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1.
Field studies in the vicinity of Saglek Bay, Labrador, demonstrated that it was possible to subdivide the Archaean gneiss complex into distinct lithologic units and erect a geologic chronology similar to that recognized in Godthaabsfjord, West Greenland. The Uivak gneisses are the oldest quartzo-feldspathic suite in the area and are distinguished from a younger gneissic suite in the field, the undifferentiated gneisses, by the presence of porphyritic basic dykes (Saglek dykes) within the Uivak gneisses. The Uivak gneisses range in composition from tonalites to granodiorites, with the two chemically distinct suites recognized: a grey granodioritic suite and an iron-rich plutonic igneous suite which locally intrudes or grades into a grey gneiss which strongly resembles the grey Uivak gneiss. Rb-Sr isotopic studies indicate an age of 3622 ± 72 m.y. (2σ) and initial Sr isotopic composition of 0.7014 ± 0.0008 (2σ) for the Uivak gneiss suite, i.e. grey gneiss plus iron-rich suite (λRb = 1.39 × 10?11 yr?1). The grey Uivak gneiss suite, treated independently, defines a Rb-Sr isochron with an age of 3610 ± 144 m.y. (2σ) and initial Sr isotopic composition of 0.7015 ± 0.0014 (2σ) which is indistinguishable from the age and initial ratio of the total Uivak gneiss suite, grey gneisses plus iron-rich suite. The undifferentiated gneisses define a Rb-Sr isochron with an age of 3121 ± 160 m.y. (2σ), and initial Sr isotopic composition of 0.7064 ± 0.0012 (2σ). The isotopic data support field observations suggesting the undifferentiated gneisses were derived by local remobilization of the grey Uivak gneisses. The Uivak gneisses resemble the Amitsoq gneisses of Godthaabsfjord both chemically and isotopically. The interpretation of the initial Sr isotopic composition of the Uivak gneisses is interpreted as the time of regional homogenization rather than the initial ratio of the plutonic igneous parents of the Uivak gneisses as suggested for the Amitsoq gneisses. Although the undifferentiated gneisses are contemporaneous with the Nuk gneisses of West Greenland, they do not form a well-defined calc-alkaline suite and may not be associated with major crustal thickening in the Labrador Archaean.  相似文献   

2.
Rb-Sr whole-rock isochron ages of gneisses from the Fiskenaesset area are considerably lower (2600–2800 m.y.) than U-Pb zircon ages for the same rocks (2880–2950 m.y.). There is a significant correlation between the isochron ages and the range in Rb/Sr ratios of the samples involved. Higher ages (and lower initial87Sr/86Sr ratios) are obtained for sample collections with a wide range in Rb/Sr ratios. Lower ages (and higher initial ratios) are obtained for sample collections with a narrow range in Rb/Sr ratios. This relationship is explained by a model of local metamorphic Sr isotope homogenisation in restricted rock volumes. This model implies that the individual isochron ages do not date specific geological events. There is a significant inverse correlation between the isochron ages and the corresponding initial ratios. It is probable that the igneous precursors of the gneisses intruded with initial87Sr/86Sr ratios well below 0.701.  相似文献   

3.
Migmatitic gneisses of overall intermediate composition occur at Vikan i Bø, North Norway. A Pb-Pb whole-rock isochron on these Vikan Gneisses yields an age of3,460 ± 70m.y. A Rb-Sr whole-rock isochron on the same rock samples yields an age of2,300 ± 150m.y., and an initial87Sr/86Sr ratio of0.7126 ± 0.0011.The geochemical implications of the discordance of the Pb-Pb and Rb-Sr whole-rock ages are discussed, and a timetable of the geological events in the early history of Lofoten-Vestera?len is proposed.  相似文献   

4.
Metamorphosed basic intrusives in the Alpine orogen of southern Spain contain relicts of unmetamorphosed gabbroic rock. Rb-Sr investigation of one of these relicts indicates that the rock and constituent minerals remained closed to Rb-Sr during the plurifacial Alpine metamorphism. The Rb-Sr isochron age is 146 ± 3 Ma with initial87Sr/86Sr of 0.7028 ± 0.0001 (λ87Rb = 1.42 × 10?11 a?1), which is taken as approaching the intrusion age. All investigated relicts of unmetamorphosed rock are characterized by excess40Ar contents between about 6 × 10?6 and 14 × 10?6 cm3 NTP/g, whereas the metamorphosed parts of the basic intrusives rarely show this phenomenon. It is argued that this argon, which is very inhomogeneously distributed, was acquired during the Alpine metamorphism. A comparison of the excess40Ar contents of the pyroxene and the plagioclase with published data suggests that in all cases the crystals have acquired the argon under about the same prevailing40Ar partial pressure, even when the geologic conditions were different.  相似文献   

5.
Whole-rock Rb-Sr age measurements on rocks from the Rhodesian Archaean craton of Southern Africa demonstrate that (a) the Mashaba area gneisses, which are typical of the Rhodesian Basement Complex, are approximately 3600 m.y. old, (b) volcanic rocks from the main greenstone belts, assigned to the Bulawayan Group, were extruded approximately 2600–2700 m.y. ago, (c) a cross-cutting pluton, the Sesombi tonalite, was emplaced2690 ± 70 m.y. ago, (d) the Gwenoro Dam migmatites, which field evidence suggests could be older than the main greenstone belts, were emplaced2780 ± 30 m.y. ago.The initial87Sr/86Sr ratios of most of the rock units of groups b, c and d are in close agreement at about 0.701, suggesting that the later granitic (sensu lato) and andesitic rocks so far analysed were not produced by remelting of, or contamination with, ancient gneissic basement.  相似文献   

6.
RbSr isotopic analyses of the intensely deformed Hebron gneiss, Labrador, yield an isochron of approximately 3.6 b.y. with an initial87Sr/86Sr ratio of 0.7044, and chemical analyses show these rocks to be granodioritic in composition. It is believed that the isochron reflects a metamorphic event and that the Hebron gneiss was either derived from a compositionally anomalous zone in the mantle or from previously existing sialic crustal material. The Hebron gneiss is compositionally similar to some of the oldest rocks in the Archean cratons of Labrador, West Greenland, Rhodesia, South Africa and Minnesota (U.S.A.).  相似文献   

7.
Sodium-rich trondhjemites and quartz keratophyres, retained as pebbles in Archaean metaconglomerates, represent some of the oldest components (up to 3 by) of the Kalgoorlie System, Western Australia. New determinations indicate high Si, Na, Na/K, Ba/Rb, Ba/K, Sr/Ca+K, Ni, Ni/Mg, and Cr/Mg, and low K, Rb, Rb/Sr, and Y, in comparison to Taylor's (1965) [1] and other' average granites and to Nockolds and Allen's (1953) [2] acid calc-alkaline rocks. Syngeosynclinal quartz keratophyres (2.6 by) studied by O'Beirne (1968) [3] show similar comparisons. Similarly, high-grade acid and intermediate gneisses of the Lewisian, Scotland, have high Na/K, K/Rb, Ba/Rb, Ba/K, Sr/Ca+K, Ni, Ni/Mg, Cr, Cr/Mg, and low K, Rb, Rb/Sr, Y and Y/Ca. Secular decrease in Na/K, K/Rb and Sr/Rb is shown by granite series in Western Australia, Transvaal, Rhodesia and the Canadian Shield. The earliest members of these series compare with sodic acid plutons of island arcs, in particular Fiji, an archipelago showing a pattern of evolution analogous to that of the proposed model (Glikson, 1971) [4]. Evidence for an origin of Archaean sodic granites from below a basic crust is furnished by the abundance of basic xenoliths, by relationships in agmatite-gneiss terrains in Greenland, Scotland, and India, by K levels lower than those of Archaean metasediments, and by low initial87Sr/86Sr ratios. The abundance of euhedral quartz phenocrysts in the quartz keratophyres supports origin through partial melting of eclogite [5]. In high-grade belts, the originally low abundances of K and Rb were accentuated by synmetamorphic depletion, with which the formation of late-stage K-granites may have been associated. The correlation of chemical data enables an insight into spatial and temporal relationships between low- and high-grade early Precambrian terrains, and on the geochemical evolution of shields.  相似文献   

8.
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9.
RbSr and UPb isotope analyses are reported for two pre-metamorphic Caledonian granites which intrude Dalradian rocks in the Central Highlands of Scotland. These data indicate that the origin of the granitic magmas involved partial fusion of old crustal material.UPb systems of zircon size and magnetic fractions from the Ben Vuirich granite are strongly discordant. However, U/Pb isotopic ratios precisely define a chord which intersects concordia at 514?7+6 m.y. and 1316?25+26 m.y. Geological constraints suggest that the lower intersection records the post-F2, pre-M3 emplacement age of the granite. The upper intersection reflects the presence of old zircon xenocrysts incorporated into the granite magma without complete isotopic resetting. The ultimate source of these xenocrysts is probably a metamorphic basement complex which formed about 1320 m.y. ago, but the immediate source region of the granites could have been Dalradian sediments derived therefrom.RbSr whole-rock systems of the Ben Vuirich granite are also strongly discordant, although 8 out of 13 data points scatter about an “errorchron” of 564 ± 24 m.y. with an initial87Sr/86Sr ratio of about 0.716. This is interpreted as a spurious result due to incomplete homogenization of Sr isotopes in the source region during partial fusion. Initial87Sr/86Sr ratios at the time of emplacement indicated by the zircon data ranged from 0.7173 to 0.7191. Whole-rock samples from the Dunfallandy Hill granite have Rb/Sr ratios 2–3 times higher than those from Ben Vuirich and define a reasonably good isochron age of 491 ± 15 m.y. with an initial87Sr/86Sr of 0.7185 ± 0.0008. This may date granite emplacement or subsequent resetting of the high Rb/Sr rocks during Caledonian metamorphism. RbSr systematics indicate that the crustal source regions of these and other Caledonian granites separated from the upper mantle at least ca. 800 m.y. ago and probably ca. 1300 m.y. ago, thus confirming the interpretation of the upper intersection age of the zircon UPb data.  相似文献   

10.
The five diogenites, Johnstown, Roda, Ellemeet, Shalka and Tatahouine, give scattered data in the87Rb/86Sr,87Sr/86Sr diagram. This can result from a disturbance which occurred later than 4.45 Ga ago. However, it is shown that if samples of sufficient size were analyzed, there meteorites could plot on the eucrite isochron and are thereby in agreement with a genetic relation between eucrites, howardites and diogenites. The age of eucrite differentiation from diogenites has been computed using data from the two families yielding an age of 4.47±0.1Ga(2σ) (λ=1.42×10?11a?1), the initial87Sr/86Sr ratio being BABI.  相似文献   

11.
New rare earth element (REE) data, Rb and Sr analyses and Sr isotope measurements are presented for pumice clasts collected from some North Chilean ignimbrites of dacite and rhyolite composition. The samples are light-REE enriched with chondrite-normalised Ce (CeN) of 17–98, YbN of 4–14 and CeN/YbN of 2.6–15. While some samples have no Eu anomalies, others do have anomalies with inferred Eu/Eu* values of down to ca. 0.4. Eleven samples have present-day87Sr/86Sr ratios between 0.7053 and 0.7100, and noting that some samples are up to 12 Ma old, initial87Sr/86Sr ratios are below ca. 0.709. These trace element and Sr isotope characteristics resemble those of contemporaneous andesite and dacite lavas, suggesting a common origin for all these rock types. The higher Rb/Sr ratios and larger Eu anomalies in most of the dacitic and rhyolitic ignimbrites are consistent with an origin by plagioclase-dominated fractional crystallization of mantle-derived andesite magma.  相似文献   

12.
40Ar-39Ar and Rb-Sr ages have been measured on separated minerals from the potassic volcanics of the Roman Comagmatic Region to test the ability of these methods to accurately data Quaternary geological events. The very high K and Rb contents of the Roman magmas present particularly favorable situations in which the very high concentrations of the radioactive nuclides40K and87Rb result in well resolved in situ enrichments of the daughter isotopes despite the very young ages. Six leucite separates contained Ar with very high bulk40/36 ratios (above 1000) and in which the40Ar and the39Ar were very well correlated, yielding well-defined ages averaging3.38±0.08×105 years. Two leucites contained Ar with lower bulk40/36 ratios (～400), and in at least two release steps from these leucites the40Ar/36Ar ratio was significantly lower than atmospheric. Despite the uncertainty in the composition of the trapped component, these two leucites have ages that do not differ significantly from the ages of the other leucites. For the biotites, it was not possible to obtain through stepwise degassing a good separation of in situ radiogenic40Ar from trapped40Ar and therefore the calculated ages are not as precise as those of the leucites. In three cases the biotite age agrees with the age of the cogenetic leucite, but in the remaining two cases discordant ages are obtained, suggesting caution when using biotites as Quaternary age indicators.Rb-Sr measurements on leucite, biotite, and pyroxene separates hand-picked from each of three tuff samples yielded a dispersion in87Sr/86Sr as large as 16 parts in 104 and87Rb/86Sr as high as 218 for leucites, and permitted the determination of internal isochron ages. The ages obtained range from3.8±0.2×105to3.3±0.2×105 years and are in good agreement with the40Ar-39Ar ages on the leucites. The data for each tuff sample yield a well-defined uniform initial87Sr/86Sr. However, different tuffs show small differences in initial87Sr/86Sr pointing to distinct sources or to assimilation of different materials during the extrusion of the tuffs. These measurements demonstrate the possibility of dating Quaternary materials by both the40Ar-39Ar method and the Rb-Sr method. The observation of concordant ages with a precision of a few percent represents a powerful tool in Quaternary stratigraphy.  相似文献   

13.
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   

14.
The Pampean Ranges of northwest Argentina are a basin-and-range tectonic province with a late Precambrian to Paleozoic basement and extensive Miocene-Recent calc-alkaline volcanism. The volcanoes include the large resurgent Cerro Galan caldera, and Recent scoria cones and lava flows. Miocene-Recent volcanic rocks of basalt to dacite composition from the Cerro Galan area exhibit a range of Rb/Sr ratios of 0.043–1.092 and initial87Sr/86Sr ratios of 0.7057–0.7115 with a clear positive correlation between87Sr/86Sr and87Rb/86Sr, indicating an apparent age of ca. 130 Ma. This relationship is interpreted to indicate that the Sr isotope variation in the Cerro Galan volcanic rocks results from mixing of a mantle-derived component with low87Sr/86Sr (<0.7057) and high Sr (>700 ppm) with a crustal component characterized by higher87Sr/86Sr (>0.7115) and lower Sr (<240 ppm). It is concluded that the mixing is best explained as a result of a small degree of selective crustal Sr contamination (ca. 10%) of a range of subsequently erupted magmas produced largely by fractional crystallization within the continental crust. We propose that the mantle-derived end-member is derived by partial melting of sub-Andean mantle with an87Sr/86Sr ratio of ca. 0.704, and that such an Sr isotope ratio characterizes the source region for calc-alkaline volcanic rocks throughout the Andes.  相似文献   

15.
The northwestern portion of the Tibesti Massif in south-central Libya consists of a complex granitic batholith (the Ben Ghnema batholith) surrounded by medium- to low-grade metamorphic wall rocks. Rb-Sr whole-rock analyses of a suite of calc-alkaline rocks from the batholith give an age of about 560 m.y. and an initial87Sr/86Sr ratio of about 0.706.This age is within the accepted range of the Pan-African orogeny which caused extensive plutonism throughout much of northwestern Africa. The relatively low initial ratio suggests that the magma was derived in the lower crust-upper mantle region and perhaps experienced some degree of mixing with older sialic material.  相似文献   

16.
Petrographic and chemical evidence suggests that boulders from a conglomeratic unit in the Isua supracrustal succession were derived by the erosion of an acid volcanogenic sediment. Six samples of the boulders and surrounding matrix yield a Rb-Sr whole rock isochron with a slope corresponding to an age of 3860 ± 240 m.y. (2 sigma error), but consideration of the initial87Sr/86Sr ratio constrains the possible age of formation of 3710 ± 900 m.y. This is in general agreement with a published Pb/Pb age of 3760 ± 70 m.y. on Isua banded ironstones.Pb isotope compositions as well as highly fractionated, heavy element depleted, rare earth element abundance patterns for the boulders suggest that their igneous precursors were derived from a source region with a similar geochemical history to that of some components of the 3700–3800 m.y. old Ami?tsoq gneisses, involving fractionation of garnet during their evolution.A Pb/Pb whole-rock isochron for Ami?tsoq gneisses from Isua yields an age of 3800 ± 120 m.y. (2σ), in good agreement with previously published Rb-Sr age data on the same rocks. The rock leads are highly unradiogenic and demonstrate substantial U depletion at least 3800 ± 120 m.y. ago. A two-stage model for the U-Pb system yields an average238U/204Pb (μ1) value of 9.3 ± 0.2 for the source region, which is significantly different from the published value of 9.9 ± 0.1 for the Isua iron formation. This indicates the existence of U-Pb heterogeneities between the source regions of plutonic and supracrustal rocks by about 3700–3800 m.y. ago. Attempts to apply U-Pb whole-rock dating to the Ami?tsoq gneisses were unsuccessful because of geologically recent U loss, possibly due to groundwater leaching.A Rb-Sr whole-rock isochron on a suite of Ami?tsoq gneiss samples from a different locality in the Isua region has yielded an age of 3780 ± 130 m.y.In contrast to the Godthaab area, there is no geochronological evidence at Isua for major rock-producing or tectonothermal events after about 3700 m.y. ago. The entire gneiss-supracrustal system developed within the approximate interval 3900–3700 m.y. ago.  相似文献   

17.
The Cordillera Darwin, a structural culmination in the Andes of Tierra del Fuego, exposes an orogenic core zone that has undergone polyphase deformation and metamorphism. Some of the classic problems of orogenic zones have remained unanswered in the Cordillera Darwin: the age of deformed plutonic rocks, the distinction of structurally reactivated basement and metamorphosed cover rocks, and the timing of orogenic events. This study addresses and partially answers these questions.A well-constrained Rb-Sr isochron age of157±8m.y. and an initial87Sr/86Sr ratio of 0.7087 obtained from a pre-tectonic granitic suite suggest a genetic relation between this suite and Upper Jurassic silicic volcanic rocks in the cover sequence (Tobifera Formation), and also suggest involvement of continental crust in formation of these magmas. A poorly constrained Rb-Sr isochron age of240±40m.y. obtained from supposed basement schists is consistent with field relations in the area which suggest a late Paleozoic/early Mesozoic metamorphism for these pre-Late Jurassic rocks. However, because of scatter in the data and the uncertainties involved in dating metasedimentary rocks, the significance of the isotopic age is dubious. Compilation of previously published ages in the area [9] with new mineral ages reported here indicate that “early Andean” orogenic events occurred between 100 and 84 m.y. ago, and that subduction-related magmatism has contributed, probably discontinuously, to the crustal evolution of the region throughout the Mesozoic.  相似文献   

18.
Initial87Sr/86Sr ratios have been determined for a representative suite of Upper Cretaceous granodiorites and associated rocks from the Above Rocks composite stock in central Jamaica and the Terre-Neuve pluton in northwestern Haiti. The average initial87Sr/86Sr ratio for severn samples of the Terre-Neuve intrusion is 0.7036, with a range of 0.7026–0.7047. For two samples of the Above Rocks the initial ratios are 0.7033 and 0.7034. A third sample from this intrusive has an initial ratio of 0.7084, which is tenatively attributed to contamination. The initial87Sr/86Sr ratios indicate that neither ancient sialic crust nor sediments carried down a Benioff zone can be the primary source of the granodioritic magma. K/Rb ratios for these rocks range from 178 to 247, which are much lower than the average values (≥1000) for tholeiitic basalts. It is concluded that the magmas originated primarily by melting of downthrust oceanic crust or adjacent mantle material.  相似文献   

19.
Lu-Hf total-rock data for the Amîtsoq gneisses of West Greenland yield an age of 3.55±0.22Gy(2σ), based on the decay constant λ176Lu=1.96×10?11y?1, and an initial176Hf/177Hf ratio of 0.280482±33. The result is in good agreement with Rb-Sr total-rock and U-Pb zircon ages. In spite of severe metamorphism of the area at 2.9 Gy, zircons from two of the samples have remained on the total-rock line, and define points close to the initial Hf ratio. The initial176Hf/177Hf lies close to a chondritic Hf isotopic evolution curve from 4.55 Gy to present. This is consistent with the igneous precursors to the Amîtsoq gneisses having been derived from the mantle at or shortly before 3.6 Gy. Anomalous relationships between Hf concentration and the176Lu/177Hf ratio may suggest that trace element abundances in the Amîtsoq gneisses are partly controlled by processes related to metamorphism.  相似文献   

20.
Analyses of rim-to-interior samples of fresh tholeiitic pillow basalts, deuterically altered holocrystalline basalts, and older, weathered tholeiitic basalts from the deep sea indicate that 87Sr/86Sr ratios of the older basalts are raised by low temperature interaction with strontium dissolved in sea water. 87Sr/86Sr correlates positively with H2O in these basalts; however, there is little detectable modification of the strontium isotope composition in rocks with H2O contents less than 1%. The isotope changes appear to be a function of relatively long-term, low-temperature weathering, rather than high-temperature or deuteric alteration. Strontium abundance and isotopic data for these rocks suggest that strontium content is only slightly modified by interaction with sea water, and it is a relatively insensitive indicator of marine alteration. Average Rb-Sr parameters for samples of apparently unaltered basalt are: Rb= 1.11ppm; Sr= 132ppm; 87Sr/86Sr= 0.70247.  相似文献   

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