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1.
Four 40Ar/39Ar dates on mineral separates from fresh and hydrothermally altered volcanic and plutonic rocks from the Ngatamariki geothermal field indicate that andesitic volcanism took place in the eastern portion of the Taupo Volcanic Zone (TVZ) prior to 1.2 Ma and probably considerably earlier. These data significantly extend the onset and duration of andesitic volcanism in the east-central TVZ over previous estimates. Intrusive activity is represented at Ngatamariki by a dioritic pluton, the only such pluton yet recognized in the entire TVZ. Hornblende from the pluton yields a crystallization age of near 550 ka. Hydrothermal alteration spatially associated with the pluton produced sericite of a similar age. Overlying and postdating the most intense hydrothermal alteration zone is the Whakamaru Ignimbrite (or its equivalent) which was emplaced at 330 ka. Two distinct geothermal systems may have been active at nearly the same site from 550 ka to present. The most intense activity occurred before 330 ka and was associated with emplacement of the Ngatamariki diorite. This was followed by the less intense system that is currently active. The geothermal regime at Ngatamariki has, therefore, probably been active intermittently for at least 550 ka.  相似文献   

2.
 Outflow sheets of the Hiko tuff and the Racer Canyon tuff, which together extend over approximately 16 000 km2 around the Caliente caldera complex in southeastern Nevada, have long been considered to be products of simultaneous or near-simultaneous eruptions from inset calderas in the west and east ends, respectively, of the caldera complex. New high-precision 40Ar/39Ar geochronology and paleomagnetic data demonstrate that emplacement of the uppermost part of the Racer Canyon tuff at 18.33±0.03 Ma was nearly synchronous with emplacement of the single outflow cooling unit of the much larger overlying Hiko tuff at 18.32±0.04 Ma. Based on comparison with the geomagnetic polarity time scale derived from the sea-floor spreading record, we conclude that emplacement of the first of several outflow cooling units of the Racer Canyon tuff commenced approximately 0.5 m.y. earlier. Only one paleomagnetic polarity is found in the Hiko tuff, but at least two paleomagnetic reversals have been found in the Racer Canyon tuff. The two formations overlap in only one place, at and near Panaca Summit northeast of the center of the Caliente caldera complex; here the Hiko tuff is stratigraphically above the Racer Canyon tuff. This study demonstrates the power of combining 40Ar/39Ar and paleomagnetic data in conjunction with phenocryst compositional modes to resolve problematic stratigraphic correlations in complex ash-flow sequences where use of one method alone might not eliminate ambiguities. Received: 13 January 1997 / Accepted: 7 May 1997  相似文献   

3.
Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Triassic alkaline magmatic belt along the northern margin of the North China Craton(NCC). Zircon U-Pb age, petrological, mineralogical and geochemical data of the pluton were obtained in this paper, to constrain its origin and mantle source characteristics. The pluton is composed of nepheline-clinopyroxene syenite and alkali-feldspar syenite, with zircon U-Pb age of 214.7±1.1 Ma. Diopside(cores)-aegirine-augite(rims), biotite, orthoclase and nepheline are the major minerals. The Baotoudong syenites have high contents of rare earth elements(REE), and are characterized by enrichment in light rare earth elements(LREE) and large ion lithophile elements(LILE; e.g., Rb, Ba, Sr), depletion in heavy rare earth elements(HREE) and high field strength elements(HFSE). They show enriched Sr-Nd isotopic compositions with initial ~87Sr/~86Sr ranging from 0.7061 to 0.7067 and ε_Nd(t) values from –9.0 to –11.2. Mineralogy, petrology and geochemical studies show that the parental magma of the syenites is SiO_2-undersaturated potassic-ultrapotassic, and is characterized by high contents of Ca O, Fe_2O_3, K_2O, Na_2O and fluid compositions(H_2O), and by high temperature and high oxygen fugacity. The syenites were originated from a phlogopite-rich, enriched lithospheric mantle source in garnet-stable area(80 km). The occurrence of the Baotoudong syenites, together with many other ultrapotassic, alkaline complexes of similar ages on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids from the subducted, altered paleo-Mongolian oceanic crust, and the northern margin of the craton has entered into an extensively extensional regime as a destructive continental margin in Late Triassic.  相似文献   

4.
The groundwaters of the Great Artesian Basin (Australia) have been previously shown to be accumulating in-situ production helium for groundwaters ages < 50 kyr and an external helium flux equivalent to whole crustal production for groundwater ages > 100 kyr [1,2]. New helium isotope measurements show that the observed in-situ production helium (3He/4He 1.6 × 10−8) is isotopically distinct from the crustal degassing helium flux (3He/4He 6.6 × 10−8). Furthermore, the crustal degassing helium isotope ratio is marginally in excess of the whole crustal production ratio (3He/4He= 3.5 × 10−8) and the production ratio in a variety of continental rock types. This suggests that the upper limit on volatile transport across the mantle-crust boundary beneath the (relatively) stable and “complacent” Australian continent can be characterized by a “conductive-diffusive” helium/heat flux ratio of 2.6 × 1064He atoms mW−1 s−1 which is two orders of magnitude less than the “intrusive-volcanic” ratio of 2.9 × 1084He atoms mW−1 s−1 measured at the Galapagos [16]. These results constrain the transcrustal mantle degassing fluxes of4He and40Ar to be much less than the mid-ocean ridge degassing fluxes; which are much less than the degassing of4He and40Ar from continental crust. Thus, the degassing of the Earth's interior is dominated by magmatic processes but the dominant fluxes of4He and40Ar to the atmosphere must come from the continental crust.  相似文献   

5.
The eruptive history of the Tequila volcanic field (1600 km2) in the western Trans-Mexican Volcanic Belt is based on 40Ar/39Ar chronology and volume estimates for eruptive units younger than 1 Ma. Ages are reported for 49 volcanic units, including Volcán Tequila (an andesitic stratovolcano) and peripheral domes, flows, and scoria cones. Volumes of volcanic units 1 Ma were obtained with the aid of field mapping, ortho aerial photographs, digital elevation models (DEMs), and ArcGIS software. Between 1120 and 200 kyrs ago, a bimodal distribution of rhyolite (~35 km3) and high-Ti basalt (~39 km3) dominated the volcanic field. Between 685 and 225 kyrs ago, less than 3 km3 of andesite and dacite erupted from more than 15 isolated vents; these lavas are crystal-poor and show little evidence of storage in an upper crustal chamber. Approximately 200 kyr ago, ~31 km3 of andesite erupted to form the stratocone of Volcán Tequila. The phenocryst assemblage of these lavas suggests storage within a chamber at ~2–3 km depth. After a hiatus of ~110 kyrs, ~15 km3 of andesite erupted along the W and SE flanks of Volcán Tequila at ~90 ka, most likely from a second, discrete magma chamber located at ~5–6 km depth. The youngest volcanic feature (~60 ka) is the small andesitic volcano Cerro Tomasillo (~2 km3). Over the last 1 Myr, a total of 128±22 km3 of lava erupted in the Tequila volcanic field, leading to an average eruption rate of ~0.13 km3/kyr. This volume erupted over ~1600 km2, leading to an average lava accumulation rate of ~8 cm/kyr. The relative proportions of lava types are ~22–43% basalt, ~0.4–1% basaltic andesite, ~29–54% andesite, ~2–3% dacite, and ~18–40% rhyolite. On the basis of eruptive sequence, proportions of lava types, phenocryst assemblages, textures, and chemical composition, the lavas do not reflect the differentiation of a single (or only a few) parental liquids in a long-lived magma chamber. The rhyolites are geochemically diverse and were likely formed by episodic partial melting of upper crustal rocks in response to emplacement of basalts. There are no examples of mingled rhyolitic and basaltic magmas. Whatever mechanism is invoked to explain the generation of andesite at the Tequila volcanic field, it must be consistent with a dominantly bimodal distribution of high-Ti basalt and rhyolite for an 800 kyr interval beginning ~1 Ma, which abruptly switched to punctuated bursts of predominantly andesitic volcanism over the last 200 kyrs.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial responsility: J. Donnelly-NolanThis revised version was published online in January 2005 with corrections to Tables 1 and 3.An erratum to this article can be found at  相似文献   

6.
The difficulty of isolating intact, mineralogically pure pedogenic crystals from cemented soil is one of the most significant obstacles to quantifying rates of soil formation, geomorphic processes, and climate change in arid regions. We evaluate the applicability of vacuum encapsulated 40Ar/39Ar geochronology to pedogenic palygorskite and sepiolite extracted from the 4 to 5 Ma, extant Mormon Mesa petrocalcic soil-geomorphic surface of southern Nevada, and from the 780 ka to 2 Ma Jornada Experimental Range La Mesa soil-geomorphic surface near Las Cruces, New Mexico. Selective dissolution of cements using NaOAc and Tiron, accompanied by particle size fractionation, was used to isolate the pedogenic Mg-phyllosilicates. Scanning electron microscopy, inductively-coupled plasma spectrometry, X-ray diffraction, gas chromatograph mass spectrometry, and Ar isotope analysis were used to determine whether extraction impacted palygorskite/sepiolite suitability for 40Ar/39Ar geochronology. We found no adverse morphological or mineralogical effects, but meaningful ages could not be obtained due to small amounts of old, detrital phyllosilicates in the samples. While the potential of pedogenic palygorskite and/or sepiolite for geochronology now seems limited, results from this study may prove relevant for samples from other, non-pedogenic surface environments. It is hoped that this work will encourage further research towards successful 40Ar/39Ar geochronology of pedogenic phyllosilicates, as well as inform future geochemical or isotopic studies of individual pedogenic mineral species.  相似文献   

7.
Amphibolitic metamorphic rocks are associated with the Thetford, Asbestos and Orford ophiolites as well as the Mont Albert and Pennington Sheet peridotites of the Québec Appalachians. An augmented compilation of the existing data on their field relations, mineralogy, geochronology, structural features and geothermobarometry is presented in order to help reconstruct the timing and processes of marginal basin closure and ophiolite emplacement during the Ordovician. A new, refined 40Ar/39Ar incremental release spectrum and isochron age is presented for the Thetford Mines ophiolite dynamothermal sole. The new 477 ± 5 Ma age resolves the existing disparity between the crustal (plagiogranite) and sole ages. The sole was formed shortly after crustal formation, suggesting that the ophiolite was decoupled at or near a spreading centre. A hitherto undescribed ultramafic-mafic amphibolitic sole beneath the Asbestos ophiolite was decoupled and rotated during the continental emplacement of the overlying ophiolite. Dating of the sole at Asbestos was hampered by presence of low K2O amphiboles, but an Acadian (377 Ma) age was obtained from orthoclase. The Orford sole was dismembered and incorporated within a serpentinitic mélange that contains other ophiolitic lithologies. Sheared amphibolites from alongside the Pennington Sheet in the Flintkote Mine are reinterpreted as a dynamothermal sole, rather than a metasomatically generated amphibole-bearing metasediment.  相似文献   

8.
The Marangudzi ring complex, Rhodesia, consists essentially of a gabbro mass intruded by ring dykes of quartz syenite and cone sheets of nepheline syenite. Five intrusive units (gabbro, two quartz syenite and two nepheline syenite units) have been studied using Rb-Sr and K-Ar methods. A total of 24 whole rock samples define a Rb-Sr isochron which gives an age of 186 ± 3m.y. and an initial (87Sr/86Sr)0 ratio of 0.70769 ± 0.00006 (±2sigma; based on λ = 1.42 × 10?11yr?1). K-Ar and Rb-Sr analyses on biotite and hastingsite separates are consistent with this age assignment. Whole rock Rb-Sr isochrons for the different units treated individually agree with the above age and initial Sr ratio within analytical uncertainties. This is believed to indicate that the different rock types are comagmatic forming by fractional crystallization of a parental, mantle-derived, K2O-rich basaltic magma, having an initial Sr ratio of 0.7077, without appreciable assimilation of the Precambrian country rock. The entire differentiation, emplacement and crystallization processes took place over a rather short time span.  相似文献   

9.
Data reported in 40Ar/39Ar geochronology studies are commonly insufficient to allow computation of ages. This deficiency renders it difficult to compare ages based on different standards or constants, and often hinders critical evaluation of the results. Herein are presented an enumeration of the data that should be reported in all 40Ar/39Ar studies, including a discussion in support of these requirements. The minimum required data are identified and distinguished from parameters that are useful but may be derived from them by calculation. Finally, recommendations are made for metadata needed to document age calculations (e.g., from age spectrum or isochron analyses).  相似文献   

10.
Five samples from a biotite-hornblende granodiorite phase of the 42.5 Ma Quxu pluton, Gangdese batholith, southern Tibet, have been collected at 250 m vertical intervals. Biotite from these rocks yields monotonically decreasing40Ar/39Ar isochron ages with decreasing elevation of 26.8 ± 0.2, 23.3 ± 0.5, 19.7 ± 0.3, 18.4 ± 0.4,and17.8 ± 0.1Ma (Tc = 335°C). Coexisting K-feldspars have virtually identical minimum apparent40Ar/39Ar ages of 17.0 ± 0.4Ma (Tc = 285°C). These data indicate parts of southern Tibet experienced a pulse of uplift in the early Miocene with the rate of uplift rising from 0.07 to 4.4 mm/year in the interval 20 to 17 Ma. An apatite fission track age of 9.9 ± 0.9Ma from this locality constrains the average uplift rate at this site to about 0.81 mm/year between 17 and 9.9 Ma and 0.30 mm/year from 9.9 Ma to present. K-feldspar from the Dagze granite, 30 km to the east, near Lhasa, yields a minimum apparent40Ar/39Ar age of 35.9 ± 0.9Ma (Tc = 227°C) which indicates an average uplift rate there of 0.21 mm/year since then. The marked pulse of uplift of the Quxu granodiorite and the difference in uplift history between the Dagze and Quxu plutons suggests southern Tibet has experienced discrete pulses of uplift variable in both space and time. These data are not consistent with models which require a large proportion of uplift of the Tibetan plateau to have occurred in the last 2 Ma. The data support the suggestion that convergence between India and Asia was largely accommodated by tectonic escape during the opening of the South China Sea 32 to 17 Ma ago and permit distributed shortening as a mechanism for crustal thickening and uplift of this part of the Tibetan plateau subsequent to 20 Ma.  相似文献   

11.
New 40Ar-39Ar geochronology, bulk rock geochemical data, and physical characteristics for representative stratigraphic sections of rhyolite ignimbrites and lavas from the west-central Snake River Plain (SRP) are combined to develop a coherent stratigraphic framework for Miocene silicic magmatism in this part of the Yellowstone ‘hotspot track’. The magmatic record differs from that in areas to the west and east with regard to its unusually large extrusive volume, broad lateral scale, and extended duration. We infer that the magmatic systems developed in response to large-scale and repeated injections of basaltic magma into the crust, resulting in significant reconstitution of large volumes of the crust, wide distribution of crustal melt zones, and complex feeder systems for individual eruptive events. Some eruptive episodes or ‘events’ appear to be contemporaneous with major normal faulting, and perhaps catastrophic crustal foundering, that may have triggered concurrent evacuations of separate silicic magma reservoirs. This behavior and cumulative time-composition relations are difficult to relate to simple caldera-style single-source feeder systems and imply complex temporal-spatial development of the silicic magma systems. Inferred volumes and timing of mafic magma inputs, as the driving energy source, require a significant component of lithospheric extension on NNW-trending Basin and Range style faults (i.e., roughly parallel to the SW–NE orientation of the eastern SRP). This is needed to accommodate basaltic inputs at crustal levels, and is likely to play a role in generation of those magmas. Anomalously high magma production in the SRP compared to that in adjacent areas (e.g., northern Basin and Range Province) may require additional sub-lithospheric processes. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.  相似文献   

12.
Lacustrine sediments of the Wilson Creek Formation in the Mono Basin, California, record a paleomagnetic field excursion constrained by 14C and 40Ar/39Ar geochronology to have occurred within the last 50 ka. However, 14C and 40Ar/39Ar ages are discordant, making it difficult to distinguish which of two possible excursions during this period, the Mono Lake or Laschamp, is recorded in the Mono Basin. New 40Ar/39Ar age determinations from sanidine, as well as the first biotite and obsidian ages, for three of the nineteen rhyolitic ashes intercalated with these sediments are presented and compared to previous 14C and 40Ar/39Ar data sets. Although the sanidine ages of the three ashes are stratigraphically consistent with each other and previously determined 40Ar/39Ar ages for other ashes in the Wilson Creek Formation, each is significantly older than 14C ages obtained from stratigraphically equivalent beds, relative paleointensity field correlations, oxygen isotope records, and glacial histories. These data indicate an absence of juvenile, eruptive crystals and most likely reflect the incorporation of crystals from older volcanic centers or underlying sediment. We examine the strengths and weaknesses of all available geochronologic data for the section exposed at Wilson Creek to arrive at an internally consistent set of age constraints. Using these constraints we propose two new relative paleointensity correlations for the section, both of which indicate that the excursion recorded in the Mono Basin occurred at ～30–34 ka on the Greenland Ice Sheet Project 2 (GISP2) ice core time scale.  相似文献   

13.
The back-arc region of the Izu-Bonin arc has complex bathymetric and structural features, which, due to repeated back-arc rifting and resumption of arc volcanism, have prevented us from understanding the volcano-tectonic history of the arc after 15 Ma. The laser-heating 40Ar/39Ar dating technique combined with high density sampling of volcanic rocks from the back-arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back-arc rifting. Based on the new 40Ar/39Ar dating results: (1) Back-arc rifting initiated at around 2.8 Ma in the middle part of the Izu-Bonin arc (30°30′N–32°30′N). Volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north–south-trending fissures and/or lines of vents. (2) Following this earliest stage of volcanism, at ca. 2.5 Ma, compositionally bimodal volcanism occurred and formed small cones in the wide area. This volcanism and rifting continued until about 1 Ma in the region west of the currently active rift zone. (3) After 1 Ma, active volcanism ceased in the area west of the currently active rift zone, and volcanism and rifting were confined to the currently active rift zone. The volcano-tectonic history of the back-arc region of the Izu-Bonin arc is an example of the earliest stage of back-arc rifting in the oceanic island arc. Age data on volcanics clearly indicate that volcanism changed its mode of activity, composition and locus along with a progress of rifting.  相似文献   

14.
Full chemical analyses, including some trace elements and both oxygen and strontium isotope abundance data are presented for samples collected from a traverse across the outcrop of the early Tertiary Loch Uisg Granophyre. Chemically, the body is rhyodacite with very uniform major and trace element composition. In contrast, depleted δ18O values vary widely from +1.5‰ in the south to ?3.7‰ in the north (a distance of about 21/2 km), a range comparable to that for the intrusive rocks of Mull as a whole. This indicates more extensive groundwater interaction (i.e. higher water/rock ratios and/or higher temperatures of isotope exchange) towards the focus of the central intrusive complex. There is some degree of correlation between δ18O and iron oxidation ratios but no other evidence that the primary igneous geochemistry of these rocks has been significantly modified by hydrothermal alteration after emplacement of the pluton. Initial87Sr/86Sr ratios range from 0.71350 ± 9 to 0.71624 ± 6 and correlate with both Rb content and Rb/Sr ratio, the latter correlation yielding a pseudo-isochron of 260 ± 54 Ma at the time of emplacement. These results confirm a major contribution from an old crustal source region, ruling out formation of the granophyre solely from a basic parent magma. However, Rb-Sr data are presented for the Moine schists exposed in Mull and Morvern which also appear to rule out their involvement in the petrogenesis of the granophyre, either as a source region for melting, or as a bulk contaminant for a mantle-derived magma. The only viable hypotheses are assimilation at depth of? Lewisian into a basaltic fractionation sequence or partial melting of a Proterozoic basement such as that involved in the production of Caledonian granites in the Scottish Highlands.  相似文献   

15.
Intrusions of ultramafic bodies into the lower density continental crust are documented for a large variety of tectonic settings spanning continental shields, rift systems, collision orogens and magmatic arcs. The intriguing point is that these intrusive bodies have a density higher by 300-500 kg m−3 than host rocks. Resolving this paradox requires an understanding of the emplacement mechanism. We have employed finite differences and marker-in-cell techniques to carry out a 2D modeling study of intrusion of partly crystallized ultramafic magma from sublithospheric depth to the crust through a pre-existing magmatic channel. By systematically varying the model parameters we document variations in intrusion dynamics and geometry that range from funnel- and finger-shaped bodies (pipes, dikes) to deep seated balloon-shaped intrusions and flattened shallow magmatic sills. Emplacement of ultramafic bodies in the crust lasts from a few kyr to several hundreds kyr depending mainly on the viscosity of the intruding, partly crystallized magma. The positive buoyancy of the sublithospheric magma compared to the overriding, colder mantle lithosphere drives intrusion while the crustal rheology controls the final location and the shape of the ultramafic body. Relatively cold elasto-plastic crust (TMoho = 400 °C) promotes a strong upward propagation of magma due to the significant decrease of plastic strength of the crust with decreasing confining pressure. Emplacement in this case is controlled by crustal faulting and subsequent block displacements. Warmer crust (TMoho = 600 °C) triggers lateral spreading of magma above the Moho, with emplacement being accommodated by coeval viscous deformation of the lower crust and fault tectonics in the upper crust. Strong effects of magma emplacement on surface topography are also documented. Emplacement of high-density, ultramafic magma into low-density rocks is a stable mechanism for a wide range of model parameters that match geological settings in which partially molten mafic-ultramafic rocks are generated below the lithosphere. We expect this process to be particularly active beneath subduction-related magmatic arcs where huge volumes of partially molten rocks produced from hydrous cold plume activity accumulate below the overriding lithosphere.  相似文献   

16.
Isochron burial dating with cosmogenic nuclides is used in Quaternary geochronology for dating sediments in caves, terraces, basins, and other depositional environments. However, the method has seldom been rigorously tested against an independent chronology. Here, we report a direct comparison of isochron burial dating with K-Ar and 40Ar/39Ar bracketing ages on volcanic flows that sandwich a fluvial gravel layer in the Xinjiang province of northwestern China. The ages agree to within analytical uncertainty, validating the assumptions and physical constants used in the isochron burial dating method.  相似文献   

17.
The Indosinian Orogeny plays a significant role in tectonic background and magmatic evolution in Indochina and surrounding regions. Being a part product of the Indosinian magmatism in northwest Vietnam during late Permian–middle Triassic period, Muong Luan granitoid pluton dominantly consists of granodiorite, less diorite and granite. This pluton is located in the Song Ma suture and assigned to the Dien Bien complex. Geochemically, the Muong Luan granitoid rocks are characterized by a wide range of SiO2 contents (59.9–75.1 wt%) and high K2O contents. They display typical features of I‐type granites. The presence of hornblende and no muscovite and cordierite in the rocks further supports for I‐type character of granitoids. The emplacement age of the Muong Luan pluton obtained by LA–ICP–MS U–Pb zircon is at 242–235 Ma, corresponding to Indosinian time. Zircon εHf values of –5.6 to –10.4, in combination with moderate Mg values of 34–45 suggested that the Muong Luan granitoid was derived from partial melting of mafic crustal source rocks, which are probably Paleoproterozoic in age as revealed by Hf model ages (TDM2 = 1624–1923 Ma).  相似文献   

18.
Eighty-nine basaltic lava flows from the northwest wall of Haleakala caldera preserve a concatenated paleomagnetic record of portions of the Matuyama-Brunhes (M-B) reversal and the preceding Kamikatsura event as well as secular variation of the full-polarity reversed and normal geomagnetic field. They provide the most detailed volcanic record to date of the M-B transition. The 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles (VGPs) that move from reverse to normal along the Americas, concluding with an oscillation in the Pacific Ocean to a cluster of VGPs east of New Zealand and back finally to stable polarity in the north polar region. All but one of the 16 Kamikatsura VGPs cluster in central South America. The full-polarity flows, with 40Ar/39Ar ages spanning a total of 680 kyr, pass a reversal test and give an average VGP insignificantly different from the rotation axis, with standard deviation consistent with that for other 0-5 Ma lava flows of similar latitude. Precise 40Ar/39Ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of 775.6±1.9 and 900.3±4.7 ka for the M-B and Kamikatsura transitional flows, respectively. This Matuyama-Brunhes age is ∼16 kyr younger than ages for M-B flows from the Canary Islands, Tahiti and Chile that were dated using exactly the same techniques and standards, suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals.  相似文献   

19.
40Ar/39Ar age determinations have been carried out on eight samples of melt rocks and one of the maskelynite from Mistastin Lake impact crater, Labrador. The observed40Ar* evolution spectra of the impact melts fall into distinct groups which correlate with petrographic variations. The release patterns of six of the melt rock samples define an age plateau in the range 34–41 m.y.; the other two have complex spectra which indicate incomplete equilibration of inclusions. Four of the samples with well-defined plateaux exhibit a high-temperature sag in their40Ar/39Ar ratio similar to that observed in some lunar samples. Maskelynite gives a partially overprinted spectrum which rises monotonically to a final age near 700 m.y., approximately half the age of the country rocks. The data from the melt samples are interpreted as indicating an age of 38 ± 4 m.y. for the Mistastin Lake impact event. Previously, it had been considered that this crater was 202 ± 25 m.y. old.  相似文献   

20.

Widely-distributed lamprophyres in the Laowangzhai gold deposit were associated closely with gold ores. Phlogopite40Ar/39Ar dating suggests that the emplacement age of lamprophyric magma ranges from (30.8±0.4) to (34.3±0.2) Ma, and gold mineralization took place at (26.4±0.2) Ma. PGE geochemical tracing indicates that gold in the gold deposit did not come from the primitive lamprophyric magma. The tempo-spatial paragenesis between lamprophyres and mesothermal gold deposits along the Jinsha-Red River belt may be attributed to the fact that they formed in the same tectonic setting.

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