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1.
Summary Leucosomes, mesosomes and melanosomes, including 12 leucosome-host rock pairs have been studied for Sr and Nd isotopic systematics in migmatitic metapelites of the Serre (southern Calabria) which experienced granulite facies conditions in Hercynian times. Most samples came from the deep to middle part of a metapelitic unit, which is 5–6 km thick. The leucosomes presently occurring in the migmatites are mainly K-poor peraluminous leucotonalites; the host rocks (metapelites, metagreywackes and opx-bearing rocks) are more or less depleted in “granitic” component and have molar Al2O3/(CaO + Na2O + K2O) (A/CNK) ratios up to 15; the highest values relate to garnet + sillimanite-rich metapelites. Sm/Nd ratios are generally lower in the leucosomes and Rb/Sr ratios are very low (≪ 1) and lower than in related mesosomes and melanosomes. The isotopic Sr and Nd values vary in a wide range within the various kinds of host-rocks and within the leucosomes. A correlation of Nd- and Sr- isotopic compositions between coexisting leucosomes and host-rocks has not been found. The isotopic Sr and Nd values of the leucosomes are less variable than the host-rocks; melanosomes having A/CNK ratios > 3 concentrate at the lowest εNd290 Ma values, owing to retention of garnet and accessories and depletion of feldspars. The leucosomes define a hyperbolic trend where most samples concentrate in the middle part. These features are interpreted as reflecting melting of several rock types generating melts having a weighted mean composition from the contributing sources. The prevailing leucotonalites and their very low Rb/Sr ratios indicate that plagioclase played a more important role than mica in the melting processes which produced the studied leucosomes. Since assemblages lacking or very poor in biotite and K-feldspar are common in metapelitic restites, potassic melts, as one should expect from melting of micas, must have been extracted before the leucosomes under study formed. Zusammenfassung Aufschmelzung im Ungleichgewicht dokumentiert an granulitfaziellen Metasedimenten der n?rdlichen Serre (Kalabrien, Süditalien) Die Sr und Nd Isotopensystematik von Leuko-, Meso- und Melanosomen, einschlie?lich von 12 Leukosom- und Nebengesteinspaaren, wurde untersucht. Sie treten in migmatitischen Metapeliten der Serre (südliches Kalabrien) auf, deren granulitfazielle Metamorphose als hercynisch eingestuft wird. Die meisten Proben stammen aus dem unteren Teil der mittleren Metapeliteinheit, die eine M?chtigkeit von 5–6 km erreicht. Die in den Migmatitien auftretenden Leukosome sind haupts?chlich K-arme, peralumin?se Leukotonalite. Die Nebengesteine (Metapelite, Metagrauwacken, und Opx-führende Gesteine) sind mehr oder weniger an “granitischer” Komponente verarmt und haben molare A/CNK Verh?ltnisse bis 15, mit den h?chsten Werten in Granat- Sillimanit- reichen Metapeliten. Die Sm/Nd- Verh?ltnisse sind generell niedriger in den Leukosomen. Rb/Sr ist sehr niedrig (≪ 1) und niedriger als in den Meso- und Melanosomen. Die Sr- und Nd-Isotopenzusammensetzungen variieren stark innerhalb der verschiedenen Nebengesteine und ihrer assoziierten Leukosome. Eine Korrelation der Nd- und Sr- Isotopenzusammensetzungen zwischen koexistierenden Leukosomen und Nebengesteinen konnte nicht festgestellt werden. Die Sr- und Nd-Isotopie der Leukosome ist weniger variabel als die der Nebengesteine. Die Melanosome, sie haben A/CNK > 3, zeigen die niedrigsten εNd290 Ma Werte als Folge der Retention von Granat und Akzessorien und der Verarmung an Feldspat. Die Leukosome definieren einen hyperbolischen Trend, wobei die meisten Proben in der Mitte zu liegen kommen. Dies wird so interpretiert, dass mehrere Gesteinstypen, die eine gewichtete mittlere Zusammensetzung der an der Aufschmelzung beteiligten Ausgangsmaterialien darstellen, aufgeschmolzen wurden. Die vorherrschenden Leukotonalite und ihre sehr niedrigen Rb/Sr Verh?ltnisse zeigen, dass Plagioklas eine wichtigere Rolle bei den leukosombildenden Schmelzprozessen spielte als Glimmer. Da Mineralvergesellschaftungen, die arm an Biotit und Alkalifeldspat sind, h?ufig in metapelitischen Restiten auftreten, müssen K-reiche Schmelzen – sie sind zu erwarten wenn Glimmer aufgeschmolzen werden – vor der Bildung der Leukosome extrahiert worden sein. Received November 11, 1998; revised version accepted October 21, 1999  相似文献   

2.
Advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes. Incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the Rb-Sr and Sm-Nd isotope systems in anatectic melts from a muscovite-bearing metasedimentary source. Modeling results show that (1) the Sm/Nd ratios and Nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt, and (2) the relative proportion of micas (muscovite and biotite) and feldspars (plagioclase and K-feldspar) that enter the melt is a key parameter determining the Rb/Sr and 87Sr/86Sr ratios of the melt. Furthermore, these two factors are not, in practice, independent. In general, nonmodal partial melting of a pelitic source results in melts following one of two paths in εNd-87Sr/86Sr ratio space. A higher temperature, fluid-absent path (Path 1) represents those partial melting reactions in which muscovite/biotite dehydration and apatite but not monazite dissolution play a significant role; the melt will have elevated Rb/Sr, 87Sr/86Sr, Sm/Nd, and εNd values. In contrast, a lower temperature, fluid-fluxed path (Path 2) represents those partial melting reactions in which muscovite/biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue; the melt will have lower Rb/Sr, 87Sr/86Sr, Sm/Nd, and εNd values than its source. The master variables controlling both accessory phase dissolution (and hence the Sm-Nd system), and melting reaction (and hence the Rb-Sr systematics) are temperature and water content. The complexity in Sr-Nd isotope systematics in metasediment-derived melts, as suggested in this study, will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component.  相似文献   

3.
 The Urach volcanic field is unique within the Tertiary–Quaternary European volcanic province (EVP) due to more than 350 tuffaceous diatremes and only sixteen localities with extremely undersaturated olivine melilitite. We report representative Pb-Sr-Nd isotopic compositions and incompatible trace element data for twenty-two pristine augite, Cr-diopside, hornblende, and phlogopite megacryst samples from the diatremes, and seven melilitite whole rocks. The Pb isotopic compositions for melilitites and comagmatic megacrysts have very radiogenic 206Pb/204Pb ratios of 19.4 to 19.9 and plot on the northern hemisphere mantle reference line (NHRL). The data indicate absence of an old crustal component as reflected in the high 207Pb/204Pb ratios of many basalts from the EVP. This inference is supported by 206Pb/204Pb ratios of ∼17.6 to 18.3 and ɛNd of ∼−7.8 to +1.6 for five phlogopite xenocryst samples reflecting a distinct and variably rejuvenated lower Hercynian basement. The 87Sr/86Sr ratios of 0.7033 to 0.7035 in the comagmatic megacrysts are low relative to their moderately radiogenic Nd isotopic compositions (ɛNd +2.2 to +5.1) and consistent with a long-term source evolution with a low Rb/Sr ratio and depletion in light rare-earth elements (LREE). The melilitite whole-rock data show a similar range in Nd isotopic ratios as determined for the megacrysts but their Sr isotopic compositions are often much more radiogenic due to surface alteration. The REE patterns and incompatible trace element ratios of the melilitites (e.g. Nb/Th, Nb/U, Sr/Nd, P/Nd, Ba/Th, Zr/Hf) are similar to those in ocean island basalts (OIB); negative anomalies for normalized K and Rb concentrations support a concept of melt evolution in the lithospheric mantle. Highly variable Ce/Pb ratios of 29 to 66 are positively correlated with La/Lu, La/K2O, and Ba/Nd and interpreted to reflect melting in the presence of residual amphibole and phlogopite. The data suggest an origin of the melilitites from a chemical boundary layer very recently enriched by melts from old OIB sources. We suggest that the OIB-like mantle domains represent low-temperature melting heterogeneities in an upwelling asthenosphere under western Europe. Received: 9 March 1995/Accepted: 24 July 1995  相似文献   

4.
Isotopic (Nd and Sr) and chemical compositions of the 177 Ma Kirkpatrick Basalt and Ferrar Dolerite from north Victoria Land, Antarctica, are examined in order to address the role of crustal assimilation and the characteristics of their mantle source. Results for the Scarab Peak chemical type (SPCT) that constitutes the flow unit capping the lava sequence [Mg-number, Mg/(Mg+Fe+2)=24, MgO=2.4%, SiO2=57.1%, initial87Sr/86Sr=0.7087–0.7097, (εNd=−4.3) conform previous reports that attribute variations in the concentrations of the more mobile elements and calculated initial87Sr/86Sr to mid-Cretaceous alteration and elevated δ18O to low-temperature interaction with meteoric water. The underlying lavas and the sills that are of the Mt. Fazio chemical type (MFCT) display a much wider range of both chemical and isotopic compositions (Mg-number=40–65, MgO=3.7 7.5%; SiO2=52.6–58.3%, initial87Sr/86Sr=0.7087–0.7117, εNd=−5.6 to −4.8). The effects of rock alteration on apparent initial87Sr/86Sr are demonstrated by large differences between the initial ratio of mineral separates or leached fractions and whole rocks. Cretaceous alteration produced Rb and Sr redistribution within the lava sequence that results in erroneous calculated initial87Sr/86Sr ratios. These effects are responsible for the large initial87Sr/86Sr variations previousl7 proposed which, combined with the large range in whole-rock δ18O, were purported to show very large degrees of crustal assimilation. The variations in εNd are restricted and indicate much smaller degrees of assimilation. The least altered of the MFCT rocks show good chemical and isotopic correlations that can be integrated into a model involving fractionation of pyroxene and plagioclase coupled with assimilation of material similar to early Paleozoic basement. The lower87Sr/86Sr and higher εNd of the SPCT suggest that they were derived by extensive fractionation of a more primitive, less contaminated, precursor of the MFCT. The most isotopically primitive Ferrar rocks from the region still have a high initial87Sr/86Sr and low initial143Nd/144Nd; this may reflect either earlier assimilation or an enriched source. The chemical and isotopic similarities, as well as the close geographic correspondence of the Ferrar Group to granitoids produced during the early Paleozoic Ross Orogeny suggest that in either case Ross-type material may have been involved in the development of the enriched isotopic signature. Editorial responsibility: I. Parsons  相似文献   

5.
 Latest Devonian to early Carboniferous plutonic rocks from the Odenwald accretionary complex reflect the transition from a subduction to a collisional setting. For ∼362 Ma old gabbroic rocks from the northern tectonometamorphic unit I, initial isotopic compositions (εNd=+3.4 to +3.8;87Sr/86Sr =0.7035–0.7053;δ18O=6.8–8.0‰) and chemical signatures (e.g., low Nb/Th, Nb/U, Ce/Pb, Th/U, Rb/Cs) indicate a subduction-related origin by partial melting of a shallow depleted mantle source metasomatized by water-rich, large ion lithophile element-loaded fluids. In the central (unit II) and southern (unit III) Odenwald, syncollisional mafic to felsic granitoids were emplaced in a transtensional setting at approximately 340–335 Ma B.P. Unit II comprises a mafic and a felsic suite that are genetically unrelated. Both suites are intermediate between the medium-K and high-K series and have similar initial Nd and Sr signatures (εNd=0.0 to –2.5;87Sr/86Sr=0.7044–0.7056) but different oxygen isotopic compositions (δ18O=7.3–8.7‰ in mafic vs 9.3–9.5‰ in felsic rocks). These characteristics, in conjunction with the chemical signatures, suggest an enriched mantle source for the mafic magmas and a shallow metaluminous crustal source for the felsic magmas. Younger intrusives of unit II have higher Sr/Y, Zr/Y, and Tb/Yb ratios suggesting magma segregation at greater depths. Mafic high-K to shoshonitic intrusives of the southern unit III have initial isotopic compositions (εNd=–1.1 to –1.8;87Sr/86Sr =0.7054–0.7062;δ18O=7.2–7.6‰) and chemical characteristics (e.g., high Sr/Y, Zr/Y, Tb/Yb) that are strongly indicative of a deep-seated enriched mantle source. Spatially associated felsic high-K to shoshonitic rocks of unit III may be derived by dehydration melting of garnet-rich metaluminous crustal source rocks or may represent hybrid magmas. Received: 7 December 1998 / Accepted: 27 April 1999  相似文献   

6.
Strontium and Nd isotopic compositions and trace element abundances were determined for Cretaceous to late Cenozoic igneous rocks from the Japan Sea side of Southwest Japan in order to investigate the effect of the opening of the Japan Sea on igneous activity. The 87Sr/86Sr ratios for both high and low silica rocks decrease with decreasing age since the middle Miocene, when the opening occurred. Similarly, 143Nd/144Nd values for these rocks increase with decreasing age, and are negatively correlated with 87Sr/86Sr ratios. A two-component mixing process can best account for these isotopic and chemical characteristics. One end-member is likely the subcontinental lithospheric mantle (SCLM) and its derivative mafic to intermediate materials which had ɛNd values of around +3. The other endmember consists of mafic to intermediate rocks with low ɛNd values (e.g., −8), probably located in the lower crust. The mantle upwelling associated with the opening of the Japan Sea did not supply typical MORB or MORB-source materials to the crust, but did provide the heat that caused the melting of lithospheric mantle and lower crust. Received: 29 August 1996 / Accepted: 6 May 1997  相似文献   

7.
High-K mafic alkalic lavas (5.4 to 3.2 wt% K2O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; 87Sr/86Sr ratios decrease and ɛNd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; 87Sr/86Sr ratios range from 0.7121 to 0.7105 and ɛNd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, 206Pb/204Pb ∼17.2, 207Pb/204Pb ∼15.5, and 208Pb/204Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛNd values and unradiogenic 206Pb/204Pb ratios have relatively low 87Sr/86Sr ratios (the EM I array), whereas basalts with low ɛNd values and high 87Sr/86Sr ratios have radiogenic 206Pb/204Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high 87Sr/86Sr and 208Pb/204Pb ratios and low ɛNd values and 206Pb/204Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998  相似文献   

8.
A detailed Sr−Nd isotopic study of primary apatite, calcite and dolomite from phoscorites and carbonatites of the Kovdor massif (380 Ma), Kola peninsula, Russia, reveals a complicated evolutionary history. At least six types of phoscorites and five types of carbonatite have been identified from Kovdor by previous investigators based on relative ages and their major and accessory minerals. Isotopic data from apatite define at least two distinct groups of phoscorite and carbonatite. Apatite from the earlier phoscorites and carbonatites (group 1) are characterized by relatively low87Sr/86Sr (0.70330–0.70349) and143Nd/144Nd initial ratios (0.51230–0.51240) with F=2.01–2.23 wt%, Sr=2185–2975 ppm, Nd=275–660 ppm and Sm=31.7–96.2 ppm. Apatite from the second group has higher87Sr/86Sr (0.70350–0.70363) and143Nd/144Nd initial ratios (0.51240–0.51247) and higher F (2.63–3.16 wt%), Sr (4790–7500 ppm), Nd (457–1074 ppm) and Sm (68.7–147.6 ppm) contents. This group corresponds to the later phoscorites and carbonatites. One apatite sample from a carbonatite from the earlier group fits into neither of the two groups and is characterized by the highest initial87Sr/86Sr (0.70385) and lowest143Nd/144Nd (0.51229) of any of the apatites. Within both groups initial87Sr/86Sr and143Nd/144Nd ratios show negative correlations. Strontium isotope data from coexisting calcite and dolomite support the findings from the apatite study. The Sr and Nd isotopic similarities between carbonatites and phoscorites indicate a genetic relationship between the two rock types. Wide variations in Sr and Nd isotopic composition within some of the earlier carbonatites indicate several distinct intrusive phases. Oxygen isotopic data from calcite and dolomite (δ18O=+7.2 to +7.7‰ SMOW) indicate the absence of any low-temerature secondary processes in phoscorites and carbonatites, and are consistent with a mantle origin for their parental melts. Apatite data from both groups of phoscorite plot in the depleted quadrant of an εNd versus εSr diagram. Data for the earlier group lie along the Kola Carbonatite Line (KCL) as defined by Kramm (1993) and data from the later group plot above the KCL. The evolution of the phoscorites and carbonatites cannot be explained by simple magmatic differentiation assuming closed system conditions. The Sr−Nd data can best be explained by the mixing of three components. Two of these are similar to the end-members that define the Kola Carbonatite Line and these were involved in the genesis of the early phoscorites and carbonatites. An additional component is needed to explain the isotopic characteristics of the later group. Our study shows that apatite from rocks of different mineralogy and age is ideal for placing constraints on mantle sources and for monitoring the Sr−Nd evolution of carbonatites. Editorial responsibility: W. Schreyer  相似文献   

9.
A detailed geochemical and geochronological study of anatectic migmatites from the Namche Barwa Massif (NBM), southern Tibet, has been carried out to place important constraints on the thermal and tectonic evolution of the eastern Himalayan syntaxis. SHRIMP zircon U/Pb dating indicates that the granulite-facies metapelite underwent metamorphism at 21.8 ± 0.7 Ma and 24.5 ± 0.7 Ma, respectively. The latter is similar to the timing of partial melting and the formation of Ca-rich leucosomes at ~ 24-25 Ma. These leucosomes are characterized by (1) high CaO, Na2O, and Na/K ratios; (2) radiogenic Sr (87Sr/86Sr(t) = 0.7407-0.7904) but unradiogenic Nd (εNd(t) = − 7.0 to − 21.2) isotope compositions; (3) depleted HFSE, and (3) variable but depleted HREE relative to their host pelites. Some of the leucosomes show large degrees of Nd isotopic disequilibrium, up to 10 epsilon units with respect to their hosts. These high CaO and Na2O leucosomes were derived from fluxing melting of metapelite at high pressures. A similar process could have operated during the formation of the Himalayan leucogranites and contributes to the heterogeneities in such granites.  相似文献   

10.
The Francistown plutonic rocks at the south-western margin of the Zimbabwe craton consist of three igneous suites: Sanukitoid, Tonalite–Trondhjemite–Granite (TTG) suites and High-K granites. The TTG suite is subdivided into High Aluminum-TTG (HA-TTG) and Low Aluminum-TTG (LA-TTG) sub-suites. Their Rb–Sr isotope systems were partially homogenized by post-crystallization thermo–tectonic events, in which hydrothermal solutions and migmatization played an important role. Therefore, the Rb–Sr isochron age of 2427±54 Ma can only be regarded as a lower limit to the Francistown plutonic rock age. The large errors in the Sm–Nd isochron dates of Francistown granitoids indicate that these dates are not really constrained. In this study we compared the rock types of Francistown and adjacent areas, adopting the precise U, Th–Pb single zircon SHRIMP ages from the Vumba area as references. For TTG and Sanukitoid suites, the age we adopted is ca. 2.7 Ga, which is close to their depleted-mantle Sm–Nd model ages (T DM). For High-K granites, the age adopted is ca. 2.65 Ga, which is also close to their Sm–Nd isochron age. The highest ε Nd t values of Sanukitoids and TTG are +2.1 and +2.3, respectively. The positive ε Nd t values and trace element geochemistry support partial melting of a depleted mantle and young oceanic crust for the genesis of Sanukitoid and the TTG suites respectively. The lowest ε Nd t values of Sanukitoids and TTGs are −1.0 and −1.1, respectively, indicating contamination by continental crust, up to 10 and 14%, respectively. The ε Nd t values of TTG decrease with decreasing Al2O3 and Sr contents and increasing Eu negative anomalies (Eu*–Eu), suggesting that the TTG magmas underwent a coupled fractionation crystallization and crustal contamination, and that the LA-TTG was the product of the fractionation and contamination of the HA-TTG sub-suite. In contrast, negative ε Nd t values for the High-K granites (from −0.4 to −3.5) indicate the involvement of LA-TTG and some materials from an old continental crust in their genesis. The products of partial melting of both oceanic and continental crusts at the south-western margin of the Zimbabwe craton occurred within a short time interval (from 2.7 to 2.65 Ga ago) suggesting that the Francistown plutonic rocks were formed in a active continental margin environment, where a young ocean plate (Limpopo oceanic plate) subducted underneath an old continental plate (Zimbabwe craton).  相似文献   

11.
 Nd and Sr isotopic data are presented for the 2449–2441 Ma Olanga and Burakovka layered mafic complexes in the eastern Baltic Shield. These complexes have similar tectonic position, but differ in two aspects: the age of the enclosing crust and the post-crystallization metamorphic history. The Sm–Nd isotopic results for the Kivakka and Lukkulaisvaara intrusions, Olanga Complex, are consistent with the model of closed-system crystallization of a single magma without significant wallrock assimilation. The Rb–Sr systems of minerals were disturbed by late Rb addition during 1.75–1.50 Ga metamorphism. The Nd and Sr isotopic systems in the Burakovka complex show no metamorphic disturbance and indicate mixing of at least four isotopically distinct components. Isotopic variations in the Burakovka Complex can be explained by a 4–20 per cent contamination of a primary komatiitic or picritic magma with a Mesoarchean crust, similar to that exposed in the region. A similar model, applied to the Olanga Complex using a Neoarchean crustal isotopic composition, cannot reproduce the observed isotopic signature. The nearly uniform initial ɛNd values between −1 and −2.3, observed in the Kivakka and Lukkulaisvaara intrusions of the Olanga Complex, as well as in the other 2.50–2.44 Ga layered mafic intrusions throughout the eastern Baltic Shield, are better explained by a mantle plume model with small amounts of crustal contamination and minor involvement of asthenospheric material. This model is also consistent with the geological observations and the temporal distribution of the Paleoproterozoic mafic magmatism in the eastern Baltic Shield. As an alternative, the enriched isotopic characteristics may be explained by melting of a metasomatically modified lithospheric mantle source. Received: 4 August 1994/Accepted: 5 April 1996  相似文献   

12.
Recent zircon dating identified several late Carboniferous to early Permian hornblende gabbro–diorite–quartz diorite–granodiorite–tonalite–granite plutons in lithological assemblages at the northern margin of the North China Block (NCB) that were previously regarded as Archaean to Palaeoproterozoic. Our geochronological results indicate that emplacement of these plutons was a continuous process during the late Carboniferous to early Permian, from 324 ± 6 to 274 ± 6 Ma, and lasted for at least 50 Ma. In this paper, the early Permian components with compositions from gabbro to granite within the intrusive complex were studied. The early Permian plutons exhibit calc-alkaline or high-K calc-alkaline, metaluminous geochemical features and highly variable SiO2 contents. They have no significant Eu anomaly in their REE patterns, and in primitive-mantle-normalized spidergrams they display depletion in Th, U, Nb, Ta, P and Ti, and enrichment in Ba, K, Pb and Sr. The granitoid bodies within these plutons display I-type and adakitic geochemical signatures. The early Permian rocks exhibit low whole-rock initial 87Sr/86Sr ratios from 0.70520 to 0.70615 and have negative whole-rock ε Nd(t) values ranging from −17.4 to −9.3 and zircon ε Hf(t) values of −23.2 to −10.5. The gabbros exhibit higher ε Nd(t) values from −11.1 to −9.3 and ε Hf(t) values from −16.5 to −10.5, and one granodiorite exhibits an even lower ε Nd(t) value of −17.4 and zircon ε Hf(t) values of −23.2 to −15.1. Geochemical, Sr–Nd and in situ zircon Hf isotopic compositions suggest that the hornblende gabbros were derived from a metasomatized lithospheric mantle, and the diorite and quartz diorite were generated from a gabbroic magma by fractional crystallization, coupled with differential assimilation of ancient lower crustal material. The granodiorite was likely derived from partial melting of ancient lower crust with involvement of some mantle components. Involvement of both lithospheric mantle and ancient lower crust in the generation of the early Permian plutons indicates strong crust–mantle interaction in the northern NCB. Petrological associations as well as geochemical and Sr–Nd–Hf isotopic results show that the early Permian plutons were emplaced along an Andean-type active continental margin during southward subduction of the Palaeo-Asian oceanic plate beneath the NCB. Integration of our results with previously published data for late Carboniferous and late Permian to middle Triassic intrusions suggests that the continental arc on the northern margin of the NCB existed for at least 50 Ma during the late Palaeozoic, and final amalgamation of the Mongolian arc terranes with the northern NCB likely occurred during a period from ~270 to ~250 Ma, i.e, in the late Permian to earliest Triassic.  相似文献   

13.
We report petrological, chemical and Os–Nd–Sr isotopic data for the Gaositai ultramafic complex from northern North China craton (NCC) to reveal its petrogenesis. The complex shows features of Alaskan-type intrusions, including (1) the concentric zoning from dunite core, to clinopyroxenite and hornblendite in the rim, and the common cumulative textures; (2) the abundance of olivine, clinopyroxene and hornblende, and the scarcity of orthopyroxene and plagioclase, and (3) the systematic decrease in Mg# of ferromagnesian phases from core to rim, accompanied by the Fe-enrichment trend of accessory spinel. The different rock types show highly varied, radiogenic Os isotopic ratios (0.129–5.2), and unradiogenic Nd isotopic composition (εNd(t) = −8 to −15), but are homogeneous in ISr ratios (0.7054–0.7066). The (187Os/188Os)i ratios are found to be anti-correlated with εNd(t) values and whole-rock Mg# as well. These data suggest significant crustal contamination during magma evolution. The crustal contaminants are dominantly Archean mafic rocks in the lower crust, and subordinate TTG gneisses at shallower crustal levels. The parental magma was hydrous picritic in composition, derived from an enriched lithospheric mantle source above a subduction zone. The zoned pattern of the complex formed probably through “flow differentiation” of a rapidly rising crystal mush along a fracture zone that was developed as a result of lithospheric extension in a back-arc setting in the northern margin of the NCC at ca. 280 Ma.  相似文献   

14.
Summary The Tyrrhenian border of the Italian peninsula has been the site of intense magmatism from Pliocene to recent times. Although calc-alkaline, potassic and ultrapotassic volcanism overlaps in space and time, a decrease of alkaline character in time and space (southward) is observed. Alkaline ultrapotassic and potassic volcanic rocks are characterised by variable enrichment in K and incompatible elements, coupled with consistently high LILE/HFSE values, similar to those of calc-alkaline volcanic rocks from the nearby Aeolian arc. On the basis of mineralogy and major and trace element chemistry two different arrays can be recognised among primitive rocks; a silica saturated trend, which resulted in formation of leucite-free mafic rocks, and a silica undersaturated trend, charactrerised by leucite-bearing rocks. Initial 87Sr/86Sr and 143Nd/144Nd values of Italian ultrapotassic and potassic mafic rocks range from 0.70506 to 0.71672 and from 0.51173 to 0.51273, respectively. 206Pb/204Pb values range between 18.50 and 19.15, 207Pb/204Pb values range between 15.63 and 15.70, and 208Pb/204Pb values range between 38.35 and 39.20. The general εSr vs. εNd array, along with crustal lead isotopic values, clearly indicates that a continental crustal component has played an important role in the genesis of these magmas. The main question is where this continental crustal component has been acquired by the magmas. Volcanological and petrologic data indicate continental crustal contamination to be a leading process along with fractional crystallisation and magma mixing. Considering, however, only the samples thought to represent primary magmas, which have been in equilibrium with their mantle source, a clearer picture emerges. A large variation of εSr vs. εNd is still observed, with εSr from −2 to +180 and εNd from + 2 to −12. A bifurcation of this array is observed in the samples that plot in the lower right quadrant, with mafic leucite-bearing Roman Province rocks buffered at εSr = + 100 whereas the mafic leucite-free potassic and ultrapotassic rocks point to strongly radiogenic Sr compositions. We may argue that mafic leucite-bearing Roman Province rocks point to εSr and εNd values similar to those of Miocene carbonate sediments whereas mafic leucite-free potassic and ultrapotassic rocks point to a silicate upper crust end-member. Lead isotopes plot well inside the field of island arcs, overlapping the values of pelagic sediments as well, but bifurcation between the samples north and south of Rome is observed. The main characteristic for the mantle source of Italian potassic and ultrapotassic magmas is the clear upper crustal signature acquired prior to partial melting through metasomatic agents released by the subducted slab. In addition, one lithospheric mantle source in the north and an asthenospheric mantle source, pointing to an HIMU reservoir, in the south were recognised. The chemical and isotopic differences observed between the northern and southern sectors of the magmatic region were possibly due to the presence of a carbonate-rich component in the crustal enriching agent in the south. One crustal component might have been generated by melting of silicate metasedimentary rocks or sediments from an ancient subducted slab. The second one might reflect the activity of mostly CO2-rich fluid released more recently by the incipient subduction of carbonate sedimentary rocks. Received February 16, 2000; revised version accepted September 6, 2001  相似文献   

15.
《Lithos》2007,93(1-2):17-38
A suite of schists, gneisses, migmatites, and biotite granitoids from the Puerto Edén Igneous and Metamorphic Complex (PEIMC) and biotite–hornblende granitoids of the South Patagonian batholith (southern Chile) has been studied. For that purpose, the chemistry of minerals and the bulk rock composition of major and trace elements including Rb–Sr and Sm–Nd isotopes were determined. Mineralogical observations and geothermobarometric calculations indicate high-temperature and low-pressure conditions (ca. 600–700 °C and 3 to 4.5 kbar) for an event of metamorphism and partial melting of metapelites in Late Jurassic times (previously determined by SHRIMP U–Pb zircon ages). Structures in schists, gneisses, migmatites and mylonites indicate non-coaxial deformation flow during and after peak metamorphic and anatectic conditions. Andalusite schists and sillimanite gneisses yield initial 87Sr/86Sr ratios of up to 0.7134 and εNd150 values as low as − 7.6. Contemporaneous biotite granitoids and a coarse-grained orthogneiss have initial 87Sr/86Sr ratios between 0.7073 and 0.7089, and εNd150 values in the range − 7.6 to − 4.4. This indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source. Thus, a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids. The PEIMC is considered as a segment of an evolving kilometre-sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in Late Jurassic times. A biotite–hornblende granodiorite and a muscovite–garnet leucogranite show initial 87Sr/86Sr ratios of 0.7048 and 0.7061, and εNd100 values of − 2.6 and − 1.8, respectively, and are thus probably related to Early Cretaceous magmas not involved in the anatexis of the metasedimentary rocks.  相似文献   

16.
We report major and trace element composition, Sr–Nd isotopic and seismological data for a picrite–basalt–rhyolite suite from the northern Tarim uplift (NTU), northwest China. The samples were recovered from 13 boreholes at depths between 5,166 and 6,333 m. The picritic samples have high MgO (14.5–16.8 wt%, volatiles included) enriched in incompatible element and have high 87Sr/86Sr and low 143Nd/144Nd isotopic ratios (εNd (t) = −5.3; Sri = 0.707), resembling the Karoo high-Ti picrites. All the basaltic samples are enriched in TiO2 (2.1–3.2 wt%, volatiles free), have high FeOt abundances (11.27–15.75 wt%, volatiles free), are enriched in incompatible elements and have high Sr and low Nd isotopic ratios (Sri = 0.7049–0.7065; εNd (t) = −4.1 to −0.4). High Nb/La ratios (0.91–1.34) of basalts attest that they are mantle-derived magma with negligible crustal contamination. The rhyolite samples can be subdivided into two coeval groups with overlapping U–Pb zircon ages between 291 ± 4 and 272 ± 2 Ma. Group 1 rhyolites are enriched in Nb and Ta, have similar Nb/La, Nb/U, and Sr–Nd isotopic compositions to the associated basalts, implying that they are formed by fractional crystallization of the basalts. Group 2 rhyolites are depleted in Nb and Ta, have low Nb/La ratios, and have very high Sr and low Nd isotopic ratios, implying that crustal materials have been extensively, if not exclusively, involved in their source. The picrite–basalt–rhyolite suite from the NTU, together with Permian volcanic rocks from elsewhere Tarim basin, constitute a Large Igneous Province (LIP) that is characterized by large areal extent, rapid eruption, OIB-type chemical composition, and eruption of high temperature picritic magma. The Early Permian magmatism, which covered an area >300,000 km2, is therefore named the Tarim Flood Basalt.  相似文献   

17.
Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.707069–0.707138, and εNd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive εHf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7041–0.7061, and εNd(t) = −3.1 to −4.3] and high contents of K2O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative εHf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive εHf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive εHf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.  相似文献   

18.
We report geochronological, geochemical and isotopic data for the Mesozoic Shangshuiquan granite from the northern margin of the North China craton. The granite is highly fractionated, with SiO2 > 74%. Occurrence of annitic biotite, high contents of alkalis (K2O + Na2O), Rb, Y, Nb and heavy rare earth elements, high FeOt/MgO, low contents of CaO, Al2O3, Ba, and Sr, and large negative Eu anomalies, makes it indistinguishable from typical A-type granites. A mantle-derived origin for the rocks of the granite is not favored because their high initial 87Sr/86Sr (≥0.706) and low εNd (t) (<−15) are completely different from either those of the lithospheric or asthenospheric mantle. In fact, their Sr–Nd isotopes fall within the range of Sr–Nd isotopic compositions of the Archean granulite terrains and are comparable to those of Mesozoic crustal-derived I-type granitoids in the region. Therefore, the Shangshuiquan granite is considered to be dominantly derived from partial melting of the ancient lower crust. Its parental magmas prove to be similar to I-type magmas and to have undergone extensive fractionation during its ascent. This is supported by the fact that some of the nearby Hannuoba feldspar-rich granulite xenoliths can be indeed regarded as the early cumulates in terms of their mineralogy, chemistry, Sr–Nd isotopes and zircon U–Pb ages and Hf isotopes. It is furthermore argued that some of highly fractionated granites worldwide, especially those with A-type characteristics and lacking close relationship with unfractionated rocks, may in fact be fractionated I-type granites. This suggestion can explain their close temporal and spatial associations as well as similar Sr–Nd isotopes with I-type granites. Our study also sheds new light on the petrogenesis of deep crustal xenoliths.  相似文献   

19.
Summary Two co-existing plutonic rocks (diorite and granodiorite) were studied from an intrusion of Variscan age in the Raztocna Valley – Nízke Tatry Mountains, Western Carpathians. Geochemical analyses of major and trace elements constrain a volcanic arc as emplacement environment and give the first hints of a mixture of two magmatic end-members: the so-called Prasivá granodiorite and the Raztocna diorite. The 87Sr/86Sr(0) ratios vary between 0.7075 and 0.7118, the ε Nd(0) values range from −1.4 to −5.0. Common Pb isotopes reveal a dominant crustal source with minor influences from a mantle and a lower crustal source. Modelling based on Sr and Nd isotope data and using three component mixing calculations indicates that mixing of 2/3 of upper mantle material with 1/3 upper crustal material can produce the isotopic composition of the Raztocna diorite. Very minor amounts of lower crust were incorporated in the diorite. For the Prasivá granodiorite, the mixing ratio of upper mantle and upper crust is similar, but a lower crustal reservoir contributed about 5–10% of the source material.  相似文献   

20.
Granitoids from the central Mawson Escarpment (southern Prince Charles Mountains, East Antarctica) range in age from Archaean to Early Ordovician. U–Pb dating of zircon from these rocks indicates that they were emplaced in three distinct pulses: at 3,519 ± 20, 2,123 ± 12 Ma and between 530 and 490 Ma. The Archaean rocks form a layer-parallel sheet of limited extent observed in the vicinity of Harbour Bluff. This granitoid is of tonalitic-trondhjemitic composition and has a Sr-undepleted, Y-depleted character typical of Archaean TTG suites. εNd and TDM values for these rocks are −2.1 and 3.8 Ga, respectively. Subsequent Palaeoproterozoic intrusions are of granitic composition (senso stricto) with pronounced negative Sr anomalies. These rocks have εNd and TDM values of −4.8 and 2.87 Ga, indicating that these rocks were probably melted from an appreciably younger source than that tapped by the Early Archaean orthogneiss. The remaining intrusions are of Early Cambrian to Ordovician age and were emplaced coincident with the major orogenic event observed in this region. Cambro–Ordovician intrusive activity included the emplacement of layer-parallel pre-deformational granite sheets at approximately 530 Ma, and the intrusion of cross cutting post-tectonic granitic and pegmatitic dykes at ca. 490 Ma. These intrusive events bracket middle- to upper-amphibolite facies deformation and metamorphism, the age of which is constrained to ca. 510 Ma—the age obtained from a syn-tectonic leucogneiss. Nd–Sr isotope data from the more felsic Cambro–Ordovican intrusions (SiO2 > 70 wt%), represented by the post-tectonic granite and pegmatite dykes, suggest these rocks were derived from Late Archaean or Palaeoproterozoic continental crust (TDM ∼ 3.5–2.3 Ga, εNd ∼ −21.8 to −25.9) not dissimilar to that tapped by the Early Proterozoic intrusions. In contrast, the compositionally more intermediate rocks (SiO2 < 65 wt%), represented by the metaluminous pre-tectonic Turk orthogneiss, appear to have melted from a notably younger lithospheric or depleted mantle source (TDM = 1.91 Ga, εNd ∼ −14.5). The Turk orthogneiss additionally shows isotopic (low 143Nd/144Nd and low 87Sr/86Sr) and geochemical (high Sr/Y) similarities to magmas generated at modern plate boundaries—the first time such a signature has been identified for Cambrian intrusive rocks in this sector of East Antarctica. These data demonstrate that: (1) the intrusive history of the Lambert Complex differs from that observed in the adjacent tectonic provinces exposed to the north and the south and (2) the geochemical characteristics of the most mafic of the known Cambrian intrusions are supportive of the notion that Cambrian orogenesis occurred at a plate boundary. This leads to the conclusion that the discrete tectonic provinces observed in the southern Prince Charles Mountains were likely juxtaposed as a result of Early Cambrian tectonism.  相似文献   

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