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1.
The post-Svecofennian tectonic development of southern Finland is controlled by intrusion of rapakivi granites (and associated rocks), reactivation of Svecofennian wrench zones, formation of sedimentary basins and successive intrusion of olivine dolerite dykes and sills. Relative age determinations have revealed that fault reactivation acted before, simultaneously and after intrusion of the rapakivi granites. Results of 40 Ar/39 Ar geochronometry of the Porkkala–Mäntsälä fault (30 km west of Helsinki) reveal ages predominantly in the range 950–1300 Myr. These ages are all significantly younger than the intrusion age of the rapakivi granites. It is suggested that these ages represent tectonic events related to the intrusion of olivine dolerite dykes and sills in SW Finland and the Sveconorwegian Orogeny active further west. 40 Ar/39 Ar ages of a sample taken from the Obbnäs granite (U–Pb zircon ages of 1645 ± 5 Myr) show ages predom-inantly in the range of 1400–1550 Myr. These ages are suggested to represent either cooling ages of the granite or ages associated with the formation of the sedimentary grabens. 相似文献
2.
Abstract Five whole-rock 40 Ar/39 Ar plateau ages from low-grade sectors of the Sambagawa belt (Besshi nappe complex) range between 87 and 97 Ma. Two whole-rock phyllite samples from the Mikabu greenstone belt record well-defined 40 Ar/39 Ar plateau ages of 96 and 98 Ma. Together these ages suggest that a high-pressure metamorphism occurred in both the Sambagawa and Mikabu belts at c. 90–100 Ma. The northern Chichibu sub-belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north ( c. 110 Ma) to south ( c. 215 Ma). The northern Chichibu sub-belt is correlated with the Kuma nappe complex (Sambagawa belt). Two whole-rock phyllite samples from the Kurosegawa terrane display markedly older metamorphic ages than either the Sambagawa or the Chichibu belts.
Accretion of Sambagawa-Chichibu protoliths began prior to the middle Jurrasic. Depositional ages decrease from middle Jurassic (Kuma-Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units. The Kurosegawa belt and the geological units to the south belong to distinctly different terrances than the Sambagawa-Chichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous. 相似文献
Accretion of Sambagawa-Chichibu protoliths began prior to the middle Jurrasic. Depositional ages decrease from middle Jurassic (Kuma-Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units. The Kurosegawa belt and the geological units to the south belong to distinctly different terrances than the Sambagawa-Chichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous. 相似文献
3.
B. R. HACKER S. R. WALLIS M. O. MCWILLIAMS P. B. GANS 《Journal of Metamorphic Geology》2009,27(9):827-844
New 40 Ar/39 Ar ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane and surrounding areas. Combined with U-Pb ages, Sm-Nd ages, Rb-Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino–Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K-feldspar 40 Ar/39 Ar ages. Subsequent unroofing at rates of 5–25 km Myr−1 brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by 40 Ar/39 Ar ages in the 219–210 Ma range, implying cooling at crustal depths at rates of 50–200 °C Myr−1 . Ages in the 210–170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166–149 Ma plutonism was followed by cooling at rates of c. 15 °C Myr−1 , suggesting relatively deep crustal conditions, whereas Cretaceous 129–118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr−1 , suggesting relatively shallow crustal depths. 相似文献
4.
New U–Pb detrital zircon ages from Triassic metasandstones of the Torlesse Terrane in New Zealand are compared with 40 Ar/39 Ar muscovite data and together, reveal four main source components: (i) major, Triassic–Permian (210–270 Myr old) and (ii) minor, Permian–Carboniferous (280–350 Myr old) granitoids (recorded in zircon and muscovite data); (iii) minor, early middle Palaeozoic, metamorphic rocks, recorded mainly by muscovite, 420–460 Myr old, and (iv) minor, Late Precambrian–Cambrian igneous and metamorphic complexes, 480–570 Myr old, recorded by zircon only. There are also Proterozoic zircon ages with no clear grouping (580–1270 Myr). The relative absence of late Palaeozoic (350–420 Myr old) components excludes granitoid terranes in the southern Lachlan Fold Belt (Australia) and its continuation into North Victoria Land (East Antarctica) and Marie Byrd Land (West Antarctica) as a potential source for the Torlesse. The age data are compatible with derivation from granitoid terranes of the northern New England Orogen (and hinterland) in NE Australia. This confirms that the Torlesse Terrane of New Zealand is a suspect terrane, that probably originated at the NE Australian, Permian–Triassic, Gondwanaland margin and then (200–120 Ma) moved 2500 km southwards to its present New Zealand position by the Late Cretaceous (90 Ma). This sense of movement is analogous to that suggested for Palaeozoic Mesozoic terranes at the North American Pacific margin. 相似文献
5.
Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high-spatial-resolution 40 Ar/39 Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S2 , S3 , and S4 ) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S3 and S4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40 Ar/39 Ar geochronology. Mean 40 Ar/39 Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S3 and 345.5 ± 1.7 Myr for S4 . Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S3 and S4 , these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region. 相似文献
6.
In the Western French Massif Central, the Argentat fault is a major structure through which As–Au fluids percolated in the Late Carboniferous along brittle fractures. New petrostructural investigations show that an early ductile normal-dextral faulting, coeval to leucogranite emplacement took place during the Late Visean syncollisional extension of the belt and was accompanied by a hydrothermal event marked by the growth of muscovites whose 40 Ar/39 Ar ages cluster around 335 Ma. This early fluid channelling is associated with brittle deformation only in the hangingwall of the Argentat fault, whereas ductile deformation is restricted to the footwall. These results provide new evidence for the upper crust implication during the syncollisional extension in the French Massif Central. This study stresses the interest of a detailed multimethod analysis to characterize hydrothermal processes, especially in basement areas where the tectonic, plutonic and metamorphic evolution is polyphased. 相似文献
7.
The alkalic Scituate Granite was emplaced into crystalline sequences within the New England Esmond–Dedham terrane in the Late Devonian ( c. 370 Ma). Variably recrystallized amphibole (iron-rich, hastingsite–hastingsitic hornblende) from four variably deformed samples of the pluton record south-westerly younging 40 Ar/39 Ar plateau ages ranging between 276 and 263 Ma. These are interpreted to date diachronous cooling through temperatures appropriate for intracrystalline retention of argon following late Palaeozoic orogenic activity. Iron-rich biotite concentrates from the samples record only slightly younger ages, and therefore suggest relatively rapid post-metamorphic cooling. The 40 Ar/39 Ar ages indicate that the late Palaeozoic tectonothermal overprint was much more regionally pervasive than was previously considered. The apparent timing of this activity is similar to previous estimates for the chronology of high-grade metamorphism throughout the adjacent Hope Valley terrane and for phases of ductile movement on the intervening Lake Char–Honey Hill fault system. 相似文献
8.
《Tectonophysics》1999,301(1-2):145-158
The Mersin ophiolite is located on the southern flank of the E–W-trending central Tauride belt in Turkey. It is one of the Late Cretaceous Neotethyan oceanic lithospheric remnants. The Mersin ophiolite formed in a suprasubduction zone tectonic setting in southern Turkey at the beginning of the Late Cretaceous. The Mersin ophiolite is one of the best examples in Turkey in order to study reconstruction of ophiolite emplacement along the Alpine–Himalayan orogenic belt. 40Ar/39Ar incremental-heating measurements were performed on seven obduction-related subophiolitic metamorphic rocks. Hornblende separates yielded isochron ages ranging from 96.0±0.7 Ma to 91.6±0.3 Ma (all errors ±1σ). Five of the seven hornblende age determinations are indistinguishable at the 95% confidence level and have a weighted mean age of 92.6±0.2 (2σ) Ma. We interpret these ages as the date of cooling below 500°C. Intraoceanic thrusting occurred (∼4 Ma) soon after formation of oceanic crust. The sole was crosscut by microgabbro–diabase dikes less than 3 m.y. later. The final obduction onto the Tauride platform occurred during the Late Cretaceous–Early Paleocene. Our new high-precision ages constrain intraoceanic thrusting for a single ophiolite (Mersin) in the Tauride belt. 相似文献
9.
Abstract 40 Ar/39 Ar step-heating and single-grain laser fusion ages from phengites from the polydeformed and polymetamorphosed blueschist-greenschist facies Nome Group fall into two groups. Samples from the upper part of the structural section that have experienced a relatively weak metamorphic and deformational post-blueschist facies overprint and one sample from the Cape Nome orthogneiss yield plateau ages of 116-125 Ma. More intensely overprinted samples yield hump-shaped spectra with minimum ages of 123 Ma and maximum ages of 334 Ma. Samples with hump-shaped spectra are derived from a greater structural depth than most samples with plateau ages. Unreasonably old maximum ages from some of the disturbed spectra suggest that the hump-shaped spectra result from the incorporation of excess 40 Ar. This interpretation conflicts with previous interpretations of similarly disturbed spectra from the Brooks Range, which have been argued to provide minimum ages for blueschist facies metamorphism. Since the maximum temperatures achieved by all samples were probably above the blocking temperature of Ar in phengite, the 116-125 Ma plateau ages are a minimum age for blueschist facies metamorphism on the Seward Peninsula, Alaska. 相似文献
10.
Yasushi Watanabe 《Resource Geology》2003,53(3):221-226
Abstract. 40 Ar-39 Ar analyses of two alunite samples from phreatic craters in the Pliocene Muine volcano in southwest Hokkaido, Japan, were carried out. The alunite with 17.4 permil δ34 SV_CDT value in hydrothermal breccia from the Nagaoyama crater and that with 14.3 permil δ34 SV_CDT value in silicified andesite from the Konuma crater give total fusion ages of 1.40 ± 0.04 Ma (la uncertainty) and 1.24 ± 0.08 Ma, respectively. However, the spectra of these samples indicate they have been effected by thermal overprinting and/or the existence of excess argon. These preliminary 40 Ar-39 A analyses suggest that the alunite underwent multiple hydrothermal activity by magmatic gas and vapor subsequent to the main hydrothermal activity. 相似文献
11.
12.
T. Simon-Labric Y. Rolland T. Dumont T. Heymes C. Authemayou M. Corsini M. Fornari 《地学学报》2009,21(2):127-136
Direct absolute dating of the Penninic Frontal Thrust tectonic motion is achieved using the 40 Ar/39 Ar technique in the Pelvoux Crystalline Massif (Western Alps). The dated phengites were formed syn-kinematically in shear zones. They underline the brittle-ductile stretching lineation, pressure-shadow fibres and slickensides consistent with underthrusting of the European continental slab below the propagating Penninic Thrust. Chlorite–phengite thermobarometry yields 10–15 km and T ∼280 °C, while 40 Ar/39 Ar phengite ages mainly range between 34 and 30 Ma, with one younger age at 27 Ma. This Early Oligocene age range matches a major tectonic rearrangement of the Alpine chain. Preservation of prograde 40 Ar/39 Ar ages is ascribed to passive exhumation of the Pelvoux shear zone network, sandwiched between more external thrusts and the Penninic Front reactivated as an E-dipping detachment fault. Partial resetting in the Low Temperature part of argon spectra below 24 Ma is ascribed to brittle deformation and alteration of phengites. 相似文献
13.
Abstract 40 Ar/39 Ar ages from white mica in rocks of the internal zone of the Brooks Range contractional orogen indicate that the Nanielik antiformal duplex developed at about 120 Ma and was remobilized on its southern boundary at c . 108 Ma. Blueschist facies metamorphism accompanied development of the antiform. The timing of the blueschist facies event and creation of the antiform overlap the period of shallow-seated deformation in the foreland fold and thrust belt and sedimentation in the foreland basin of the Brooks Range. Blueschist facies P-T conditions may therefore characterize the thicker parts of orogenic wedges in some orogenic systems; ancient blueschists need not necessarily be interpreted as indicators of active subduction or continent-continent collision.
Microprobe analysis using quantitative wavelength-dispersive and electron backscattered electron imaging methods was used to characterize the composition of white micas in the dated samples. None of the samples was compositionally homogeneous; many contained 2-3 populations of white mica, including both potassic and sodic varieties. Samples which had undergone (in sequence) amphibolite, albite-epidote amphibolite and blueschist facies metamorphic events retained muscovites relict of the amphibolite facies event. Samples that had undergone only the blueschist facies event also contained multiple populations of mica, some probably from detrital sources. 相似文献
Microprobe analysis using quantitative wavelength-dispersive and electron backscattered electron imaging methods was used to characterize the composition of white micas in the dated samples. None of the samples was compositionally homogeneous; many contained 2-3 populations of white mica, including both potassic and sodic varieties. Samples which had undergone (in sequence) amphibolite, albite-epidote amphibolite and blueschist facies metamorphic events retained muscovites relict of the amphibolite facies event. Samples that had undergone only the blueschist facies event also contained multiple populations of mica, some probably from detrital sources. 相似文献
14.
We present the first trace element and age data combined with new Sr, Nd, and Pb isotope ratios on lavas from San Felix Island in the Southeast Pacific. A 40 Ar/39 Ar plateau age of 421 ± 18 ka implies young intraplate volcanic activity in this region relative to the ∼22 Ma old volcanism on the neighbouring Easter seamount chain (ESC). The incompatible element compositions of the San Felix magmas are similar to those of EM1-type basalts from Gough, although the isotopic compositions differ. San Felix formed some 20 Ma after the ESC plume affected the plate in this region but no chemical signature of the ESC material is observed in the young volcanic rocks. The composition of the San Felix basalts indicates a mantle source containing old continental lithospheric material from either metasomatized mantle or recycled sediments, which ascends in a weak mantle plume. 相似文献
15.
ABSTRACT This work presents a detailed 87 Sr/86 Sr isotope curve for the interval 7.5–9.7 Ma obtained by a high-resolution analysis (sampling spacing of about 40 kyr) of an astronomically calibrated land-based sedimentary sequence exposed in the central Mediterranean area (Gibliscemi section, southern Sicily). The main aim is to verify a synchronous response of the Mediterranean seawater Sr isotope record to the oceanic forcing on the basis of multiple comparisons of the Gibliscemi record with published coeval 87 Sr/86 Sr curves. A good correlation with the 87 Sr/86 Sr data from the ODP site 926 (equatorial Atlantic ocean), considered to be the Sr chemostratigraphic reference section for the Late Miocene, and from the Pacific DSDP site 590B was registered. Conversely, the comparison of the Gibliscemi Sr isotope data with 87 Sr/86 Sr ratios from the coeval segment of the land-based Sardella section (eastern Mediterranean) shows important differences highlighting a local control on the seawater Sr isotope changes in semi-isolated subbasins within the Late Miocene Mediterranean. 相似文献
16.
Counterclockwise P-T-t trajectory from the metamorphic sole of a Neo-Tethyan ophiolite (Turkey) 总被引：2，自引：0，他引：2
The Kiziltepe ophiolitic thrust sheet in the Bolkar Mountains of Turkey occurs between two subparallel ophiolite belts bounding the Tauride carbonate platform and represents a remnant of the Cretaceous Neo-Tethyan oceanic lithosphere. It is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra-oceanic subduction zone in the Neo-Tethys 92-90 Ma. Blueschist-facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing P/T with cooling. Regional tectonic constraints suggest a Maastrichtian age for the timing of this blueschist-facies metamorphism. Sodic amphibole-rich veins and crossite/Mg-riebeckite rims on hornblende suggest that growth of blueschist-facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries. We infer a counterclockwise P-T-t trajectory during which metamorphism was accompanied/succeeded by rapid uplift along the northern edge of the Tauride belt in Late Cretaceous-early Tertiary time. 相似文献
17.
Palaeo- and Neo-Tethyan-related magmatic and metamorphic units crop out in Konya region in the south central Anatolia. The Neotethyan assemblage is characterized by mélange and ophiolitic units of Late Cretaceous age. They tectonically overlie the Middle Triassic–Upper Cretaceous neritic to pelagic carbonates of the Tauride platform. The metamorphic sole rocks within the Konya mélange crop out as thin slices beneath the sheared serpentinites and harzburgites. The rock types in the metamorphic sole are amphibolite, epidote-amphibolite, garnet-amphibole schist, plagioclase-amphibole schist, plagioclase-epidote-amphibole schist and quartz-amphibole schist. The geochemistry of the metamorphic sole rocks suggests that they were derived from the alkaline (seamount) and tholeiitic (E-MORB, IAT and boninitic type) magmatic rocks from the upper part of the Neotethyan oceanic crust. Four samples from the amphibolitic rocks yielded 40Ar/39Ar isotopic ages, ranging from 87.04 ± .36 Ma to 84.66 ± .30 Ma. Comparison of geochemistry and geochronology for the amphibolitic rocks suggests that the alkaline amphibolite (seamount-type) cooled below 510 ± 25 °C at 87 Ma whereas the tholeiitic amphibolites at 85 Ma during intraoceanic thrusting/subduction. When all the evidence combined together, the intraoceanic subduction initiated in the vicinity of an off-axis plume or a plume-centered spreading ridge in the Inner Tauride Ocean at 87 Ma. During the later stage of the steady-state subduction, the E-MORB volcanics on the top of the down-going slab and the arc-type basalts (IAT/boninitic) detached from the leading edge of the overriding plate, entered the subduction zone after ~2 my and metamorphosed to amphibolite facies in the Inner Tauride Ocean. Duration of the intraoceanic detachment (~87 Ma) and ophiolite emplacement onto the Tauride-Anatolide Platform (Tav?anl? Zone), followed by subsequent HP/LT metamorphism (~82 Ma) spanned ~5 my in the western part of the Inner Tauride Ocean. 相似文献
18.
Kathrin Abraham Sophie Opfergelt François Fripiat Anne-Julie Cavagna Jeroen T.M. de Jong Stephen F. Foley Luc André Damien Cardinal 《Geostandards and Geoanalytical Research》2008,32(2):193-202
We report silicon isotopic determinations for USGS rock reference materials BHVO-1 and BHVO-2 using a Nu Plasma multi-collector (MC)-ICP-MS, upgraded with a new adjustable entrance slit, to obtain medium resolution, as well as a stronger primary pump and newly designed sampler and skimmer cones ("B" cones). These settings, combined with the use of collector slits, allowed a resolution to be reached that was sufficient to overcome the 14 N16 O and 14 N2 interferences overlying the 30 Si and the 28 Si peaks, respectively, in an earlier set-up. This enabled accurate measurement of both δ30 Si and δ29 Si. The δ value is expressed in per mil variation relative to the NBS 28 quartz reference material. Based on data acquired from numerous sessions spread over a period of six months, we propose a recommended average δ30 Si of −0.33 ± 0.05‰ and −0.29 ± 0.11‰ (2se) for BHVO-1 and BHVO-2, respectively. Our BHVO grand mean silicon isotope composition (δ30 Si =−0.31 ± 0.06‰) is significantly more negative than the only published value for BHVO-2, but is in very good agreement with the recently established average value of ocean island basalts (OIB), confirming the conclusion that the OIB reservoir has a distinct isotopic composition from the solar reservoir as sampled by chondrites. 相似文献
19.
Stanislaw Halas Janina Szaran Maciej Czarnacki Ahmad Tanweer 《Geostandards and Geoanalytical Research》2007,31(1):61-68
Refinements have been made to achieve over 99% yield in the conversion of CO to CO2 in order to improve the reproducibility and accuracy of δ18 O measurements in sulfates. BaSO4 (10-15 mg) was mixed with an identical amount of spectrographic-grade graphite and loaded into a Pt boat. The mixture was gradually heated to 1100 °C to reduce sulfate to CO and CO2 ; the former gas was simultaneously converted to CO2 by a glow discharge between Pt electrodes immersed in a magnetic field (produced by a pair of external neodymium magnets). A small memory effect was noticed during the analysis (less than 0.3‰ per 10‰ difference in δ18 O between two subsequently analysed samples). The memory effect, however, was suppressed by repetitive preparation of the same specimen. CO2 produced in this way from sulfate reference samples was analysed on a dual inlet and triple collector mass spectrometer along with CO2 equilibrated with VSMOW, GISP and SLAP water reference samples. To avoid large departures of measured isotope ratios from 18 O/16 O of the working calibrator we used CO2 gas prepared from ocean water sulfate for this purpose. The calibrated δ18 O values (in ‰) obtained in this way for NBS-127, IAEA SO-5 and IAEA SO-6 reference materials were 8.73 ± 0.05, 12.20 ± 0.07 and -10.43 ± 0.12, respectively. 相似文献
20.
Marinella A. Laurenzi Maria Laura Balestrieri Giulio Bigazzi Julio C. Hadler Neto Pedro J. Iunes Pio Norelli Massimo Oddone Ana Maria Osorio Araya José G. Viramonte 《Geostandards and Geoanalytical Research》2007,31(2):105-124
New analyses have been performed in order to enhance the data-set on the independent ages of four glasses that have been proposed as reference materials for fission-track dating. The results are as follows. Moldavite - repeated 40 Ar/39 Ar age determinations on samples from deposits from Bohemia and Moravia yielded an average of 14.34 ± 0.08 Ma. This datum agrees with other recent determinations and is significantly younger than the 40 Ar/39 Ar age of 15.21 ± 0.15 Ma determined in the early 1980s. Macusanite (Peru) -four K-Ar ages ranging from 5.44 ± 0.06 to 5.72 ± 0.12 Ma have been published previously. New 40 Ar/39 Ar ages gave an average of 5.12 ± 0.04 Ma. Plateau fission-track ages determined using the IRMM-540 certified glass and U and Th thin films for neutron fluence measurements agree better with these new 40 Ar/39 Ar ages than the previously published ages. Roccastrada glass (Italy) - a new 40 Ar/39 Ar age, 2.45 ± 0.04 Ma, is consistent with previous determinations. The Quiron obsidian (Argentina) is a recently discovered glass that has been proposed as an additional reference material for its high spontaneous track density (around 100 000 cm-2 ). Defects that might produce "spurious" tracks are virtually absent. An independent 40 Ar/39 Ar age of 8.77 ± 0.09 Ma was determined and is recommended for this glass. We believe that these materials, which will be distributed upon request to fission-track groups, will be very useful for testing system calibrations and experimental procedures. 相似文献