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1.
Systematic shifts of oxygen isotopic compositions in the higher grade parts of the high temperature-low pressure Hercynian metamorphic sequence, exposed in the Trois Seigneurs Massif, have previously been explained as a result of an influx of surface-derived water during the prograde part of the metamorphic cycle. It has been suggested that this caused a regional lowering of 87Sr/86Sr in the metamorphic sequence. Mapping of strontium isotopic compositions across a 15 m meta-carbonate horizon in the higher grade pelite-psammite sequence shows that strontium isotopic compositions were homogenised over length scales of metres or less during the Hercynian metamorphism, which brought the carbonate and pelite-psammite to oxygen isotopic equilibrium with a common fluid. Comparison of model pre-Hercynian 87Sr/86Sr profiles across the carbonate (based on a depositional/diagenetic age of 450 Ma and initial 87Sr/86Sr ratio of 0.7086 given by 10 m length scale averaging) with the post-Hercynian 87Sr/86Sr profile (calculated from analysed 87Sr/86Sr and Rb/Sr compositions) implies strontium isotopic diffusion distances of ca. 0.4 m in the carbonate and ca. 7 m in the pelite-psammite. The limited Sr-isotopic diffusion distance of 0.4–0.7 m within the carbonate is compatible with pervasive oxygen-isotopic exchange over distances restricted to 4–15 m if fluid strontium concentrations were between 4 and 50 ppm. The strontium isotopic transport distances are not compatible with pervasive oxygen isotopic alteration over the observed 5 km regional scale. Either the flow was perfectly layer-parallel or, more probably, the regional-scale alteration of oxygen took place by fluid circulation in the brittle regime early in, or prior to, the Hercynian metamorphic event. Flow along cracks with incomplete diffusive exchange between fluid and wall rock would allow greater decoupling of oxygen and strontium isotopic transport than pervasive advective transport with local fluid-solid equilibrium.  相似文献   

2.
Theδ18O (SMOW) values of the Kirkpatrick Basalt (Jurassic) on Mt. Falla, Queen Alexandra Range, vary between +6.3‰ and +8.6‰ The apparent enrichment of these rocks in18O excludes the possibility that they were altered by interaction with aqueous solutions of meteoric origin. Theδ18O values of the flows correlate significantly with the initial87Sr/86Sr ratios and all major elements. These correlations confirm the hypothesis that the basalt magma was contaminated by rocks of the continental crust through which it was extruded. Estimates of the chemical composition of the basalt magma and the contaminant, based on extrapolations of the new oxygen data, generally confirm earlier estimates based on extrapolations of initial87Sr/86Sr ratios. The87Sr/86Sr ratio of the uncontaminated basalt was 0.7093 which indicates that magma may have originated by melting either in old Rb-enriched lithospheric mantle under Antarctica or in the overlying crust, or both.  相似文献   

3.
The Mesozoic diabase dikes of Liberia are tholeiites whose 87Sr/86Sr and 87Rb/86Sr ratios scatter widely on the Rb-Sr isochron diagram. The problem is attributed to differences in the initial 87Sr/86Sr ratios of these rocks which range from 0.70311 to 0.70792, assuming a uniform age of 186 Ma for the dikes and using (87Rb)=1.42 × 10–11y–1. The range of values is similar to that observed in the Mesozoic basalt flows and dikes of other Gondwana continents.New whole-rock K-Ar dates confirm previous conclusions that the diabase dikes in the Liberian and Pan-African age provinces of Liberia absorbed extraneous 40Ar after intrusion. Only the dikes in the Paynesville Sandstone have K-Ar dates that range from 117 Ma to 201 Ma and may not contain extraneous 40Ar. However, dikes from all three age provinces of Liberia have elevated initial 87Sr/86Sr ratios. These results indicate that contamination with radiogenic 87Sr occurred primarily before intrusion of the magma whereas the addition of extraneous 40Ar occurred after emplacement and reflects the age and mineral composition of the country rock.The 18O values of the Liberian diabase range from +5.6/% to +9.10/% and correlate positively with initial 87Sr/86Sr ratios. The data can be modeled by fractional crystallization and simultaneous assimilation of crustal rocks by the magma. However, samples containing amphibole and biotite replacing pyroxene deviate from the Sr-O isotope trajectories of the model and appear to have been depleted in 18O and enriched in 87Sr by interactions with groundwater at high temperature.Laboratory for Isotope Geology and Geochemistry Contribution No. 76  相似文献   

4.
Two major episodes are evident in the metamorphic and igneous Precambrian basement of the Llano Uplift, central Texas. Dynamothermal metamorphism was accompanied by minor basaltic and tonalitic syntectonic plutonism. This was followed by a second period of thermal overprinting accompanying emplacement of high-K2O, high-level major granite plutons. Extensive isotopic age work by Zartman, published in the mid-1960s, suggests that development of the basement complex, spanning an interval of 150 m.y. or more, began with deposition of Valley Spring Gneiss (the lowest unit) and terminated about 1,050 m.y. ago with final postmetamorphic cooling (indicated by retention ages of Ar and Sr in biotite). We have supplemented these data with more than 50 new K-Ar and Rb-Sr analyses.Two foliated plutons in the southeast are 1,167±12m.y. (2) old, with distinctly different initial 87Sr/86Sr ratios. Field relationships and isotopic data indicate that these plutons are the earliest yet known in the Uplift. Metamorphosed basalt dikes and gabbro bodies were emplaced immediately preceding and following the syntectonic plutons. Eleven of these rocks had extremely uniform initial 87Sr/ 86Sr=0.7029±0.0005. A Rb-Sr whole-rock isochron of the unfoliated Enchanted Rock pluton indicates an age of 1,048±34 m.y. with initial 87Sr/86Sr= 0.7048±0.0007. One of the northern unfoliated granites, the Lone Grove pluton, gives a whole-rock isochron age of 1,056±12 m.y., with initial 87Sr/86Sr = 0.7061±0.0003. All of the intrusive rocks have initial 87Sr/86Sr ratios consistent with a source in the mantle or lower crust, but not in ancient remobilized continental crust. Six K-Ar hornblende ages from metabasalts are 1,078±19 m.y. (1), in general agreement with K-Ar and Rb-Sr mineral ages elsewhere in the eastern Llano Uplift. A metasedimentary Valley Spring Gneiss sample from the western Uplift has a whole rock-muscovite Rb-Sr age of 1,129±9 m.y. Field and isotopic data are now sufficiently numerous to permit a moderately detailed reconstruction of the Precambrian history of the area.  相似文献   

5.
Many granitic bodies intrude the basement gneisses in Meghalaya Plateau, Northeast India. Rb-Sr whole-rock isotopic ages of the granitoids range from 881 to 479 Ma while the ages of the basement orthogneisses vary from 1714 to 1150 Ma. All the plutons are dominantly metaluminous and show geochemical variation. Oxygen isotopic compositions in the granitoids and gneisses are concordant (d18O: + 5.78% to + 8.70%). However, the gneisses from high-grade terrain have low d18O value of +2.52% to +5.31%. Initial 87Sr/86Sr (ISr) ratios of the plutons vary from 0.70459 to 0.71487 and tend to increase with progressive younging in age. The geochemical characters suggest derivation of the granites from lower crustal source. The fractionated rare earth patterns observed in the granitoids can be obtained by partial melting of gneisses or diorites. Some gneiss samples have experienced interaction with hydrothermal fluids resulting in lowering d18O. The isotopic ages of granite plutonism in Meghalaya are similar to the plutonic and tectonothermal events in other parts of India, southwestern Australia and document final amalgamation events of the Gondwana Supercontinent.  相似文献   

6.
An isotopic dating investigation (66 K-Ar and 34 Rb-Sr analyses) provided the geochronological framework for the Alpine events of metamorphism and granitic magmatism on Naxos. The oldest phase of high-pressure/medium-temperature metamorphism, M1, was dated by Rb-Sr and K-Ar analyses of paragonites, phengitic muscovites and muscovites at 45±5 Ma (Middle Eocene). Most of the record of the M1 phase has been eraded by a second phase of medium-pressure/high-temperature metamorphism, M2, which induced a metamorphic zonation with anatectic melting in the highest-grade part, the migmatite dome. Most K-Ar dates of M2 hornblendes, muscovites, biotites and tourmalines range from about 21 Ma in the lower-grade part (biotite-chloritoid zone) to about 11 Ma in the migmatite dome. From the pattern of K-Ar mineral dates it is concluded that the M2 phase took place 25±5 Ma ago (Late Oligocene/Early Miocene) and was followed by a prolonged cooling history until about 11 Ma ago (Late Miocene), when the ambient temperature in the migmatite dome had decreased to below 400–360 °C. A Rb-Sr whole-rock isochron analysis of a granodioritic mass dated the intrusion (and the associated M3 phase of contact-metamorphism) at 11.1±0.7 Ma (Late Miocene), with an initial 87Sr/86Sr ratio of 0.7112 ±0.0001. A local phase of low-grade retrograde metamorphism, M4, probably related to Late Alpine overthrusting, was dated at about 10 Ma (Late Miocene).  相似文献   

7.
The oxygen isotope composition of 56 Hercynian granites and 42 pre-Hercynian gneisses has been investigated. In addition some mineral δ 18O data and 5 δD values of whole rocks have been obtained. The granites from the N-Schwarzwald show, in general, relatively uniform δ 18O values between 11.5 and 13.5‰, those from the S-Schwarzwald are less uniform and lighter in 18O and range from 2.3 to 11.5‰. The gneisses from the pre-Hercynian basement exhibit more or less the same variation and range from 1.7 to 10.4‰. δ 18O values <6‰ only occur in the S-Schwarzwald and obviously indicate hydrothermal interactions of meteoric waters, which probably took place after the emplacement and solidification of the granites and which equally affected granites and gneisses. Due to the nearly identical 18O/16O ratios of S-Schwarzwald granites and gneisses, it is proposed that such gneisses in the pre-Hercynian basement qualify as the precursor rocks of the S-Schwarzwald, granites whereas for the N-Schwarzwald granites crustal rocks with heavier δ 18O values, unknown from the present surface, have to be postulated. This distribution is also reflected on a 87Sr/86Sr-18O/16O diagram.  相似文献   

8.
The formation of Parzán fluorite-lead «extension-type» vein (Spanish Central Pyrenees) is related to the post-Hercynian hydrothermal activity widespread not only in the Pyrenees but in Western Europe as well. The lode was later strongly deformed during the Alpine orogeny with textural modifications and destruction of primary fluid inclusions. In order to determine if geochemical signatures of the vein were disturbed during Alpine deformation, post-ore fluid inclusions and quartz oxygen-isotopes have been addressed. Microthermometry and X-ray microanalysis of frozen inclusions confirm the Na-Ca-Cl character of post-ore fluid circulations and indicate that they were originally derived from a single source: Triassic connate waters. Oxygen isotope data on quartz formed during deformation along with trace element geochemistry suggest that the geochemical signatures of the vein (mainly REE and 87Sr/86Sr) were apparently not reset during deformation. The REE content of fluorites is very high (up to 0.12%); the chondrite normalized REE patterns display a continuous decrease from La to Lu. Sulphur isotope data for the Parzán vein is outside the typical “deep seated” range, suggesting dominantly crustal sources of sulphur for sulphides, while formation waters may be proposed for barite. Parzán fluorites have the most radiogenic Sr values of all the investigated fluorite deposits in Central Pyrenees. They scatter in the range of both the igneous rocks and the Cambrian-Ordovician and Triassic detrital sequences of the Parzán area. If REE contents and 87Sr/86Sr ratios reflect the primary stage of vein filling as suggested by the good correlation between both geochemical signatures, the linear array between 87Sr/86Sr and 1/ΣREE is suggestive of a mixing process. The δ34S, δ18O and 87Sr/86Sr parameters in barite, which postdates the fluorite, also provide strong evidence that barite deposition is a result of fluid mixing. Such an interpretation is in line with present-day ideas for the genesis of many of the F-Ba-Pb-Zn vein deposits in the Hercynian belt of Iberia.  相似文献   

9.
Water samples from cold and geothermal boreholes, hot springs, lakes and rivers were analyzed for δD, δ18O and 87Sr/86Sr compositions in order to investigate lake water–groundwater mixing processes, water–rock interactions, and to evaluate groundwater flow paths in the central Main Ethiopian Rift (MER) of the Ziway–Shala basin. Different ranges of isotopic values were recorded for different water types: hot springs show δ18O −3.36 to +3.69 and δD −15.85 to +24.23, deep Aluto-Langano geothermal wells show δ18O −4.65 to −1.24 and δD −12.39 to −9.31, groundwater wells show δ18O −3.99 to +5.14 and δD −19.69 to +32.27, whereas the lakes show δ18O and δD in the range +3.98 to +7.92 and +26.19 to +45.71, respectively. The intersection of the Local Meteoric Water Line (LMWL: δD = 7 δ18O + 11.2, R2 = 0.94, n = 42) and the Local Evaporation Line (LEL: δD = 5.63δ18O + 8, n = 14, R2 = 0.82) was used to estimate the average isotopic composition of recharge water into the basin (δD = −5.15 and δ18O = −2.34). These values are depleted if compared with the modern-day average precipitation, presumably indicating paleo-groundwater components recharged during previous humid climatic phases. The measured stable isotope values indicate that the geothermal wells, some of the hot springs and groundwater wells mainly consist of meteoric water. The Sr isotopic signatures in all waters are within the range of the Sr isotopic composition of the rift basalts and rhyolites. The variability of Sr isotopic data also pinpoints complex water–rock interaction and mixing processes in groundwater and surface water. The 87Sr/86Sr ratio ranges from 0.70445 to 0.70756 in the hot springs, from 0.70426 to 0.70537 in two deep geothermal wells, and from 0.70673 to 0.70721 in the rift lakes Ziway, Langano, Shala and Awasa. The radiogenic composition recorded by the lakes indicates that the input water was predominantly affected by progressive interaction with rhyolitic volcanics and lacustrine sediments.  相似文献   

10.
Pillow basalts from the early Archean (3.7 to 3.8 Ga) Isua greenstone belt, West Greenland, are characterized by well-preserved rims and concentric core structures. The pillow rims and cores have different mineral assemblages, and chemical and isotopic compositions. The rims have systematically higher contents of Fe2O3, MgO, MnO, K2O, Rb, Ba, Ga, Y, and transition metals than the cores. In contrast, the cores possess higher concentrations of SiO2, Na2O, P2O5, Sr, Pb, U, Nb, and the light rare earth elements (REEs than the rims). These compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration. Variations of many major and trace element concentrations between the rims and cores of the Isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration. Al2O3, TiO2, Th, Zr, and the heavy REEs display similar values in both rims and cores, suggesting that these elements were relatively immobile during postemplacement alteration.In addition, the rims and cores have distinctive Sm-Nd and Rb-Sr isotopic compositions in that the rims are characterized by higher 143Nd/144Nd and 87Sr/86Sr ratios than the cores. The pillow basalts yield 2569 ± 170 Ma and 1604 ± 170 Ma errorchron ages on 143Nd/144Nd vs. 147Sm/144Nd and 87Sr/86Sr vs. 87Rb/86Sr diagrams, respectively. The Sm-Nd errorchron age may correspond, within errors, to a late Archean tectonothermal metamorphic event recorded in the region. The Sm-Nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of Nd, relative to Sm, during late Archean metamorphism. Although the Rb-Sr errorchron age overlaps with the timing of an early to mid-Proterozoic tectonothermal metamorphic event recorded in the region, because of a considerably large mean square of weighted deviates value and scatter in 86Sr/87Sr and 87Rb/86Sr ratios, this age may not have a precise geological significance. The 1.6 Ga Rb-Sr errorchron is likely to have resulted from the loss of radiogenic 87Sr. Collectively, the Sm-Nd and Rb-Sr data obtained from the 3.7-3.8 Ga Isua pillow basalt rims and cores are consistent with disturbances of the Sm-Nd and Rb-Sr systems by tectonothermal metamorphic events long after their eruption.In contrast to the Sm-Nd and Rb-Sr systems, the Lu-Hf system appears to be largely undisturbed by metamorphism. Five core samples and three rim samples yield a 3935 ± 350 Ma age, within error of the approximate age of eruption (3.7 to 3.8 Ga). Two rim samples that have gained Lu give an age of 1707 ± 140 Ma, within error of the Rb-Sr errorchron age. Initial 176Hf/177Hf ratios of the undisturbed samples at 3.75 Ga lie within ±1 ε-unit of the chondritic value, suggesting no long-term depletion in the mantle source of the basalts.  相似文献   

11.
Rb-Sr and U-Pb isotopic studies of the two contrasting granite types of the Daguzhai and Luobuli massifs in South China provide new constraints on the interpretation of isotopic age data for plutonic igneous rocks. A Rb-Sr internal isochron age of 146±7Ma for the Luobuli adamellite is interpreted to represent the age of magma crystallization, whereas the whole rock Rb- Sr isochron yields an older apparent age of 161±10Ma which is regarded as resulting from contamination processes affecting the petrogenesis of this adamellite. In the Daguzhai granite the marked scatter of whole- rock Rb-Sr data in isochron diagram is ascribed to the open system behavior of Rb during postmagmatic autometasomatism. Uniformity of initial87Sr /86Sr ratio in this granite is indicated in a plot of87Sr versus86Sr. The autometasomatism has also affected zircon U-Pb system, resulting in a spread of data along the concordia curve between 165 and 125Ma. This spread is regarded as indicating the duration of the autometasomatism.  相似文献   

12.
The Lepontine Gneiss Complex of southern Switzerland and northern Italy is characterized by high-grade metamorphism and intensive deformation of Alpine age with migmatites prevalent in the area with the highest metamorphic grade. Petrological and structural observations are generally inconclusive but indicate in some places an Alpine age for the migmatite formation. To determine the time of migmatite formation a geochronologic study was undertaken in one of the best exposed areas, the Valle Bodengo, Italy. Rb-Sr whole-rock errorchrons of intrusive migmatite phases and of two rather homogeneous granitoid gneiss bodies yield apparent ages between 280 and 350 m.y. They suggest a Hercynian or older igneous history for these rocks. The U-Pb ages of the euhedral zircons are highly discordant, but they do point to the presence of zircon components more than 450 m.y. old. The concordia-intercept ages are incompatible with the Rb-Sr data and the low initial 87Sr/86Sr ratios of about 0.706. These low initial ratios suggest that either the bulk of the granitoid material is not much older than Hercynian, or older crustal material was isotopically homogenized on a regional scale with rocks that had low Rb/Sr and 87Sr/86Sr ratios (e.g. the lower crust or upper mantle) during a Hercynian metamorphism. Rb-Sr small-scale whole-rock isochrons and tie lines of adjacent, lithologically different rock phases give Alpine ages, the best isochron yielding 22 m.y. This coincides with concordant U-Pb ages of monazites of 23 to 24 m.y. Rb-Sr mineral isoohrons (muscovite, biotite, feldspars, apatite) give ages of 18–21 m.y. Our interpretation is that this age pattern resulted due to rapid cooling after the climax of the last phase of the Alpine metamorphism and we conclude that high-grade metamorphic conditions existed during the upper Oligocene or early Miocene. Other investigators have suggested that the Alpine metamorphism had a climax 35–40 m.y. ago and that the younger mineral ages are a result of simple continuous cooling due to uplift. Based on this study and other recent geochronological studies in the Lepotine Gneiss Complex we suggest that there had to be a thermal maximum at about 20–25 m.y. The example of Valle Bodengo demonstrates that the areal coincidence of the zone of highest-grade metamorphism with the occurrence of migmatites does not necessarily mean that metamorphism and migmatite formation were coeval and related to each other.  相似文献   

13.
Geochemical observations, including major ion and trace element analysis, and isotopic tracing have been carried out in the Subarnarekha River system (northeastern India) during a surface-water- and groundwater-monitoring program aimed at evaluating impacts of mining. The aquifer is of fracture type. Groundwater flow conditions and pollutant transfer were observed through a network of 69 wells. δ18O and δ2H results suggest that transfer from rainfall towards groundwater storage through soils and the unsaturated zone is fast, without any major transformation like evaporation. The scatter of 87Sr/86Sr signatures in surface water and groundwater are explained by three end-members. One is compatible with rainwater inputs. The most mineralised end-member represents anthropogenic inputs (agricultural practices and ore processing). The third end-member, characterised by a high 87Sr/86Sr signature, is believed to be controlled by natural geochemical processes, although affected by human activities (e.g. drainage of mine waste). Potential flow paths, investigated north of the area, reveal that all groundwater types seem to evolve more in pockets than along a flow path. The limited extent of transfer and the predominance of natural phenomena help to explain the moderate level of groundwater contamination and the characteristics of surface water contamination by mining and the metallurgy industry.  相似文献   

14.
Well-dated Precambrian is mostly developed in the north of the Armorican Massif, in which area voluminous Cadomian magmatism is dated at between 650 and 550 Ma. Much older relicts occur at Cap de la Hague, in Guernsey and in the Tregor, and are also found in the northern continental margin of the Iberian Peninsula. In all these occurrences, whole-rock systems have been opened, so that the true ages cannot be determined by the Rb-Sr whole rock isochron method. Four U-Pb zircon ages are between 1.8 and 2 Ga (in Guernsey: Icart orthogneisses; in Tregor: Port Beni, Trebeurden, Morguignen orthogneisses).There is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 Ma to 270 Ma ago. On the contrary, the evolution of initial 87Sr/86Sr ratios with time shows that the observed mid- and west European continental crust is probably not older than 700 Ma. The increase of the initial 87Sr/86Sr ratios of the magmas with time suggests that, after its formation in Cadomian times, this segment of continental crust evolved virtually as a closed system and Hercynian magmatism arose principally from re-melting of relatively young sialic components.  相似文献   

15.
莫干山花岗岩体位于东天目山晚中生代火山盆地东端,用LA-ICPMS进行锆石U-Pb定年得到年龄为128.1±2.1Ma,全岩Rb Sr等时线定年结果为135.4±4.3 Ma,表明其属燕山晚期岩浆活动产物.莫干山花岗岩的Sr-Nd-O同位素分析结果为:初始87Sr/86Sr=0.70933;εNd(t)=-3.75～ - 6.4;δ18O=8.86‰～10.78‰,表明其成因类型属Ⅰ型花岗岩,是壳-幔物质混合形成的.按Sr Nd双变量二元混合模型计算得出源区物质中地壳端员和亏损地幔端员的贡献份额分别为47％～49％、51％～53％.莫干山花岗岩与建德群黄尖组火山岩的锆石U-Pb年龄、全岩Rb Sr等时线年龄基本一致,其Nd-Sr同位素组成也很相似,表明它们来自同一岩浆源.  相似文献   

16.
Developed in the southeast coast of te East Shandong Peninsula,the Mesozoic fault-magma belt consists of five rock series:the syenite series;the monzonite series;the megaporphyritic monzogranite series;the biotite-granite series;and the alkali granite seres.Based on their Rb-Sr isochron ages(122-220Ma),these rock series may be divided into three magma subcycles dated at Triassic,Late Jurassic and Early Cretaceous.The initial ^87Sr/^86Sr ration in these rock series range from 0.70436 to 0.7155.The starting points of the Rb-Sr isochrons exhibit four different distribution trends on the(^87Sr/^86Sr)i-^87Rb/^86Sr diagram.These characteristics show that the multiple granitic rock series are different in genesis and derivation.The syenite series might be derived from the combination of mantle-derived magma and crustal material,and the others could be derived from granulite-facies and amphibilite-facies rocks in the deep crust.  相似文献   

17.
Northwestern Fujian contains abundant well-studied Precambrian basement, and was a composite terrane in Cathaysia during the Neoproterozoic; however, its magmatic activity, petrogenesis, and tectonic evolution remain controversial. This article focuses on the geochronology and geochemistry of the Neoproterozoic Group in order to resolve the above problems. We provide new SHRIMP U-Pb zircon dating for the Mamianshan Group: 851.9 ± 9.2 to 825.5 ± 9.8 Ma for the Longbeixi Formation, 796.5 ± 9.3 Ma for the Dongyan Formation, and 756.2 ± 7.2 Ma for the Daling Formation. These ages document the existence of Neoproterozoic magmatism in the northwestern Cathaysia Block. Dongyan Nd-Sr isotopic data show that mafic amphibolite schists, mafic greenschists, and quartzofeldspathic schists were derived from a more depleted mantle (initial εNd ? +5.5 and 87Sr/86Sr ratio 0.703409643), a mixture of depleted mantle and crustal components (initial εNd ? ?1 and 87Sr/86Sr ratio 0.702045282–0.704147714), and late Palaeoproterozoic continental crustal materials (initial εNd < ?1 and 87Sr/86Sr ratio 0.71083603), respectively. These new data, together with previous studies, suggest a bi-subduction-collision orogenic model for the Neoproterozoic evolution of the Yangtze and Cathaysia blocks. Our plate tectonic scenario involves earlier NW-dipping subduction during 1.0 Ga–860 Ma along the southeastern margin of the Yangtze Block and later NW-dipping subduction near the northwestern margin of the Cathaysia Block starting at ca. 850 Ma. The 796.5 ± 9.3 Ma age of the volcanic Dongyan Formation suggests that the final assembly of the Yangtze and Cathaysia blocks probably occurred after ca. 800 Ma. The 756.2 ± 7.2 Ma age of the Daling Formation indicates that post-orogenic extensional magmatism took place after 800 Ma along the northwestern margin of Cathaysia.  相似文献   

18.
Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial 87Sr/86Sr ratio of 0.70391±4. Nd values (at 120 Ma) show a relatively wide range of from –0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial 87Sr/86Sr ratios of from 0.70380 to 0.70430 and Nd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured Nd(0) value of –10.2, 87Sr/86Sr ratio of 0.73679 and a T Nd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below 300° C by 100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration.Unmetamorphosed leucogabbros from the Early Cretaceous Darran Complex of eastern Fiordland have significantly higher Nd values (3.9 to 4.6) and slightly lower 87Sr/ 86Sr (0.70373 to 0.70386) than the western Fiordland granulites. This indicates that the western and eastern Fiordland complexes are not correlative although both have geochemical similarities to Phanerozoic calc-alkaline island-arc suites. The Fiordland granulites are LREE enriched (LaN/ YbN=12 to 40) and have trace element characteristics (e.g. high K/Rb and low Rb/Sr ratios) typical of many Rb-depleted Precambrian granulite terrains. The Fiordland trace element trends, however are attributed to magmatic, not metamorphic processes, reflecting the character of the Early Cretaceous magma sources. The range of Nd values, but uniform initial 87Sr/86Sr of the western Fiordland granulites is consistent with derivation of the parent Early Cretaceous magmas at least in part from a LREE enriched, low Rb/Sr protoliths of mid-to late-Paleozoic age. Partial melting of this protolith occurred during or immediately preceding a period of great crustal thickening culminating in rapid thickening of existing crust by 20 km following emplacement of the granulitic rocks. The rapid crustal thickening was probably a consequence of a collisional event in which an Early Cretaceous magmatic arc was over-ridden by one or more thrust sheets.  相似文献   

19.
In the present work we studied Mg-ilmenite megacrysts from the Arkhangelsk kimberlites (the Kepino kimberlite field and mantle xenoliths from the Grib pipe). On the basis of isotopic (Rb/Sr, Sm/Nd, δ18O) and trace-element data we argue that studied Mg-ilmenite megacrysts have a genetic relation to the “protokimberlitic” magma, which was parental to the host kimberlites. Rb-Sr ages measured on phlogopite from ilmenite-clinopyroxenite xenoliths and the host Grib kimberlite overlap within the error (384 Ma and 372 ± 8 Ma, respectively; Shevchenko et al., 2004) with our estimation of the Kotuga kimberlite emplacement (378 ± 25 Ma). Sr and Nd isotopic compositions of megacrysts are close to the isotopic composition of host kimberlites (Mg-ilmenites from kimberlites have 87Sr/86Sr(t = 384) = 0.7050–0.7063, ?Nd(t = 384) = + 1.7, +1.8, ilmenite from ilmenite-garnet clinopyroxenite xenolith has 87Sr/86St(t = 384) = 0.7049, ?Nd(t = 384) = +3.5). Oxygen isotopic composition of ilmenites (δ18O = +3.8–+4.5‰) is relatively “light” in comparison with the values for mantle minerals (δ18O = +5–+6‰). Taking into account ilmenite-melt isotope fractionation, these values of δ18O indicate that ilmenites could crystallize from the “protokimberlitic” melt. Temperatures and redox conditions during the formation of ilmenite reaction rims were estimated using ilmenite-rutile and titanomagnetite-ilmenite thermo-oxybarometers. New minerals within the rims crystallized at increasing oxygen fugacity and decreasing temperature. Spinels precipitated during the interaction of ilmenite with kimberlitic melt at T = 1000–1100°C and oxygen fugacity $\Delta \log f_{O_2 }$ [QFM] ≈ 1. Rims comprised with rutile and titanomagnetite crystallized at T ≈ 1100°C, $\Delta \log f_{O_2 }$ [NNO] ≈ 4 and T = 600–613°C, $\Delta \log f_{O_2 }$ [QFM] ≈ 3.7, respectively. Rutile lamellae within ilmenite grains from clinopyroxenitic xenolith were formed T ≥ 1000–1100°C and oxygen fugacity $\Delta \log f_{O_2 }$ [NNO] = ?3.7. Since the pressure of clinopyroxene formation from this xenolith was estimated to be 45–53 kbar, redox conditions at 135–212 km depths could be close to $\Delta \log f_{O_2 }$ [NNO] = ?3.7.  相似文献   

20.
The Dongping gold deposit, situated on the northern margin of the North China Platform, is a composite deposit composed of auriferous quartz vein-type and altered rock-type ore bodies. It is hosted in the inner contact zone of an alkaline intrusion which was intruded into Archean metamorphic rocks and was formed not later than the Hercynian period. Auriferous quartz veins of the deposit are dated with the fluid inclusion Rb-Sr isochron method at 103 ± 4 Ma, indicating that the gold deposit was formed in the Yenshanian period.87Sr/86Sr sourcetracing shows the ore forming materials came dominantly from alkaline intrusions. These results, combined with other isotope and REE data, suggest that the Dongping gold deposit is not a traditional magmatic hydrothermal deposit, but a reworked hydrothermal deposit related to heated and evolved meteoric water. This project (49372105) is financially supported by the National Natural Science Foundation of China.  相似文献   

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