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1.
A comparison between conventional KAr (biotite) ages and fission track (zircon and apatite) and UPb (zircon) ages obtained from stratigraphically well-constrained Priabonian (Late Eocene) volcano-sedimentary deposits of northern Italy is presented. Two sections at Priabona (one level) and Possagno (two levels) were dated. The application of fission track dating appears fruitful for obtaining reasonably precise (±4 to 5% 2σ errors) ages useful for time-scale calibration. The concordancy of apatite and zircon fission track ages, and the reproducibility of results provide the time of volcanic eruption and deposition. The UPb analysis of the zircons has not been unsuccessful, but discordancy does not permit accurate dating. Significant dates obtained from Possagno are: KAr method, 35.0 ± 0.5 Ma (duplicate analysis on K-rich biotite from the same level); fission track dating method, 35.8 ± 1.4 Ma (weighted mean age on 2 apatite and 3 zircon separates from the same level); UPb method, 36.7 ± 1.0 Ma (maximum age of discordant zircons from the same level). The comparison between the present results and recent multi-method and multi-laboratory results obtained from time equivalent Priabonian (Late Eocene) biotite-rich layers from the Apennines shows perfect agreement and supports the location of a Priabonian stage between about 37.5 Ma and about 33.7 (±0.5) Ma; the alternative ages preferred by the Decade of North American Geology convention should be abandoned and a large portion of this scale revised accordingly.  相似文献   

2.
UPb geochronology provides an absolute time framework for the evolution of the Sigma gold deposit and surrounding rocks at Val d'Or, southern Abitibi subprovince. The Bourlamaque batholith, the largest pluton in the area, gives a 2699.8 ± 1.0 Ma UPb zircon age. This pluton cuts the Val d'Or Formation which hosts the mineralization. A UPb zircon age of 2704.9 ± 1.1 Ma on a felsic volcanic rock, the Colombière “rhyolite”, 13 km east of the mine dates that formation. The gold-bearing quartz vein system at Sigma is hosted by andesites and two generations of porphyry intrusions, all metamorphosed to the greenschist facies. The oldest porphyry (“porphyritic diorite”) shows the same deformation as the volcanic rocks, and has a 2703.7 ± 2.5 Ma UPb zircon age. The porphyritic diorite and volcanic rocks are cut by feldspar-porphyry dykes which post-date regional folding and have a 2694.0 ± 2.2 Ma UPb zircon age.Regional greenschist metamorphism has been dated directly, with a UPb date of 2684 ± 7 Ma on rutile in the Colombière “rhyolite”. The mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals. Hydrothermal rutile, from an alteration halo around the veins in andesite, has a 2599 ± 9 Ma UPb age. Textural evidence clearly indicates that the wall-rock alteration and vein filling are contemporaneous, and hence the vein system and gold mineralization appear to have developed at least 80 m.y. after the formation and metamorphism of host greenstones.  相似文献   

3.
UPb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the Ancient Gneiss Complex. The oldest episode at3644 ± 4Ma(2σ) produced the dominant type of zircon, characteristically purplish-brown and massive in texture; this probably constitutes unaltered zircon precipitated from the original magma. Recrystallization accompanied (and obscured) by early Pb loss took place within the oldest grains at3504 ± 6Ma and3433 ± 8Ma. Whole new grains grew at these times also. We interpret the post-3644 Ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss. In addition, many grains are visibly overgrown by two layers of younger zircon of different colour and texture, dated at2986 ± 20Ma and2867 ± 30Ma. Euhedral, finely-zoned whole grains having the 2986 Ma age are present also, evidently contributed by very thin felsic veins associated with the nearby Lochiel granite. The age of3644 ± 4Ma combined with precise zircon UPb dating of volcanics from the Onverwacht Group reported elsewhere demonstrates that at least part of the Ancient Gneiss Complex is older than the Barberton Greenstone Belt.  相似文献   

4.
Garnets in an amphibolite-facies metasediment from Sulitjelma, North Norway yield precise and concordant SmNd, UPb and RbSr ages that relate directly to the pressure (P) and temperature (T) conditions of mineral growth. Differential mineral reaction between graphitic and non-graphitic layers within this sample preserves a record of theP-T and time (t) history experienced during Barrovian regional metamorphism. Garnets in graphitic layers grew during prograde metamorphism at462 ± 16°C and5.2 ± 0.5 kbar under conditions of lowaH2O, and yield indistinguishable147Sm143Nd and238U206Pb ages of434.1 ± 1.2 Ma and433.9 ± 1.0 Ma, respectively. In contrast, garnet growth in adjacent graphite-free layers did not occur untilP-T conditions of540 ± 18°C and8.0 ± 1.0 kbar were attained, with continued growth in response to minor heating and decompression with final matrix equilibration at544 ± 16°C and7.0 ± 1.0 kbar. The inclusion-free garnet rims in this assemblage record indistinguishable147Sm143Nd and238U206Pb ages of424.6 ± 1.2 Ma and423.4± 1.7 Ma, respectively. These results provide precise estimates for average heating and burial rates during prograde metamorphism of 8.6−4.4+7.5°C Ma−1 and 0.8−0.5+0.9 km Ma−1, respectively. Rb and Sr exchange between coexisting silicates in the graphite-free assemblage continued for some 37 Ma after the “peak” of metamorphism, and require an average cooling rate of about 4.0°C Ma−1 during uplift. These results illustrate a clear relationship between reaction history and the timing of mineral growth and provide definitive constraints on the rates of thermal and tectonic processes accompanying regional metamorphism.  相似文献   

5.
The concentrations and isotopic compositions of argon, krypton and xenon have been determined in a grain size suite of zircons separated from pyroxene syenite of the Botnavatn Igneous Complex, southwestern Norway. The UPb systematics of these zircons has been studied previously.Kr and Xe are mixtures of fissiogenic gas from the spontaneous fission of238U and a component with atmospheric isotopic composition. From correlation diagrams the fissiogenic component is determined to be:83Kr :84Kr :86Kr = (4.6 ± 1.3) : (11.0 ± 2.0) : 100 and129Xe :131Xe :132Xe :134Xe :136Xe = (0.6 ± 0.3) : (8.8 ± 0.2) : (56.8 ± 0.3) : (82.8 ± 0.4) : 100. The fissiogenic136Xe/86Kr is 6.0 ± 0.4.The Ar isotopic composition shows radiogenic40Ar and a small excess of38Ar. The excess38Ar of about 1 × 10−11 cm3 STP/g can be explained by reactions of α-particles with chlorine. Asymmetric fission of238U which has been postulated to cause argon isotope anomalies in U-rich minerals is unnecessary to explain the observed38Ar concentrations.UXe ages are (1.19 ± 0.07) Ga, in agreement with UPb ages. However, if the recoil loss of fissiogenic Xe is considered the UXe ages of these zircons are about 1.53 Ga, which is comparable with the KAr ages and some RbSr ages observed in basement rocks in this region. The uncertainty of the product of fission yield times spontaneous fission decay constant of238U prevents to decide which age is the true crystallization age.  相似文献   

6.
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   

7.
The Jemez Mountains volcanic field (JMVF), located in north-central New Mexico, has been a site of basaltic to rhyolitic volcanism since the mid-Miocene with major caldera forming eruptions occurring in the Pleistocene. Eruption of the upper Bandelier Tuff (UBT) is associated with collapse of the Valles Caldera, whereas eruption of the lower Bandelier Tuff (LBT) resulted in formation of the Toledo Caldera. These events were previously dated by K-Ar at 1.12 ± 0.03 Ma and 1.45 ± 0.06 Ma, respectively. Pre-Bandelier explosive eruptions produced the San Diego Canyon (SDC) ignimbrites. SDC ignimbrite “B” has been dated at 2.84 ± 0.07 Ma, whereas SDC ignimbrite “A”, which underlies “B”, has been dated at 3.64 ± 1.64 Ma. Both of these dates are based on single K-Ar analyses.40Ar/39Ar dating of single sanidine crystals from these units indicates revision of the previously reported dates. Isochron analysis of 26 crystals from the UBT gives a common trapped 40Ar/36Ar component of 304.5, indicating the presence of excess 40Ar in this unit, and defines an age of 1.14 ± 0.02 Ma. Isochron analysis of 26 crystals from the LBT indicates an atmospheric trapped component and an age of 1.51 ± 0.03 Ma. An age of 1.78 ± 0.04 Ma, based on the weighted mean of 5 individual analyses, is indicated for SDC ignimbrite “B”, whereas 3 analyses from SDC ignimbrite “A” give a weighted mean age of 1.78 ± 0.07 Ma. Evidence for xenocrystic contamination in the SDC ignimbrites comes from analyses of a correlative air-fall pumice unit in the Puye Formation alluvial fan giving ages of 1.75 ± 0.08 and 3.50 ± 0.09 Ma. The presence of xenocrysts in bulk separates used for the original K-Ar analyses could account for the significantly older ages reported.Geochemical data indicate that SDC ignimbrites are early eruptions from the magma chamber which evolved to produce the LBT, as compositions of SDC ignimbrite “B” are virtually identical to least evolved LBT samples. Differentiation during the 270-ka interval between eruption of SDC ignimbrite “B” and the LBT produced an array of high-silica rhyolite compositions which were erupted to form the LBT. Mixed pumices associated with eruption of the LBT indicated an influx of more mafic magma into the system which produced shifts in some incompatible trace-element ratios. Lavas and tephras of the Cerro Toledo Rhyolite record the geochemical evolution of the Bandelier magma system during the 370-ka interval between eruption of the LBT and the UBT.The combined geochronologic and geochemical data place the establishment and evolution of the Bandelier silicic magma system within a precise temporal framework, beginning with eruption of the SDC ignimbrites at 1.78 Ma, and define a periodicity of 270–370 ka to ash-flow eruptions in the JMVF. These intervals are comparable to those in other multicyclic caldera complexes and are a measure of the timescales over which substantial fractionation of large silicic magma bodies occur.  相似文献   

8.
The chronology and isotope geochemistry of a selection of Proterozoic Scourie dykes has been investigated in order to specify both their time of emplacement within the thermal history of the Archaean crust of N.W. Scotland, and to attempt to characterise the evolution of continental lithosphere. SmNd, RbSr and UPb isotope analyses are presented. Primary, major igneous minerals separated from four well preserved dykes yield SmNd ages of 2.031 ± 0.062Ga, 2.015 ± 0.042Ga, 1.982 ± 0.044Ga and 2.101 ± 0.078Ga, which are interpreted as crystallisation ages.The initial Nd isotope compositions in the dykes at their emplacement age of 2.0 Ga, range from +3.4 to −6.8, indicating the presence of an older lithospheric component. SmNd whole-rock isotope data for fifteen dykes, if interpreted to have age significance, yield an “age” of 3.05 ± 0.27 Ga. SmNd crustal residence ages for the same dykes average 2.95 Ga, which is interpreted as the time that small melts were added to the Lewisian lithosphere. The possibility that correlated147Sm/144Nd and143Nd/144Nd ratios are an artifact of mixing between depleted mantle melts generated at 2.0 Ga, and an older enriched lithospheric component is not eliminated by the data, but the relationship between 1/Nd and143Nd/144Nd ratios rules out any simple mixing. UPb isotope data for plagioclase feldspars and whole-rock samples of dykes provide useful estimates of initial Pb-isotope composition of the dykes at the time of their emplacement. Initial206Pb/204Pb and207Pb/204Pb ratios vary considerably and range from 13.98 to 15.78, and 14.72 to 15.56 respectively, and suggest that the UPb fractionation responsible must have occurred at least 2.5 Ga ago.The Scourie dykes have inherited a trace element enriched component from the Lewisian lithosphere, which has resided there since ca. 3 Ga ago. Whether the dykes inherited this material from the crust or the mantle portions of the lithosphere or both, it seems likely that small basaltic melts derived from asthenospheric mantle were ultimately responsible for the enrichment. The simplest view is that these small melt fractions had been resident in the mantle part of the Lewisian lithosphere. In this case the Archaean trace-element enrichment and element fractionation in the Lewisian lithospheric mantle sampled by the dykes was closely associated in time with the generation of the 2.9 Ga old crustal portion of the lithosphere [36,37].  相似文献   

9.
The Tadhak alkaline ring-complex of Permian age provides two whole rock UPb isochrons giving concordant ages in agreement within relative errors with the RbSr isochron age:235U207Pb isochron: 271 ± 32Ma(MSWD= 0.3);238U206Pb isochron: 254 ± 18Ma(MSWD= 7.8), both on 8 whole-rock samples. The existence of these isochrons indicates that in favorable conditions U (and Pb) can be immobile. This can be due either to the lack of hard oxidizing conditions and/or to the location of U, in very low concentrations, in weathering-resistant minerals. The initial ratios (206Pb/204Pb = 18.714 ± 70and207Pb/204Pb = 15.589 ± 16), corrected for their Permian age, lie in the range observed for oceanic island basalts or continental alkali basalts and indicate an origin in a similar mantle, without any significant crustal contamination. This was also suggested by the initial87Sr/86Sr ratio of 0.70457 ± 4. Moreover, these Sr and Pb isotopic characteristics belong to the field of the so-called “Dupal” anomaly and indicate that it existed already 270 Ma ago. This study shows the potential interest of isotopic investigations of within-plate alkaline ring-complexes to characterize subcontinental mantle compositions, particularly in the past.  相似文献   

10.
A geochronological study utilized the unspiked potassium–argon (K–Ar) technique to obtain ages from the two main volcanic members of the shield stage of the Waianae Volcano, HI. These new dates are further constrained using a combination of stratigraphic relationships, magnetostratigraphy and major element geochemistry. Exposed shield lavas encompass 0.85 Ma, with reliably dated tholeiitic lavas from the main shield ranging from 3.93±0.08 to 3.54±0.04 Ma, and a later shield stage ranging in age from 3.57±0.04 to 3.08±0.04 Ma. These data suggest that the total extent of Waianae shield activity was significantly more than 1 Ma. The age of faulting in two flank zones is constrained to be about 3.4 Ma. Preliminary estimates of lava accumulation rates vary from about 0.3 to 2.0 mm/a; calculated rates show no systematic variation with location in the volcano or with time.  相似文献   

11.
The geochronologic history of the northern portion of the Western Gneiss Region, south-central Norway has been investigated by UPb analyses of zircon and titanite from various basement units and one supracrustal gneiss. A six-point (four zircon and two titanite) discordia line from a migmatite melt-pod and its host-gneiss (Åstfjord migmatite gneiss) defines upper- and lower-intercept ages of 1659.1 ± 1.8 Ma and 393.6 ± 3.6 Ma, respectively. The upper-intercept age is interpreted as a time of tonalite emplacement and migmatite formation in the region.A seven-point (three zircon and four titanite) discordia line from the Ingdal granite gneiss has similar upper- and lower-intercept ages (1652.9 ± 1.7 Ma and 396.1 ± 4.9 Ma, respectively) and the upper-intercept age is interpreted as the time of granite crystallization. The lower-intercept age from both theÅstfjord migmatite gneiss and the Ingdal granite gneiss is interpreted as the time of regional metamorphic resetting and is evidence for Caledonian influence in the region.Titanite from these two gneiss units, as well as from other tonalite, granite, and supracrustal gneisses throughout the Western Gneiss Region displays a remarkably uniform pattern of discordance. Titanite discordance is variable from 6% to 100% and, in general, the degree of discordance is proportional to the Caledonian metamorphic grade of nearby supracrustal schists. Episodic, diffusional lead loss from titanite during Caledonian metamorphism probably caused the discordance pattern, but a combined mechanism of mixing and diffusional lead loss cannot be ruled out. In any case, the seventeen-point titanite and zircon discordia line clearly indicates that: (1) the bulk of the granitoid terrane in this portion of the Western Gneiss Region was emplaced, migmatized, and cooled in a short time interval about 1657 Ma ago; (2) a second, short-lived thermal event that exceeded the blocking temperature of titanite occurred about 395 Ma ago; and (3) titanite and zircon in the region were not isotopically disturbed by comparable geologic events in the period from 1657 to 395 Ma, or at any time after 395 Ma.  相似文献   

12.
SmNd isotopic data indicate that differential REE mobility occurred on a whole-rock scale during transitional amphibolite- to granulite-facies regional metamorphism ( 700°C, 7 kbar) in early Archaean rocks ( 3930 Ma) of the Napier Complex of Enderby Land, Antarctica. The degree of mobility is independent of metamorphic grade but correlates directly with development of tectonic fabric. Whole-rock samples with D3-M3 internal fabrics lie along an array corresponding to an age of 2410 ± 100Ma, whereas samples preserving only earlier fabrics preserve an older, albeit imprecisely defined isochron age. In contrast to a widely held belief, such changes did not require the presence of a large hydrous fluid flux. If the mechanism responsible for SmNd resetting at this locality (where TCHUR ages range from 1990 Ma to 6090 Ma) is more widespread than is currently recognised, isolated SmNd model ages, particularly in complex terrains should be treated with caution.  相似文献   

13.
Rabaul Caldera is the most recently active (1937–1943) of four adjoining volcanic centres aligned north-south through the northern extremity of eastern New Britain. Geological mapping after the 1983–1985 Rabaul seismic and deformation crisis has partially revealed a long and complex eruption history dominated by numerous explosive eruptions, the largest accompanied by caldera collapse. The oldest exposed eruptives are the basaltic pre-caldera cone Tovanumbatir Lavas K/Ar dated at 0.5 Ma. The dacitic Rabaul Quarry Lavas exposed in the caldera wall and K/Ar dated at 0.19 Ma, are overlain by a sequence of dacitic and andesitic pyroclastic flow and fall deposits. Uplifted coral reef limestones, interbedded within the pyroclastic sequence on the northeast coast, suggest that explosive eruptions in the Rabaul area had commenced prior to the 0.125 Ma last interglacial high sea level stand. The pyroclastic sequence includes the large Boroi Ignimbrites and Malaguna Pyroclastics both 40Ar/39Ar dated at about 0.1 Ma, and the Barge Tunnel Ignimbrite 40Ar/39Ar dated at around 0.04 Ma. Few reliable ages exist for the many younger eruptives. These include Holocene ignimbrites of the latest caldera-forming eruptions—the Raluan Pyroclastics variously dated (14C) at either about 3500 or 7000 yr B.P., and the ca. 1400 yr B.P. Rabaul Pyroclastics. At least eight intracaldera eruptions have occurred since the 1400 yr B.P. collapse, building small pyroclastic and lava cones within the caldera.A major erosional episode is evident as a widespread unconformity in the upper pyroclastic stratigraphy at Rabaul. Lacking relevant radiometric ages, this episode is assumed to have occurred during last glaciation low sea levels and is here arbitarily dated at ca. ?20 ka. At least five, possibly nine, significant ignimbrite eruptions have occurred at Rabaul during the last ?20 ka. The new eruptive history differs considerably from that previously published, which considered ignimbrite eruption and caldera collapse to have first occurred at 3500 yr B.P.Rabaul volcanism has been dominated by two main types: (a) basaltic and basaltic andesite cone building eruptions; and (b) dacitic, and rarely andesitic or rhyolitic, plinian/ignimbrite eruptions of both high- and low-aspect ratio types. The 1400 yr B.P. Rabaul Ignimbrite is a type example of a low-aspect ratio, high-energy, and potentially very damaging eruption. Fine vitric ash deposits, common in the Rabaul pyroclastic sequence, demonstrate the frequent modification of eruptions by external water probably related to early caldera lakes or bays. Interbedding of these fine ashes with plinian pumice lapilli beds suggests that many early eruptions occurred from multiple vents, located in both wet and dry areas.  相似文献   

14.
Zircon U–Pb ages of two acidic tuff and two turbidite sandstone samples from the Nakanogawa Group, Hidaka Belt, were measured to estimate its depositional age and the development of the Hokkaido Central Belt, northeast Japan. In the northern unit, homogeneous zircons from pelagic acidic tuff from a basal horizon dated to 58–57 Ma, zircons from sandstone from the upper part of the unit dated to 56–54 Ma, and zircons from acidic tuff from the uppermost part dated to 60–56 Ma and 69–63 Ma. Both of the tuff U–Pb ages are significantly older than the youngest radiolarian fossil age (66–48 Ma). Therefore, the maximum depositional age of the turbidite facies in the northern unit is 58 Ma and the younger age limit, estimated from the fossil age, is 48 Ma. In the southern unit, homogeneous zircons from turbidite sandstone dated to 58–57 Ma. Thus the depositional age of this turbidite facies was interpreted to be 66–56 Ma from the fossil age, probably close to 57 Ma. Most of the zircon U–Pb ages from the Nakanogawa Group are younger than 80 Ma, with a major peak at 60 Ma. This result implies that around Hokkaido volcanic activity occurred mainly after 80 Ma. Older zircon ages (120–80 Ma, 180–140 Ma, 340–220 Ma, 1.9 Ga, 2.2 Ga, and 2.7 Ga) give information about the provenance of other rocks in the Hidaka Belt. It is inferred that the Nakanogawa Group comprises protoliths of the upper sequence of the Hidaka Metamorphic Zone, which therefore has the same depositional age as the Nakanogawa Group (66–48 Ma). The depositional ages of the lower sequence of the Hidaka Metamorphic Zone and the Nakanogawa Group are probably the same.  相似文献   

15.
High spatial resolution U–Pb dates of zircons from two consanguineous ignimbrites of contrasting composition, the high-silica rhyolitic Toconao and the overlying dacitic Atana ignimbrites, erupted from La Pacana caldera, north Chile, are presented in this study. Zircons from Atana and Toconao pumice clasts yield apparent 238U/206Pb ages of 4.11±0.20 Ma and 4.65±0.13 Ma (2σ), respectively. These data combined with previously published geochemical and stratigraphic data, reveal that the two ignimbrites were erupted from a stratified magma chamber. The Atana zircon U–Pb ages closely agree with the eruption age of Atana previously determined by K–Ar dating (4.0±0.1 Ma) and do not support long (>1 Ma) residence times. Xenocrystic zircons were found only in the Toconao bulk ignimbrite, which were probably entrained during eruption and transport. Apparent 238U/206Pb zircon ages of 13 Ma in these xenocrysts provide the first evidence that the onset of felsic magmatism within the Altiplano–Puna ignimbrite province occurred approximately 3 Myr earlier than previously documented.  相似文献   

16.
The Medina Wrenth in the central Mediterranean is a transform fault connecting the plate collision in northwest Africa and northern Sicily with that occurring at the Aegean plate boundary, south of Greece. The more than 800 km long crescent-shaped wrench zone is currently seismically quiet but exhibits major deformation since 5 Ma within a belt 30–100 km wide. It forms the southern boundary of two microplates moving eastward with respect to Africa and Europe. A simple plate rotation model constrained by recent paleomagnetic data indicates that a continental Iblean microplate and a hybrid continental/oceanic Ionian microplate, separated along the Malta Escarpment, have rotated anticlockwise by 11° and 12°, respectively, around poles in southern Italy. These rotations involved some 100 km of dextral eastward movement relative to Africa of the Ionian Basin north of the Medina Wrench since 5 Ma. Combining the published 26° clockwise rotation of the Peloponnesus and northwest half of the Aegean with the 12° anticlockwise rotation of the Ionian microplate results in (a) a 99% agreement between the length of the seismic Benioff Zone beneath Greece and the total convergence of the microplates, and (b) an average rate of convergence across the Aegean plate boundary southwest of the Peloponnesus of 6.6 ± 1cm a−1 since the Miocene. Relative motion between microplates in a collision zone thus may be as much as 6 times faster than convergence between the major plates which spawned them, and they can be considered rigid to the first order over the time span involved.  相似文献   

17.
Sr-isotopic data from the Main and Upper Zones of the Bushveld Complex show that the evolution of the Upper Zone started with a large influx of magma close to the level of the “Pyroxenite Marker”, a distinctive orthopyroxenite layer in otherwise relatively uniform gabbronorites. Whole rock samples, which span the complete stratigraphic succession (ca. 2100 m) above this layer, fall on a single RbSr isochron (2066 ± 58Ma) and hence have a common initial ratio of 0.7073 ± 1. This ratio is significantly lower than those of the Main Zone (ca. 0.7085), below the level of the Pyroxenite Marker.The entire Upper Zone crystallized from a mixed magma which was thoroughly blended before crystallization. This magma had an isotopic ratio intermediate between that of the Main Zone and the added magma which had an initial ratio of ca. 0.7067. Further significant magma additions during crystallization are precluded unless they were of the same isotopic composition as the blended magma, which is considered improbable. Hence the layering and mineralogical diversity of the Upper Zone was produced by internal processes and not produced by magma influxes during crystallization.The lithological, compositional and isotopic changes at the Pyroxenite Marker and the petrological coherence of all rocks above this horizon support the placing of the Upper Zone boundary at this point in the stratigraphy.  相似文献   

18.
Seventy sites of sills, flows and dikes from Northeastern Paraná Magmatic Province (PMP), were submitted to paleomagnetic, chemical and radiometric analyses. The rocks are high in TiO2 content, and similar in composition to the rocks from the northern region of PMP. The sills intrude mainly Paleozoic sediments, and can be subdivided into two domains; the northern being characterized by sills showing reversed polarities, and the southern essentially by sills of normal polarities. 40Ar/39Ar dating of three distinct sills gave plateau ages (129.9 ± 0.1, 130.3 ± 0.1 and 131.9 ± 0.4 Ma) that are similar to surface-outcropping flows of the Northern Paraná Basin, and the Ponta Grossa dikes. The new paleomagnetic data combined with existing data from the northern PMP allowed the calculation of a paleomagnetic pole at 71.4° E and 83.0° S (N = 92; α95=2.4°; k = 39). This pole is in good agreement with poles for central and southern PMP, which are slightly older than the northern PMP, as well as for the contemporaneous Central Alkaline Province (Paraguay) on the western side of PMP. In contrast, the coeval pole for the Ponta Grossa dikes (eastern border of PMP), however, is slightly displaced from that group of poles, suggesting that dikes in that area may have undergone some tectonic tilting.  相似文献   

19.
The Hangingwall Basalt at Kambalda, Western Australia, contains zircons that have been shown by ion microprobe analyses to have very high U and Th contents and a wide variety of crystallization ages. Nearly all of these zircons certainly are xenocrysts; a few might relate to intrusive veinlets. The age of the youngest xenocrysts, 2693 ± 50Ma(2 σ), shows that the eruptive age of the basalt cannot exceed 2743 Ma. This confirms that the apparent SmNd isochron giving 3200 Ma [1,2] for Kambalda mafic and ultramafic rocks is a mixing-line [2] between unrelated components enriched and depleted in light rare earth elements. Mixing probably occurred at depth by erosion of 3200–3500 Ma old felsic crust from the walls of the HWB conduits. The zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern Yilgarn Block. The latter implies that the Kalgoorlie-Norseman greenstone sequences were formed in a continental rather than an oceanic environment.  相似文献   

20.
Coexisting fine-grained (0.1–20 μm) authigenic silicate minerals separated from altered tuffs in Miocene and Plio-Pleistocene lacustrine deposits were characterized petrographically and using X-ray powder diffraction. The authigenic minerals are dominated by clinoptilolite, erionite, phillipsite, K-feldspar, silica, calcite, smectite, and randomly interstratified illite/smectite. Minor accessories of opal-CT, cristobalite, and barite are present with the major alteration minerals. Authigenic minerals from altered tuffs were dated using the K/Ar method to evaluate the utility of these minerals for determining the time of alteration in low-temperature diagenetic environments. The eruption ages of some of these zeolite-rich tuffs were determined using the 40Ar/39Ar method on single sanidine and plagioclase minerals. The K/Ar isotopic ages of the fine-grained K-feldspar show minimal variation compared with results from the clinoptilolite separates. The isotopic ages from the authigenic K-feldspar (15-13.8 Ma) and some of the zeolites (16.-6.7 Ma) are similar to the eruption ages of the tuffs and indicate early alteration. Despite their open-framework structure, zeolites apparently can retain part or all of their radiogenic argon under favorable conditions (e.g., saturated environment). How much of the radiogenic argon is retained is estimated from the isotopic ages of other coexisting secondary minerals that are commonly dated by the K/Ar method. Although zeolite isotopic ages should be interpreted with caution, they may be useful to constrain temporal relations of low-temperature diagenetic processes when used in conjunction with other dateable minerals.  相似文献   

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