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1.
Kohistan Sequence has been considered as island arc formed during the subduction of oceanic lithosphere at the leading edge of northward moving Indian continent.. Sedimentary sequences indicate that formation of the intra\|oceanic Kohistan arc began in early Cretaceous time. The isotopic data demonstrate the involvement of enriched, DUPAL type mantle, suggesting that Kohistan arc was formed at or south of the present equator (Khan et al., 1997). The Intra oceanic phase of Kohistan lasted until sometime between 102 and 85 Ma, when Kohistan collided with Asia. From this time until collision with India about 50 Ma ago, Kohistan existed as Andean\|type margin. This paleomagnetic study is from the volcanic and plutonic rocks exposed in Gupis\|Shamran area (west of Gilgit) in northern part of the Kohistan arc. According to geochronological data these rocks were formed 61～55Ma ago (Treloar et al., 1989), when Kohistan was existing as Andean\|type margin. Seven to nine samples were collected from nine sites of Shamran volcanics (58±1)Ma and from five sites of Pingal, Gupis, and Yasin plutons (Ar\|Ar hornblende ages ranges from 61～52Ma). On the basis of one Rb\|Sr age of (59±2)Ma from these plutons, the above\|mentioned Ar/Ar ages may be regarded as reasonable intrusion ages of these plutons (Searle, 1991).  相似文献   

2.
The paper presents results of geochronological and paleomagnetic studies of the volcanogenicsedimentary sequence of Henrietta Island in the East Siberian Sea. Our 40Ar/39Ar investigations confirm existing ideas that the bottom part of the section formed in the Ediacaran (~565 Ma) and that the basalts in the top of the section formed before the middle Cambrian (~520 Ma). Calculated paleomagnetic data confirm that during the rocks formation the territory of present-day Henrietta Island was located close to the 20° latitude, which lets us adjust some information published earlier on the age and natural remanent magnetization of the dolerite dikes of the nearby Jeannette Island. The new data also let us propose that a regional tectonothermal event, probably caused by accretion-related processes, took place at the beginning of the Ordovician.  相似文献   

3.
DIFFUSION MODELLING IN GARNET FROM TSO MORARI ECLOGITE AND IMPLICATIONS FOR EXHUMATION MODELS1　ChakrabortyS ,GangulyJ.ContribMineralPetrol,1992 ,111:74～ 86 . 2　ChemendaAI ,BurgJ P ,MattauerM .EarthPlanetSciLett ,2 0 0 0 ,174:397～ 40 9. 3　ChemendaAI ,MattauerM ,MalavieilleJ ,etal.EarthPlanetSciLett ,1995 ,132 :2 2 5～ 2 32 . 4　GirardM ,BussyF .TerraNostra ,1999,99/ 2 ,5 7～ 5 8. 5　GuillotS ,SigoyerdeJ ,LardeauxJM ,etal.Contr…  相似文献   

4.
This paper presents the results of geochronological studies of the Tomtor alkaline-ultramafic pluton, one of the largest Nb, Y, Sc, and TR deposits. A new scheme of its magmatism is given. The current K-Ar and Rb-Sr ages of different igneous rocks of the Tomtor pluton range from 800 to 250 Ma [Zaitsev et al., 1992; Frolov et al., 2003]. Such dispersion is probably related to the intense carbonatization of the rocks. The U-Pb zircon and 40Ar/39Ar mica ages indicate two stages of the formation of the pluton (700 and 400 Ma), which agrees well with the age of cycles of rift-related tectonogenesis of the Siberian platform.  相似文献   

5.
Petrological and geochronological investigations were carried out on metamorphic rocks of the Veporic unit (Inner Western Carpathians) in northern Hungary. K/Ar and Ar/Ar data on micas and amphibole show only Alpine ages (mostly in the range of 87-95 Ma) in this basement unit. Thermobarometric calculations yield lower amphibolite facies peak conditions (ca. 550냴 °C and 9ǃ kbar) for the Eoalpine metamorphic event. Complex evolution of gneissic rocks is reflected by the presence of discontinuously zoned garnets, the cores of which may represent relics of a pre-Alpine (presumably Variscan) thermal event. Zircon fission track (FT) data in the narrow range of 75-77.5 Ma indicate that this portion of the Veporic unit was emplaced to shallow crustal levels already during the Senonian time. The relative minor difference between zircon FT and K/Ar or Ar/Ar ages suggests very rapid cooling during the Late Cretaceous, most probably related to the extensional unroofing of the Veporic core complex. The obtained cooling ages do not support previous models of Tertiary uplift and exhumation of the Veporic unit along the Hurbanovo-Diósjeni Line.  相似文献   

6.
TWO TYPES OF CENOZOIC HIGH-K MAGMATINSM IN EASTERN TIBET:IMPLICATIONS FOR THE NATURE OF MANTLE SOURCES1　ArnaudNO ,VidalP ,TapponnierP ,etal.ThehighK2 OvolcanismofnorthwesternTibet:geochemistryandtectonicim plications[J] :EarthPlanetSciLett,1991,11:351～ 36 7. 2　DengW .CenozoicvolcanicrocksinthenorthernNgaridistrictoftheTibet Discussionontheconcurrentsubduction[J] .Ac taPetrolSinica ,1989( 3) :1～ 11. 3　HarrisonTM ,LeloupPH ,RyersonFJ,…  相似文献   

7.
"昆仑-黄河运动"是发生在早、中更新世之交的一次强烈的构造运动,它使青藏高原现代构造地貌格局基本定型。以往对昆仑-黄河运动时限的研究,主要由黄河干流发育的阶地、昆仑山垭口地区羌塘组与望昆冰碛层等古地磁、ESR、TL等分析测试得出,缺少其他年代学约束。在塔里木盆地,昆仑-黄河运动表现为西域砾岩与乌苏群之间的角度不整合接触。本文通过对塔里木盆地南缘西域砾岩顶部的普鲁上层火山岩4个样品的全岩~(40)Ar/~(39)Ar定年,获得~(40)Ar/~(39)Ar坪年龄为1.03±0.05 Ma~1.20±0.05 Ma,加权平均年龄为1.1±0.1 Ma(n=4,MSWD=2.3)。这是首次应用~(40)Ar/~(39)Ar定年方法,获得昆仑-黄河运动的时代下限为1.1±0.1 Ma,与前人确定的昆仑-黄河运动时代下限在误差范围内一致,为我国西部新构造运动提供了重要的年代学约束。  相似文献   

8.
A combined paleomagnetic and geochronological study is reported of Paleogene basalt lavas and an intercalated red bed succession, comprising a minimum of 14 basalt flows and 10 red bed horizons in the Tuoyun Basin of the southwest Tian Shan Range, China. Two basalt matrix samples yield 40Ar / 39Ar isochron ages of 58.5 ± 1.3 Ma (2σ, MSWD = 0.9) and 60.4 ± 1.3 Ma (2σ, MSWD = 1.7). These compare well with a previously published K–Ar dilution age of 61.7 ± 2.3 Ma for comparable Paleogene basalts and confirm that the younger pulse of magmatism in this basin is represented by both intrusive and extrusive activity. Demagnetization and component analysis identify a stable characteristic remanence (ChRM) with predominantly reversed polarity following removal of secondary remanence by peak demagnetization steps below 250–350 °C or 5 mT. Rock magnetic analysis identifies pseudo-single domain magnetite or titanomagnetite as carriers. The stable ChRM passes a fold test; it was probably acquired at the time of lava emplacement. Results from the bulk of the collection imply that paleomagnetic data from the upper and lower ( 115 Ma) basalt series in the Tuoyun Basin are not distinguishable at the 95% significance level and indicate that this tectonic domain remained essentially stationary with respect to the Earth's spin axis for 50 Ma prior to onset of the India/Asia collision in early Eocene times. It is therefore probable that no paleomagnetically detectable crustal shortening occurred in the southwest Tian Shan prior to collision. Paleomagnetic data sets from the Tuoyun Basin also show that little or no paleolatitude difference is present between the Tian Shan and the reference latitude of Eurasia at 60 Ma. This supports previous evidence suggesting that central Asian blocks in the vicinity of the Tian Shan are unlikely to have experienced appreciable northward convergence relative to Eurasia since onset of the India/Asia collision and initiation of the Himalaya.  相似文献   

9.
Volcanic pipes with K-Ar ages 65-49 Ma (Zubkov et.al., 1989) are widespread in the Minusinsk depression. We present the results of the first paleomagnetic investigation on four volcanic pipes (42 samples), accompanied by 40Ar/39Ar dating. All four pipes yield a high temperature ChRM component of reversed polarity with mean direction D = 178.8, I = −62, n = 33, k = 160.7. a95 = 7.3. However, some samples from the Bele pipe demonstrate another direction (D = 244.3, I = −58.2, n = 4, k = 407.4, a95 = 4.6). 40Ar/39Ar method gave plateau ages of pipes ranging from 74 to 81 Ma. These results cast some doubt on previous K-Ar dating. The paleomagnetic poles corresponding to both directions of magnetization are statistically different from the APWP of Eurasia given by Besse and Courtillot (1991). Paleomagnetic poles from basaltic rocks of the near-Baikal region (Khramov, 1984) are consistent with the first direction identified from Minusinsk depression, but again are different from the APWP of Eurasia. These data, therefore, call into question the validity of the Eurasian APWP to Northern Asia.  相似文献   

10.
LOW TEMPERATURE DATING OF HIGH MOUNTAIN ROCKS:(U-Th)/He AGES FROM HIGHER HIMALAYAN SAMPLES, EASTERN NEPAL1　HouseMA ,WernickeBP ,FarleyKA .DatingtopographyoftheSierraNevada ,California ,usingapatite (U Th) /Heages[J].Nature,1998,396 (5 ) :6 6～ 6 9. 2　HubbardMS ,Harrison .4 0 Ar/ 3 9ArageconstraintsondeformationandmetamorphismintheMainCentralThrustzoneandTibetanSlab ,EasternNepalHimalaya[J].Tectonics,1989,8(4) :86 5～ 880 . 3　HubbardMS …  相似文献   

11.
The paleogeography of the Earth, including the East European Platform, is very inaccurately defined for the interval 500–700 Ma. The quantity and quality of Late Precambrian–Cambrian paleomagnetic data on this platform are absolutely insufficient for reliable paleogeographical or paleotectonic reconstructions. Since there are almost no unstudied objects in the platform that could be used for paleomagnetic studies, it seems reasonable to consider the deformed platform margins. Of particular interest is the Bashkir anticlinorium (South Urals) with numerous Ediacaran sedimentary sections, some of which contain tuff beds suitable for isotope dating. We present paleomagnetic and geochronological data on the Upper Ediacaran Zigan Formation, sampled in the western part of the western limb of the Bashkir anticlinorium. The East European Platform must have been at near-equatorial latitudes at ～550 Ma.  相似文献   

12.
The age of spreading of the Liguro–Provençal Basin is still poorly constrained due to the lack of boreholes penetrating the whole sedimentary sequence above the oceanic crust and the lack of a clear magnetic anomaly pattern. In the past, a consensus developed over a fast (20.5–19 Ma) spreading event, relying on old paleomagnetic data from Oligo–Miocene Sardinian volcanics showing a drift-related 30° counterclockwise (CCW) rotation. Here we report new paleomagnetic data from a 10-m-thick lower–middle Miocene marine sedimentary sequence from southwestern Sardinia. Ar/Ar dating of two volcanoclastic levels in the lower part of the sequence yields ages of 18.94±0.13 and 19.20±0.12 Ma (lower–mid Burdigalian). Sedimentary strata below the upper volcanic level document a 23.3±4.6° CCW rotation with respect to Europe, while younger strata rapidly evolve to null rotation values. A recent magnetic overprint can be excluded by several lines of evidence, particularly by the significant difference between the in situ paleomagnetic and geocentric axial dipole (GAD) field directions. In both the rotated and unrotated part of the section, only normal polarity directions were obtained. As the global magnetic polarity time scale (MPTS) documents several geomagnetic reversals in the Burdigalian, a continuous sedimentary record would imply that (unrealistically) the whole documented rotation occurred in few thousands years only. We conclude that the section contains one (or more) hiatus(es), and that the minimum age of the unrotated sediments above the volcanic levels is unconstrained. Typical back-arc basin spreading rates translate to a duration ≥3 Ma for the opening of the Liguro–Provençal Basin. Thus, spreading and rotation of Corsica–Sardinia ended no earlier than 16 Ma (early Langhian). A 16–19 Ma, spreading is corroborated by other evidences, such as the age of the breakup unconformity in Sardinia, the age of igneous rocks dredged west of Corsica, the heat flow in the Liguro–Provençal Basin, and recent paleomagnetic data from Sardinian sediments and volcanics. Since Corsica was still rotating/drifting eastward at 16 Ma, it presumably induced significant shortening to the east, in the Apennine belt. Therefore, the lower Miocene extensional basins in the northern Tyrrhenian Sea and margins can be interpreted as synorogenic “intra-wedge” basins due to the thickening and collapse of the northern Apennine wedge.  相似文献   

13.
A review of the available radiometric and paleomagnetic data from the Deccan Flood Basalt Province (DFBP) suggests that the volcanism was episodic in nature and probably continued over an extended duration from 69 Ma to 63 Ma between 31R and 28N. It is likely that the most intense pulse of volcanism at 66.9 ± 0.2 Ma preceded the Cretaceous Tertiary Boundary (KTB, 65.2 ± 0.2Ma) events by R∼1.7Ma. The magnetostratigraphic record in the Deccan lava pile is incomplete and it is therefore possible that the lava flows constituting the reverse polarity sequence were erupted in more than one reversed magnetic chron.  相似文献   

14.
The thrust nappe played an important role in the Mesozoic tectonic evolution of the middle part of the Central Asian Orogenic Belt(CAOB). However, the timing, structural style and kinematic processes of the thrust nappe remain controversial, particularly the detail of the thrust nappe in the Guaizihu region(110 km east of Ejinaq). In this study, we investigate new field mapping, seismic sections, geochronology and low-temperature thermochronometric dating to provide constraints on the history of...  相似文献   

15.
The Illapel Plutonic Complex (IPC), located in the Coastal Range of central Chile (31°–33° S), is composed of different lithologies, ranging from gabbros to trondhjemites, including diorites, tonalites and granodiorites. U/Pb geochronological data shows that the IPC was amalgamated from, at least, four different magmatic pulses between 117 and 90 Ma (Lower to mid-Cretaceous). We present new paleomagnetic results including Anisotropy of Magnetic Susceptibility (AMS) from 62 sites in the plutonic rocks, 10 sites in country rocks and 7 sites in a mafic dyke swarm intruding the plutonic rocks.Remanent magnetizations carried by pyrrhotite in deformed country rock sediments nearby the intrusive rocks indicate that tilting of the sedimentary rocks occurred prior or during the intrusion. The paleomagnetic study shows no evidence for either a measurable tilt of the IPC or a significant rotation of the forearc at this latitude range. Moreover, new 40Ar/39Ar ages exclude any medium- to low-temperature post-magmatic recrystallization/deformation event in the studied samples. AMS data show a magnetic foliation that is often sub-vertical. Despite an apparent N–S elongated shape of the IPC, the large variations in the orientation of the AMS foliation suggests that this plutonic complex could be made of several units distributed in a N–S trend rather than N–S elongated bodies.Previous works have suggested for this area a major shift on tectonic evolution from highly extensional during Lower Cretaceous to a period around 100 Ma, associated with exhumation and compressive deformation to conform the present day Coastal Range. The low degree of anisotropy and the lack of evidence for a tectonic fabric in the intrusive rocks indicate that the shift from extensional to compressional should postdate the emplacement of the IPC, i.e. is younger than 90Ma.  相似文献   

16.
New structural, geochronological and paleomagnetic data were obtained on dolerite dikes of the Nola region (Central African Republic) at the northern border of the Congo craton. In this region, metavolcanic successions were thrust southward onto the craton during the Panafrican orogenic events. Our structural data reveal at least two structural klippes south of the present-day limits of the Panafrican nappe suggesting that it has once covered the whole Nola region, promoting the pervasive hydrothermal greenschist metamorphism observed in the underlying cratonic basement and also in the intrusive dolerite dikes. Paleomagnetic measurements revealed a stable dual-polarity low-inclination magnetization component in nine dikes (47 samples), carried by pyrrhotite and magnetite. This component corresponds to a paleopole at 304.8°E and 61.8°S (dp = 5.4, dm = 10.7) graded at Q = 6. Both metamorphism and magnetic resetting were dated by the 40Ar/39Ar method on amphibole grains separated from the dikes at 571 ± 6 Ma. The Nola pole is the first well-dated paleomagnetic pole for the Congo craton between 580 and 550 Ma. It marks a sudden change in direction of the Congo craton apparent polar wander path at the waning stages of the Panafrican orogenic events.  相似文献   

17.
40Ar-39Ar analyses of one alkali pyroxenite whole rock and two phlogopite separates of calcite carbonatites from the Sung Valley carbonatite-alkaline complex, which is believed to be a part of the Rajmahal-Bengal-Sylhet (RBS) flood basalt province, yielded indistinguishable plateau ages of 108.8 ± 2.0Ma, 106.4 ± 1.3Ma and 107.5 ± 1.4Ma, respectively. The weighted mean of these ages, 107.2 ± 0.8 Ma, is the time of emplacement of this complex. This implies that Sung Valley complex and probably other such complexes in the Assam-Meghalaya Plateau postdate the main flood basalt event (i.e., the eruption of tholeiites) in the RBS province by ∼10Ma.  相似文献   

18.
对分布在攀西古裂谷带内的禄丰县鸡街和大向坪超基性碱性环状杂岩体中霞辉岩，以及西昌太和层状辉长岩等两类岩石进行全岩^40Ar-^39Ar坪年龄测定，结果表明鸡街岩体和大向坪岩体中霞辉岩年龄范围在214～193Ma，相当于晚三叠世，属于印支期。总体上，超基性碱性环状杂岩的侵位时间略晚于含矿辉长岩；太和层状辉长岩^40Ar-^39Ar坪年龄，由于过剩Ar等原因的扰动，使其年龄值远远高于成岩年龄。暗示全岩或辉石单矿物等不适合作为攀西地区层状辉长岩定年的对象。  相似文献   

19.
According to geologic reconstructions, the motion of the Sierran-Great Valley block with respect to the Colorado Plateau was mainly westerly at more than 20 mm/yr from 16 to 10 Ma, changing to northwest or NNW since 8 to 10 Ma, at an average rate of 15 mm/yr. These kinematics are consistent with two other independent methods of determining the position of the block since 20 Ma–reconstructions based on paleomagnetic data from range blocks that bound the Basin and Range on the west, and a revised history of Pacific-North America plate motion based on a global plate circuit (Atwater and Stock, 1998, this issue). The plate-tectonic reconstruction shows a change to more northerly motion between the Pacific and North American plates at ～8 Ma, in concert with the motion of the Sierran-Great Valley block. Moreover, the northeast limit of extant oceanic crust (as indicated by the reconstruction of the continental geology) tracks closely with the southwest limit of extant continental crust (as indicated by the positions of oceanic plates) since 20 Ma. The coordination between plate motions and the intraplate geology suggests that plate-boundary forces strongly influenced deformation within the continent.  相似文献   

20.
The Huautla volcanic field (HVF), in the Sierra Madre del Sur (SMS), is part of an extensive record of Palaeogene magmatism reflecting subduction of the Farallon plate along the western edge of North America. Igneous activity resulting from Farallon subduction is also exposed to the north, in the Sierra Madre Occidental (SMO) and Mesa Central (MC) provinces. We present the results of a stratigraphic and K–Ar, Ar–Ar, and U–Pb geochronological study of the Huautla volcanic successions, in order to refine our knowledge on the petrologic and temporal evolution of the northern SMS and gain insights on magmatic–tectonic contrasts between the SMS and the SMO–MC provinces. The HVF is made up of lava flows and pyroclastic successions that overlie marine Cretaceous sequences and post-orogenic continental deposits of Palaeogene age. In the study area, the main Oligocene succession is pre-dated by the 36.7 million years its caldera west of the Sierra de Huautla. The HVF succession ranges in age from ～33.6 to 28.1 Ma and comprises a lower group of andesitic–dacitic lava flows, an intermediate sequence of ignimbrites and dacitic lavas, and an upper group of andesitic units. The silicic succession comprises a crystal-poor ignimbrite unit (i.e. the Maravillas ignimbrite; 31.4 ± 0.6, 32.0 ± 0.4 Ma; ～260 km3), overlain by a thick succession of dacitic lavas (i.e. the Agua Fría dacite; 30.5 ± 1.9, 31.0 ± 1.1 Ma). Integration of the new stratigraphic and geochronological data with prior information from other explosive centres of the north-central SMS allows us to constrain the temporal evolution of a silicic flare-up episode, indicating that it occurred between 37–32 Ma; it consisted of three major ignimbrite pulses at ～36.5, ～34.5, and ～33–32 Ma and probably resulted from a progressive, mantle flux-driven thermomechanical maturation of the continental crust, as suggested in the HVF by the transition from andesitic to voluminous siliceous volcanism. The information now available for the north-central sector of the SMS also allows recognition of differences between the temporal and spatial evolution of magmatism in this region, and of that documented in the southern SMO and MC provinces, suggesting that such contrasts are probably related to local differences in configuration of the subduction system. At ～28 Ma, the MC and southern SMO provinces experienced a trenchward migration of volcanism, associated with slab rollback; on the other hand, the broad, more stable distribution of Oligocene magmatism in the central and north oceanic plate was subducting at a low angle.  相似文献   

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