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1.
The Erro-Tobbio peridotites (Voltri Massif, Ligurian Alps) represent subcontinental lithospheric mantle tectonically exhumed during Permo–Mesozoic extension of the Europe–Adria lithosphere. Previous studies have shown that exhumation started during Permian times, and occurred along kilometer-scale lithospheric shear zones which enhanced progressive deformation and recrystallization from spinel- to plagioclase-facies conditions. Ongoing field and petrologic investigations have revealed that the peridotites experienced, during uplift, a composite history of diffuse melt migration and multiple episodes of ultramafic–mafic intrusions. In this paper we present the results of field, structural and petrologic–geochemical investigations into a sector of the Erro-Tobbio peridotite unit that preserves well this multiple intrusion history. Melt impregnation in the peridotites is evidenced by significant plagioclase enrichment and crystallization of unstrained orthopyroxene replacing kinked mantle olivine and clinopyroxene; impregnating melts were thus opx-saturated. Melt–rock interaction caused chemical changes in mantle minerals (e.g. Al decrease and REE increase in cpx; Ti and Cr# enrichment in spinel). Nevertheless, clinopyroxenes still exhibit LREE depletion (CeN/SmN = 0.006–0.011), indicating a depleted signature for the percolating melts. Melt impregnation was thus related to diffuse porous flow migration of depleted MORB-type melt fractions that modified their compositions towards opx saturation by mantle–melt interaction during ascent. The impregnated peridotites are intruded by a hectometer-scale stratified cumulate body, mostly consisting of troctolites and plagioclase wehrlites, showing gradational, interfingered contacts with the host mantle rocks. Subsequent intrusion events are revealed by the occurrence of olivine gabbros as decameter-wide lenses, variably thick (centimeter- to meter-scale) dykes and thin dykelets, which crosscut both the peridotite foliation and the magmatic layering in the cumulates. Overall, major and trace element compositions of minerals in the intrusives indicate that they represent variably differentiated cumulus products crystallized from rather primitive N-MORB-type aggregated melts. Slightly more evolved compositions are shown by olivine gabbros, relative to the troctolites and plagioclase wehrlites of the cumulate body. Mineral chemistry features (e.g. the Fo–An correlation and high Na, Ti, Mg# in cpx) indicate that the studied intrusive rocks crystallized at moderate pressure conditions (3–5 kbar, i.e. 9–15 km depth). Our study thus points to a progressive transition from porous flow melt migration to emplacement of magmas in fractures, presumably related to progressive change of lithospheric mantle rheology during extension-related uplift and cooling.  相似文献   

2.
This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and dunites crop out at Mt. Arpone and Mt. Musinè. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that <1–5% fractional melting of spinel-facies DMM produced the injected melts. Plagioclase peridotites are hybrid rocks resulting from pre-existing spinel peridotites and variable enrichment of plagioclase and micro-gabbroic material by percolating melts. The impregnating melts attained silica-saturation, as testified by widespread orthopyroxene replacement of olivine, during open system migration in the lithosphere. At Mt. Musinè, coarse granular spinel harzburgite and dunite bodies replace the plagioclase peridotites. Most of these replacive, refractory peridotites have interstitial magmatic clinopyroxene with trace element compositions in equilibrium with MORB, while some Cpx have REE-depleted patterns suggesting transient geochemical features of the migrating MORB-type melts, acquired by interaction with the ambient plagioclase peridotite. These replacive spinel harzburgite and dunite bodies are interpreted as channels exploited for focused and reactive migration of silica-undersaturated melts with aggregate MORB compositions. Such melts were unrelated to the silica-saturated melts that refertilized the pre-existing plagioclase peridotites. Finally, MORB melt migration occurred along open fractures, now recorded as gabbroic dikes.

Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.  相似文献   


3.
This paper reports the results of a mineralogical study of 14 mantle peridotite samples dredged in 2009 from the eastern slope of the northwestern segment of the Stalemate Ridge in the northwestern Pacific during cruise SO201-KALMAR Leg 1b of the R/V Sonne. The sample collection included four serpentinized and silicified dunites and ten variably serpentinized lherzolites. The compositions of primary minerals (clinopyroxene, orthopyroxene, and spinel) change systematically from the lherzolites to dunites. Spinel from the lherzolites shows higher Mg# and lower Cr# values (0.65–0.68 and 0.26–0.33, respectively) compared with spinel from the dunites (Mg# = 0.56–0.64 and Cr# = 0.38–0.43). Clinopyroxene from the lherzolites is less magnesian (Mg# = 91.7–92.4) than clinopyroxene from dunite sample DR37-3 (Mg# = 93.7). Based on the obtained data, it was concluded that the lherzolites of the Stalemate Fracture Zone were derived by 10–12% near-fractional melting of a DMM-type depleted mantle reservoir beneath the Kula-Pacific spreading center. The dunites were produced by interaction of residual lherzolites with sodium- and titaniumrich melt and are probably fragments of a network of dunite channels in the shallow mantle. The moderately depleted composition of minerals clearly distinguishes the lherzolites from the strongly depleted peridotites of the East Pacific Rise and indicates the existence of slow-spreading mid-ocean ridges in the Pacific Ocean during the Cretaceous-Paleogene.  相似文献   

4.
Mantle petrology and mineralogy of the Thetford Mines Ophiolite Complex   总被引：4，自引：0，他引：4  
The Ordovician Thetford Mines ophiolite complex (TMOC) formed by boninite-fed seafloor-spreading, probably in a fore-arc environment. The mantle section is dominated by foliated harzburgite (≤ 5–6% clinopyroxene), cut by dunitic (± chromitite cores) and orthopyroxenitic veins and dykes. Contrasting structures, textures and mineral compositions allow us to subdivide the mantle. The granular-textured rocks of the Duck Lake Block (DLB) have two steeply-dipping foliations. The older foliation strikes NW, is sub-perpendicular to the Moho, and is interpreted to have resulted from upflow of the asthenosphere beneath the spreading ridge. This fabric is overprinted by a 2nd ductile foliation striking ENE, oriented sub-parallel to the Moho, which we interpreted as having formed by crust–mantle shear as the lithosphere migrated away from the spreading ridge. The DLB mantle has a limited range of spinel Cr# (100Cr / (Cr + Al) = 51–71). Comparison with experimentally determined residual spinel compositions (equilibrium melting) implies a maximum loss of 27–38% melt if the protolith had a fertile MORB mantle composition. However, interstitial-textured clinopyroxene may have high TiO2 (< 0.04wt.%) and Na2O (< 0.27wt.%), and some interstitial spinel has higher TiO2 (< 0.09wt.%), suggesting interaction with (or crystallization from) an “impregnating” melt. Interstitial tremolitic amphibole also indicates the passage of late hydrous fluids. The harzburgite in the Caribou Mountain Block (CMB) has a porphyroclastic texture, with a strong, locally mylonitic foliation striking roughly N–S, parallel to the orientation of seafloor-spreading related paleo-normal faults in the crust. These fabrics and textures imply a colder, lithospheric deformation, possibly related to tectonic denudation (oceanic core complex). This would explain problematic lava/mantle contacts, favour infiltration of seawater, serpentinization, and reduced fO2 conditions. The CMB mantle shows a wider range of mineral compositions than the DLB, with spinel Cr# (28–86) implying ≤ 15–45% of equilibrium melting. Locally higher TiO2 in spinel (< 0.05wt.%) and clinopyroxene (< 0.11wt.%), a local rimward decrease in spinel Cr#, clinopyroxene Cr#, and olivine Fo-content, and traces of interstitial amphibole, are attributed to the circulation of an evolved hydrous melt during peridotite deformation. This suggests that the lower limit to the extent of melting inferred for the CMB (15%), established on the basis of Al-rich spinel rims and neoblasts, is probably too low. On the other hand, the higher inferred degree of depletion of the CMB is probably unaffected by the metasomatic overprint and is a more robust conclusion.  相似文献   

5.
Coarse-grained, granular spinel lherzolites xenoliths from the Premier kimberlite show evidence of melt extraction and metasomatic enrichment, documenting a complex history for the shallow mantle beneath the Bushveld complex. Compositions of orthopyroxene, clinopyroxene and spinel indicate equilibration within the spinel–peridotite facies of the upper mantle, at depths from 80 to 100 km and temperatures from 720 to 850 °C. Bulk compositions have lower Mg-number [atomic 100 Mg/(Mg + Fe*)] than previously studied spinel peridotites from Premier, and have higher Mg/Si than low-temperature coarse grained garnet lherzolites, suggesting shallower melting conditions or metasomatic enrichment. Clinopyroxene in one sample is highly LREE-depleted indicating very minor modification of a residue of 20% melt extraction, whereas the calculated REE pattern for a melt in equilibrium with a mildly LREE-depleted sample is similar to MORB or late Archean basalt, possibly related to the Bushveld Complex. Bulk and mineral compositions suggest minimal refertilization by silicate melts in four out of six samples, but REE patterns indicate introduction of a LIL-enriched component that may be related to kimberlite.  相似文献   

6.
Deep-seated xenoliths entrained in the Hannuoba basalts of the northern Sino-Korean Craton include mafic and felsic granulites, mantle wall-rock from spinel– and garnet–spinel peridotite facies, and basaltic crystallisation products from the spinel-pyroxenite and garnet-pyroxenite stability fields. The mineral compositions of the xenoliths have been used to estimate temperatures and, where possible, pressures of equilibration, and to construct a geothermal framework to interpret the upper mantle and lower crustal rock-type sequences for the region. The xenolith-derived paleogeotherm is constrained in the depth interval of 45–65 km and like others from areas of young basalt magmatism, is elevated and strongly convex toward the temperature axis. Two-pyroxene granulites give the lowest temperatures and garnet pyroxenites the highest, while the spinel lherzolites fall between these two groups. The present-day Moho beneath the Hannuoba area is defined at 42 km by seismic data, and coincides with the deepest occurrence of granulite. Above this boundary, there is a lower crust–upper mantle transition zone about 10-km thick, in which spinel lherzolites and mafic granulites (with variable plagioclase contents) are intermixed. It is inferred that this underplating has resulted in a lowering of the original pre-Cenozoic Moho (then coinciding with the crust–mantle boundary, CMB) from about 30 km to its present-day position and was due to intrusions of basaltic magmas that displaced peridotite mantle wall-rock and equilibrated to mafic granulites. Trace element patterns of the diopsides (analysed by laser ablation-ICPMS) from the Cr-diopside series spinel lherzolites and associated layered xenoliths (spinel lherzolites and pyroxenites) indicate a fertile uppermost mantle with moderate depletion by low degrees of partial melting and little evidence of metasomatic activity. The similarity in major and trace element compositions of the minerals in both rock types suggests that the layered ultramafic xenoliths formed by mantle deformation processes (metamorphic segregation), rather than by melt veining or metasomatism.  相似文献   

7.
《Gondwana Research》2001,4(3):427-436
The ‘Three Phenocryst Basalts’ (TPB) from Pavagadh hill constitute one of the most primitive basalts in Deccan traps. Electron microprobe analyses of phenocryst minerals from the TPB reveal that the Fo % of olivine varies from 89 to 68, the clinopyroxene grains are diopside/salite or augite and the Cr# [Cr/(Cr+Al)] in spinel is about 0.65. The An content of feldspar varies from 63 to 67. The mineral chemical data allow us to infer that olivine and spinel crystallized early, and when olivine attained Fo76–73%, the crystallization of clinopyroxene was initiated. Plagioclase crystallization occurred at the late stage. It is indicated that the source region of the TPB may possibly be undepleted mantle (lherzolite zone) at about 85 km depth and 27 kbar pressure, where Cr# of spinel is about 0.47.  相似文献   

8.
Upper mantle xenoliths from Wikieup, AZ, provide abundant evidence for magmatic modification of the uppermost mantle beneath the Transition Zone between the Colorado Plateau and the southern Basin and Range province. Upper mantle lithologies in this xenolith suite are represented by spinel peridotite, wehrlite, plagioclase peridotite, and Al-augite group pyroxenites. Isotopic data for these xenoliths yield relatively uniform values and suggest a common petrogenesis. Al-augite-bearing gabbro and pyroxenite xenoliths from this locality are interpreted to have formed by crystal fractionation processes from parent alkali basalts similar to the Wikieup host basalt. Mineral and whole rock compositions show consistent trends of increasing incompatible element contents (Fe, Al, Ca, Na, K, LIL, and LREE), and decreasing compatible element contents (Mg, Cr, Ni) from spinel peridotite to wehrlite to plagioclase peridotite to the host basalt composition. These compositional trends are interpreted as resulting from varying degrees of magma-mantle wall rock interaction as ascending mafic magmas infiltrated upper mantle peridotite. Small degrees of melt infiltration resulted in slightly modified spinel peridotite compositions while moderate degrees metasomatized spinel peridotite to wehrlite, and the highest degrees metasomatized it to plagioclase peridotite. Whole rock compositions and clinopyroxene, plagioclase, and whole rock isotopic data suggest that the infiltrating magmas were the same as those from which the gabbros and pyroxenites crystallized, and that they were alkalic in composition, similar to the Wikieup host alkali olivine basalts. Relatively uniform 143Nd/144Nd for the mineral separates and whole rocks in spite of the significantly wide range in their 147Sm/144Nd (0.71–0.23 in clinopyroxene) suggests that the Wikieup xenoliths including gabbro, pyroxenite, peridotite, wehrlite, and plagioclase peridotite, are all relatively young rocks formed or metasomatized by a relatively recent magmatic episode. Received: 21 May 1996 / Accepted: 23 December 1996  相似文献   

9.
Quaternary basalts in the Cerro del Fraile area contain two types of mantle xenoliths; coarse-grained (2–5 mm) C-type spinel harzburgites and lherzolites, and fine-grained (0.5–2 mm) intensely metasomatized F-type spinel lherzolites. C-type xenoliths have high Mg in olivine (Fo = 90–91) and a range in Cr# [Cr/ (Cr + Al) = 0.17–0.34] in spinel. Two C-type samples contain websterite veinlets and solidified patches of melt that is now composed of minute quenched grains of plagioclase + Cr-spinel + clinopyroxene + olivine. These patches of quenched melts are formed by decompression melting of pargasitic amphibole. High Ti contents and common occurrence of relic Cr-spinel in the quenched melts indicate that the amphibole is formed from spinel by interaction with the Ti-rich parental magma of the websterite veinlets. The fO2 values of these two C-type xenoliths range from ΔFMQ −0.2 to −0.4, which is consistent with their metasomatism by an asthenospheric mantle-derived melt. The rest of the C-type samples are free of “melt,” but show cryptic metasomatism by slab-derived aqueous fluids, which produced high concentrations of fluid-mobile elements in clinopyroxenes, and higher fO2 ranging from ΔFMQ +0.1 to +0.3. F-type lherzolites are intensely metasomatized to form spinel with low Cr# (∼0.13) and silicate minerals with low MgO, olivine (Fo = ∼84), orthpyroxene [Mg# = Mg/(Mg + ΣFe) = ∼0.86] and clinopyroxene (Mg# = ∼0.88). Patches of “melt” are common in all F-type samples and their compositions are similar to pargasitic amphibole with low TiO2 (<0.56 wt%), Cr2O3 (<0.55 wt%) and MgO (<16.3 wt%). Low Mg# values of silicate minerals, including the amphibole, suggest that the metasomatic agent is most likely a slab melt. This is supported by high ratios of Sr/Y and light rare earth elements (REE)/heavy REE in clinopyroxenes. F-type xenoliths show relatively low fO2 (ΔFMQ −0.9 to −1.1) compared to C-type xenoliths and this is explained by the fusion of organic-rich sediments overlying the slab during the slab melt. Trench-fill sediments in the area are high in organic matter. The fusion of such wet sediments likely produced CH4-rich fluids and reduced melts that mixed with the slab melt. High U and Th in bulk rocks and clinopyroxene in F-type xenoliths support the proposed interpretation.  相似文献   

10.
Summary Crust-derived xenoliths hosted by Miocene basaltic diatremes in the Hyblean Plateau (south-eastern Sicily, Italy) provide new information regarding the nature of a portion of the central Mediterranean lower crust. These xenoliths can be divided into three groups: gabbros (plagioclase + clinopyroxene + Fe–Ti oxides ± apatite ± amphibole ± Fe-rich green spinel), diorites (An-poor plagioclase, clinopyroxene ± Fe–Ti oxides ± orthopyroxene) and mafic granulites (plagioclase + clinopyroxene + green spinel ± orthopyroxene ± Fe–Ti oxides). Gabbros form the main subject of this paper. They represent cumulates whose igneous texture has been locally obliterated by metamorphic recrystallization and shearing. They were permeated by Fe–Ti-rich melts related to tholeiitic-type fractional crystallisation. Incompatible element ratios (Zr/Nb = 5–26; Y/Nb = 1.4–11) indicate that these cumulate gabbros derived from MORB liquids. Late-stage and hydrothermal fluids caused diverse, sometimes important, metasomatic trasformations. Petrographic and geochemical comparison with gabbroids from well-known geodynamic settings show that the Hyblean lower crustal xenoliths were probably formed in an oceanic or oceanic-continent transition environment.  相似文献   

11.
The paper discusses the results of mineralogical and petrographic studies of spinel lherzolite xenoliths and clinopyroxene megacrysts in basalt from the Jixia region related to the central zone of Cenozoic basaltic magmatism of southeastern China. Spinel lherzolite is predominantly composed of olivine (Fo89.6–90.4), orthopyroxene (Mg# = 90.6–92.7), clinopyroxene (Mg# = 90.3–91.9), and chrome spinel (Cr# = 6.59–14.0). According to the geochemical characteristics, basalt of the Jixia region is similar to OIB with asthenospheric material as a source. The following equilibrium temperatures and pressures were obtained for spinel peridotite: 890–1269°C and 10.4–14.8 kbar. Mg# of olivine and Cr# of chrome spinel are close to the values in rocks of the enriched mantle. It is evident from analysis of the textural peculiarities of spinel lherzolite that basaltic melt interacted with mantle rocks at the xenolith capture stage. Based on an analysis of the P–T conditions of the formation of spinel peridotite and clinopyroxene megacrysts, we show that mantle xenoliths were captured in the course of basaltic magma intrusion at a significantly lower depth than the area of partial melting. However, capture of mantle xenoliths was preceded by low-degree partial melting at an earlier stage.  相似文献   

12.
The lherzolites have recrystallized to plagioclase lherzolites consisting of olvine, pyroxenes, chromian spinel, plagioclase and pargasite at a depth of 20 to 25 km in the uppermost part of the mantle. It is believed that the garnet lherzolites and spinel lherzolites were originally derived from depths of 50–75 km and 30–50 km respectively. The clinopyroxenes contained about 10 mol. % of jadeite and Tschermak's molecules, respectively and the orthopyroxenes also included about 5–10% of Tschermak's component. Transported upward, the garnet was transformed through pyroxene-spinel symplectite to olivine, plagioclase and spinel aggregates, and most of the jadeite amd some Tschermak's components in the pyroxenes formed secondary pyroxenes and pargasite, and finally plagioclase under isochemical conditions.  相似文献   

13.
Remnants of a fossil continent–ocean transition similar to that of the modern non-volcanic continental margins are preserved in the Jurassic External Liguride units. They consist of fertile lherzolites of subcontinental origin, MOR-type basalts and rare gabbroic intrusives, together with continental crust bodies exhumed during the rifting phases preceding the oceanization. The gabbroic rocks include troctolites, (olivine) gabbros, Fe–Ti oxide-bearing gabbros and diorites. Trace element and Nd isotope compositions indicate that these rocks were derived from N-MORB melts variably evolved through fractional crystallisation. In the gabbroic rocks, high-temperature ( 900 °C) shearing along ductile shear zones is locally overprinted by amphibolite-facies recrystallization (T  650 °C), which was most likely assisted by seawater-derived fluids. Basalts crop out as lava flows and as dykes crosscutting mantle lherzolites and gabbroic rocks. They display nearly flat REE patterns and high Y/Nb values (5–14), similar to modern N-MORB. Basalts are also characterised by weak Zr enrichment relative to neighbouring REE (Zr/Zr = 1.1–1.7) and high (Sm/Yb)DM ratios (1.5–1.8). Their Nd isotope compositions are close to typical depleted mantle (initial Nd = +7.6 to + 9.4). The geochemical features of parental melts of basaltic and gabbroic rocks may be attributed to melting of a MORB-type asthenospheric source. Trace element modelling shows that low-degree (≤ 6%) fractional melting of a depleted spinel peridotite cannot account for the elevated Sm/Yb ratios of basalts. Low-degree melting of a mixed source of spinel peridotite with small amounts of garnet pyroxenite has been proposed to explain the trace element signature of basalts.  相似文献   

14.
The Pliocene (7 Ma) Nb-enriched arc basalts of the ValovayamVolcanic Field (VVF) in the northern segment of the Kamchatkaarc, Russia, host abundant mantle xenoliths, including spinelIherzolites. Textural and microstructural evidence for high-temperature,multi-stage, creep-related deformations in spinel Iherzolitessupports a sub-arc mantle derivation. Pyroxene chemistry indicatesthe existence of two compositional suites: (1) a Cr-diopsidesuite with low-Tt, moderate-Al clinopyroxene compositions, and(2) an Al-augite suite with high Al and Tt, and low Cr concentrationsin clinopyroxene. Some spinel lherzolite xenoliths contain metasomaticAl-augite-type clinopyroxene, Al-Tt spinel, and felsic veinssimilar to trondhjemite melt. The Al-augite series xenolithstypically contain high-Na plagioclase, Cr-poor, Al-Fe-Mg andAl-Tt-Fe spinels, with occasional almandine-grossularite garnetand high-Al and -Na pargasitic amphibole. Pyroxene and spinel compositional trends suggest that the Crdiopsideseries xenoliths from the VVF Nb-enriched arc basalts representan island-arc mantle affected by a metasomatic event. Occurrenceof high-Na plagioclase and trondhjemitic veins favors the additionof a metasomatic component with high Na, Al and Si to the northernKamchatka arc mantle. Trondhjemitic veins, representing siliceousslab melts, compositionally exemplify the metasomatic component.Na metasomatism by peridotite-slab melt interaction is an importantmantle hybridization process responsible for arc-related alkalinemagma generation from a veined sub-arc mantle. KEY WORDS: metasomatism; island arc; mantle xenoliths; Kamchatka; mantle  相似文献   

15.
ODP Leg 209 Site 1274 mantle peridotites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg# (up to 0.92) and spinel Cr# (∼0.5), suggesting high degree of partial melting (>20%). Detailed studies of their microstructures show that they have extensively reacted with a pervading intergranular melt prior to cooling in the lithosphere, leading to crystallization of olivine, clinopyroxene and spinel at the expense of orthopyroxene. The least reacted harzburgites are too rich in orthopyroxene to be simple residues of low-pressure (spinel field) partial melting. Cu-rich sulfides that precipitated with the clinopyroxenes indicate that the intergranular melt was generated by no more than 12% melting of a MORB mantle or by more extensive melting of a clinopyroxene-rich lithology. Rare olivine-rich lherzolitic domains, characterized by relics of coarse clinopyroxenes intergrown with magmatic sulfides, support the second interpretation. Further, coarse and intergranular clinopyroxenes are highly depleted in REE, Zr and Ti. A two-stage partial melting/melt–rock reaction history is proposed, in which initial mantle underwent depletion and refertilization after an earlier high pressure (garnet field) melting event before upwelling and remelting beneath the present-day ridge. The ultra-depleted compositions were acquired through melt re-equilibration with residual harzburgites. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.  相似文献   

16.
Geological and geophysical evidence indicates that at least100 km of Archaean to Proterozoic lithospheric mantle has beenremoved from beneath large areas of eastern and southeasternChina during late Mesozoic to Cenozoic time. Mantle-derivedxenoliths in Tertiary basalts from several localities acrossthis region have been studied by X-ray fluorescence, electronmicroprobe and laser ablation microprobe–inductively coupledplasma-mass spectrometry to characterize this thinner lithosphere.Trace element patterns of clinopyroxenes in the peridotitesfrom southeastern China can be divided into four groups: fertilegarnet lherzolites, fertile spinel (± garnet) lherzolites,and depleted and enriched peridotites. The addition of Nb, Sr,light rare earth elements, but not of Ti and Zr, suggests ametasomatizing agent containing both H2O and CO2. This studyalso demonstrates that the negative Ti anomaly commonly observedin clinopyroxene from mantle peridotites cannot be balancedby the Ti in coexisting orthopyroxene, but can be explainedby small degrees of partial melting, using appropriate distributioncoefficients. Most of the peridotites from southeastern China,whether spinel or garnet facies, are highly fertile in termsof Al2O3 and CaO contents and mg-number; many resemble commonlyused primitive mantle compositions. Modelling of trace elementpatterns in clinopyroxene indicates that most spinel and garnetperidotites from the Nushan, Mingxi and Niutoushan localitiesexperienced less than 5%, and many less than 2%, partial melting.A few depleted spinel peridotites from Nushan, and all spinelperidotites from Mingxi, require 10–25% fractional partialmelting; almost all spinel peridotites from the Qilin localityshow evidence of higher degrees (6–25%) of fractionalpartial melting. At both Nushan and Mingxi, the more depletedcompositions occur in the upper part of the lithospheric mantle,which now is  相似文献   

17.
The Dramala massif, located in the Dinarides–Hellenides orogenic belt, forms the mantle section of the Neotethyan Pindos ophiolite complex in NW continental Greece. Its southern domain is comprised mainly of voluminous harzburgite masses with variable clinopyroxene and olivine modal abundances, ranging from clinopyroxene‐bearing to typical and olivine‐rich harzburgites. The harzburgite varieties are characterized by elevated Cr# [Cr/(Cr + Al)] in Cr‐spinel (0.43–0.79), high forsterite (Fo) content in olivine (0.90–0.93), low Al2O3 content in clinopyroxene (≤1.77 wt.%) and poor whole‐rock abundances of Al2O3 (≤0.68 wt.%), CaO (≤0.68 wt.%), Sc (≤11 ppm) and REE, which are indicative of their refractory nature. In terms of fO2 values, the southern Pindos harzburgites plot between the FMQ‐2 (Fayalite–Magnetite–Quartz) and FMQ + 2 buffers. Simple batch and fractional melting models are not sufficient to explain their depleted composition. Their Ni/Yb ratios vs. Yb bulk‐rock abundances can be reproduced by up to 22–31% closed‐system non‐modal dynamic melting of an assumed spinel‐bearing lherzolite source. Cr‐spinel chemistry data suggest that the southern Dramala harzburgites were formed in an oceanic centre and then were reworked in the mantle wedge above a subducted slab. Combined petrographic and compositional data indicate that the studied harzburgites interacted with arc‐derived tholeiitic melts. This interaction resulted in substantial olivine and minor Cr‐spinel addition to the studied harzburgites, thus enhancing their refractoriness. Cryptic metasomatism was plausibly responsible for the demolition of any strong geochemical signatures suggestive of a previous melting event in a spreading centre. Comparable observations from the neighbouring Vourinos suite imply that the southern Dramala harzburgites probably represent an arc/fore‐arc mantle region within the mutual Pindos–Vourinos, Mesohellenic lithospheric mantle. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

18.
Composition of chromiferous spinel included in olivines of Mg-rich basalts and gabbros of the Deccan Traps (Gujarat and Western Ghats) are reported here. They vary from Al-rich compositions [Al2O3 = 53wt.%; Cr#, 100Cr/(Cr + Al) = 12] to Cr-rich compositions [Cr2O3 = 51wt.%; Cr# = 84], and from Cr-Al rich compositions towards Cr-rich Ti-magnetite (TiO2 up to 23 wt.%, ulvöspinel up to 67mol.%). The Mg# [100Mg/(Mg + Fe2+)] of spinel decreases from 81 to nearly zero. The highest Cr# has been found in the Bushe Fm., Thakurvadi Fm., and some high-Ti basalts of the Pavagadh section, whereas some of the low-Ti basalts of Saurashtra have Al-rich compositions typical of spinels found in mid-ocean ridge basalts. The chemical composition of the Deccan Trap spinels is completely different compared to that observed in mantle spinel suites, with very few exceptions. The decreasing Al and increasing Fe and Ti of spinel seems to be mainly the result of decrease of Mg in the locally coexisting melts and favourable cationic substitutions in the lattice. There is barely any evidence of general relationships between the composition of the Deccan spinels and inferred mantle sources of the host magmas. Pyroxene inclusions in spinels may witness a high-pressure stage of crystallization, but the possibility of non-equilibrium crystallization, or even magma mixing, cannot be ruled out. Overall, the compositional ranges of chromiferous spinel in the Deccan Traps closely match those observed in the other Large Igneous Provinces having mafic/ultramafic intrusions and mafic magma compositions (e.g., Siberian Traps, Karoo, Emeishan).  相似文献   

19.
A wide range of trace elements have been analysed in mantle xenoliths (whole rocks, clinopyroxene and amphibole separates) from alkaline lavas in the Eastern Carpathians (Romania), in order to understand the process of metasomatism in the subcontinental mantle of the Carpatho-Pannonian region. The xenoliths include spinel lherzolites, harzburgites and websterites, clinopyroxenites, amphibole veins and amphibole clinopyroxenites. Textures vary from porphyroclastic to granoblastic, or equigranular. Grain size increases with increasing equilibrium temperature of mineralogical assemblages and results from grain boundary migration. In peridotites, interstitial clinopyroxenes (cpx) and amphiboles resulted from impregnation and metasomatism of harzburgites or cpx-poor lherzolites by small quantities of a melt I with a melilitite composition. Clinopyroxenites, amphibole veins and amphibole clinopyroxenites are also formed by metasomatism as a result of percolation through fracture systems of large quantities of a melt II with a melanephelinite composition. These metasomatic events are marked by whole-rock enrichments, relative to the primitive mantle (PM), in Rb, Th and U associated in some granoblastic lherzolites and in clinopyroxene and amphibole veins with enrichments in LREE, Ta and Nb. Correlations between major element whole-rock contents in peridotites demonstrate that the formation of interstitial amphibole and clinopyroxene induced only a slight but variable increase of the Ca/Al ratio without apparent modifications of the initial mantle composition. Metasomatism is also traced by enrichments in the most incompatible elements and the LREE. The Ta, Nb, MREE and HREE contents remained unchanged and confirm the depleted state of the initial but heterogeneous mantle. Major and trace element signature of clinopyroxene suggests that amphibole clinopyroxenites and some granoblastic lherzolites have been metasomatized successively by melts I and II. Both melts I and II were Ca-rich and Si-poor, somewhat alkaline (Na > K). Melt I differed from melt II in having higher Mg and Cr contents offset by lower Ti, Al, Fe and K contents. Both were highly enriched in all incompatible trace elements relative to primitive mantle, showing positive anomalies in Rb, Ba, Th, Sr and Zr. They contrasted by their Ta, Nb and LREE contents, lower in melt I than in melt II. Melts I and II originate during a two-stage melting event from the same source at high pressure and under increasing temperature. The source assemblage could be that of a metasomatized carbonated mantle but was more likely that of an eclogite of crustal affinity. Genetic relationships between calc-alkaline and alkaline lavas from Eastern Carpathians and these melts are thought to be only indirect, the former originating from partial melting of mantle sources respectively metasomatized by the melts I and II. Received: 17 March 1997 / Accepted: 14 July 1997  相似文献   

20.
ABSTRACT

The Neo-Tethys-related Chaldoran ophiolite peridotites in NW Iran are remnants of mantle lithosphere, exhumed tectonically during the Late Cretaceous. Harzburgite is the predominant peridotite type, associated with oceanic lower crust cumulate gabbros occasionally. The ophiolite rocks are unconformably overlain by Late Cretaceous-Paleocene sediments. New whole-rock geochemistry of the variably serpentinized harzburgites shows a depleted nature, exemplified by low Al2O3, CaO, TiO2, V and Y and high Ni, Cr and Mg and also low rare earth element (REE) contents. The harzburgites present LREE enrichment. Positive correlations between some LREEs and high field strength elements (HFSE) suggest enrichment of LREEs by melt re-fertilization processes. Cr-spinels have Cr number of [Cr# = Cr/(Cr + Al) = 0.53–0.67], showing medium to high degree of partial melting (F = ~17-20%). Both whole-rock and mineral chemistry data show a supra-subduction zone setting and progressive depletion along with increase in spinel Cr# (MOR to fore arc). The cumulate gabbros have high MgO and SiO2, low TiO2 and Ti/V < 10 and also low chondrite normalized Dy (<8.5). The gabbro samples show enriched LREEs and LILEs and depleted HREEs and HFSEs with respect to MORBs.

Subduction initiation (SI) model in a fore-arc/proto-fore-arc environment is suggested for the upper mantle evolution of the Chaldoran ophiolite. The rocks have experienced depletion in a second melting process at the later stages of SI and compositions were probably modified by extraction of island arc tholeiitic (IAT) and possibly boninitic (BON) melts. The chemostratigraphic progression for ‘subduction initiation rule (SIR)’ is likely traceable in Chaldoran mafic-ultramafic sequence, which corresponds to the most Neo-Tethyan ophiolites and is similar to MOR to supra-subduction zone (SSZ) evolution of most Iranian ‘Inner’ and ‘Outer Zagros’ ophiolitic peridotites.  相似文献   

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