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1.
Archean komatiites, high-Mg basalts and tholeiites from the North Star Basalt and the Mount Ada Basalt formations of the Talga-Talga Subgroup, Warrawoona Group, Pilbara Block, Western Australia, define a linear correlation on the normal143Nd/144Nd vs.147Sm/144Nd isochron plot. The data give an age of 3712 ± 98 Ma and initialεNd(T) of +1.64 ± 0.40. The 3712 ± 98 Ma date is consistent with the regional stratigraphic sequence and available age data and the SmNd linear array may be interpreted as an isochron giving the eruption age of the Talga-Talga Subgroup. An alternative interpretation is that the isochron represents a mixing line giving a pre-volcanism age for the Subgroup. Consideration of geochemical and isotopic data indicates that the true eruptive age of the Talga-Talga Subgroup is possibly closer to about 3500 Ma. Regardless of the age interpretation, the new Nd isotopic data support an existence of ancient LREE-depleted reservoirs in the early Archean mantle, and further suggest that source regions for the Pilbara volcanic rocks were isotopically heterogeneous, withεNd(T) values ranging from at least 0 to +4.0.  相似文献   

2.
LaCe ages are reported for two sets of Finnish pegmatites, Lövböle and Mustikkamäki, and for an Amiˆtsoq gneiss, Greenland. When λβ138La value (2.29 × 10−12 yr−1) obtained by radioactivity measurement [1] is used for the chronological calculation, the LaCe ages (2129, 2325, 3271 Myr) evaluated for these rocks are 18–35% older than the SmNd ages for the same samples. To make the LaCe age fit to the SmNd age for the same sample, a new value of (2.77 ± 0.21) × 10−12 yr−1 is evaluated for λβ138La. In this calculation, the LaCe and SmNd ages reported for a Bushveld gabbro [2] have been also taken into account together with those for the Lövböle pegmatite and the Mustikkamäki pegmatite, while the Amiˆtsoq gneiss (GGU110999) has been omitted because of the complicated thermal history of this sample.  相似文献   

3.
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   

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.
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   

6.
High initial parent/daughter element ratios and a unique dual decay scheme make UPb zicron ages more precise and reliable than most isotopic ages, and thus inherently superior for time-scale calibration. Employing improved techniques to the conventional method of UPb dating, we have analyzed microgram-size (2–12 × 10−8 g) zircon fractions from biostratigraphically controlled volcanic ashes and dated key Paleozoic time-markers with a precision better than 1% (±2Ma). Four of the stratotype samples from Britain for which fission-track ages [1] were previously reported have yielded improved ages of:438.7 ± 2.0Ma for the lower Silurian zone of Coronograptus cyphus from Llandovery strata at Dob's Linn, southern Scotland;457.5 ± 2.2 Ma for a Middle Ordovician Caradoc (Longvillian) ash near Bala, North Wales, and;465.7 ± 2.1and464.6 ± 1.8 Ma for the Didymograptus artus Zone and the type Didymograptus Murchisoni Zone, respectively, of the Llanvirn Series at Arenig Fawr and Abereiddi Bay, Wales. Another sample from the zone of Dicellograptus anceps (P. pacificus Subzone) of the Ashgill Series at Dob's Linn has been dated at445.7 ± 2.4Ma, suggesting placement of the Ordovician-Silurian time boundary at approximately 441 Ma. A sixth bentonite from Caradocian age strata of North America (Spechts Ferry Shale, Decorah Formation, Missouri) is453.7 ± 1.8Ma old, indicating that the Rocklandian Stage of the Mohawkian Series is only slightly younger than the Longvillian Stage of the Caradoc Series in Britain.  相似文献   

7.
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   

8.
Up to now the age of granulite gneisses intruded by the Zabargad mantle diapir has been an unsolved problem. These gneisses may represent either a part of the adjacent continental crust primarily differentiated during the Pan African orogeny, or new crust composed of Miocene clastic sediments deposited in a developing rift, crosscut by a diabase dike swarm and gabbroic intrusions, and finally metamorphosed and deformed by the mantle diapir. Previous geochronological results obtained on Zabargad island and Al Lith and Tihama-Asir complexes (Saudi Arabia) suggest an Early Miocene age of emplacement for the Zabargad mantle diapir during the early opening of the Red Sea rift. In contrast, SmNd and RbSr internal isochrons yield Pan African dates for felsic and basic granulites collected 500–600 m from the contact zone with the peridotites. Devoid of evidence for retrograde metamorphic, minerals from a felsic granulite provide well-defined RbSr and SmNd dates of 655 ± 8 and 699 ± 34 Ma for the HP-HT metamorphic event (10 kbar, 850°C). The thermal event related to the diapir emplacement is not recorded in the SmNd and RbSr systems of the studied gneisses; in contrast, the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the RbSr isotopic system of the mafic granulite. The initial143Nd/144Nd ratio of the felsic granulite is higher than the contemporaneous value for CHUR and is in agreement with other Nd isotopic data for samples of upper crust from the Arabian shield. This result suggests that source rocks of the felsic granulite were derived at 1.0 to 1.2 Ga from either an average MORB-type mantle or a local 2.2 Ga LREE-depleted mantle. Zabargad gneisses represent a part of the disrupted lower continental crust of the Pan African Afro-Arabian shield. During early stages of the Red Sea rifting in the Miocene, these Precambrian granulites were intruded and dragged upwards by a rising peridotite diapir.  相似文献   

9.
The geological evolution of the Mesozoic Troodos Ophiolite Complex in Cyprus, and the tectonic nature and timing of the palaeomagnetically indicated anticlockwise rotation of Cyprus of some 80° and ca. 15° northward translation, have been open for debate for some time. New palaeomagnetic data from 18 sites ( 180samples) in the post-ophiolite sediments, ranging in age from Upper Cretaceous to Upper Miocene, are presented. Most of the sites are of normal geomagnetic polarity, but indications of reversed polarity have been found in an older group of sediments (the Lefkara Formation of Upper Palaeocene age).Six sites from the older group of sediments (Upper Cretaceous to Eocene in age) give a site mean direction of the AF cleaned sediments of (D, I) = (323°, 29°) with α95 = 18°, while 5 sites from a younger group of sediments (Oligocene to Miocene in age) give a cleaned site mean direction of (D, I) = (334°, 58°) with α95 = 9°. These and published data suggest that an anticlockwise rotation of Cyprus of 60 ± 10° occurred early during the post-igneous evolution of the Cyprus oceanic crust between 90 and 50Ma, leaving only a minor anticlockwise rotation of 20 ± 10° to occur during the last 50 Ma. It is furthermore concluded that the northward translation of Cyprus of 15° mostly took place during the last 30Ma.It thus appears that a fairly rapid rotation of the Cyprus microplate first took place in the Late Cretaceous and Early Tertiary time with an average angular velocity of 1–2°/Ma, during which the northward translation was minor or negligible. In the latter half of the Tertiary, the sense of movement appears to have radically changed, the northward translation now being dominant with an average velocity of 5–6cm/yr. This temporal evolution is found to be in good agreement with the Mesozoic and Tertiary movements of the African lithospheric plate relative to Europe, as evidenced from the Atlantic sea-floor magnetic anomaly spreading history.  相似文献   

10.
Ion microprobe zircon ages, a Nd model age and RbSr whole-rock dates are reported from the high-grade gneiss terrain at Sabaloka on the River Nile north of Khartoum, formally considered to be part of the Archaean/early Proterozoic Nile craton. The granulites, which are of both sedimentary and igneous derivation, occur as remnants in migmatites. Detrital zircon ages range from ≈ 1000 to ≈ 2650 Ma and prove the existence of Archaean to late Proterozoic continental crust in the sedimentary source region. The Nd model age for one sedimentary granulite is between 1.26 (TCHUR) and 1.70 (TDM) Ga and provides a mean crustal residence age for the sedimentary precursor. Igneous zircons in enderbitic gneiss crystallized at 719 ± 81 Ma ago, an age that also corresponds to severe Pb loss in the detrital zircons and which probably reflects the granulite event at Sabaloka. The RbSr data indicate isotopic homogenization at about 700 Ma ago in the granulites and severe post-granulite disturbance at ≈ 570 Ma in the migmatites. We associate this disturbance with hydration, retrograde metamorphism and anatexis that produced undeformed granites ≈ 540 Ma ago. The ≈ 700 Ma granulite event at Sabaloka suggests that this part of the Sudan belongs to the Pan-African Mozambique belt while the ancient Nile craton lay farther west. The gneisses studied here may represent the infrastructure of the ancient African continental margin onto which the juvenile arc assemblage of the Arabian-Nubian shield was accreted during intense horizontal shortening and crustal interstacking of a major collision event.  相似文献   

11.
The Qinling Mountains in central China are the joint orogenic zone between the Sino-Korean (or North China) and the Yangtze craton blocks. The age and genesis of the Danfeng mafic volcanics in the north of the Shangzhou-Danfeng fault zone, i.e. the main suture zone in the Qinling orogenic belt, have been controverted for a long time because their age is closely related to the converged time of two blocks. The ages and the geochemical data of the Heihe pillow lavas for the Danfeng mafic volcanics in the Heihe River area in the Qinling orogen are reported in this paper. The obtained isochron age by the Sm-Nd isotopic data of the 13 whole-rock samples for the mafic pillow lavas is 963±130 (2σ) Ma, corresponding to INd = 0.51173±16 (2σ),ɛ Nd(T)= +6.6, MSWD0.57. However, the Rb-Sr isotopic analytical results for the same samples as the Sm-Nd whole-rock ones are disperse. For the Sm-Nd isotopic systems were interfered during the later geological functions, the Sm-Nd isochron age for the whole-rock sample (Q9511WR) and the mineral phenocrystal samples: amphiboles (Hb) and plagioclases (Plag) presents the better uncertainty, whereas isochron ages of 930 Ma and 437 Ma are given if the WR-Plag and WR-Hb are calculated respectively, and their Rb-Sr isochron age is 268±47(2σ) Ma, Isr = 0.70475±11 (2σ), MSWD0.96. The major and trace elements for the lavas show that they were formed in the quasi-N-MORB setting.  相似文献   

12.
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   

13.
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   

14.
Santo Antão, the northernmost island of the Cape Verde Archipelago, consists entirely of silica-undersaturated volcanic products and minor intrusions. 40Ar–39Ar incremental heating experiments have been carried out on 24 samples that cover the entire exposed chronological sequence. The oldest lavas (7.57±0.56 Ma), representing an older volcanic basement, are exposed about 620 m above mean sea level. After an interval of quiescence of up to 4.3 Ma the volcanic activity resumed and continued at low eruption rates. The older basement is unconformably overlain by a ca. 810-m-thick lava sequence that spans an age range from 2.93±0.03 to 1.18±0.01 Ma. This sequence is cut by many dykes and sills. Simultaneous volcanic activity occurred in the northeastern, central and eastern part of the island. A phonolitic pumice deposit that forms a noteworthy feature over most of the island has an estimated age of 0.20 Ma. This predates volcanic activity that formed the highest point of the island (Tope de Coroa) which has an age of 0.17±0.02 Ma. The most recent eruption on the island formed nephelinitic lavas in the Porto Novo region at 0.09±0.03 Ma. The oldest volcanism exposed on Santo Antão, which took place about 7.6 Ma ago, was simultaneous with waning activity on Maio at the eastern end of the Cape Verde Archipelago.  相似文献   

15.
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   

16.
The Quaternary Takidani Granodiorite (Japan Alps) is analogous to the type of deep-seated (3–5 km deep) intrusive-hosted fracture network system that might support (supercritical) hot dry/wet rock (HDR/HWR) energy extraction. The I-type Takidani Granodiorite comprises: porphyritic granodiorite, porphyritic granite, biotite-hornblende granodiorite, hornblende-biotite granodiorite, biotite-hornblende granite and biotite granite facies; the intrusion has a reverse chemical zonation, characterized by >70 wt% SiO2 at its inferred margin and <67 wt% SiO2 at the core. Fluid inclusion evidence indicates that fractured Takidani Granodiorite at one time hosted a liquid-dominated, convective hydrothermal system, with <380°C, low-salinity reservoir fluids at hydrostatic (mesothermal) pressure conditions. ‘Healed’ microfractures also trapped >600°C, hypersaline (35 wt% NaCleq) fluids of magmatic origin, with inferred minimum pressures of formation being 600–750 bar, which corresponds to fluid entrapment at 2.4–3.0 km depth. Al-in-hornblende geobarometry indicates that hornblende crystallization occurred at about 1.45 Ma (7.7–9.4 km depth) in the (marginal) eastern Takidani Granodiorite, but later (at 1.25 Ma) and shallower (6.5–7.0 km) near the core of the intrusion. The average rate of uplift across the Takidani Granodiorite from the time of hornblende crystallization has been 5.1–5.9 mm/yr (although uplift was about 7.5 mm/yr prior to 1.2 Ma), which is faster than average uplift rates in the Japan Alps (3 mm/yr during the last 2 million years). A temperature–depth–time window, when the Takidani Granodiorite had potential to host an HDR system, would have been when the internal temperature of the intrusive was cooling from 500°C to 400°C. Taking into account the initial (7.5 mm/yr) rate of uplift and effects of erosion, an optimal temperature–time–depth window is proposed: for 500°C at 1.54–1.57 Ma and 5.2±0.9 km (drilling) depth; and 400°C at 1.36–1.38 Ma and 3.3±0.8 km (drilling) depth, which is within the capabilities of modern drilling technologies, and similar to measured temperature–depth profiles in other active hydrothermal systems (e.g. at Kakkonda, Japan).  相似文献   

17.
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   

18.
Optically stimulated luminescence (OSL) dating of perennially frozen loess was tested on quartz grains extracted from deposits associated with the late Pleistocene Dawson tephra in western Yukon Territory, Canada. OSL samples were obtained from ice-rich loess bracketing the Dawson tephra, while radiocarbon (14C) samples were collected from the bulk sediments directly underlying the tephra and from a ground-squirrel burrow 2.7 m below the tephra. Here we report the OSL characteristics and ages of the extracted quartz grains, as well as additional radiocarbon ages for samples described in Froese [2002. Age and significance of the late Pleistocene Dawson tephra in eastern Beringia. Quaternary Science Reviews 21, 2137–2142; 2006. Seasonality of the late Pleistocene Dawson tephra and exceptional preservation of a buried riparian surface in central Yukon Territory, Canada. Quaternary Science Reviews 25, 1542–1551]. We refine the time of Dawson tephra deposition to between 25,420±70 and 25,290±80 14C a BP. Bayesian analysis of constraining radiocarbon ages places the deposition of the Dawson tephra at between 30,433 and 30,032 cal a BP. Linear modulation (LM) OSL analysis of multi-grain aliquots of quartz showed that the initial part of the decay curve is dominated by a rapidly bleached (‘fast’) component; these samples, however, had relatively dim continuous wave (CW) OSL signals at the multi-grain aliquot (each composed of 80 grains) and single-grain scales of analysis. The single-aliquot regenerative-dose protocol was applied to multi-grain aliquots and single grains to obtain equivalent dose (De) values for samples collected from below and above the Dawson tephra. The De values were examined graphically and numerically, the latter using the central age, minimum age, and finite mixture models. For multi-grain aliquots, the central age model gave weighted mean De values between 30 and 50 Gy, which greatly underestimated the expected De of 74–81 Gy for both samples studied. Possible reasons for these underestimations are discussed, and a solution proposed based on single-grain analysis. Measurements of single grains produced De values in agreement with the expected De, and yielded OSL ages of 28±5 and 30±4 ka for the samples taken from above and below the Dawson tephra, respectively. Examination of individual grains with differing luminescence behaviors showed that a significant number of the measured quartz grains exhibited anomalous luminescence properties that would have compromised the results obtained from multi-grain aliquots. We therefore recommend analysis of individual grains to overcome the age-shortfall from multi-grain analysis of these and similar samples of quartz.  相似文献   

19.
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   

20.
Cosmogenic exposure dating of moraines during the last two decades has vastly improved knowledge on the timing of glaciation worldwide. Due to a variety of geologic complications, such as moraine degradation, snow cover, bedrock erosion and isotopic inheritance, samples from multiple large boulders (>1–2 m) often lead to the most accurate moraine age assignments. However, in many cases, large boulders are not available on moraines of interest. Here, I test the suitability of pebble collections from moraine crest surfaces as a sample type for exposure dating. Twenty-two 10Be ages from two Pleistocene lateral moraine crests in Pine Creek valley in the upper Arkansas River basin, Colorado, were calculated from both pebble and boulder samples. Ten 10Be ages from a single-crested Bull Lake lateral moraine range between 3 and 72 ka, with no statistical difference between pebble (n = 5) and boulder (n = 5) ages. The lack of a cluster of 10Be ages suggests that moraine degradation has led to anomalously young exposure ages. Twelve 10Be ages from a single-crested Pinedale lateral moraine have a bimodal age distribution; one mode is 22.0 ± 1.4 ka (three boulders, two pebble collections), the other is 15.2 ± 0.9 ka (two boulders, five pebble collections). The interpretation of the two age modes is that two glacier maxima of similar extent were attained during the late Pleistocene. Regardless of moraine age interpretations, that 10Be ages from pebble collections and boulders are indistinguishable on moraines of two different ages, and in two different age modes of the Pinedale moraine, suggests that pebble collections from moraine crests may serve as a suitable sample type in some settings.  相似文献   

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