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1.
New xenolith occurrences in the Cenozoic alkali basalts of north-eastern Brazil have been studied in order to constrain the possible imprint on the continental mantle lithosphere of its passage over the Fernando de Noronha plume and the regional mantle processes. Texturally, the lherzolite and harzburgite xenoliths define three groups: group 1, porphyroclastic; group 2, protogranular; group 3, transitional between groups 1 and 2. Equilibrium temperatures are highest for group 1 and lowest for group 2. Clinopyroxenes from group 1 peridotites have Primitive Mantle (PM)-normalised REE patterns varying from L-MREE-enriched convex-upward, typical of phases in equilibrium with alkaline melts, to LREE-enriched, spoon-shaped, to LREE-enriched, steadily fractionated in a wehrlite. Group 2 clinopyroxenes show patterns slightly depleted in LREE to nearly flat. The M-HREE are at 3–5 ×PM concentration level, as typical in fertile lithospheric lherzolites. Most of group 3 clinopyroxenes show LREE-depleted patterns similar to the group 2 ones, but in two samples the clinopyroxenes are characterised by LREE-enriched, spoon-shaped profiles. Sr and Nd isotopes of the group 1 clinopyroxenes form an array between DM and EMI-like components, both of them are also present in the host basalts. Melts estimated to be in equilibrium with the group 1 clinopyroxenes having L-MREE-enriched, convex-upward patterns are similar to the Cenozoic alkaline magmas. The groups 2 and 3 clinopyroxenes define two distinct compositional fields at higher 143Nd/144Nd values, correlated with their LREE composition. The isotopes of the groups 2 and 3 LREE-depleted clinopyroxenes form an array from DM towards the isotopic composition of Mesozoic tholeiitic basalts from north-eastern Brazil. Melts in equilibrium with these clinopyroxenes are similar to these basalts, thus suggesting that such xenoliths record geochemical imprint from older melt-related processes.

The LREE-enriched spoon-shaped group 3 clinopyroxenes are characterised by the highest 143Nd/144Nd values at any given 87Sr/86Sr composition. These results are interpreted in terms of a lithospheric mantle section which underwent thermo-chemical and mechanical erosion by infiltration of asthenospheric alkali basalts having EMI-like isotope characteristics during Cenozoic time. At that time, the lithospheric mantle consisted of fertile lherzolites and harzburgites recording the geochemical imprint of Mesozoic mantle processes. The onset of the interaction between lithospheric peridotites and alkaline melts was characterised by the porous flow percolation of small melt volumes that induced chromatographic enrichments in highly incompatible elements and the isotope signature of the spoon-shaped, group 3 clinopyroxenes. Group 1 peridotites represent the base of the lithospheric column eroded by the ascending alkaline melts, whereas the group 2 documents the shallower lithospheric section, with group 3 being the transition. The similarity of processes and isotope components in the protogranular xenoliths from Fernando de Noronha area and north-eastern Brazil supports the hypothesis that the lithosphere beneath Fernando de Noronha is a detached portion of the continental one. Furthermore, the similarity in terms of textural and geochemical features documented by the mantle samples coming from the two different regions seems to confirm the interference of the two regions with the same plume.  相似文献   


2.
The crystal chemistry of six clinopyroxenes enclosed in protogranular spinel-peridotite mantle xenoliths from Lake Nji (Cameroon, W Africa) was studied by means of single-crystal X-ray diffraction and electron microprobe. These spinel-peridotite xenoliths are characterised by clinopyroxene contents distinctly higher than those reported by Lee et al. (1996) for spinel-peridotite xenoliths of the same region (19–11% and 15–8% respectively) and by high clinopyroxene/orthopyroxene ratios: 0.7–2.7 (present study) and about 0.4 (Lee et al. 1996). The clinopyroxene crystal chemistry indicates that the xenoliths show the compositional features of a mantle source contaminated by small-volume melts, responsible for the high clinopyroxene content. The protogranular texture of the Cameroon xenoliths (upper portion of lithospheric mantle) testifies that the spinel peridotite and the associated small-volume melts completely re-equilibrated at the spinel-peridotite facies. This is also supported by the petrological and geochemical data of Lee et al. (1996) on the Etinde–Biu Plateau spinel-peridotite xenoliths, which underwent Late Proterozoic–Early Paleozoic enrichment in incompatible trace elements. Received: 6 October 1999 / Accepted: 27 March 2000  相似文献   

3.
Twenty spinel peridotite xenoliths from Pliocene alkali basaltic tuffs and lavas of the western Pannonian Basin (Hungary) have been analysed for bulk rock major and trace elements, electron probe mineral compositions, and REE and Sr, Nd isotopes on separated and leached clinopyroxenes. The xenoliths are texturally diverse, including protogranular, porphyroclastic, equigranular and poikilitic textures which can generally be correlated with geochemical features. Protogranular xenoliths are relatively undepleted in Ca, Al, Ti and Na, whereas poikilitic xenoliths are more refractory. LREE-depleted patterns. and MORB-like Nd and Sr values are associated with protogranular peridotites. In contrast, xenoliths with complex textures are generally LREE-enriched. Much of the isotopic variation in the suite (Sr=–20.4 to +10.4, +Nd=+1.8 to +13.7) can be related to interaction between protogranular mantle and melts resembling the host alkali basalts, but a third (high Sr) component may be due to Miocene subduction beneath the region.  相似文献   

4.
Anhydrous spinel peridotite xenoliths from the Ray Pic Quaternary alkali basalt volcano (French Massif Central) show a wide range of mineralogical and geochemical compositions, reflecting significant heterogeneities in the shallow sub-continental lithospheric mantle. Variations in modal mineralogy, mineral chem istry, REE patterns and radiogenic isotope data suggest that depletion by partial melting and enrichment by cryptic metasomatism were the major mantle processes which caused the heterogeneity. The lithospheric mantle beneath Ray Pic contains two contrasting types of peridotite: (i) lherzolites with LREE-depleted compositions, high 143Nd/144Nd, low 87Sr/86Sr and unradiogenic Pb isotope ratios; (ii) lherzolites, harzburgites and a wehrlite with LREE-enriched patterns, low 143Nd/144Nd, high 87Sr/86Sr and radiogenic Pb isotope ratios. The former closely resemble depleted MORB-source mantle. The latter are related to enrichment by recent infiltration of small degree partial melts or fluids from the asthenospheric mantle, possibly related to the “low velocity component” observed by Hoernle et al. (1995) in European Neogene alkaline magmas. Thus, the Ray Pic peridotite xenoliths represent interaction between asthenospheric mantle-derived melts/fluids and depleted lithospheric mantle. This is probably linked to the upwelling mantle plume imaged beneath the Massif Central (Granet et al. 1995). A relationship between textural deformation, equilibration temperature and geochemistry of the xenoliths suggests that the hotter (> 900 °C) undeformed regions are LREE-enriched and tend to have more enriched isotope ratios, whereas the cooler (< 900 °C) regions have undergone more deformation and are more depleted both in LREE and in isotope compositions. Received: 27 July 1996 / Accepted: 25 November 1996  相似文献   

5.
Spinel peridotite xenoliths from the Atsagin-Dush volcanic centre, SE Mongolia range from fertile lherzolites to clinopyroxene(cpx)-bearing harzburgites. The cpx-poor peridotites typically contain interstitial fine-grained material and silicate glass and abundant fluid inclusions in minerals, some have large vesicular melt pockets that apparently formed after primary clinopyroxene and spinel. No volatile-bearing minerals (amphibole, phlogopite, apatite, carbonate) have been found in any of the xenoliths. Fifteen peridotite xenoliths have been analysed for major and trace elements; whole-rock Sr isotope compositions and O isotope composition of all minerals were determined for 13 xenoliths. Trace element composition and Sr-Nd isotope compositions were also determined in 11 clinopyroxene and melt pocket separates. Regular variations of major and moderately incompatible trace elements (e.g. heavy-rare-earth elements) in the peridotite series are consistent with its formation as a result of variable degrees of melt extraction from a fertile lherzolite protolith. The Nd isotope compositions of LREE (light-rare-earth elements)-depleted clinopyroxenes indicate an old (≥ 1 billion years) depletion event. Clinopyroxene-rich lherzolites are commonly depleted in LREE and other incompatible trace elements whereas cpx-poor peridotites show metasomatic enrichment that can be related to the abundance of fine-grained interstitial material, glass and fluid inclusions in minerals. The absence of hydrous minerals, ubiquitous CO2-rich microinclusions in the enriched samples and negative anomalies of Nb, Hf, Zr, and Ti in primitive mantle-normalized trace element patterns of whole rocks and clinopyroxenes indicate that carbonate melts may have been responsible for the metasomatic enrichment. Low Cu and S contents and high δ34S values in whole-rock peridotites could be explained by interaction with oxidized fluids that may have been derived from subducted oceanic crust. The Sr-Nd isotope compositions of LREE-depleted clinopyroxenes plot either in the MORB (mid-ocean-ridge basalt) field or to the right of the mantle array, the latter may be due to enrichment in radiogenic Sr. The LREE-enriched clinopyroxenes and melt pockets plot in the ocean island-basalt field and have Sr-Nd isotope signatures consistent with derivation from a mixture of the DMM (depleted MORB mantle) and EM (enriched mantle) II sources. Received: 18 January 1996 / Accepted: 23 August 1996  相似文献   

6.
An alkali basalt near Glen Innes, northeastern New South Wales, contains a suite of Cr-diopside group ultramafic xenoliths which includes some spinel peridotites but which is dominated by a diverse spinel pyroxenite assemblage. Pyroxenite xenoliths range from subcalcic clinopyroxenites (composed largely of unmixed prismatic subcalcic clinopyroxene megacrystals and lesser orthopyroxene megacrystals) to equant mosaic textured websterites (orthopyroxene and Ca-rich clinopyroxene ± spinel). Rare orthopyroxenite xenoliths also occur. The pyroxenite xenoliths are characterised by high 100Mg/(Mg + Fe2+) ratios (M˜ 90) and low concentrations of Ti, K, P, La, Ce and Zr. The websterites are mineralogically and chemically similar to many spinel pyroxenites occurring as layers or dykes in peridotite massifs such as those at Ronda in southern Spain and at Ariège (French Pyrénées). T / P estimates indicate crystallization temperatures of 1250–1350 °C for subcalcic clinopyroxene-orthopyroxene megacrystal pairs and 900–1000 °C for the equilibrated mosaic textured websterites and associated peridotites at pressures of 9–13 kbar. Subcalcic clinopyroxene megacrystals, websterites and orthopyroxenites have LREE-depleted chondrite-normalised REE abundances with (La/Yb)CN < 1 and their convex-upwards REE patterns are typical of subcalcic clinopyroxene-dominated cumulates. The pyroxenites are not residua from partially melted pyroxenite layers or dykes in mantle peridotites nor are they completely crystallized protobasaltic or protopicritic magmas. They are interpreted as high-pressure crystal segregations from basaltic magmas (probably mildly alkaline or transitional) flowing within narrow mantle conduits (the flow crystallization model of Irving, 1980). The parental magma(s) was Ti-poor (0.6–0.7% TiO2) and relatively Mg-rich (M˜ 74 − 70). Pyroxenite genesis was a two-stage process involving crystallization of tschermakitic subcalcic clinopyroxenes and orthopyroxenes  ±spinel as liquidus or near-liquidus phases at 1250–1350 °C and 9–13 kbar to yield “primary” subcalcic clinopyroxenites which then re-equilibrated at 900–1000 °C and similar pressures to produce the mosaic textured “secondary” websterites. The pyroxenites show a wide range of 143Nd/144Nd and 87Sr/86Sr values (0.513298–0.512473 and 0.702689–0.704659, respectively). Their isotopic ratios appear to have been variably modified by exchange with adjacent mantle peridotites or migrating basaltic melts. Received: 11 December 1995 / Accepted: 3 December 1996  相似文献   

7.
High temperature (1150–1250 °C), coarse-grained olivine-bearing clinopyroxenites occur in the ash-tuffs of the Dreiser Weiher maar-type volcano (West Eifel, Germany) as discrete xenoliths or as 1-5-cm-broad veins crosscutting anhydrous spinel peridotite host xenoliths. The clinopyroxenes (cpx) of these xenoliths have been analysed for trace element and Nd-Sr isotope compositions in order to document intra-suite variations and to constrain the processes involved in the formation of heterogeneities within a relatively well defined upper mantle section beneath the West Eifel. The patterns formed by cpx from the pyroxenites on multi-element diagrams are subparallel and convex-upward, showing troughs for high-field-strength elements (Nb, Zr, Hf, Ti) and Sr. Trace element modelling indicates that these pyroxenites represent high pressure precipitates of magmas that are more primitive or similar in compositions to the most undifferentiated Cenozoic alkali basaltic lavas from the West Eifel. The cpx cover the whole spectrum of Nd-Sr isotope compositions shown by the primitive lavas from the entire West Eifel volcanic field suggesting isotopic heterogeneity on the scale of an individual volcanic centre. Due to incomplete re-equilibration between the vein melts and the peridotitic wall rocks, cpx of the host peridotites of the composite xenoliths (that belong to the 1b-group of Stosch and Seck, 1980) have in some cases retained relics of a pre-vein host composition. The relic cpx range from LREE-depleted to LREE-enriched with isotope signatures indicating a time-integrated higher enrichment (lower 143Nd/144Nd and higher 87Sr/86Sr) than the cpx of the corresponding veins. The trace element and isotope compositions of the xenoliths support the perception that magmas generated from sub-lithospheric mantle sources beneath the West Eifel formed a system of narrow dike networks and differentiated during their ascent through the lithosphere (Duda and Schmincke 1985). The data provide evidence that: (1) melts parental to the Dreiser Weiher pyroxenites are genetically related to the young alkali basaltic volcanics; (2) these melts can be derived from distinct domains of the mantle beneath Dreiser Weiher ranging in Sr-Nd isotope signatures from HIMU-like to Bulk-Silicate-Earth values; (3) the enrichment process associated with the upwards migration of these magmas was spatially limited to a cm-scale in the case of the studied composite xenoliths; (4) parts of the Dreiser Weiher lithosphere have experienced an enrichment prior to the vein interaction by a metasomatic agent that is isotopically unrelated to the primitive West Eifel lavas. Received: 25 August 1997 / Accepted: 25 November 1997  相似文献   

8.
Upper mantle xenoliths from the southern Rio Grande rift axis (Potrillo and Elephant Butte) and flank (Adam’s Diggings) have been investigated to determine chemical depletion and enrichment processes. The variation of modal, whole rock, and mineral compositions reflect melt extraction. Fractional melting is the likely process. Fractional melting calculations show that most spinel peridotites from rift axis locations have undergone <5% melting versus 7–14% melting for xenoliths from the rift shoulder, although the total range of fractional melting overlaps at all three locations. In the rift axis, deformed (equigranular and porphyroclastic texture) spinel peridotites are generally characterized by significantly less fractional melting (2–5%) than undeformed (protogranular) xenoliths (up to 16%). This difference may reflect undeformed xenoliths being derived from greater depths and higher temperatures than deformed rocks. Spinel peridotites from the axis and shoulder of the Rio Grande rift have undergone mantle metasomatism subsequent to melt extraction. Under the rift shoulder spinel peridotites have undergone both cryptic and patent (modal) metasomatism, possibly during separate events, whereas the upper mantle under the rift axis has undergone only cryptic metasomatism by alkali basaltic magma.  相似文献   

9.
A total of 17 alkali basalts (alkali olivine basalt, limburgite, olivine nephelinite) and quartz tholeiites, and of 10 peridotite xenoliths (or their clinopyroxenes) were analyzed for Nd and Sr isotopes. 143Nd/144Nd ratios and 87Sr/86Sr ratios of all basalts and of the majority of ultramafic xenoliths plot below the mantle array with a large variation in Nd isotopes and a smaller variation in Sr isotopes. The tholeiites were less radiogenic in Nd than the alkali basalts. Volcanics from the Eifel and Massif Central regions contain Nd and Sr, which is more radiogenic than that of the basalts from the Hessian Depression. Nd and Sr isotopic compositions of all rocks from the latter area, with the exception of one tholeiite and one peridotite plot in the same field of isotope ratios as the Ronda ultramafic tectonite (SW Spain), which ranges in composition from garnet to plagioclase peridotite. The alkali basaltic rocks are products of smaller degrees of partial melting of depleted peridotite, which has undergone a larger metasomatic alteration compared with the source rock of tholeiitic magmas. For the peridotite xenoliths such metasomatic alteration is indicated by the correlation of their K contents and isotopic compositions. We assume that the upper mantle locally can acquire isotopic signatures low in radiogenic Nd and Sr from the introduction of delaminated crust. Such granulites low in radiogenic Nd and Sr are products of early REE fractionation and granite (Rb) separation.  相似文献   

10.
The REE and Pb, Sr, Nd isotopes in three xenoliths from limburgite and scoria-breccias, including spinel-lherzolite, spinel-garnet-lherzolite and phlogopite-gamet-lherzolite, were analysed. The REE contents of the xenoliths are 1.3 to 3.3 times those of the chondrites with their REE patterns characterized by weak LREE depletion. The143Nd/144Nd values of whole rocks and minerals range from 0.51306 to 0.51345 with εNd=+ 8.2− +15.8,206Pb/204 Pb < 18.673, and207Pb/204Pb < 15.574. All this goes to show that the upper mantle in Mingxi at the depth of 67–82 km is a depleted mantle of MORB type, with87Sr/86 Sr ratios 0.70237–0.70390. In Nd-Sr diagram the data points of whole rocks are all out of the mantle array, implying that the xenoliths from Mingxi have more radiogenic Sr isotopes than those of the mantle array.  相似文献   

11.
Mantle xenoliths hosted in Miocene-Quaternary mafic alkaline volcanic rocks from Sardinia have been investigated with electron microprobe, laser ablation microprobe-inductively coupled plasma-mass spectrometry and thermal ionization mass spectrometry techniques. The xenoliths are anhydrous clinopyroxene-poor lherzolites and harzburgites, plus very rare websterites and olivine-websterites. Glassy pods having thin subhedral to euhedral microlites of olivine, clinopyroxene and spinel have been found in harzburgites and websterites. Clinopyroxene shows trace element variability, with values of (La/Yb)N ranging from sub-chondritic (0.01) to supra-chondritic (8.6). The Sr–Nd isotopic ratios of the clinopyroxenes fall mostly in the field of the European lithospheric mantle xenoliths (87Sr/86Sr from 0.70385 to 0.70568 and 143Nd/144Nd ranging from 0.512557 to 0.512953). The geochemical characteristics of the Sardinian xenoliths testify to the variable degrees of earlier partial melt extraction, followed by metasomatic modification by alkaline melts or fluids. Websterites are considered to represent small lenses or veins of cumulitic (i.e. magmatic) origin within the mantle peridotite.  相似文献   

12.
Mantle xenoliths in within-plate Cenozoic alkaline mafic lavas from NE Spain are used to assess the local subcontinental lithospheric mantle geotherm and the influence of melting and metasomatism on its oxidation state. The xenoliths are mainly anhydrous spinel lherzolites and harzburgites and gradations between, with minor pyroxenites. Most types show protogranular textures, but transitional protogranular–porphyroclastic and equigranular lherzolites also exist. Different thermometers used in the estimates provide higher subsolidus equilibrium temperatures for harzburgites (1,062 ± 29°C) than for lherzolites (972 ± 89°C), although there is overlap; the lowest temperatures correspond to porphyroclastic lherzolites, whereas pyroxenites give the highest temperatures (up to 1,257°C). Maximum pressures for subsolidus equilibrium of peridotites are at 2.0–1.8 GPa. Later they followed adiabatic decompression and harzburgites registered lower pressures (1.02 ± 0.19 GPa) than lherzolites (1.41 ± 0.27 GPa). One pyroxenite gives values consistent with the spinel lherzolite field (1.08 GPa). The shallowest barometric data are in agreement with the highest local conductive geotherms, which implies that the lithosphere–asthenosphere boundary is at 70–60 km minimum depth. Higher equilibrium temperatures for the harzburgites could be explained by the existence of mafic magma bodies or dykes at the lower crust–mantle boundary. Paleo-fO2 conditions during partial melting as inferred from the covariation between V and MgO concentrations are mainly between QFM−1 and QFM−2 in log units. However, most thermobarometric fO2 estimates are between QFM−1 and QFM+1, suggesting oxidation caused by later metasomatism during uplift and cooling.  相似文献   

13.
Crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the Cretaceous Salta Rift at Quebrada de Las Conchas, Provincia Salta, Argentina were investigated to get information about the age and the chemical composition of the lower crust. Most of the xenoliths have a granitoid composition with quartz-plagioclase-garnet-rutile ± K-feldspar as major minerals. The exceedingly rare mafic xenoliths consist of plagioclase-clinopyroxene-garnet ± hornblende. All xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned. Thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca. 900 °C and pressures of ca. 10 kbar. The Sm-Nd mineral isochron ages are 95.1 ± 10.4 Ma, 91.5 ± 13.0 Ma, 89.0 ± 4.2 Ma (granitoid xenoliths), and 110.7 ± 23.6 Ma (mafic xenolith). These ages are in agreement with the age of basanitic volcanism (ca. 130–100 and 80–75 Ma) and are interpreted as minimum ages of metamorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature of the Sm-Nd system (>600–700 °C). The Sm-Nd and Rb-Sr isotopic signatures (147Sm/144Nd = 0.1225–0.1608; 143Nd/144Ndt 0 = 0.512000–0.512324; 87Rb/86Sr = 0.099–0.172; 87Sr/86Srt 0 = 0.708188–0.7143161) and common lead isotopic signatures (206Pb/204Pb = 18.43–18.48; 207Pb/204Pb = 15.62–15.70; 208Pb/204Pb = 38.22 –38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic basement from NW Argentina, apart from the lower 208Pb/204Pb ratio of the basement. The Sm-Nd depleted mantle model ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic basement point to a similar Proterozoic protolith. Time constraints, the well equilibrated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca. 90 Ma and to a prominent thermal anomaly in the lithosphere. The composition of the xenoliths is similar to the composition of the Early Paleozoic basement in the Andes of NW Argentina and northern Chile. A thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths. Received: 21 July 1998 / Accepted: 27 October 1998  相似文献   

14.
Lithospheric thinning beneath the North China Craton is widely recognized, but whether the Yangtze block has undergone the same process is a controversial issue. Based on a detailed petrographic study, a suite of xenoliths from the Lianshan Cenozoic basalts have been analyzed for the compositions of minerals and whole rocks, and their Sr–Nd isotopes to probe the nature and evolution of the subcontinental lithospheric mantle beneath the lower Yangtze block. The Lianshan xenoliths can be subdivided into two Types: the main Type 1 xenoliths (9–15% clinopyroxene and olivine-Mg# < 90) and minor Type 2 peridotites (1.8–6.2% clinopyroxene and olivine-Mg# > 90). Type 1 peridotites are characterized by low MgO, high levels of basaltic components (i.e., Al2O3, CaO and TiO2), LREE-depleted patterns in clinopyroxenes and whole rocks, and relatively high 143Nd/144Nd (0.513219–0.513331) and low 86Sr/87Sr (0.702279–0.702789). These features suggest that Type 1 peridotites represent fragments of the newly accreted fertile lithospheric mantle that have undergone ~ 1% of fractional partial melting and later weak silicate–melt metasomatism, similar to Phanerozoic lithospheric mantle beneath the eastern North China Craton. Type 2 peridotites may be shallow relics of the older lithospheric mantle depleted in basaltic components, with LREE-enriched and HREE-depleted patterns, relatively low 143Nd/144Nd (0.512499–0.512956) and high 86Sr/87Sr (0.703275–0.703997), which can be produced by 9–14% partial melting and subsequent carbonatite–melt metasomatism. Neither type shows a correlation between equilibration temperatures and Mg# in olivine, indicating that the lithospheric mantle is not compositionally stratified, but both types coexist at similar depths. This coexistence suggests that the residual refractory lithospheric mantle (i.e., Type 2 peridotites) may be irregularly eroded by upwelling asthenosphere materials along weak zones and eventually replaced to create a new and fertile lithosphere mantle (i.e., Type 1 xenoliths) as the asthenosphere cooled. Therefore, the subcontinental lithospheric mantle beneath the lower Yangtze block shared a common evolutional dynamic environment with that beneath the eastern North China Craton during late Mesozoic–Cenozoic time.  相似文献   

15.
87Sr/86Sr ratios of alkali olivine basalts, nepheline basanites and olivine nephelinites of Miocene age from the northern Hessian Depression vary between 0.7032 and 0.7036. Tholeiitic rocks from this area, which are possibly affected by crustal contamination, have more radiogenic Sr (0.7035 to 0.7042). Peridotite xenoliths with coarse protogranular (10 samples) and with porphyroclastic textures (2 samples) contain K- and Na-rich glasses which are products of reaction of metasomatic fluids with depleted peridotite. The Sr abundance in xenoliths is related to the amount of glass (and phlogopite).Sr ranges from 11 ppm to 147 ppm and 87Sr/86Sr ratios from 0.7033 to 0.7039. The isotopic ratios are neither correlated with Sr concentrations nor with Rb/Sr ratios. 87Sr/ 86Sr ratios of etched clinopyroxenes range from 0.7028 to 0.7040. In some xenoliths, clinopyroxenes differ from the whole rock samples significantly in their isotopic composition.If almost all of the pre-metasomatic Sr was located in the clinopyroxenes, the metasomatically introduced Sr ranges from 35 to 80% of the whole rock Sr. The calculated isotopic composition ranges from 0.7033 to 0.7040 for the majority of the xenoliths. For two pyroxenes which are not in isotopic equilibrium with the whole rock, the age of the metasomatic event could be estimated on the base of diffusion of Sr in clinopyroxene. Even assuming a diffusion coefficient as low as 10–15 cm2s–1 the time between the metasomatic alteration and the eruption of the basaltic host magma must be shorter than 1 Ma.The 87Sr/86Sr ratios of the basalts are interpreted as products of mixtures of a depleted component ( 0.7028) and metasomatic fluids (0.7035–0.7053) in their source peridotite.  相似文献   

16.
Lithium concentrations and isotopic compositions of olivine and 87Sr/86Sr and 143Nd/144Nd of coexisting clinopyroxene from peridotite xenoliths from the Quaternary Labait volcano, Tanzania, document the influence of rift-related metasomatism on the ancient cratonic mantle. Olivines show negative correlations between Fo content and both δ7Li and Li concentrations. Olivines in iron-rich peridotites (Fo85–87) have high Li concentrations (3.2–4.8 ppm) and heavy δ7Li (+5.2 to +6.6). In contrast, olivines in ancient, refractory peridotites have lower Li concentrations (∼2 ppm) and relatively light δ7Li (+2.6 to +3.5). This reflects mixing between ancient, refractory cratonic lithosphere and asthenosphere-derived rift magmas. A uniquely fertile, deformed, high-temperature garnet lherzolite, interpreted to be from the base of the lithosphere, has a 87Sr/86Sr of 0.7029 and 143Nd/144Nd of 0.51286, similar to HIMU oceanic basalts. It provides the best estimate of the Sr–Nd isotope composition of the upwelling mantle (i.e., plume, sensu lato) underlying this portion of the East African Rift, and is slightly less radiogenic compared to previous estimates of the plume that were based on rift basalts. Although elevated δ7Li are not exclusive to HIMU source regions, the data collectively indicate that the plume beneath Labait has HIMU characteristics in Sr, Nd and Li isotope composition. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

17.
Summary The Nemby mantle xenoliths are represented by spinel peridotites variably affected by “metasomatic processes” in Proterozoic times (1.2–0.5 Ga). Clinopyroxene compositional variations mainly reflect variably degrees of basalt extraction and mantle “metasomatic processes”. Clinopyroxene crystal chemistry allows to estimate the equilibration pressure by V(Cell) and V(M1) relationship. The investigated clinopyroxenes yielded a pressure range of 12–18 kb. Therefore, the Nemby xenoliths are representative of about 20 km thickness of subcontinental lithospheric mantle of the South America platform at Tertiary times (ca. 46 Ma).
Zusammenfassung Klinopyroxene aus metasomatischen Spinell-Peridotit-Mantel Xenolithen aus Nemby (Paraguay): Kristallchemie und petrologische Bedeutung Die Mantelxenolithe aus Nemby sind Spinellperidodite, die in verschiedenem Ausma? im Proterozoikum (1.2 bis 0.5 Ga) durch “metasomatische Prozesse” beeinflusst wurden. Variationen in der Zusammensetzung der Klinopyroxene lassen variable Ausma?e von Basaltextraktion und Mantelmetasomatose erkennen. Die Kristallchemie der Pyroxene erm?glicht es, den Gleichgewichtsdruck durch V(Cell) und V(M1) Beziehungen zu bestimmen. Die untersuchten Klinopyroxene zeigten Drucke im Bereich von 12 bis 18 kb. Die Nemby Xenolithe sind deshalb repr?sentativ für einen ungef?hr 20 km dicken subkontinentalen lithosph?rischen Mantel der südamerikanischen Plattform im Terti?r (ca. 46 Ma).

Received February 1, 1999; revised version accepted November 26, 1999  相似文献   

18.
Mantle xenoliths entrained in Quaternary alkaline basalts from the Turkana Depression in southern Ethiopia (the East Africa Rift) were studied for their geochemical and Sr–Nd–Pb isotopic compositions to constrain the evolution of the lithosphere. The investigated mantle xenoliths are spinel lherzolites in composition with a protogranular texture. They can be classified into two types: anhydrous and hydrous spinel lherzolites; the latter group characterized by the occurrences of pargasite and phlogopite. The compositions of whole-rock basaltic component (CaO = 3.8–5.6 wt%, Al2O3 = 2.5–4.1 wt%, and MgO = 34.7–38.1 wt%), spinel (Cr# = 0.062–0.117, Al2O3 = 59.0–64.4 wt%) and clinopyroxene (Mg# = 88.4–91.7, Al2O3 = 5.2–6.7 wt%) indicate that the lherzolites are fertile and have not experienced significant partial melting. Both types are characterized by depleted 87Sr/86Sr (0.70180–0.70295) and high 143Nd/144Nd (0.51299–0.51348) with wide ranges of 206Pb/204Pb (17.86–19.68) isotopic compositions. The variations of geochemical and isotopic compositions can be explained by silicate metasomatism induced by different degree of magma infiltrations from ascending mantle plume. The thermobarometric estimations suggest that the spinel lherzolites were derived from depths of 50–70 km (15.6–22.2 kb) and entrained in the alkaline magma at 847–1,052°C. Most of the spinel lherzolites from this study record an elevated geotherm (60–90 mW/m2) that is related to the presence of rising mantle plume in an active tectonic setting. Sm–Nd isotopic systematic gives a mean TDM model age of 0.95 Ga, interpreted as the minimum depletion age of the subcontinental lithosphere beneath the region.  相似文献   

19.
Major and trace element and 143Nd/144Nd (0.51209–0.51216) and 87Sr/86Sr (0.70879–0.71105) isotope analyses are presented on a representative group of lavas from the Vulsini district of the Roman magmatic province. Three distinct series are identified; the high-K and low-K series are similar to those described from other Italian volcanoes, while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near-primary mantle melts. Major and trace element variations within the high-K series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites. Crustal contamination resulted in an increase in 87Sr/86Sr with increasing fractionation, but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub-continental mantle. These are reviewed using the available results from Vulsini, Roccamonfina and Ernici. Transition element abundances and Ta/Yb ratios indicate that the pre-enrichment mantle was similar to that of E-type MORB, and that these elements were not mobilised by the enrichment process. Mixing calculations suggest that three components were involved in the enrichment process; mantle comparable with the source of MORB, and two other components rich in trace elements. One, the low-K component, had high Sr/Nd, Th/Ta and Ba/Nb and no europium anomaly while the second had lower Sr/Nd, a negative europium anomaly and very high Th/Ta. It was also characterised by low Nb/Ba and high Rb/Ba ratios, similar to those reported from phlogopite-rich peridotite xenoliths. The trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high-K end-member reflecting the subduction of sediments and the stabilisation of mantle phlogopite.  相似文献   

20.
The mineral chemistry, major and trace element, and Sr–Nd isotopic composition of Cr-diopside, spinel peridotite xenoliths from the Estancia Lote 17 locality in southern Patagonia document a strong carbonatitic metasomatism of the backarc continental lithosphere. The Lote 17 peridotite xenolith suite consists of hydrous spinel lherzolite, wehrlite, and olivine websterite, and anhydrous harzburgite and lherzolite. Two-pyroxene thermometry indicates equilibration temperatures ranging from 870 to 1015 °C and the lack of plagioclase or garnet suggests the xenoliths originated from between ˜40 and 60 km depth. All of the xenoliths are LILE- and LREE-enriched, but have relatively low 87Sr/86Sr (0.70294 to 0.70342) and high ɛNd (+3.0 to +6.6), indicating recent trace element enrichment (∼25 Ma, based on the low 87Sr/86Sr and high Rb concentrations of phlogopite separates) in the long-term, melt-depleted Patagonian lithosphere. Lote 17 peridotite xenoliths are divided into two basic groups. Group 1 xenoliths consist of fertile peridotites that contain hydrous phases (amphibole ± phlogopite ± apatite). Group 1 xenoliths are further subdivided into three groups (a, b, and c) based on distinctive textures and whole-rock chemistry. Group 1 xenolith mineralogy and chemistry are consistent with a complex metasomatic history involving variable extents of recent carbonatite metasomatism (high Ca/Al, Nb/La, Zr/Hf, low Ti/Eu) that has overprinted earlier metasomatic events. Group 2 xenoliths consist of infertile, anhydrous harzburgites and record cryptic metasomatism that is attributed to CO2-rich fluids liberated from Group 1 carbonatite metasomatic reactions. Extremely variable incompatible trace element ratios and depleted Sr–Nd isotopic compositions of Lote 17 peridotite xenoliths indicate that the continental lithosphere was neither the primary source nor an enriched lithospheric contaminant for Neogene Patagonian plateau lavas. Neogene plateau magmatism associated with formation of asthenospheric slab windows may have triggered this occurrence of “intraplate-type” carbonatite metasomatism in an active continental backarc setting. Received: 26 January 2000 / Accepted: 1 March 2000  相似文献   

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