic properties of the Chelyabinsk meteorite are controlled mainly by taenite and kamacite at temperatures >75 K. In the temperature range below 75 K, magnetic properties are controlled by chromite; the magnetic hardness of the samples is maximal at 10 K and equals to 606 and 157 mT for the light- and dark-colored lithologies, respectively.     

Investigating the potential of using environmental magnetism techniques as pollution proxy in urban road deposited sediment

The use of mineral magnetic techniques as pollution proxy for road deposited sediment was explored using various statistical approaches. Standard techniques were adopted for measurement of mineral magnetic and geochemical parameters. The analyses of magnetic parameters revealed that the samples were dominated by ferrimagnetic minerals and multidomain grains. This implied that the magnetic fractions in the samples might be of anthropogenic origin. Results also indicate that the samples were dominated by low coercive, magnetically soft minerals. Thermomagnetic curves confirmed magnetite as the remanence bearing magnetic mineral having a Curie point temperature of ~580 °C. The strong association observed between magnetic susceptibility, susceptibility of anhysteric remanent magnetization and saturation isothermal remanent magnetization and aluminum, titanium, manganese, iron, chromium and lead demonstrated that these metals occurred as ferrimagnetic particles of technogenic origin resulting from vehicular sources. Assessment of pollution status of the road deposited sediment identified silicon and lead as the priority pollutants of concern. Generally, pollution load index was <1 (mean, 0.66 ± 0.14), indicating that the samples were not polluted in the overall, but the metals were in the buildup stage requiring constant monitoring. The sources of pollutants from principal component and cluster analyses identified the sources of pollution to be mainly from vehicular emissions such as brake linings, exhaust materials, tire wear, corroded metal parts, abrasion of lubricating oil and road construction materials. This study found that mineral magnetic techniques offer great potential as pollution proxy for soil pollution studies.     

Meso‐Cenozoic Tectonothermal History of Permian Strata,Southwestern Weibei Uplift: Insights from Thermochronology and Geothermometry

This study provides an integrated interpretation for the Mesozoic-Cenozoic tectonothermal evolutionary history of the Permian strata in the Qishan area of the southwestern Weibei Uplift, Ordos Basin. Apatite fission-track and apatite/zircon(U-Th)/He thermochronometry, bitumen reflectance, thermal conductivity of rocks, paleotemperature recovery, and basin modeling were used to restore the Meso-Cenozoic tectonothermal history of the Permian Strata. The Triassic AFT data have a pooled age of ～180±7 Ma with one age peak and P(χ2)=86%. The average value of corrected apatite(U-Th)/He age of two Permian sandstones is ～168±4 Ma and a zircon(U-Th)/He age from the Cambrian strata is ～231±14 Ma. Bitumen reflectance and maximum paleotemperature of two Ordovician mudstones are 1.81%, 1.57% and ～210°C, ～196°C respectively. After undergoing a rapid subsidence and increasing temperature in Triassic influenced by intrusive rocks in some areas, the Permian strata experienced four cooling-uplift stages after the time when the maximum paleotemperature reached in late Jurassic:(1) A cooling stage(～163 Ma to ～140 Ma) with temperatures ranging from ～132°C to ～53°C and a cooling rate of ～3°C/Ma, an erosion thickness of ～1900 m and an uplift rate of ～82 m/Ma;(2) A cooling stage(～140 Ma to ～52 Ma) with temperatures ranging from ～53°C to ～47°C and a cooling rate less than ～0.1°C/Ma, an erosion thickness of ～300 m and an uplift rate of ～3 m/Ma;(3)(～52 Ma to ～8 Ma) with ～47°C to ～43°C and ～0.1°C/Ma, an erosion thickness of ～500 m and an uplift rate of ～11 m/Ma;(3)(～8 Ma to present) with ～43°C to ～20°C and ～3°C/Ma, an erosion thickness of ～650 m and an uplift rate of ～81 m/Ma. The tectonothermal evolutionary history of the Qishan area in Triassic was influenced by the interaction of the Qinling Orogeny and the Weibei Uplift, and the south Qishan area had the earliest uplift-cooling time compared to other parts within the Weibei Uplift. The early Eocene at ～52 Ma and the late Miocene at ～8 Ma, as two significant turning points after which both the rate of uplift and the rate of temperature changed rapidly, were two key time for the uplift-cooling history of the Permian strata in the Qishan area of the southwestern Weibei Uplift, Ordos Basin.     

Rb---Sr isotopic equilibrium during Sveconorwegian (= Grenville) deformation and metamorphism of the Orust dykes, S.W. Sweden

The Orust dykes and their regional correlatives are an important time marker in the polymetamorphic Östfold-Marstrand belt of S.E. Norway and S.W. Sweden. They were deformed and metamorphosed in the amphibolite facies during the Sveconorwegian (= Grenville) orogeny. Three narrow strongly deformed dykes yield Rb---Sr whole-rock ages (1106 ± 52 m.y., 1062 ± 90 m.y., 1000 ± 153 m.y.). The weighted mean age (1087 m.y.) is interpreted as the age of the first post-dyke deformation and the accompanying amphibolite facies metamorphism. High initial ⁸⁷Sr/⁸⁶Sr ratios (0.7074 ± 9, 0.710 ± 2, 0.713 ± 3, respectively) and other geochemical evidence point to considerable exchange of mobile elements between the dykes and the country rocks. The emplacement age is not known precisely. However, the dykes may be correlated with widespread mafic magnatism which accompanied crustal instability in the N. Atlantic region early in the Sveconorwegian (= Grenville) orogeny about 1200 m.y. ago.     

An example of multicomponent magnetization in welded tuffs: a case study of Upper Cretaceous welded tuffs of eastern Russia

Four distinct components of natural remanent magnetization were isolated from a single site in welded tuffs in the Upper Cretaceous Kisin Group of the Sikhote Alin mountain range, Russia. In order to contribute toward a basis for an interpretation of multicomponent magnetization, rock magnetic experiments were performed on the welded tuffs. All four magnetization components essentially reside in magnetite. The lowest-temperature component up to 300 °C (component A: D=349.3°, I=60.9°, α₉₅=7.3°, N=7) is a present day viscous magnetization. The third-removed component (component C: D=41.4°, I=51.8°, α₉₅=3.5°, N=8), isolated over the temperature range of 450–560 °C, is a primary remanence. The second- and fourth-demagnetized components (component B: D=174.7°, I=−53.1°, α₉₅=21.2°, N=3 and component D: D=188.1°, I=−64.5°, α₉₅=4.0°, N=8, respectively) are secondary magnetizations related to a thermal event in Sikhote Alin between 66 and 51 Ma. Components B and D were acquired through different remagnetization processes. Component B is ascribed to a thermoviscous remanent magnetization carried by single-domain magnetite, and component D is a chemical remanent magnetization.     

Uplift and denudation of Mt Sanqingshan Geopark,Jiangxi Province,China

Mt Sanqingshan, a global Geopark and world natural heritage site located in Jiangxi Province, China, is famous for its eroded granite peaks. The uplift and denudation history of the area has been reconstructed using fission track methods for the first time. Apatite fission track ages (AFTAs) cluster into three groups at ca. 25 Ma, 45–55 Ma, and 70 Ma. These ages can be related to ancient multilevel denudation planes at about 900, 1200, and 1500 m above sea level, respectively. The apatite data also reveal four cooling stages for the Mt Sanqingshan region, from ca. 90 to 65–60 Ma, 65–60 to 45 Ma, 45 to 20–15 Ma, and 20–15 Ma to the present, with cooling rates of 1.96°C, 1.18°C, 0.37°C, and 3.78°C per million years, respectively, and an average cooling rate of 1.80°C per million years. Calculated uplift rates are 0.055, 0.034, 0.011, and 0.11 mm year^?1 in the four stages, yielding uplifts of 4140, 570, 290, and 1940 m, respectively. The uplift rate of the last stage was significantly faster than that of the other three preceding stages, reflecting rejuvenation of Mt Sanqingshan, as a result of new tectonism. The average uplift rate at Mt Sanqingshan is 0.053 mm year^?1, and the average denudation rate is 0.048 mm year^?1, resulting in 3550 m of uplift and 2540 m of denudation relative to eustatic sea level. The 1010 m difference is very close to the average elevation of about 1000 m at present. A comparison of uplift–denudation histories for Mt Sanqingshan and Mt Huangshan shows that fission track results can be useful for defining geomorphological development stages.     

Phanerozoic Paleomagnetism Characteristics of the Qomolangma Area in Tibet

This paper conducts systematic test research on the 2920 paleomagnetic directional samples taken from Ordovician-Paleogene sedimentary formation in the north slope of Qomolangma in south of Tibet and obtains the primary remanent magnetization component and counts the new data of paleomagnetism the times. Based on the characteristic remanent magnetization component, it calculates the geomagnetic pole position and latitude value of Himalaya block in Ordovician-Paleogene. According to the new data of paleomagnetism, it draws the palaeomagnetic polar wander curve and palaeolatitude change curve of the north slope of Qomolangma in Ordovician-Paleogene. It also makes a preliminary discussion to the structure evolution history and relative movement of Himalaya bloc. The research results show that many clockwise rotation movements had occurred to the Himalaya block in northern slope of Qomolangmain the process of northward drifting in the phanerozoic eon. In Ordovician-late Cretaceous, there the movement of about 20.0° clockwise rotation occurred in the process of northward drifting. However, 0.4° counterclockwise rotation occurred from the end of late Devonian epoch to the beginning of early carboniferous epoch; 6.0° and 8.0° counterclockwise rotation occurred in carboniferous period and early Triassic epoch respectively, which might be related with the tension crack of continental rift valley from late Devonian period to the beginning of early carboniferous epoch, carboniferous period and early Triassic epoch. From the Eocene epoch to Pliocene epoch, the Himalaya block generated about 28.0° clockwise while drifting northward with a relatively rapid speed. This was the result that since the Eocene epoch, due to the continuous expansion of mid-ocean ridge of the India Ocean, the neo-Tethys with the Yarlung Zangbo River as the main ocean basin closed to form orogenic movement and the strong continent-continent collision orogenic movement of the east and west Himalayas generated clockwise movement in the mid-Himalaya area. According to the calculation of palaeolatitude data, the Himalaya continent-continent collusion orogenic movement since the Eocene epoch caused the crustal structure in Indian Plate-Himalaya folded structural belt-Lhasa block to shorten by at least 1000 km. The systematic research on the paleomagnetism of Qomolangma area in the phanerozoic eon provides a scientific basis to further research the evolution of Gondwanaland, formation and extinction history of paleo-Tethys Ocean and uplift mechanism of the Qinghai-Tibet Plateau.     

Holocene sediments and magnetic stratigraphy from Lakes Zug and Zurich, Switzerland

Piston cores 7 m and 6.5 m long were collected from Lakes Zug and Zurich respectively. Radiocarbon age determinations, pollen assemblage zones and varve counts indicate dates of 5000–6000 years B.P. (Zug), and 12,000–13,000 years B.P. (Zurich) for the base of the cores. Declination of the magnetic remanence varies through 70° and inclination varies through 30°. Although the correlation with the geomagnetic secular variation curve covering the last 15,000 years as determined from Lake Windermere, England (Creer et al., 1972) is not precise, the results suggest that Lake Zug and Lake Zurich sediments have been recording the broad outline of past secular variations of the earth's magnetic field. Since intensity of magnetization and susceptibility correlate markedly with lithology, a detailed sediment stratigraphy is presented. The amount and texture of the detrital input appears to be a controlling factor for the natural magnetic remanence. Intensity varies from 90 μG in zones of organic, sulphide pigmentation and those with ultra-fine laminations to 0.8 μG in impure lacustrine chalk. Susceptibility ranges from 9 μG/Oe in laminated, fine-grained glacially derived muds to 0.5 μG/Oe in impure lacustrine chalk. Intensity of magnetization also varies systematically within individual turbidites with lowest values in the coarse-grained, basal fractions. Slumped beds were identified on the basis of erratic anomalies in magnetic declination and inclination measurements.     

Ferrimagnetic minerals in red paleosols of Pleistocene Epoch, eastern China

The type,grain size and origin of ferimagnetic minerals separated from red paleosols of pleistocene Epoch(Q2)in eastern China ,were studied by using mineral magnetic measurement,X-ray powder diffraction and electron microscopy.Results showed that the iron oxider in red paleosols were composed of hematite(α-Fe2O3),maghemite(γ-Fe2O3) and goethite(α-FeOOH),Mineral magnetic parameters and X-ray diffraction patterns indicated that maghemite was the dominant remanence carrier in red paleosols,which is characterized by superparamagnetic(SP) and stable single domain(SSD) grains,The variations of magnetic susceptibility(χ） ,anhysteretic magnetic susceptibility(χRAM）and saturation isothermal remanent magnetization(SIRM) for red paleosols following heating to various temperatures showed two peak values at 700℃ and 900℃.The spherulitic magnetic particles measuring 250-1000μm in diameter in red paleosols were separated by the magnetic separation method,indicating that these magnetic particles were an assemblage of superparamagnetic and stable single domain ferrimagnetic grains,It is suggested that the ferrimagnetic minerals of red paleosols be a pedogenic ferrimagnetic component under high temperature and high humid conditions in the Pleistocene Fpoch(Q2).It is concluded that the magnetism characteristics of red paleosols can be used to evaluate the environmental changes of Quaternary in eastern China.     

Magnetic rock properties of the gabbros from the ODP Drill Hole 1105 A of the Atlantis Bank, Southwest Indian Ridge

Laboratory studies of 30 samples from 158 m long drill core of the Hole 1105 A (ODP Leg 179) of the Atlantis Bank, Southwest Indian Ridge have revealed magnetic properties of the gabbros, olivine gabbros, oxide gabbros and olivine oxide gabbros down the core. Comparison of modal proportions of the oxides, grain sizes and magnetization parameters of the rocks has confirmed that most coarse-grained oxide mineral bearing rocks record low Koenigsberger ratio (2 to 5) and median destructive fields (5 to 7 mT). Average natural remanent magnetization (J_nrm) and stable remanent magnetization (J_st) of the core samples are 5.8 A/m and 1.9 A/m, respectively. Their mean stable magnetic inclination is 66‡ ± 4‡, about 14‡ steeper than the expected dipole inclination of the area similar to the one reported at Hole 735 B. The excess inclination perhaps marks a tectonic block rotation of the reversely magnetized rocks of the bank. We interpret that gabbros and serpentinites devoid of basaltic carapace significantly contribute to seafloor spreading anomalies of the bank.     

40Ar/39Ar traverse — Grenville Front Tectonic Zone to Britt Domain,Grenville Province,Ontario, Canada*

Abstract ⁴⁰Ar/³⁹Ar data (on hornblende, muscovite and K-feldspar) are presented for samples from the western Grenville Province taken along a 140-km traverse from the Grenville Front into the Britt domain. Our interpretation is based on 28 new analyses, synthesized with 20 previously reported from the traverse area. In regions where comparisons are possible, muscovite and (large domain) K-feldspar apparent ages appear similar (at c. 920–930 Ma), but throughout the traverse, these are c. 60–70 Myr younger than the hornblende ages. The inferred cooling rate over the c. 350–500°C temperature range, c.2°C Myr^-1, is appropriate for exhumation controlled by post-orogenic erosional unroofing. At the Grenville Front Tectonic Zone (GFTZ) — Britt domain boundary there is a c. 25-Myr offset in both hornblende and muscovite/K-feldspar ages. We interpret the lower ages in the Britt domain to reflect variations in crustal thickness and geothermal gradient between the flank and interior of a thick orogen. The argon data from the GFTZ are interpreted in the context of an asymmetric crustal-scale antiformal structure developed during a late episode of convergence. Hornblende from rocks on either side of the core of the antiform has an apparent age of c. 990 Ma, our estimate of the age of the compressional event. In the west, we infer that these date the short-lived thermal event associated with the development of the crustal-scale antiform previously postulated. In the east, the ages reflect the cooling of material brought toward the surface in the flank of the antiform. Hornblendes from the antiform core appear to contain excess radiogenic argon. We suggest that this was the ambient argon in rocks transported from depth that was subsequently trapped when the rocks cooled rapidly.     

Middle Devonian Paleomagnetism of Geological Complexes of Central Tuva

New paleomagnetic data are obtained for Middle Devonian rocks of Central Tuva. The rocks contain one-, two-, or three-component magnetization. The low-temperature (LT) components of magnetization are close to the directions of the present-day or Cenozoic magnetic field in Tuva. Based on the directions of the high-temperature (HT) components of magnetization, which were distinguished in the magnetite spectrum of blocking temperatures of up to 580оС, we revealed a prefolding magnetization of different polarity. The time when Middle Devonian rocks acquired the prefolding HT component of magnetization almost does not differ from the time of rock formation. Middle Devonian sequences were formed at low latitudes (19°–25° N). We calculated the Middle Devonian paleomagnetic pole (Φ =–13°, Λ = 106°, A₉₅ = 7), which can be used to describe the movement of the Caledonian block in Central Asia, and probably Siberia, if these blocks had been tectonically coupled by the Devonian.     

Thermal history of granitic rocks from western Victoria: A fission‐track dating study

Fission‐track ages have been determined on sphene and apatite from 28 granitic intrusions across the western half of Victoria. The sphene ages compare closely with independent K‐Ar biotite ages for the same intrusions, where these are available, and are invariably older than apatite ages by 35 to 135 m.y. This is in accord with the effective geological track annealing temperatures for these two minerals which are estimated to be 260 ± 20°C and 80 ± 10°C respectively. Both sphene and apatite ages decrease from west to east across western Victoria, the sphenes ranging from 470 ± 28 to 355 ± 19 m.y. The Wando Vale granodiorite and Dergholm granite from the Dundas Tableland of far‐western Victoria have sphene ages of 470 ± 28 m.y. and 452 ±16 m.y. respectively, clearly suggesting a relationship to the Ordo‐vician granitic rocks of southeastern South Australia. Fission‐track ages from the numerous post‐tectonic granites in the Ballarat Trough fall into two distinct groups. Rocks from the western area have sphene ages in the relatively narrow range 393 ± 14 m.y. suggesting emplacement in the Early Devonian time whereas those in the east have sphene ages of 362 ± 7 m.y. (near the Devonian‐Carboniferous boundary). Over the temperature interval recorded by sphene‐apatite pairs, cooling of the granitic rocks was very slow ranging from 0.8 to 5.3°C/m.y. Cooling in this range was probably controlled by uplift and erosion of overburden during a long period of post‐tectonic relaxation. Corresponding uplift rates are estimated to be 0.03 to 0.18 km/ m.y. assuming a normal continental geothermal gradient of 30°C/km. Below 80°C average cooling and uplift rates were probably about l°C/m.y. and 0.03 km/m.y. respectively so that cooling was essentially complete within about 80 m.y. of the apatite ages.     

The age of anorthosite events and their geological implications

It is proposed by the authors in the light of isotopie age data available that anorthosite events are advisable to be assigned to two periods, i.e., the Karelian period (1,700–2,00 m.y.) and the Grenville period (1.000–1.300 m.y.), rather than simply to a time span of 1,300±200 m.y. as suggested by N. Herz in 1969. This division is in agreement with the earth history. It is noticed that anorthosites always occur in the mobile zones between plates, indicating a close relationship with deep faults. Anorthosites of the Karelian period are found principally in tectonic zones that strike approximately NWW or NEE in Eurasian (possibly North American) plate. Grenville anorthosites, constituting two (possibly three) belts running roughly in NNE or NNW direction, occur in orogenic zones marginal to the present continents resulting from the breaking up of Pangaca. This suggests that the breaking up of the ancient continent of Pangaea started to operate as early as late Precambrian and was probably responsible for the continental drift along these tectonic belts during late Palaeozoic.     

First report of coupled Early Permian paleomagnetic and geochronologic data from the Dunhuang block (NW China), and implications for the tectonic evolution of the Paleo-Asian ocean

In order to better understand the tectonic relationship between the Dunhuang block (DHB) and its adjacent blocks, and to constrain the timing of the closure of the Paleo-Asian ocean, a combined geochronologic and paleomagnetic investigation has been undertaken on Early Permian tuff, basalt flows and sandstones from the Shuangbaotang Formation in the northwestern part of the DHB. U-Pb zircon dating indicates that the age of the strata is between 280.6 ± 2.9 Ma and 291.4 ± 2.6 Ma. Thermal demagnetization of a three-component isothermal remanent magnetization (IRM), and Curie point experiment suggest that magnetite dominates in the rock samples analyzed. In addition, there is a minor amount of hematite in some sandstones. Stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization (ChRM) from 11 tuff layers, two lava flows and nine sandstone beds. Two components were isolated from all samples: a lower temperature component (LTC) and a higher temperature component (HTC). The LTC is near the direction of the present-day geomagnetic field and produced a negative fold test, indicating it is a viscous remanent magnetization in the present-day geomagnetic field. Most of the HTC are reverse polarity (Normal = 4, reverse = 158), which is in accordance with the Kiaman Reversed Superchron that spans the Late Carboniferous-Permian interval. In addition, the HTC of all studied sites passed the fold and reverse tests, suggesting that they likely represent primary remanent magnetization. The tilt-corrected mean direction from all sites (tuff, basalts and sandstones) is D_s = 1.7°, I_s = 43.1°, k_s = 403, α₉₅ = 1.5°, N = 22. The mean paleopole of the site-mean direction-corresponded VGPs lies at 74.5°N, 268.5°E with A₉₅ = 1.6°. Considering the consistent inclination values recorded among the studied tuff, basalts and sandstones, and the low degrees of anisotropy within all samples, we suggest that there is no significant inclination shallowing caused by depositional compaction in the sedimentary layers of the studied section. Taking into account the results from this study as well as previous Late Paleozoic paleomagentic studies from adjacent tectonic blocks, we conclude that the DHB formed part of the amalgamated Dunhuang-North China-Alxa-Qaidam mega-block during the Early Permian, but was separated from the Tarim block by a small ocean (here named the Qiemo-Xingxingxia paleo-ocean) at this time. A comparison of the Early Permian paleolatitudes of these and other adjacent blocks suggest that the Paleo-Asian ocean (sensu lato) was still open at this time. Combined with other geological evidence, a paleogeographic reconstruction of the Paleo-Asian ocean has been reconstructed for the Early Permian.