首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Whole-rock samples of metamorphic and granitic cobbles and boulders from the Kamiaso conglomerate in central Japan yield well-defined RbSr isochron ages of 1985 ± 25 my and 1820 ± 40 my. These ages are the oldest yet obtained for rocks in the Japanese Islands, and provide key evidence for the middle Precambrian metamorphic and igneous events in the provenance of these rocks. The age of 1985 my defined by six samples of quartzo-feldspathic gneiss may be the time of emplacement of the original granitic rocks. The 1820 my age indicates the time of extensive regional metamorphism and igneous activity. Precambrian episodes in the provenance of the Kamiaso conglomerate are summarized as follows: (1) 2000 my — formation of granitic rocks, (2) 1800–1600 my — high grade metamorphism accompanied by igneous activity, (3) 1200–1000 my — some significant thermal event.Judging from rock types and geochronological data, it can be said that metamorphic rocks in the Kamiaso conglomerate are remarkably similar to those of the Matenrei and Nangnim systems in North Korea. The Precambrian complex from which the metamorphic and granitic rocks were derived, was exposed to the north not far from the present site of the Kamiaso conglomerate in the late Paleozoic time, and it was probably a part of the large Precambrian continent in East Asia.  相似文献   

2.
Examples of positive correlations between initial 87Sr/86Sr and δ18O have now been shown to be very common in igneous rock series. These data in general require some type of mixing of mantle-derived igneous rocks with high-18O, high-87Sr crustal metamorphic rocks that once resided on or near the Earth's surface, such as sedimentary rocks or hydrothermally altered volcanic rocks. Mixing that involves assimilation of country rocks by magmas, however, is not a simple two-end-member process; heat balance requires appreciable crystallization of cumulates. In such cases, the isotopic compositions may strongly reflect this open-system behavior and indicate the process of assimilation, whereas the major element chemical compositions of the contaminated magmas will be largely controlled by crystal-melt equilibria and crystallization paths fixed by multicomponent cotectics. A variety of oxygen and strontium isotope “mixing” curves were therefore calculated for this process of combined assimilation-fractional crystallization. The positions and characteristics of the resultant curves on δ18O-87Sr/86Sr diagrams markedly diverge from simple two end-member mixing relationships. Based on the above, model calculations can be crudely fitted to two igneous rock suites (Adamello and Roccamonfina in Italy), but the shapes of the calculated curves appear to rule out magmatic assimilation as an explanation for most δ18O-87Sr/86Sr correlations discovered so far, including all of those involving calc-alkaline granitic batholiths and andesitic volcanic rocks. The isotopic relationships in such magma types must be inherited from their source regions, presumably reflecting patterns that existed in the parent rocks (or magmas) prior to or during melting.  相似文献   

3.
Experimental control in the classical igneous petrology is based on determination of the equilibrium conditions of simple — mostly 1–4 component — system. The statements based on such experiments are valid for rock products chemically not influenced by active environments. For elucidation of processes between geochemically active volatile-rich country rocks, high pressure and high temperature experiments oncomplex systems containing such active rock samples, too were performed. High pressure bombs were constructed with long and narrow container, allowing to investigate the selective migration of elements between different rock samples, even by temperature slope within the container. In these bombs different igneous, sedimentary and metamorphic rocks in contact at different temperatures and pressures were investigated. P-t relations and stability fields of different mineral facies cannot be exactly determined by phase diagrams of simple uni- or multicomponent systems, because they depend on other components and on the environments, too. The new complex experiments, however permit to assess directly the stability fields of the mineral facies. The data obtained proves that an intense selective migration of the volatiles take place even in solid samples at 550° C, in some rock associations already at 250° C, if there is a concentration gradient of volatiles between the samples. Selective migration proceeds following the principle of transvaporization, i.e. towards the partial pressures — chemical activities — of the mobile components and following the sequence of their ionic potentials. In this way, different hypomagmatic and metamorphic rocks are produced. The new kind of experiments shows that — opposite to the approaches of the classical igneous petrology — a high grade contamination of the ascending magma is possible. Contamination proceeds by transvaporization. Assimilation — Einschmelzung — mostly plays an insignificant role. Gravitative crystallization differentiation operates intensely only in the higher sections of volcanic pathways, characterized by relatively low pressures and thus by low viscosity of the magma. Contamination is intense if the ascension of magma is relatively slow as it is presumable along the oblique circumpacifique or innercarpathian andesitic structural lines. In this way, basaltic magma produces andesitic to rhyolitic rocks partly abundant in volatiles. However contamination is restricted and consequently basaltic rocks are formed if magma ascends rapidly through vertical channels as it is presumably the case with the young peripherical basalt volcanics, e.g. the innercarpathian basin system. These experiments also evidence the activation process of magma by transvaporization. Selective migration of volatiles even through solid rocks is introduced by structural pressure minimum zones towards these zones, which results in the melting of neighbouring rocks and their lifting as liquid magma until volatile supply is sufficient.  相似文献   

4.
The alkali-rich igneous rocks in China occur as fifteen linear distributed belts of each extending several hundreds to several thousands kilometers in length. These include most types of alkali-rich igneous rock categories discovered worldwide. The related Au, Cu large and superlarge deposits or metallogenic focus-areas include Dongping, Guilaizhuang, Yulong, etc. Direct and indirect genetic links have been found between alkali-rich igneous rocks and Au, Cu mineralization. The petrogenesis and metallogenesis of the alkali-rich igneous rocks are mainly controlled by (1) mantle enrichment, (2) strong interaction between mantle and crust, (3) lower contens of sulfur, high f O2 and high contents of volatiles, (4) the significant turn of regional tectonic framework from compress to extension and (5) the strong change of regional lithosphere structure.  相似文献   

5.
High-pressure (HP) and ultrahigh-pressure (UHP) eclogites exposed in collisional orogens are widely regarded to record the history of crustal rocks that were subducted to mantle depths and exhumed back to the surface. Insight into subduction and exhumation processes plays an important role in understanding the nature and evolution of subduction zones, geodynamics and plate tectonics. In contrast to continental-type HP to UHP metamorphic rocks that are dominated by felsic lithology, oceanic-type HP to UHP metamorphic rocks are dominated by mafic eclogites and thus have greater density, and their exhumation needs to overcome large barriers and may involve complicated tectonic processes. The exhumation of HP to UHP rocks is mainly influenced by the internal buoyancy, however, the external tectonic forces (such as channel flow) also act as effective exhumation drivers; in addition, effects of tectonic settings (such as slab rollback and breakoff) should take into account. The HP-UHP metamorphic terrane in Southwestern Tianshan, which mainly comprises of metasediments with interlayered metamafic lenses and blocks, represents a typical accretionary mélange associated with deep subduction of oceanic crust. However, the exhumation mechanisms of these once deeply buried HP-UHP rocks are still under discussion. Based on the field occurrences, petrographic features, peak metamorphic P-T conditions and ages of the eclogites/blueschists and their metasedimentary country rocks, a “sediment-type subduction channel” model is advocated in this study to appraise/decipher the formation and evolution of Southwestern Tianshan HP-UHP metamorphic belt. Poly-cyclic metamorphic eclogites record the multistage burial-exhumation cycling manipulated by convective flow in a channel-like interface between the plates, giving robust evidence for the presence of a subduction channel. In addition, this study summarizes some remaining geotectonic problems and research perspectives concerning the Southwestern Tianshan HP-UHP metamorphic belt.  相似文献   

6.
Twelve apatite samples from igneous and metamorphic rocks from the Black Forest and igneous rocks from the Kaiserstuhl were analysed for their REE content. The ΣREE range from 0.116 to 1.69 wt.%; the lowest values were found in the metamorphic rocks. All apatites from the various parent rocks show a general enrichment of the lighter rare earths over the heavier and their chondrite-normalized rare earth patterns exhibit a more or less pronounced negative Ce anomaly. This Ce depletion is accompanied by relatively low La and Pr values. In addition, the apatites from igneous rocks from the Black Forest show a marked negative Eu anomaly. An explanation is offered for the simultaneous occurrence of a negative Ce and a negative Eu anomaly in one investigated rock system. The negative Ce anomaly is attributed to the occurrence of phases enriched in Ce (e.g., monazite, allanite) which crystallized prior to or simultaneously with apatite. The negative Eu anomaly in these magmatic apatites most probably is caused by discrimination of Eu2+ from the apatite lattice. The result of this discrimination is a selective Eu enrichment in the later crystallizing feldspars, plagioclase and orthoclase. At least in this case, the positive Eu anomaly in feldspars is not a reliable indicator of low oxygen fugacity during their crystallization; the Eu depletion of the earlier crystallized apatites is preferable for this purpose.  相似文献   

7.
Three periods of volcanic activity connected with tectonic events form the geological history of the Valley of Mexico (Mooser 1963, 1969). An igneous rock suite from rhyodacites to andesites (but lacking rhyolites and basalts) can be observed in each period. During the Tertiary epochs — in the Oligo-Miocene and Upper Miocene-Pliocene — we have a more dacitic volcanism, in the Quaternary epoch a more andesitic volcanism. This result was verified by calculating the average of all available and stratigraphically datable chemical analyses byGunn &Mooser (1971) andNegendank (1972). Using the average chemical composition of the Oligo-Miocene, Upper Miocene-Pliocene and Quaternary products the equivalent igneous rocks were computed using theRittmann-norms in theStreckeisen-Q-A-P-F double triangle with the following result (names in parenthesis are those using the classification ofMiddlemost (1973): Quaternary : quartz-latite-andesite (andesite) Upper Miocene-Pliocene : leuco-quartz-latite-andesite (high lime dacite) Oligo-Miocene : leuco-quartz-latite-andesite (high lime dacite) The equal average composition of the two groups of Tertiary volcanic rocks seems to support the theory of a uniform primary andesite magma apart from which of the two possible theories of petrogenesis one favors. The calculated average trace element abundances show high Cr- and Ni-values which suggests that mantle material was involved if we consider the Tertiary products as partial melting products of the lower crust. A more elegant hypothesis seems to be the model ofGunn &Mooser (1971), who consider these volcanic rocks as partial melting products of oceanic tholeiites or their high pressure derivatives in the sense ofRaleigh &Lee (1969).  相似文献   

8.
While working to construct a geological map. A.Almela, an Engineer of Mines, discovered in Alhama de Murcia a previously unknown volcanic crater. This volcano erupted through the miocene layers and extended above them, thus establishing the date of this eruption as being post-miocene. Since there are no sedimentary rocks in this region of a more recent date it is impossible to fix the date of the eruption any more exactly. The crater is about 1Km. In the surface of the crater one finds slag, scoriaceous lava and compact rocks. The rock is composed of phlogopite, olivine and diopside in phenocrysts in a hyaloid cement with tiny grains of sanidine, olivine and diopside; and a thin layer of phlogopite in a glassy brownish black. A study was made of the chemical constituency of the rocks on the basis of three complete analyses and a comparison was made with the findings already made by Vera-Garrucha and those of Fortuna. All of the data as compiled in the five tables and four diagrams graphically represents the obtained results. The volcanic rocks have a close analogy to those ofOsann’s « Verita » type, which are shown not only by the analytical value of the magmatic parameters but also by the positions of the points shown in diagramsK-mg andLs-Fs-Ks of the three types already analized. In all the samples which have been studied from Barqueros the value of the K2O is superior to that of Na2O; it is also greater than that of Vera-Garrucha and less than in that of Fortuna.  相似文献   

9.
Portions of highland breccia boulder 7 collected during the Apollo 17 mission were studied using UThPb and RbSr systematics. A RbSr internal isochron age of3.89 ± 0.08b.y. with an initial87Sr/86Sr of0.69926 ± 0.00008 was obtained for clast 1 (77135,57) (a troctolitic microbreccia). A troctolitic portion of microbreccia clast 77215,37 yielded a UPb internal isochron of3.8 ± 0.2b.y. and an initial206Pb/207Pb of 0.69. These internal isochron age are interpreted as reflecting metamorphic events, probably related to impacts, which reset RbSr and UPb mineral systems of older rocks.Six portions of boulder 7 were analyzed for U, Th, and Pb as whole rocks. Two chemical groups appear to be defined by the U, Th, and Pb concentration data. Chemical group A is characterized by U, Th, and Pb concentrations and238U/204Pb values which are higher than those of group B. Group A rocks have typical232Th/238U ratios of ～ 3.85, whereas-group B rocks have unusually high Th/U values of ～ 4.1.Whole-rock UPb and PbPb ages are nearly concordant. Two events appear to be reflected in these data — one at ～ 4.4 b.y. and one at ～ 4.5 b.y. The chemical groupings show no correlation with documented ages. The old ages of ～ 4.4 b.y. and ～ 4.5 b.y. may, like the younger ～ 4.0 b.y. ages, be related to basin excavation events.  相似文献   

10.
Exceptionally well-preserved pillow lavas and inter-pillow hyaloclastites from the Barberton Greenstone Belt in South Africa contain textural, geochemical, and isotopic biomarkers indicative of microbially mediated alteration of basaltic glass in the Archean. The textures are micrometer-scale tubular structures interpreted to have originally formed during microbial etching of glass along fractures. Textures of similar size, morphology, and distribution have been attributed to microbial activity and are commonly observed in the glassy margins of pillow lavas from in situ oceanic crust and young ophiolites. The tubes from the Barberton Greenstone Belt were preserved by precipitation of fine-grained titanite during greenschist facies metamorphism associated with seafloor hydrothermal alteration. The presence of organic carbon along the margins of the tubes and low δ13C values of bulk-rock carbonate in formerly glassy samples support a biogenic origin for the tubes. Overprinting relationships of secondary minerals observed in thin section indicate the tubular structures are pre-metamorphic. Overlapping metamorphic and igneous crystallization ages thus imply the microbes colonized these rocks 3.4–3.5 Ga. Although, the search for traces of early life on Earth has recently intensified, research has largely been confined to sedimentary rocks. Subaqueous volcanic rocks represent a new geological setting in the search for early life that may preserve a largely unexplored Archean biomass.  相似文献   

11.
Cenozoic capping volcanic rocks in the Nevada portion of the Basin and Range Province of the western United States belong to the high alumina calcalkaline igneous series. Varying proportions of plagioclase (An=85 to 45 percent), pyroxene (augite, pigeonite, and hypersthene), olivine, magnetite, biotite, and oxyhornblende indicate a modal range from olivine basalt to andesite. Major element analyses made on randomly collected samples, as well as on samples from systematically measured stratigraphic sections in localities of minimum erosion show ranges in Al2O3 (from 17.5 to 22.5 percent); SiO2 (from 44.0 to 54.0 percent); MgO (from 3.47 to 8.20 percent) and CaO (from 7.19 to 11.90 percent). Na2O/K2O is always greater than 1.0. Ba++ and Sr++ abundances for some of the rocks are in agreement with those suggested, by workers in the field, for average basalt and andesite derived by melting of mantle or lower crust, but for many of the samples the values found are considerably higher. Although the presence of biotite in the earlier flows and oxyhornblende in the later ones along with the presence of much magnetite in all the rocks examined suggests that in part these rocks were derived by crystallization of a melt under conditions of high partial pressure of oxygen, the available trace element data indicates that contamination of the magma with crustal material was also a factor in their developmental history. The case for a parent magma, subsequently fractionally crystallized and contaminated in part, is strengthened by the occurrence of crystal cumulates and highly altered xenoliths in some of the flows.  相似文献   

12.
Collections of Early Triassic rocks sampled from three outcrops on the SW slope of the Pai-Khoi Ridge are studied. The magnetic polarity succession r-n-r is established from the prefolding characteristic component of natural remanence in sedimentary rocks and a basaltic nappe. According to stratigraphic data and isotopic dating of the basaltic nappe, the studied parts of the lower subformation of the Lestanshorskaya Formation correlate with the central part of magnetic zone R 1 T 1 of the European Russia magnetostratigraphic time scale. The angles of rotation in the horizontal plane are estimated for the studied Early Triassic Pai-Khoi structures with respect to the East European platform.  相似文献   

13.
The experimental arrangement for the measurement of the dielectric constant has been described in brief. The dielectric constants of numerous metamorphic, igneous and sedimentary rocks have been reported in the frequency range of 100 KHz and 20 MHz, whereas the dry samples have less dispersion in the dielectric constant, the wet samples, in turn, have greater dispersion. The role of water in increasing the values of the dielectric constant has been discussed. There is no direct correlation between the amount of water and the change of the dielectric constant. In general, basic rocks have higher values than the acidic rocks.  相似文献   

14.
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   

15.
  相似文献   

16.
Eleven whole rock Rb/Sr age determinations from the Chilean metamorphic basement — formerly considered as Precambrian — define two limiting reference isochrons of 342 and 273 my. Analized rocks are phyllites and schists of sedimentary origin with mineralogical assemblages mainly corresponding to the greenschist facies.Two metamorphic series characterize the Chilean metamorphic basement: an eastern low P/T series, and a western high P/T series. Four of the analyzed samples belong to the former and seven to the latter.The obtained age is interpreted as the age of a main metamorphic episode of the basement. This episode would have taken place in the Upper Paleozoic.  相似文献   

17.
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   

18.
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   

19.
  相似文献   

20.
The Hf isotope composition of original igneous or detrital zircons in high-grade metamorphic rocks can be used to trace protolith origin, but metamorphic effect on the Hf isotope composition of newly grown domains remains to evaluate. We report a detailed in situ combined study of intragrain U-Pb and Lu-Hf isotopes in zircons from granitic gneiss and eclogite in the Dabie orogen of China that experienced ultrahigh-pressure eclogite-facies metamorphism. The results show correlations in 206Pb / 238U age, initial Hf isotope composition, and Th / U and Lu / Hf ratios between the domains of different origins. The metamorphic domains are characterized by low Th / U and Lu / Hf ratios but high ?Hf(t) values relative to the igneous core and mantle of pre-metamorphic ages. Positive correlations are observed between Th / U and Lu / Hf ratios, pointing to the similar effect of metamorphism on both U-Th-Pb and Lu-Hf isotope systems. Thus the metamorphic domains are distinguished from the igneous core and mantle by their low Lu / Hf ratios that are less than 0.001 for the granitic gneiss and less than 0.0001 for the eclogite. Despite differences in both protolith age and geochemical source between granitic gneiss and eclogite, rim ?Hf(t) values are variably 3.1 to 13.5 greater than core ?Hf(t) values when calculated at timing of protolith formation. This indicates that the zircon overgrowth was associated with a metamorphic medium that has high 176Hf / 177Hf but low 176Lu / 177Hf ratios. While the metamorphic domains contain more radiogenic Hf isotopes than the original igneous core and mantle, their Lu / Hf ratios are significantly lower than those of core and mantle. Therefore, the metamorphic zircons acquired their initial Hf isotope ratios from metamorphic fluids that have high 176Hf / 177Hf ratios but low Lu / Hf ratios with sound variability depending on the Lu-Hf isotope compositions of pre-existing and co-precipitating phases.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号