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1.
Mesozoic and Cenozoic volcanic rocks are widely distributed in the circum-Pacific area of eastern China. These rocks have long been genetically linked to westward subduction of the paleo-Pacific oceanic plate to the eastern Asia continent[1,2]. Research in re-cent years[3―6] has attained conclusions that a simple paleo-Pacific subduction model does not work well in interpreting all the volcanic rocks in eastern China, although some of them could be attributed to circum-Pacific interaction …  相似文献   

2.
Alkali basalts and nephelinites from the southern end of the East African Rift (EAR) in northern Tanzania have incompatible trace element compositions that are similar to those of ocean island basalts (OIB). They define a considerable range of Sr, Nd and Pb isotopic compositions (87Sr/86Sr= 0.7035−0.7058,εNd = −5to+3, and206Pb/204Pb= 17.5−21.3), each of which partially overlaps the range found in OIB. However, they occupy a unique position in combined Nd, Sr and Pb isotopic compositional space. Nearly all of the lavas have radiogenic Pb, similar to HIMU with high time-integrated238U/204Pb coupled with unradiogenic Nd (+2 to −5) and radiogenic Sr (>0.704), similar to EMI. This combination has not been observed in OIB and provides evidence that these magmas predominantly acquired their Sr, Nd and Pb in the subcontinental lithospheric mantle rather than in the convecting asthenosphere. These data contrast with compositions for lavas from farther north in the EAR. The Pb isotopic compositions of basalts along the EAR are increasingly radiogenic from north to south, indicating a fundamental change to sources with higher time-integratedU/Pb, closer to the older cratons in the south. An ancient underplated OIB melt component, isolated for about 2 Ga as enriched lithospheric mantle and then remelted, could generate both the trace element and isotopic data measured in the Tanzanian samples. Whereas the radiogenic Pb in Tanzanian lavas requires a source with high time-integratedU/Pb, most continental basalts that are thought to have interacted with the continental lithospheric mantle have unradiogenic Pb, requiring a source with a history of lowU/Pb. Such lowU/Pb is readily accomplished with the addition of subduction-derived components, since the lower averageU/Pb of arc basalts (0.15) relative to OIB (0.36) probably reflects addition of Pb from subducted oceanic crust. If the subcontinental lithosphere is normally characterized by low time-integratedU/Pb it would appear that subduction magmatism is more important than OIB additions in supplying the Pb inventory of the lithospheric mantle. However,U/Pb ratios of xenoliths derived from the continental lithospheric mantle suggest that both processes may be important. This apparent discrepancy could be because xenoliths are not volumetrically representative of the subcontinental lithospheric mantle, or, more likely, that continental lithospheric mantle components in basalts are normally only identified as such when the isotopic ratios are dissimilar from MORB or OIB. Lithospheric enrichment from subaccreted OIB components appears to be more significant than generally recognized.  相似文献   

3.
Ar–Ar dating, major and trace element analyses, and Sr–Nd–Pb isotope results of two groups of Lower Cretaceous (erupted at 126 and 119 Ma, respectively) intermediate–felsic lava from the northeastern North China Block (NCB) suggest their derivation from melting of mixtures between the heterogeneous lower crust and underplated basalts. Both groups exhibit high‐K calc‐alkaline to shoshonitic affinities, characterized by light rare earth element (LREE) and large ion lithophile element (LILE) enrichment and variable high field strength element (HFSE, e.g. Nb, Ta and Ti) depletion, and moderately radiogenic Sr and unradiogenic Nd and Pb isotopic compositions. Compared with Group 2, Group 1 rocks have relatively higher K2O and Al2O3/(CaO + K2O + Na2O) in molar ratio, higher HFSE concentrations and lower Nb/Ta ratios, and higher Sr–Nd–Pb isotope ratios. Group 1 rocks were derived from a mixture of an enriched mantle‐derived magma and a lower crust that has developed radiogenic Sr and unradiogenic Nd and Pb isotopic compositions, whereas the Group 2 magmas were melts of another mixture between the same mantle‐derived component and another type of lower crust having even lower Sr, Nd, and Pb isotopic ratios. Shift in source region from Group 1 to Group 2 coincided with a change in melting conditions: hydrous melting of both the underplated basalt and the lower crust produced the earlier high‐Nb and low‐Nb/Ta melts with little or no residual Ti‐rich phases; while the younger low‐Nb and high‐Nb/Ta magmas were melted under a water‐deficient system, in which Ti‐rich phases were retained in the source. Generation of the two groups of intermediate–felsic volcanic rocks was genetically linked with the contemporaneous magma underplating event as a result of lithospheric thinning in the eastern NCB.  相似文献   

4.
143Nd/144Nd,87Sr/86Sr and trace element results are reported for volcanic and plutonic rocks of the Aleutian island arc. The Nd and Sr isotopic compositions plot within the mantle array with εNd values of from 6.5 to 9.1 and87Sr/86Sr ratios of from 0.70289 to 0.70342. Basalts have mildly enriched light REE abundances but essentially unfractionated heavy REE abundances, while andesites exhibit a greater degree of light to heavy REE fractionation. Both the basalts and andesites have significant large ion lithophile element to light rare earth element (LILE/LREE) enrichments. Variations in the isotopic compositions of Nd and Sr are not related to the spatial distribution of volcanoes in the arc, nor are they related to temporal differences. εNd and87Sr/86Sr do not correlate with major element compositions but do, however, correlate with certain LILE/LREE ratios (e.g. BaN/LaN). Plutonic rocks have isotropic and trace element characteristics identical to some of the volcanic rocks. Rocks that make up the tholeiitic, calc-alkaline and alkaline series in the Aleutians do not come from isotopically distinct sources, but do exhibit some differing LILE characteristics.Given these elemental and isotopic constraints it is shown that the Aleutian arc magmas could not have been derived directly from homogeneous MORB-type mantle, or fresh or altered MORB subducted beneath the arc. Mixtures of partially altered MORB with deep-sea sediment can in principle account for the isotopic characteristics and most of the observed LILE/LREE enrichments. However, some samples have exceedingly high LILE/LREE enrichments which cannot be accounted for by sediment contamination alone. For these samples a more complex scenario is considered whereby dehydration and partial melting of the subducted slab, containing less than 8% sediment, produces a LILE-enriched (relative to REE) metasomatic fluid which interacts with the overlying depleted mantle wedge. The isotopic and LILE characteristics of the mantle are extremely sensitive to metasomatism by small percentages of added fluid, whereas major elements are not substantially effected, Major element compositions of Aleutian magmas are dominantly controlled by the partial melting of this mantle and subsequent crystal fractionation; whereas isotopic and LILE characteristics are determined by localized mantle heterogeneities.  相似文献   

5.
Melt inclusions from four individual lava samples representing the HIMU (Mangaia Island), EMI (Pitcairn Island) and EMII (Tahaa Island) end member components, have heterogeneous Pb isotopic composition larger than that defined by the erupted lavas in each island. The broad linear trend in 207Pb/206Pb–208Pb/206Pb space produced by the melt inclusions from Mangaia, Tahaa and fPitcairn samples reproduces the entire trend defined by the Austral chain, the Society islands and the Pitcairn island and seamount groups. The inclusions preserve a record of melt composition of far greater isotopic diversity than that sampled in whole rock basalts. These results can be explained by mixing of a common depleted component with the HIMU, EMI and EMII lavas, respectively. We favor a model that considers the oceanic lithosphere to be that common component. We suggest that the Pb isotopic compositions of the melt inclusions reflect wall rock reaction of HIMU, EMI and EMII melts during their percolation through the oceanic lithosphere. Under these conditions, the localized rapid crystallization of olivine from primitive basalt near the reaction zone would allow the entrapment of melt inclusions with different isotopic composition.  相似文献   

6.
The Earth's mantle is chemically and isotopically heterogeneous, and a component of recycled oceanic crust is generally suspected in the convecting mantle [Hofmann and White, 1982. Mantle plumes from ancient oceanic crust. Earth Planet. Sci. Lett. 57, 421–436]. Indeed, the HIMU component (high µ = 238U/204Pb), one of four isotopically distinct end-members in the Earth's mantle, is generally attributed to relatively old (≥ 1–2 Ga) recycled oceanic crust in the form of eclogite/pyroxenite, e.g. [Zindler and Hart, 1986. Chemical geodynamics. Ann. Rev. Earth Planet. Sci. 14, 493–571]. Although the presence of the recycled component is generally supported by element and isotopic data, little is known about its physical state at mantle depths. Here we show that the concentrations of Ni, Mn and Ca in olivine from the Canarian shield stage lavas, which can be used to assess the physical nature of the source material (peridotite versus olivine-free pyroxenite) [Sobolev et al., 2007. The amount of recycled crust in sources of mantle-derived melts. Science 316, 412–417], correlate strongly with bulk rock Sr, Nd and Pb isotopic ratios. The most important result following from our data is that the enriched, HIMU-type (having higher 206Pb/204Pb than generally found in the other mantle end-members) signature of the Canarian hotspot magmas was not caused by a pyroxenite/eclogite constituent of the plume but appears to have been primarily hosted by peridotite. This implies that the old (older than ~ 1 Ga) ocean crust, which has more evolved radiogenic isotope compositions, was stirred into/reacted with the mantle so that there is not significant eclogite left, whereas younger recycled oceanic crust with depleted MORB isotopic signature (< 1 Ga) can be preserved as eclogite, which when melted can generate reaction pyroxenite.  相似文献   

7.
We have investigated the hypothesis that mantle Pb isotope ratios reflect continued extraction of Pb into the Earth's core over geologic time. The Pb, Sr and Nd isotopic compositions, and the abundance of siderophile and chalcophile elements (W, Mo and Pb) and incompatible lithophile elements have been determined for a suite of ocean island and mid-ocean ridge basalt samples. Over the observed range in Pb isotopic compositions for oceanic rocks, we found no systematic variation of siderophile or chalcophile element abundances relative to abundances of similarly incompatible, but lithophile, elements. The high sensitivity of theMo/Pr ratio to segregation of Fe-metal or S-rich metallic liquid (sulfide) and the observed constantMo/Pr ratio rules out the core formation model as an explanation for the Pb paradox. The mantle and crust have the sameMo/Pr and the sameW/Ba ratios, suggesting that these ratios reflect the ratio in the Earth's primitive mantle.

Our data also indicate that thePb/Ce ratio of the mantle is essentially constant, but the presentPb/Ce ratio in the mantle ( 0.036) is too low to represent the primitive value ( 0.1) derived from Pb isotope systematics. HigherPb/Ce ratios in the crust balance the lowPb/Ce of the mantle, and crust and mantle appear to sum to a reasonable terrestrialPb/Ce ratio. The constancy of thePb/Ce ratio in a wide variety of oceanic magma types from diverse mantle reservoirs indicates this ratio is not fractionated by magmatic processes. This suggests crust formation must have involved non-magmatic as well as magmatic processes. Hydrothermal activity at mid-ocean ridges may result in significant non-magmatic transport of Pb from mantle to crust and of U from crust to mantle, producing a higherU/Pb ratio in the mantle than in the total crust. We suggest that the lower crust is highly depleted in U and has unradiogenic Pb isotope ratios which balance the radiogenic Pb of upper crust and upper mantle. The differences between thePb/Ce ratio in sediments, this ratio in primitive mantle, and the observed ratio in oceanic basalts preclude both sediment recycling and mixing of primitive and depleted reservoirs from being important sources of chemical heterogeneities in the mantle.  相似文献   


8.
High 4He/3He ratios of 100 000 to 160 000 found at HIMU ocean islands (“high μ,” where μ is the U/Pb ratio) are usually attributed to the presence of recycled oceanic crust in the HIMU mantle source. However, significantly higher 4He/3He ratios are expected in recycled crust after residence in the mantle for periods greater than 1 Ga. In order to better understand the helium isotopic signatures in HIMU basalts, we have measured helium and neon isotopic compositions in a suite of geochemically well-characterized basalts from the Cook–Austral Islands. We observe 4He/3He ratios ranging from 56 000 to 141 000, suggesting the involvement of mantle reservoirs both more and less radiogenic than the mantle source for mid-ocean ridge basalts (MORBs). In addition, we find that the neon isotopic compositions of HIMU lavas extend from the MORB range to compositions less nucleogenic than MORBs. The Cook-Austral HIMU He–Ne isotopic compositions, along with Sr, Nd, Pb, and Os isotopic compositions, indicate that in addition to recycled crust, a relatively undegassed mantle end-member (e.g., FOZO) is involved in the genesis of these basalts. The association of relatively undegassed mantle material with recycled crust provides an explanation for the close geographical association between HIMU lavas and EM (enriched mantle)-type lavas from this island chain: EM-type signatures represent a higher mixing proportion of relatively undegassed mantle material. Mixing between recycled material and relatively undegassed mantle material may be a natural result of entrainment processes and convective stirring in deep mantle.  相似文献   

9.
Geochemical analyses of dikes, sills, and volcanic rocks of the Mesozoic Appalachian Tholeiite (MAT) Province of the easternmost United States provide evidence that continental tholeiites are derived from continental lithospheric mantle sources that are genetically and geochronologically related to the overlying continental crust. Nineteen olivine tholeiites and sixteen quartz tholeiites from the length of this province, associated in space and time with the last opening of the Atlantic, display significant isotopic heterogeneity: initial εNd = +3.8 to −5.7; initial 87Sr/86Sr= 0.7044−0.7072; 206Pb/204Pb= 17.49−19.14; 207Pb/204Pb= 15.55−15.65; 208Pb/204Pb= 37.24−39.11. In PbPb space, the MAT define a linear array displaced above the field for MORB and thus resemble oceanic basalts with DUPAL Pb isotopic traits. A regression of this array yields a secondary PbPb isochron age of ≈ 1000 Ma (μ1 = 8.26), similar to Sm/Nd isochrons from the southern half of the province and to the radiometric age of the Grenville crust underlying easternmost North America. The MAT exhibit significant trace element ratio heterogeneity (e.g., Sm/Nd= 0.226−0.327) and have trace element traits similar to convergent margin magmas [e.g., depletions of Nb and Ti relative to the rare earth elements on normalized trace element incompatibility diagrams, Ba/Nb ratios (19–75) that are significantly greater than those of MORB, and low TiO2 (0.39–0.69%)].Geochemical and geological considerations very strongly suggest that the MAT were not significantly contaminated during ascent through the continental crust. Further, isotope and trace element variations are not consistent with the involvement of contemporaneous MORB or OIB components. Rather, the materials that control the MAT incompatible element chemistry were derived from subcontinental lithospheric mantle. Thus: (1) the MAT/arc magma trace element similarities; (2) the PbPb and Sm/Nd isochron ages; and (3) the need for a method of introducing an ancient (> 2−3 Ga) Pb component into subcontinental mantle that cannot be much older than 1 Ga leads to a model whereby the MAT were generated by the melting of sediment-contaminated arc mantle that was incorporated into the continental lithosphere during arc activity preceding the Grenville Orogeny (≈ 1000 Ma).  相似文献   

10.
Middle Miocene to Quaternary lavas on Kunashir Island in the southern zone of the Kurile Arc were examined for major, trace, and Sr–Nd–Pb isotope compositions. The lavas range from basalt through to rhyolite and the mafic lavas show typical oceanic island arc signatures without significant crustal or sub-continental lithosphere contamination. The lavas exhibit across-arc variation, with increasingly greater fluid-immobile incompatible element contents from the volcanic front to the rear-arc; this pattern, however, does not apply to some other incompatible elements such as B, Sb, and halogens. All Sr–Nd–Pb isotope compositions reflect a depleted source with Indian Ocean mantle domain characteristics. The Nd and Pb isotope ratios are radiogenic in the volcanic front, whereas Sr isotope ratios are less radiogenic. These Nd isotope ratios covary with incompatible element ratios such as Th/Nd and Nb/Zr, indicating involvement of a slab-derived sediment component by addition of melt or supercritical fluid capable of mobilizing these high field-strength elements and rare earth elements from the slab. Fluid mobile elements, such as Ba, are also elevated in all basalt suites, suggesting involvement of slab fluid derived from altered oceanic crust. The Kurile Arc lavas are thus affected both by slab sediment and altered basaltic crust components. This magma plumbing system has been continuously active from the Middle Miocene to the present.  相似文献   

11.
Huronian basalts from central Ontario, Canada, dated at about 2450 Ma and associated with an early rifting episode, are classified as siliceous, low-TiNb tholeiites. They display strong enrichment in large-ion lithophile (LILE) and light rare earth (LREE) elements compared to modern oceanic lavas. The tectonic setting and geochemistry resemble Mesozoic rift-related low-Ti flood basalts, including the Ferrar Group of Antarctica, and the Parana and equivalent Etendeka volcanics of south Brazil and Namibia, respectively. High LILE/LREE ratios are also similar to subduction-related island arc tholeiites, and it is suggested that enrichment of the Huronian lithospheric mantle source occurred through ancient subduction of crustal material, probably during formation and consolidation of the Archean continental crust.Melting models suggest that Huronian subcontinental mantle source compositions, derived from least contaminated, aphyric, mafic end-members, had already undergone a complex evolution, including withdrawal of Archean basalts and hydrous enrichment in incompatible components. Despite several subsequent melting episodes and a second, probably magmatic, enrichment event, however, many aspects of the Huronian source signature were preserved, and appeared in later basaltic products of this mantle mass. Keweenawan volcanics, for example, dated at about 1100 Ma, preserve low P, Zr, Ti and HREE abundances.  相似文献   

12.
Quanshu Yan  Xuefa Shi 《Island Arc》2014,23(3):221-235
Major element and trace element compositions, and Sr, Nd and Pb isotopic compositions for postcollisional granites from the Laoshan granitic complex, in the eastern side of the Triassic suture between the South China and North China tectonic blocks were determined. The granites are alkaline, A‐type and can be further classified as A1 granites. The trace element composition of these granites is transitional between those of oceanic island basalt and enriched mid‐oceanic ridge basalt, with depletions in Ba, Sr, P, and Ti that can be ascribed to mineral fractionation and enrichments in Cs, Rb, Th and U possibly resulted from the involvement of slab fluids. The isotopic signature of Laoshan granites represent a mixture between an enriched mantle type 1 (EMI)‐like end‐member and lower continental crust (LCC). We propose that the magmas that formed the Laoshan A1 granites are a mixture between those derived from the EMI‐like delaminated eclogitic rocks (subsequently enriched by fluids released from Mesozoic Pacific subducted slab) and those derived from the LCC, which consists of granulites or metamorphic residues from the prior generation of I‐type granites in the region. The mixed magmas then experienced a strongly alkali feldspar‐dominated fractionation prior to their emplacements as A‐type granites in the Laoshan granitic complex.  相似文献   

13.
Analyses for major and trace elements, including REE, and Sr, Nd and Pb isotopes are reported from a suite of Siluro-Devonian lavas from Fife, Scotland. The rocks form part of a major calc-alkaline igneous province developed on the Scottish continental margin above a WNW-dipping subduction zone. Within the small area (ca. 15 km2) considered, rock types range from primitive basalts and andesites (high Mg, Ni and Cr) to lavas more typical of modern calc-alkaline suites with less than 30 ppm Ni and Cr. There is a marked silica gap between these rocks (< 62%) and the rare rhyolites (> 74%), yet the latter can be generated by fractional crystallization from the more mafic lavas. In contrast, variation in incompatible element concentrations and ratios in the mafic lavas can not be generated by fractional crystallization processes. Increasing SiO2 is accompanied by increasing Rb, K, Pb, U and Ba relative to Sr and high field strength elements, increasing LREE enrichment and increasing Sr calculated at 410 Ma, and by decreasing HREE, Eu/Eu*, Sm/Nd and Nd (410). Nd and Sr are roughly anticorrelated and have more radiogenic compositions than the mantle array, in common with data reported elsewhere from this part of the arc. The correlation extrapolates up to cross the mantle array within the composition field of the contemporary MORB source, and extrapolates down towards the probable compositional range of Lower Palaeozoic greywackes, which may form the uppermost 8 km of the crust, or may be supplied to the source by subduction. One sample, however, lies within the mantle array, and closely resembles lavas from northwestern parts of the arc, where a mantle source with mild time-integrated Rb/Sr and LREE enrichment has been inferred. The lavas have relatively high initial 207Pb/204Pb for their 206Pb/204Pb, a feature which has been interpreted elsewhere as the result of incorporation of a sediment component into arc magmas. The systematic changes with increasing SiO2 in isotopic and chemical parameters can be explained by mixing of a greywacke-derived component with depleted mantle. The various possible mixing mechanisms are discussed, and it is considered most likely that mixing occurred in the mantle source through greywacke subduction. The bulk of the Rb, K, Ba and Pb in the lavas is probably recycled from the crust, whereas less than some 40% of the Sr and Nd is recycled. The calc-alkaline chemical trends are solely a function of mixing with the sediment component.  相似文献   

14.
Late Early Paleozoic mafic-ultramafic dykes and volcanic rocks from the South Qinling belt are char- acterized by εNd( t ) = 3.28― 5.02, (87Sr/86Sr)i= 0.70341― 0.70555, (206Pb/204Pb)i = 17.256― 18.993, (207Pb/204Pb)i= 15.505―15.642, (208Pb/204Pb)i=37.125―38.968, ?8/4=21.18―774.43, ?7/4=8.11―18.82. These charac- teristics suggest that they derived from a Middle Neoproterozoic mantle with isotopic compositions of mixed HIMU, EMII and minor EMI components. We interpret that these rocks were melting products of depleted mantle modified by subducted ancient oceanic crust and continental margin sediments along the northern margin of Yangtze block during Early Neoproterozoic.  相似文献   

15.
himu, em i andem ii are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteristichimu signature is found in Tubuai, Mangaia and Rurutu,em i is present in the source of Rarotonga and Pitcairn volcanics andem ii dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands.We suggest that the three components are not independent but are physically related in the mantle. Thehimu component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction.em i andem ii are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust.The trace element pattern and the isotopic composition ofhimu lavas can be quantitatively modelled using a mixture of 25% old recycledmorb crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of theU/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those ofem i andem ii are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlyingmorb oceanic crust. Pelagic sediments (μ 5 andκ 6) account for the Pb isotopic composition ofem i whereas terrigenous sediments (μ 10 andκ 4.5) evolve towards theem ii end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of thehimu component. This, together with the low probability of isolating oceanic crust in the mantle for 2 Ga, explains why the extremehimu composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.  相似文献   

16.
Mariana Trough basalt (MTB) glasses from zones of of active seafloor volcanism have incompatible trace element compositions which are intermediate between normal MORB and basaltic rocks from the active northern Mariana Island Arc (MIAB). The chemical variation is observed in trace elemental abundances and ratios such as LIL/LIL and LIL/HFS. MTB glasses with high LIL/HFS and Ba/Sm ratios, and low K/Rb, K/Ba, and Sm/Nd ratios have more enriched Sr and Nd isotopic compositions.Comparison of the SrNd isotopic compositions of MTB and MIAB suggests that the source region within the mantle wedge is heterogeneous. The diverse trace element and isotopic compositions of MTB glasses both within and between dredge sites near 18°N imply small-scale source heterogeneity. Correlation between Sm/Nd and143Nd/144Nd of the MTB glasses is interpreted as due to recent binary mixing, rather than closed system evolution of a common homogeneous source. Mixing of melts at or near the source region between a mantle component with long-term LREE and LIL element depletion (MORB-like) and a relatively enriched component with lower integrated143Nd/144Nd (Arc-like) is suggested by trends of the MTB data on ratio-ratio, ratio-element and element-element plots.  相似文献   

17.
SEONG HEE  CHOI  JONG IK  LEE  CHUNG-HWA  PARK  JACQUES  MOUTTE 《Island Arc》2002,11(4):221-235
Abstract   Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite-normalized rare earth element, from light rare earth element (LREE)-depleted through spoon-shaped to LREE-enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid-oceanic ridge basalt (MORB)-like to near bulk-earth estimates along the MORB–oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb)N ratio and Sr-Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long-term growth and enrichment history.  相似文献   

18.
Combined elemental, and Sr and Nd isotopic data are presented for Mesozoic dolerite dikes of Liberia (Africa) which are related to the initial stage of opening of the Atlantic Ocean.The large scatter of both trace element and isotopic data allows the identification of five groups of dolerites which cannot be related to each other by simple processes of mineral fractionation from a common source. On the contrary, the observed chemical and isotopic variation within some dolerites (Groups I and II) may result either from variable degrees of melting of an isotopically heterogeneous source or mixing between enriched and depleted oceanic type mantle. For the other dolerites (Groups III–V) mixing with a third mantle source with more radiogenic Sr and with element ratios characteristic of subduction environments is suggested. This third source is probably the subcontinental lithospheric mantle.Finally, no significant modification by interaction with continental crust is apparent in most of the analyzed samples.  相似文献   

19.
Abstract The Archean to Paleo–Proterozoic Bundelkhand massif basement of the central Indian shield has been dissected by numerous mafic dykes of Proterozoic age. These dykes are low‐Ti tholeiites, ranging in composition from subalkaline basalt through basaltic‐andesite to dacite. They are enriched in light rare earth elements (LREE), large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE: Nb, P and Ti). Negative Sr anomaly is conspicuous. Nb/La ratios of the dykes are much lower compared with the primitive mantle, not much different from the average crustal values, but quite similar to those of continental and subduction related basaltic rocks. Bulk contamination of the mantle derived magma by crustal material is inadequate to explain the observed geochemical characteristics; instead contamination of the mantle/lithospheric source(s) via subduction of sediment is a better proposition. Thus, in addition to generating juvenile crust along the former island arcs, subduction processes appear to have influence on the development of enriched mantle/lithospheric source(s). The Bundelkhand massif basement is inferred to represent subduction related juvenile crust, that experienced lithospheric extension and rifting possibly in response to mantle plume activities. The latter probably supplied the required heat, material (fluids) and extensional environment to trigger melting in the refractory lithospheric source(s) and emplacement of the mafic dykes. Proterozoic mafic magmatic rocks from Bundelkhand, Aravalli, Singhbhum and Bastar regions of the Indian shield and those from the Garhwal region of the Lesser Himalaya display remarkably similar enriched incompatible trace elements characteristics, although limited chemical variations are observed in all these rocks. This may indicate the existence of a large magmatic province, different parts of which might have experienced similar petrogenetic processes and were probably derived from mantle/lithospheric source(s) with similar trace element characteristics. The minor, less enriched to depleted components of the Jharol Group of the Aravalli terrane and those from the Singhbhum terrane may represent protracted phases of rifting, that probably caused thinning and mobilization of the lithosphere, facilitating the eruption/emplacement of the asthenospheric melts (with N‐ to T‐types mid‐oceanic ridge basalts signatures) and deposition of deep water facies sediments in the younger developing oceanic basins. In contrast, Bundelkhand region did not experience such protracted rifting, although dyke swarms were emplaced and shallow water Bijawar Group and Vindhyan Supergroup sediments were deposited in continental rift basins. All these discrete Proterozoic terranes appear to have experienced similar petrogenetic processes, tectonomagmatic and possibly temporal evolution involving subduction processes, influencing the lithospheric source characteristics, followed by probably mantle plume induced ensialic rifting through to the development of oceanic basins in the Indian shield regions and their extension in the Lesser Himalaya.  相似文献   

20.
The Cenozoic basaltic province of the Vogelsberg area (central Germany) is mainly composed of intercalated olivine to quartz tholeiites and near-primary nephelinites to basanites. The inferred mantle source for the alkaline and tholeiitic rocks is asthenospheric metasomatized garnet peridotite containing some amphibole as the main hydrous phase. Trace element modelling indicates 2 to 3% partial melting for the alkaline rocks and 5 to 7% partial melting for the olivine tholeiites. Incompatible trace element abundances and ratios as well as Nd and Sr radiogenic isotope compositions lie between plume compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts (OIB) and the Central European Volcanic Province elsewhere. The mafic olivine tholeiites have similar Ba/Nb, Ba/La and Nd–Sr isotope ratios to the alkaline rocks indicating derivation of both magma types from chemically comparable mantle sources. However, Zr/Nb ratios are slightly higher in olivine tholeiites than in basanites reflecting some fractionation of Zr relative to Nb during partial melting. Quartz tholeiites have higher Ba/Nb, Zr/Nb, La/Nb, but lower Ce/Pb ratios and lower Nd isotope compositions than the alkaline rocks which can be explained by interaction of the basaltic melt with lower (granulite facies) crustal material or partial melts thereof during stagnation within the lower crust. It appears most likely that upwelling of hot, asthenospheric material results in the generation of primitive alkaline rocks at the base of the lithosphere at depths of 75–90 km. Lithospheric extension together with minor plume activity and probably lower lithosphere erosion induced melting of shallower heterogenous upper mantle generating a spectrum of olivine tholeiitic melts. These olivine tholeiitic rocks evolved via crystal fractionation and probably limited contamination to quartz tholeiites.  相似文献   

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