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1.
A wide compositional continuum of basalts has been erupted from near-ridge seamounts constructed on the Cocos Plate between the Clipperton and Orozco Francture Zones. They range from highly evolved to moderately primitive (3.0–7.8% MgO), LREE-enriched alkali basalts, to moderately evolved to near-primary (5.2–9.5% MgO) tholeiites indistinguishable from N-type MORB. The data set of 159 quench glass analyses exhibits a remarkably consistent variation in both major and trace element composition that is keyed to variations in (La/Sm). Modeling of potential liquid lines of descent at pressures ranging from 1 bar to 8 kbar shows that this covariation is partially due to systematic differences in liquid lines of descent, where the alkaline lavas have undergone substantially more high pressure clinopyroxene fractionation and substantially less low pressure plagioclase fractionation than the tholeiites. In addition, systematic variation in the composition of the more primitive glasses indicates that they were derived from mixing of discrete enriched and depleted melts in the heterogenous seamount mantle source at pressures of 8–10 kbar and greater, and that clinopyroxene may be a residual phase during partial melting. These results show that porous media flow in the seamount mantle source is minor and that melt transport is accomplished primarily through cracking and diking. This study supports suggestions that the general homogeneity of basalt along the EPR is due to mixing in sub-axial magma chambers and mush zones, with additional mixing during partial mantle melting and melt segregation.  相似文献   

2.
The basaltic lavas erupted throughout the Mojave Desert are basanites (SiO2<46%, normative nepheline>5%, and K2O>1.5%), alkali-olivine basalts (SiO2=46–48%; ne=0–5%; and K2O=1.0–1.5%), and low-alumina, sub-alkaline basalts (SiO2=48–51%; ne=0; K2O<1.0%). One volcano, Pisgah Crater, erupted five times, with lava from each successive phase containing more silica and less potash than the one proceeding it. This compositional trend is the reverse of that expected from differentiation of a single alkalic magma, and therefore, may represent a succession of magmas tapped from a zone of continuing partial melting in the mantle.These lava compositions suggest that first melting was under high water pressure and was followed by relatively dry partial melting of gamet-orthopyroxene-clinopyroxene-olivine assemblages. The successive increase in silica and alkali decrease also requires that the partial melting zone move to shallower levels.All lavas sampled in the Mojave Desert area have compositions that can best be explained by the extraction of magma from such a rising melting zone, analogous to the mantle diapirs suggested by Green and Ringwood.  相似文献   

3.
The petrogenesis of the igneous complex in the Spanish Peaks region, Colorado, is reevaluated with Sr isotopic data (published and new) and new trace element data of REE and other transition metals. These data indicate that the diverse rock types ranging from ultrabasic lamprophyres to granitic porphyry of the igneous complex have independent origins.The lamprophyric magmas could be derived from the upper mantle by small degrees of partial melting of garnet peridotite source(s). Limited intra-type crystal fractionation is responsible for the small differences in REE and other elemental abundances. The intermediate syenodiorites were also derived by partial melting of some mantle sources with garnet as a residual phase. However, the major and trace element data suggest that the source for the syenodiorites is distinctly different from that of the lamprophyres. The narrow range of K/Rb ratios observed in minettes (449–464), camptonites (384–450) and syenodiorites (370–460) suggest that amphibole fractionation is not important in the generation of these intra- and intertype rocks.The granitic rocks were derived by melting of a lower crust whose composition varied from that of a granodioritic granulite to an amphibolite. It is suggested that the high alkali contents of the granitic porphyries are due to the introduction of alkalis into the source granulites shortly prior to the melting.  相似文献   

4.
The Vulsinian lavas are dominated by a suite of undersaturated leucite-bearing basic to intermediate compositions. The remaining lavas are mainly oversaturated and have shoshonitic affinities. One hundred and thirty-five samples have been analysed for major elements and most for 20 trace elements. Twenty-seven lavas have been analysed for REE. They are all perpotassic (for the undersaturated lavas: K2O/Na2O=2–8) and have very high LIL element concentrations, (e.g. Rb=400–800 ppm, Th=25–150 ppm, REE/REEcho=c.200, (LREE/HREE)cho=c.20) even in the most basic rocks.The undersaturated lavas appear to be interrelated by fractional crystallization of cpx±olivine (from 14 to 11 wt.% CaO), cpx+leu±plg±mica (from 11 to 8 wt.% CaO), cpx+leu+plg+apa+magnetite±mica (from 8 to 5 wt.% CaO), and additional sanidine (or hyalophane)±haüyne (from 5 to 3 wt.% CaO). The saturated lavas and the few slightly undersaturated shoshonite basalts are thought to be evolved from the undersaturated magma(s) by crustal contamination or mixing with silica-rich magmas. The parental Vulsinian magma having: Mg-value=c.73, Cr=300–700 ppm, Ni=100–125 ppm, Sc= 40–50 ppm, Fo89–92, Di77–97 approximates a primary, mantle-derived liquid. Enrichment in LIL elements (incl. REE) and LREE/HREE suggest a small degree of partial melting from fertile mantle; whereas the low concentrations of Na, Ti and P suggest larger degrees of partial melting. This indicates that either the primary magma or the parental mantle was metasomatized by a fluid, which previously equilibrated with subducted continental material. This model agrees with published high 18O, high 87Sr/86Sr and low 143Nd/144Nd.  相似文献   

5.
Partitioning of elements between majorite garnet and ultrabasic melt has been studied at 16 GPa and 1950° C. Ca, Ti, La, Sm, Gd, Zr, Hf, Fe, Ni, Mn, K, and Na are enriched in the melt, whereas Al, Cr, V, Sc and Yb are concentrated in majorite garnet. Thus, majorite garnet fractionation by partial melting could produce chemical heterogeneities in these elements deviating from chondritic abundance. Using the partitioning behaviour of elements between majorite garnet and ultrabasic melt, the petrogenesis of komatiite is discussed. A simple model to explain the chemical varieties of komatiites is as follows. Aluminadepleted komatiite was generated by partial melting of the primitive mantle at 200–650 km depth, and alumina-enriched komatiite is the product of remelting of the residual solid at the same depths, whereas alumina-undepleted komatiite was formed by partial melting of the primitive upper mantle at depths shallower than 200 km. We suggest the possibility of large-scale chemical layering or heterogeneity in the early Archean upper mantle as an alternative model for komatiite genesis; shallower mantle depleted in majorite garnet and the underlying mantle enriched in majorite garnet. Alumina-depleted and alumina-enriched komatiites in the early Archean might be generated by a high degree of partial melting of the layered mantle. Such chemical layering could have been homogenized by the late Archean. This explains the observations that alumina-depleted and alumina-enriched komatiites were generally formed in the early Archean but alumina-undepleted komatiite was erupted in the late Archean.  相似文献   

6.
An ophiolite complex includes three major members: basaltic volcanics, mafic-ultramafic cumulates, and residual peridotite. From the aspect of igneous petrology, three distinct types are recognized among the associations of the three members: (Liguria type) alkalic basalt, plagioclase-type cumulates, and lherzolite; (Yakuno type) high-alumina tholeiite, clinopyroxene-type cumulates, and clinopyroxene-bearing harzburgite; (Papua type) low-alumina tholeiite, orthopyroxene-type cumulates, and clinopyroxene-free harzburgite. In the light of recent experimental studies, the three types represent cogenetic, complementary products of low (<15%), moderate, and high (>30%) degrees of partial melting in the lherzolitic source mantle, respectively. The cumulates of the Yakuno ophiolite show structural and chemical continuity to the underlying residual peridotite, and were recrystallized at high pressures (5–10 kb). They originated in a deep, soft-floored magma chamber directly overlying the partially melted residual harzburgite, from which the magma was extracted. The three members of the Yakuno ophiolite were cogenetically formed through a magmatic event induced by a moderate degree of partial melting in the mantle.  相似文献   

7.
The Neogene Yamadağ volcanics occupy a vast area between Sivas and Malatya in eastern Anatolia, Turkey. These volcanic rocks are characterized by pyroclastics comprising agglomerates, tuffs and some small outcrops of basaltic–andesitic–dacitic rocks, overlain upward by basaltic and dacitic rocks, and finally by basaltic lava flows in the Arapkir area, northern Malatya Province. The basaltic lava flows in the Arapkir area yield a 40Ar/39Ar age of 15.8 ± 0.2 Ma, whereas the dacitic lava flows give 40Ar/39Ar ages ranging from 17.6 through 14.7 ± 0.1 to 12.2 ± 0.2 Ma, corresponding to the Middle Miocene. These volcanic rocks have subalkaline basaltic, basaltic andesitic; alkaline basaltic trachyandesitic and dacitic chemical compositions. Some special textures, such as spongy-cellular, sieve and embayed textures; oscillatory zoning and glass inclusions in plagioclase phenocrysts; ghost amphiboles and fresh biotite flakes are attributable to disequilibrium crystallization related to magma mixing between coeval magmas. The main solidification processes consist of fractional crystallization and magma mixing which were operative during the soldification of these volcanic rocks. The dacitic rocks are enriched in LILE, LREE and Th, U type HFSE relative to the basaltic rocks. The basaltic rocks also show some marked differences in terms of trace-element and REE geochemistry; namely, the alkaline basaltic trachyandesites have pronounced higher HFSE, MREE and HREE contents relative to the subalkaline basalts. Trace and REE geochemical data reveal the existence of three distinct magma sources – one subalkaline basaltic trachyandesitic, one alkaline basaltic and one dacitic – in the genesis of the Yamadağ volcanics in the Arapkir region. The subalkaline basaltic and alkaline basaltic trachyandesitic magmas were derived from an E-MORB type enriched mantle source with a relatively high- and low-degree partial melting, respectively. The magmatic melt of dacitic rocks seem to be derived from an OIB-type enriched lithospheric mantle with a low proportion of partial melting. The enriched lithospheric mantle source reflect the metasomatism induced by earlier subduction-derived fluids. All these coeval magmas were generated in a post-collisional extensional geodynamic setting in Eastern Anatolia, Turkey.  相似文献   

8.
Melting experiments on a mantle-derived nodule assemblage consisting of clinopyroxene, phlogopite and minor titanomagnetite, sphene and apatite have been done at 20 and 30 kbar between 1,175 and 1,300° C. The nodule composition was selected on the basis of modal and chemical analyses of 84 mantle derived nodules with metasomatic textures from the Katwe-Kikorongo and Bunyaruguru volcanic fields of south-west Uganda. At 30 kbar, 1,225 and 1,250° C, representing 20–30% partial melting, the compositions of glasses compare favourably to those of the average composition of 26 high potassic mafic lavas from the same region. Glasses produced by sufficiently low degrees of partial melting at 20 kbar could not be analysed. Glass compositions obtained for 20–30% melting at 30 kbar have high K2O (3.07–5.05 wt.%), low SiO2 (35.0–39.2 wt.%), high K/K + Na (0.54–0.71), K + Na/Al (0.99–1.08) and Mg/ Mg + FeT of 0.59–0.62. These results support the suggestion of Lloyd and Bailey (1975) that the nodules represent the source material for the high K-rich lavas of south-west Uganda. If this conclusion is correct it implies that anomalous mantle source of phlogopite clinopyroxenite composition could produced the Ugandan lavas by relatively higher degrees of partial melting than that normally considered for highly alkaline mafic magmas derived from a pyrolitic mantle source. Higher degrees of melting are considered likely from such a different source region, rich in alkalis, water and radioactive elements. Steeper geotherms and increased fluxing of sub-rift mantle by degassing would also produce higher degrees of partial melting.  相似文献   

9.
《Geochimica et cosmochimica acta》1999,63(23-24):4119-4138
Measurements of 238U-230Th-226Ra and 235U-231Pa disequilibria in a suite of tholeiitic-to-basanitic lavas provide estimates of porosity, solid mantle upwelling rate and melt transport times beneath Hawaii. The observation that (230Th/238U) > 1 indicates that garnet is required as a residual phase in the magma sources for all of the lavas. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Hawaiian basalts. For chromatographic porous flow, the calculated maximum porosity in the melting zone ranges from 0.3–3% for tholeiites and 0.1–1% for alkali basalts and basanites, and solid mantle upwelling rates range from 40 to 100 cm yr−1 for tholeiites and from 1 to 3 cm yr−1 for basanites. For dynamic melting, the escape or threshold porosity is 0.5–2% for tholeiites and 0.1–0.8% for alkali basalts and basanites, and solid mantle upwelling rates range from 10 to 30 cm yr−1 for tholeiites and from 0.1 to 1 cm yr−1 for basanites. Assuming a constant melt productivity, calculated total melt fractions range from 15% for the tholeiitic basalts to 3% for alkali basalts and basanites.  相似文献   

10.
The genesis of basaltic magmas   总被引：29，自引：2，他引：29  
This paper reports the results of a detailed experimental investigation of fractionation of natural basaltic compositions under conditions of high pressure and high temperature. A single stage, piston-cylinder apparatus has been used in the pressure range up to 27 kb and at temperatures up to 1500° C to study the melting behaviour of several basaltic compositions. The compositions chosen are olivine-rich (20% or more normative olivine) and include olivine tholeiite (12% normative hypersthene), olivine basalt (1% normative hypersthene) alkali olivine basalt (2% normative nepheline) and picrite (3% normative hypersthene). The liquidus phases of the olivine tholeiite and olivine basalt are olivine at 1 Atmosphere, 4.5 kb and 9 kb, orthopyroxene at 13.5 and 18 kb, clinopyroxene at 22.5 kb and garnet at 27 kb. In the alkali olivine basalt composition, the liquidus phases are olivine at 1 Atmosphere and 9 kb, orthopyroxene with clinopyroxene at 13.5 kb, clinopyroxene at 18 kb and garnet at 27 kb. The sequence of appearance of phases below the liquidus has also been studied in detail. The electron probe micro-analyser has been used to make partial quantitative analyses of olivines, orthopyroxenes, clinopyroxenes and garnets which have crystallized at high pressure.These experimental and analytical results are used to determine the directions of fractionation of basaltic magmas during crystallization over a wide range of pressures. At pressures corresponding to depths of 35–70 km separation of aluminous enstatite from olivine tholeiite magma produces a direct fractionation trend from olivine tholeiites through olivine basalts to alkali olivine basalts. Co-precipitation of sub-calcic, aluminous clinopyroxene with the orthopyroxene in the more undersaturated compositions of this sequence produces derivative liquids of basanite type. Magmas of alkali olivine basalt and basanite type represent the lower temperature liquids derived by approximately 30% crystallization of olivine-rich tholeiite at 35–70 km depth. At depths of about 30 km, fractionation of olivine-rich tholeiite with separation of both olivine and low-alumina enstatite, joined at lower temperatures by sub-calcic clinopyroxene, leads to derivative liquids with relatively constant SiO2 (48 to 50%) increasingly high Al2O3 (15–17%) contents and retaining olivine + hypersthene normative chemistry (5–15% normative olivine). These have the composition of typical high-alumina olivine tholeiites. The effects of low pressure fractionation may be superimposed on magma compositions derived from various depths within the mantle. These lead to divergence of the alkali olivine basalt and tholeiitic series but convergence of both the low-alumina and high-alumina tholeiites towards quartz tholeiite derivative liquids.The general problem of derivation of basaltic magmas from a mantle of peridotitic composition is discussed in some detail. Magmas are considered to be a consequence of partial melting but the composition of a magma is determined not by the depth of partial melting but by the depth at which magma segregation from residual crystals occurs. Magma generation from parental peridotite (pyrolite) at depths up to 100 km involves liquid-crystal equilibria between basaltic liquids and olivine + aluminous pyroxenes and does not involve garnet. At 35–70 km depth, basaltic liquids segregating from a pyrolite mantle will be of alkali olivine basalt type with about 20% partial melting but with increasing degrees of partial melting, liquids will change to olivine-rich tholeiite type with about 30% melting. If the depth of magma segregation is about 30 km, then magmas produced by 20–25% partial melting will be of high-alumina olivine tholeiite type, similar to the oceanic tholeiites occurring on the sea floor along the mid-oceanic ridges.Hypotheses of magma fractionation and generation by partial melting are considered in relation to the abundances and ratios of trace elements and in relation to isotopic abundance data on natural basalts. It is shown that there is a group of elements (including K, Ti, P, U, Th, Ba, Rb, Sr, Cs, Zr, Hf and the rare-earth elements) which show enrichment factors in alkali olivine basalts and in some tholeiites, which are inconsistent with simple crystal fractionation relationships between the magma types. This group of elements has been called incompatible elements referring to their inability to substitute to any appreciable extent in the major minerals of the upper mantle (olivine, aluminous pyroxenes). Because of the lack of temperature contrast between magma and wall-rock for a body of magma near to its depth of segregation in the mantle, cooling of the magma involves complementary processes of reaction with the wall-rook, including selective melting and extraction of the lowest melting fraction. The incompatible elements are probably highly concentrated in the lowest melting fraction of the pyrolite. The production of large overall enrichments in incompatible elements in a magma by reaction with and highly selective sampling of large volumes of mantle wall-rock during slow ascent of a magma is considered to be a normal, complementary process to crystal fractionation in the mantle. This process has been called wall-rock reaction. Magma generation in the mantle is rarely a simple, closed-system partial melting process and the isotopic abundances and incompatible element abundances of a basalt as observed at the earth's surface may be largely determined by the degree of reaction with the mantle or lower crustal wall-rocks and bear little relation to the abundances and ratios of the original parental mantle material (pyrolite).Occurrences of cognate xenoliths and xenocrysts in basalts are considered in relation to the experimental data on liquid-crystal equilibria at high pressure. It is inferred that the lherzolite nodules largely represent residual material after extraction of alkali olivine basalt from mantle pyrolite or pyrolite which has been selectively depleted in incompatible elements by wall-rock reaction processes. Lherzolite nodules included in tholeiitic magmas would melt to a relatively large extent and disintegrate, but would have a largely refractory character if included in alkali olivine basalt magma. Other examples of xenocrystal material in basalts are shown to be probable liquidus crystals or accumulates at high pressure from basaltic magma and provide a useful link between the experimental study and natural processes.  相似文献   

11.
Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhön, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr–Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of D with 87Sr/86Sr in amphibole and phlogopite and a slightly larger 18O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.  相似文献   

12.
The Tianshan Carboniferous–Permian rift-related volcanism in northwestern China represents a newly recognized large igneous province extending over at least 1.5 × 106 km2. The volcanic successions comprise thick piles of basaltic lavas and subordinate intermediate and silicic lavas and pyroclastics, and are interpreted to result from a mantle plume head with component of εNd(t) ≈ +5, 87Sr/86Sr(t) ≈ 0.704 and La/Nb ≈ 0.9. On the basis of petrogeochemical data, the Carboniferous basic lavas can be generally incorporated into low-Ti/Y (LT, Ti/Y < 500) magma type that can be further divided into three subtypes: LT1, LT2 and LT3. The chemical evolution of the LT1, LT2 (in central Tianshan) and LT3 (in western Tianshan and Jungar) lavas is controlled by an olivine (ol) + clinopyroxene (cpx) fractionation, but gabbroic fractionation accounts for the chemical variation of the LT3 lavas from eastern Tianshan. Elemental and isotopic data suggest that the chemical variation of Tianshan Carboniferous basic lavas cannot be explained by crystallization from a common parental magma.The Sr–Nd isotopic variation of the crustally contaminated LT3 lavas is related to the nature of lithosphere through which the plume-derived melts have erupted. The involvement of an older (Precambrian) lithosphere led the LT3 lavas in western Tianshan to have lower to negative εNd(t) (−1.2 to +6.1) and variable 87Sr/86Sr(t) (0.7036–0.7061), whereas the LT3 lavas from eastern Tianshan and Jungar are characterized by high εNd(t) (+4.2 to +9.7) and low 87Sr/86Sr(t) (0.7035–0.7044), that are related to the contamination of upper crust containing early Paleozoic and Devonian arc-basin volcanic rocks and/or to a pre-Carboniferous subduction enrichment of the lithospheric mantle source region. The observed geochemical variations in the Tianshan data are consistent with an AFC process.The Tianshan Carboniferous rift-related volcanic rocks display a spatial petrogeochemical variation in which predominantly uncontaminated LT1 and less-contaminated LT2 tholeiitic lavas erupted in central Tianshan rift and predominantly the strongly contaminated LT3 tholeiites erupted in the circumjacent regions of the central Tianshan rift. The LT1 and LT2 lavas were generated by a higher degree (10–30%) of partial melting in the garnet stability field of the mantle plume compared to the LT3 lavas. The lower degree (<10%) of partial melting in the spinel–garnet transition zone of the mantle plume, as is characteristic of the LT3 lavas, may be the result of a relatively lower geotherm.  相似文献   

13.
This study presents Sr and Pb isotopic ratios and Rb, Sr, U, Th, and Pb concentrations of an ultrapotassic basaltic suite and related rocks from the central Sierra Nevada, California. The ultrapotassic suite yields a narrow range of Sr and Pb isotopic compositions (87Sr/86Sr=0.70597–0.70653; 206Pb/ 204Pb=18.862–19.018; 207Pb/204Pb=15.640–15.686; 208Pb/ 204Pb=38.833–38.950). Associated basalts containing ultramafic nodules have less radiogenic Sr (87Sr/86=0.70430–0.70521) and generally higher Rb/Sr ratios than the ultrapotassic suite. Leucitites from Deep Springs Valley, California, contain high 87Sr/86Sr (71141–0.71240) and low 206Pb/204Pb (17.169–17.234) ratios, reflecting contamination by crustal granulite.The isotopic relationships support an origin of the ultrapotassic basaltic suite by partial melting of an enriched upper mantle source. Dehydration of a gently inclined oceanic slab beneath the Sierra Nevada may have provided Ba, K, Rb, Sr, and H2O, which migrated into the overlying upper mantle lithosphere. The end of subduction 10 m.y. ago allowed increased asthenospheric heat flow into the upper mantle lithosphere. The increased heat flow enhanced fluid movement in the upper mantle and contributed towards isotopic homogenization of the upper mantle source areas. Continued heating of the enriched upper mantle caused partial melting and subsequent eruption of the ultrapotassic lavas.  相似文献   

14.
A 4 km2 exposure of shallowly-emplaced leucogranite on the Atlantic coast at Cabo de Santo Agostinho, 30 km south of Recife, Brazil has been extensively studied chemically and isotopically. Twenty-three major-element analyses indicate that the Cabo granite ranges from peralkaline to peraluminous; Na2O+K2O is very high (7.4 to 10.4 wt.%), with CaO low (0.3%) and MgO vanishing (<0.06%). Microprobe analyses confirm the presence of arfvedsonite (biotite absent), and nearly total absence of plagioclase. The rocks are moderately to highly enriched in LREE (La 45 to 350 times chondritic), with extremely pronounced negative Eu anomalies (Eu/Eu*=0.02 to 0.07). Whole-rock 18O is consistent at +8.5±0.3%.oSMOW A Rb-Sr isochron age of isotopically slightly disturbed samples is 104.8±1.8 Ma, with initial87Sr/86Sr=0.7084±0.0011. Sr is depleted (2–20 ppm) but Ba is 200–750 ppm.Crystallization path calculations and petrographic observations suggest that magma formed at a pressure close to 6 kbar but rose to a crustal level equivalent to roughly 1 kbar. Quartz, the liquidus phase at moderate H2O concentrations and pressures above 2 kbar, was resorbed during decompression as the magma moved upwards. Ultimately, quartz and alkali feldspar coprecipitated. Feldspar was not retained in the source rock nor removed early from the fractionating magma. Therefore the strong negative Eu anomaly and low Sr abundance are characteristics inherited from the source. A high H2O concentration necessary for a large degree of melting was lacking, hence the Cabo magma composition must reflect a small degree of partial melting of a rather quartz-rich rock such as a feldspathic arenite.In a pre-drift reconstruction of Gondwanaland, the Cabo granite fits on the southernmost and youngest end of the trend of the Niger-Nigerian igneous centers with which it has close affinity. The Cabo granite occupies the western end of the trace of the ancestral Ascension mantle plume which presumably served as the heat source.  相似文献   

15.
We report analyses of noble gases and Nd–Sr isotopes in mineral separates and whole rocks of late Pleistocene (< 0.2 Ma) monzonites from Ulleungdo, South Korea, a volcanic island within the back arc basin of the Japan island arc. A Rb–Sr mineral isochron age for the monzonites is 0.12 ± 0.01 Ma. K–Ar biotite ages from the same samples gave relatively concordant ages of 0.19 ± 0.01and 0.22 ± 0.01 Ma. 40Ar/39Ar yields a similar age of 0.29 ± 0.09 Ma. Geochemical characteristics of the felsic plutonic rocks, which are silica oversaturated alkali felsic rocks (av., 12.5 wt% in K2O + Na2O), are similar to those of 30 alkali volcanics from Ulleungdo in terms of concentrations of major, trace and REE elements. The initial Nd–Sr isotopic ratios of the monzonites (87Sr/86Sr = 0.70454–0.71264, 143Nd/144Nd = 0.512528–0.512577) are comparable with those of the alkali volcanics (87Sr/86Sr = 0.70466–0.70892, 143Nd/144Nd = 0.512521–0.512615) erupted in Stage 3 of Ulleungdo volcanism (0.24–0.47 Ma). The high initial 87Sr/86Sr values of the monzonites imply that seawater and crustally contaminated pre-existing trachytes may have been melted or assimilated during differentiation of the alkali basaltic magma.A mantle helium component (3He/4He ratio of up to 6.5 RA) associated with excess argon was found in the monzonites. Feldspar and biotite have preferentially lost helium during slow cooling at depth and/or during their transportation to the surface in a hot host magma. The source magma noble gas isotopic features are well preserved in fluid inclusions in hornblende, and indicate that the magma may be directly derived from subcontinental lithospheric mantle metasomatized by an ancient subduction process, or may have formed as a mixture of MORB-like mantle and crustal components. The radiometric ages, geochemical and Nd–Sr isotopic signatures of the Ulleungdo monzonites as well as the presence of mantle-derived helium and argon, suggests that these felsic plutonic rocks evolved from alkali basaltic magma that formed by partial melting of subcontinental lithospheric mantle beneath the back arc basin located along the active continental margin of the southeastern part of the Eurasian plate.  相似文献   

16.
Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for post-collisional late Cenozoic shoshonitic volcanic rocks from the western Kunlun Mountains, NW China. They are distributed in two approximately E–W striking sub-belts, with the lavas in the southern sub-belt having been generated earlier than those in the northern sub-belt. The mineralogy of the rocks reflects crystallization from moderate temperature magmas (700–1000 °C) with high oxygen and water fugacities. They are geochemically characterized by relatively low TiO2, Al2O3 and FeO and high alkalies coupled with very high contents of incompatible element concentrations. Remarkably negative Nb, Ta and Ti anomalies are displayed on primitive mantle-normalized incompatible element patterns. In addition, they show a relatively broad range of low εNd (−1.8 to −8.7) at more restricted 87Sr/86Sr ratios (0.7081–0.7090). Pb isotopes are characterized by a range of 207Pb/204Pb (15.48–15.74) and 208Pb/204Pb (38.30–39.12) ratios at relatively invariant 206Pb/204Pb (18.60–18.83) values, except one sample with a ratio of 18.262, leading to near-vertical arrays. The lavas from the northern sub-belt have relatively high 87Sr/86Sr ratios. All lavas have extremely high La/Yb ratios, probably reflecting that the magmas were derived from a metasomatized lithospheric mantle source containing phlogopite–hornblende garnet peridotite affected by subducted sediments and hydrous fluids, rather than from a depleted asthenopheric mantle source or mantle plume source. However, the lavas from the southern sub-belt were derived from a lower degree of melting of more highly metasomatized sub-lithospheric mantle in comparison with those from the northern sub-belt. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in the western Kunlun could be a consequence of continuously conductive heating of upwelling, hot asthenospheric mantle following the delamination subsequent to thickening, which is consistent with the spatial and temporal geochemical variations in shoshonitic rocks in Tibet.  相似文献   

17.
Primary basanitoids from Ross Island, Antarctica have REE patterns and Pb isotope ratios similar to those for primary alkali basalts and nephelinites on ocean islands. The lead data from all volcanics on Ross Island have a spread of 4% in the 206/204 ratio and give a two-stage model lead age of 1500 m.y. The age is interpreted to be the time since the development of the chemical heterogeneity of the mantle source, presumably during an earlier melting process. Comparison of REE, K, Rb, Sr, Ba and P2O5 concentrations for alkali basalts and nephelinites shows that the chondrite normalized mantle source is enriched in light REE with average La/Sm=3.4, Ce/Sm=2.6, Nd/Sm=1.6. Assuming a mantle source with heavy REE abundances of three times chondrites, nephelinites require 3 to 7% partial melting of the mantle source and alkali basalts require 7 to 15% partial melting. The patterns of K, Cu, V and Ti abundances suggest that phlogopite is a residual mineral for most nephelinite, but not alkali basalt mantle sources, and that a sulfide phase and a titanium-rich mineral are in the residual mantle source for both alkali basalts and nephelinites. Small positive Eu anomalies (2–5%) in near primary alkali basalts and nephelinites suggest that the xxx of the mantle sources is 10?6 to 10?9 atm. The progressive enrichment of light REE and incompatible elements in the mantle sources for nephelinites and alkali basalts is proposed to result by intrusion of veins of basaltic melt due to very low percentages of melting 1 000 to 3 000 m.y. ago when this part of the deeper mantle was previously involved in convection and partial melting.  相似文献   

18.
Mantle xenoliths from Hainan and Qilin, South China have been studied to constrain the nature of the upper mantle and mantle processes beneath a continental margin. The extremely low Ti (160–245 ppm) contents in clinopyroxenes from some spinel lherzolites, indicative of high degrees of partial melting are inconsistent with the relatively high clinopyroxene modes (7.4–13%) in these samples. This inconsistency could be due to polybaric melting that started in the garnet stability field, then, after the breakdown of garnet to pyroxene and spinel, continued in the spinel stability field. Polybaric melting, due to adiabatic decompression of upwelling mantle, would leave a residual mantle in which the degree of depletion decreases with depth. The predicted stratified lithospheric mantle is evidenced by the negative correlation between the forsterite content in olivine and the equilibration temperature, proportional to the depth in the lithosphere from which the xenolith was derived. The lower part of the lithospheric mantle beneath South China consists predominantly of fertile and moderately depleted peridotites, which are either devoid of LREE enrichment, or show the trace element signature of incipient metasomatism, and plot within the Phanerozoic mantle domain. In contrast, the upper part of the mantle contains harzburgite and cpx-poor lherzolite, which are strongly affected by metasomatism of melt/fluid of highly variable composition. The anomalously high orthopyroxene mode (up to 47%) makes some of these refractory samples compositionally similar to the Proterozoic/Archean mantle. Their low equilibrium temperature (800–900 °C) points to the presence of old lithospheric relicts in the uppermost mantle beneath South China. Such lithosphere architecture may have resulted from partial replacement of Archean–Proterozoic lithosphere by asthenosphere that rose adiabatically subsequent to lithospheric thinning during the Cenozoic.  相似文献   

19.
The Hirschberg and Rödern diatremes, within the Permian Saar-Nahe trough, SW Germany, are composed chiefly of basaltic tuffs, with associated small intrusions of K-rich tholeiites. Several tholeiite bodies carry 2–20 mm crystals of magnesian clinopyroxene and orthopyroxene, the latter containing up to 5.5% Al2O3 and often extensively resorbed and rimmed by fine-grained olivine and clinopyroxene. Experimental duplication of these pyroxenes has been achieved under conditions of Pload=6–10 kb, T=1280–1080° C and 2–4 wt.-% H2O, confirming that they represent a rare occurence of high pressure phenocrysts in tholeiitic basalts.These conditions of pyroxene crystallization also place constraints on processes of magma generation, indicating that the tholeiites originated by partial melting of unusually hydrous peridotite mantle (0.4–0.8% H2O) beneath a relatively thin continental crust (maximum thickness approximately 30 km). Water present in the mantle at the site of magma generation may have been derived from the dehydration of oceanic lithosphere prior to the formation of the Saar-Nahe trough. This lithosphere probably underwent subduction at the margin of the Palaeozoic European continent during the Hercynian cycle of sedimentation, andesitic volcanism and folding. The termination of this cycle was followed by a period of basin-range type tensional faulting, leading to the formation of the Permian basins of present-day Central Europe, and widespread bimodal basalt/rhyolite volcanism.  相似文献   

20.
Faruk Aydin  Orhan Karsli  Bin Chen 《Lithos》2008,104(1-4):249-266
Whole-rock geochemistry, Sr–Nd–Pb isotopes and K–Ar data are reported for alkaline samples collected from the Neogene alkaline volcanics (NAVs) in the Eastern Pontides, northeastern Turkey, in order to investigate their source and petrogenesis and geodynamic evaluation of the region. The NAVs were made of three groups that comprise of basanite–tephrite (feldspar-free; Group A), tephrite–tephriphonolite (feldspar and feldspathoid-bearing; Group B) and alkaline basalt–rhyolite (feldspathoid-free; Group C) series. These rocks cover a broad compositional range from silica-undersaturated to silica-oversaturated types, almost all of which are potassic in character. They show enrichment of LREE and LILE and depletion of HFSE, without a Eu anomaly in most of the mafic samples. Textural features and calculated pressures based on the Cpx-barometer in each series indicate that the alkaline magma equilibrated at shallow crustal depths under a pressure of about 3–4.5 kbar and approximating a crystallization depth of 9–14 km. The NAVs are slightly depleted in isotopic composition, with respect to 87Sr/86Sr (ranging from 0.705018 to 0.705643) and 143Nd/144Nd (ranging from 0.512662 to 0.512714) that indicate young Nd model ages (0.51–059 Ga). This may indicate that the parent melts tapped a homogeneous and young lithospheric mantle source which was metasomatized by subduction-derived sediments during the Late Mesozoic. Pb isotopic compositions (206Pb/204Pb = 18.85–18.95; 207Pb/204Pb = 15.60–15.74; 208Pb/204Pb = 38.82–39.25) may also be consistent with a model for an enriched subcontinental lithospheric mantle source. Lithospheric thinning and resultant upwelling of asthenosphere induced by lithospheric delamination may have favoured partial melting of chemically enriched, young lithospheric mantle beneath the Eastern Pontides. Then, the melt subsequently underwent a fractional crystallization process along with or without minor amounts of crustal assimilation, generating a wide variety of rock types in a post-collision extensional regime in the Eastern Pontides during the Neogene.  相似文献   

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