首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Uncontaminated volcanic rocks from the 60 Ma Vaigat Formation,West Greenland, contain 6·5–30 wt % MgO, averaging15·5 wt % MgO. Olivine (mg-number 77·4–93·3)forms diverse assemblages of zoned phenocrysts and xenocrystsshowing evidence for equilibrium and fractional crystallization,oxidation, partial to complete re-equilibration, as well asmagma mixing. The olivine crystals contain glass inclusionsand have high contents of Ca and Cr, indicating that all olivineswith up to mg-number 93·0 crystallized from melts. Associatedchromites (mg-number 45·4–77·2) are essentiallyunzoned and in equilibrium with the olivines. Matrix glassesfrom pillow breccias have 6·7–8·8 wt % MgOand quenched close to 1200°C with oxidation states one log-unitabove the NNO (nickel–nickel oxide) buffer. Compositionaldifferences between the glasses from different volcanic membersare inherited from the primary melts. The magmas erupted ascrystal-charged melts, and liquids with more than  相似文献   

2.
The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (143Nd/144Nd = 0·51267–0·51283,87Sr/86Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar  相似文献   

3.
Picritic lava flows near Lijiang in the late Permian Emeishanflood basalt province are associated with augite-phyric basalt,aphyric basalt, and basaltic pyroclastic units. The dominantphenocryst in the picritic flows is Mg-rich olivine (up to 91·6%forsterite component) with high CaO contents (to 0·42wt %) and glass inclusions, indicating that the olivine crystallizedfrom a melt. Associated chromite has a high Cr-number (73–75).The estimated MgO content of the primitive picritic liquidsis about 22 wt %, and initial melt temperature may have beenas high as 1630–1690°C. The basaltic lavas appearto be related to the picritic ones principally by olivine andclinopyroxene fractionation. Age-corrected Nd–Sr–Pbisotope ratios of the picritic and basaltic lavas are indistinguishableand cover a relatively small range [e.g. Nd(t) = –1·3to +4·0]. The higher Nd(t) lavas are isotopically similarto those of several modern oceanic hotspots, and have ocean-island-likepatterns of alteration-resistant incompatible elements. Heavyrare earth element characteristics indicate an important rolefor garnet during melting and that the lavas were formed byfairly small degrees of partial melting. Rough correlationsof isotope ratios with ratios of alteration-resistant highlyincompatible elements (e.g. Nb/La) suggest modest amounts ofcontamination involving continental material or a relativelylow-Nd component in the source. Overall, our results are consistentwith other evidence suggesting some type of plume-head originfor the Emeishan province. KEY WORDS: Emeishan; flood basalts; picrites; mantle plumes; late Permian  相似文献   

4.
Plume-Associated Ultramafic Magmas of Phanerozoic Age   总被引：19，自引：12，他引：19  
A parameterization of experimental data in the 0·2–7·0GPa pressure range constrains both forward models of potentialprimary magma compositions that exit the melting regime in themantle and inverse models for computing the effects of olivinefractionation for any olivine-phyric lava suite. This is usedto infer the MgO contents of primary magmas from Gorgona, Hawaii,Baffin Island and West Greenland. They typically contain 18–20%MgO for wide variations in assumed peridotite source compositions,but MgO can drop to 14–17% for Fe-enriched sources, andincrease to 24–26% for fractional melts from Gorgona.Primary magmas with 18–20% MgO have potential temperaturesof 1520–1570°C. For Gorgona picrites with 24% MgO,the potential temperature and initial melting pressure wereabout 1700°C and 8·0 GPa, respectively; melting washot and deep, consistent with the plume model. There are importantrestrictions to magma mixing in mantle plumes. Primary magmasthat exit the melting regime are both well-mixed aggregate fractionalmelts and isolated fractional melts. The latter can originatefrom a hot plume axis and be in equilibrium with olivines havingmg-numbers of 93·0–93·6, but they have MgOcontents and thermal characteristics that are difficult to constrain. KEY WORDS: komatiite; picrite; basalt; MORB; olivine; mantle plumes; primary magmas; equilibrium melting; accumulated fractional melting  相似文献   

5.
Roca Redonda volcano is a mostly submarine shield volcano that rises nearly 3 km from the adjacent seafloor. Over twenty lava flows and palagonite tuff are exposed in a 60 meter high oblong outcrop above sea level, and several other flows are exposed in the shallow water surrounding the islet. Thick, slightly alkaline picritic flows form the base of the section. Thinner picrites interbedded with sparsely porphyritic alkali-olivine basaltic pahoehoe toes characterize the upper section. The subaerial section probably records the filling of a palagonite tuff cone with younger lavas. Numerous fumaroles that may have a magmatic component are present in the shallow (<30 m) submarine zone and indicate that the volcano is probably still active. Three lava types are exposed: the basal picrites with 19% > MgO > 14%, high-Mg basalts with MgO of about 9%, and low-Mg basalts with MgO of about 6%. The Sr and Nd isotopic ratios of the three lava types are within analytical uncertainty. Olivine compositions indicate that the picrites are basaltic liquids that have accumulated olivine whose composition is in equilibrium with the host basaltic liquid. Apparently, basaltic magmas percolated through dunite and troctolite that had crystallized from slightly older Roca Redonda basaltic magma. Lavas from Roca Redonda have enriched trace element contents and isotopic ratios relative to nearby Wolf volcano, but they are quite similar to lavas from Cerro Azul and Ecuador volcanoes. The common characteristic of these volcanoes is that they lie on the periphery of the archipelago and are in a stage of subaerial growth. This suggests that Galápagos volcanoes may go through a juvenile alkaline stage before a mature tholeiitic stage, analogous to the Loihi stage of Hawaiian volcanism. A low 3He/4He ratio in olivine from one of the picrites indicates a small contribution by the Galápagos mantle plume. Received: 15 December 1997 / Accepted: 6 May 1998  相似文献   

6.
Olivines with a forsterite minal of up to 99.5 mol % were discovered in ultra-potassium lavas of Pian di Celle volcano (Italy). Based on high-precision analyses of these olivines and the available literature data, a new type of extreme-Mg olivines with a forsterite content of >96 mol % was distinguished. These olivines could have formed as a result of oxidation of lower Mg olivines, crystallization from the carbonatite melt, or they may represent earlier crystallized phenocrysts in skarn. Oxidized olivines are easily diagnosed on the content of typomorphic admixtures, in particular, high MgO and NiO contents. It is difficult to recognize olivines from skarns and carbonatites due to the lack of high-precision analytical data. Extreme-Mg olivines from Pian di Celle (Italy) are close to the carbonatite subtype of olivines.  相似文献   

7.
The Antampombato–Ambatovy complex is the largest intrusionin the central–eastern part of the Cretaceous flood basaltprovince of Madagascar, with an exposed surface area of about80 km2. It has an 40Ar/39Ar incremental heating age of 89·9± 0·4 Ma and a U–Pb age of 90 ± 2Ma. The outcropping plutonic rocks range from dunite and wehrlite,through clinopyroxenite and gabbro, to sodic syenite. A dykeswarm cross-cutting some of the above lithologies (and the nearbyPrecambrian basement rocks) is formed of picritic basalts, alkalito transitional basalts, benmoreites and rhyolites; some ofthe latter are peralkaline. A few basaltic dykes have cumulateolivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereasothers have characteristics more akin to those of primitiveliquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200ppm Ni). These basalts have relatively high TiO2 (2·2wt %) and total iron (14 wt % as Fe2O3), and moderate contentsof Nb (10–11 ppm) and Zr (c. 100 ppm). Initial (at 90Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basaltdykes are 0·7030–0·7037 and 0·51290–0·51283,respectively. Syenites and peralkaline rhyolites have Sr- andNd-isotope ratios of 0·7037–0·7039 and 0·51271–0·51274,respectively. The data suggest derivation of the parental magmasfrom a time-integrated depleted mantle source, combined withsmall amounts of crustal contamination in the petrogenesis ofthe more evolved magmas. The isotopic compositions of the mafic–ultramaficrocks are most similar to those of the mid-ocean ridge basalt(MORB)-like igneous rocks of eastern Madagascar, and suggestthe existence of an isotopically ‘depleted’ componentin the source of the entire Madagascar province, even thoughthe Antampombato basalts are chemically unlike the lavas anddykes with the same depleted isotopic signature found in westernMadagascar. If this depleted component is plume-related, thissuggests that the plume has a broadly MORB-source mantle composition.The existence of isotopically more enriched magma types in theMadagascan province has several possible petrogenetic explanations,one of which could be the interaction of plume-related meltswith the deep lithospheric mantle beneath the island. KEY WORDS: geochronology; flood basalts; Antampombato–Ambatovy intrusion; Cretaceous; Madagascar  相似文献   

8.
The picritic (MgO >13·5%) lavas of Grenada providea unique opportunity to evaluate the platinum group elements(PGE) and Os isotope compositions of primitive subduction-generatedmelts. Compared with other arc lavas they have undergone verylimited crustal contamination (  相似文献   

9.
The Betts Cove ophiolite, Newfoundland, consists of cumulate ultramafics, gabbro/clinopyroxenites, sheeted dikes and pillow lavas. The pillow lavas are divisible into three compositional groups: lower lavas (<0.25 wt.% TiO2), intermediate lavas (0.25–0.50 wt.% TiO2) and upper lavas (>0.7 wt.% TiO2). The lower and intermediate lavas are very depleted in Ti, Zr, Y, P, and REE and have high Al2O3/TiO2 ratios relative to normal oceanic tholeiite. The extreme depletion of these lavas and their dike equivalents (diabase and picrites) suggests they were derived by melting a severely depleted lherzolite. Conversely, the upper lavas, a volumetrically small part of the ophiolite, are compositionally similar to fractionated oceanic tholeiite and thus, their source material may be like that postulated for modern ocean floor basalts. Whereas the majority of basalts in the Betts Cove ophiolite are depleted in incompatible elements, most dikes and lavas from the Blow-MeDown ophiolite, western Newfoundland, are not and have incompatible element concentrations similar to modern oceanic tholeiite. The chemical differences between the two ophiolite massifs are related to melting of ultramafic source materials which are in different states of depletion brought about by previous melting episodes.  相似文献   

10.
Mineral and melt inclusions in olivines from the most Mg-richmagma from the southern West Sulawesi Volcanic Province indicatethat two distinct melts contributed to its petrogenesis. Thecontribution that dominates the whole-rock composition comesfrom a liquid with high CaO (up to 16 wt %) and low Al2O3 contents(CaO/Al2O3 up to 1), in equilibrium with spinel, olivine (Fo85–91;CaO 0·35–0·5 wt %; NiO 0·2–0·30wt %) and clinopyroxene. The other component is richer in SiO2(>50 wt %) and Al2O3 (19–21 wt %), but contains significantlyless CaO (<4 wt %); it is in equilibrium with Cr-rich spinelwith a low TiO2 content, olivine with low CaO and high NiO content(Fo90–94; CaO 0·05–0·20 wt %; NiO0·35–0·5 wt %), and orthopyroxene. Boththe high- and low-CaO melts are potassium-rich (>3 wt % K2O).The high-CaO melt has a normalized trace element pattern thatis typical for subduction-related volcanic rocks, with negativeTa–Nb and Ti anomalies, positive K, Pb and Sr anomalies,and a relatively flat heavy rare earth element (HREE) pattern.The low-CaO melt shows Y and HREE depletion (Gdn/Ybn 41), butits trace element pattern resembles that of the whole-rock andhigh-CaO melt in other respects, suggesting only small distinctionsin source areas between the two components. We propose thatthe depth of melting and the dominance of H2O- or CO2-bearingfluids were the main controls on generating these contrastingmagmas in a syn-collisional environment. The composition ofthe low-CaO magma does not have any obvious rock equivalent,and it is possible that this type of magma does not easily reachthe Earth's surface without the assistance of a water-poor carriermagma. KEY WORDS: melt inclusions; mineral chemistry; olivine; syn-collisional magmatism; ankaramites; low-Ca magma  相似文献   

11.
The Laki eruption involved 10 fissure-opening episodes thatproduced 15·1 km3 of homogeneous quartz-tholeiite magma.This study focuses on the texture and chemistry of samples fromthe first five episodes, the most productive period of the eruption.The samples comprise pumiceous tephra clasts from early falloutdeposits and lava surface samples from fire-fountaining andcone-building activity. The fluid lava core was periodicallyexposed at the surface upon lobe breakout, and its characteristicsare preserved in glassy selvages from the lava surface. In allsamples, plagioclase is the dominant mineral phase, followedby clinopyroxene and then olivine. Samples contain <7 vol.% of euhedral phenocrysts (>100 µm) with primitivecores [An* = 100 x Ca/(Ca + Na) >70; Fo > 75; En* = 100x Mg/(Mg + Fe) >78] and more evolved rims, and >10 vol.% of skeletal, densely distributed groundmass crystals (<100µm), which are similar in composition to phenocryst rims(tephra: An*58–67, Fo72–78, En*72–81; lava:An*49–70, Fo63–78, En57–78). Tephra and lavahave distinct vesicularity (tephra: >40 vol. %; lava: <40vol. %), groundmass crystal content (tephra: <10 vol. %;lava: 20–30 vol. %), and matrix glass composition (tephra:5·4–5·6 wt % MgO; lava: 4·3–5·0wt % MgO). Whole-rock and matrix glass compositions define atrend consistent with liquid evolution during in situ crystallizationof groundmass phases. Plagioclase–glass and olivine–glassthermometers place the formation of phenocryst cores at 10 kmdepth in a melt with 1 wt % H2O, at near-liquidus temperatures(1150°C). Phenocryst rims and groundmass crystals formedclose to the surface, at 10–40°C melt undercoolingand in an 10–20°C cooler drier magma (0–0·1wt % H2O), causing an 10 mol % drop in An content in plagioclase.The shape, internal zoning and number density of groundmasscrystals indicate that they formed under supersaturated conditions.Based on this information, we propose that degassing duringascent had a major role in rapidly undercooling the melt, promptingintensive shallow groundmass crystallization that affected themagma and lava rheology. Petrological and textural differencesbetween tephra and lava reflect variations in the rates of magmaascent and the timing of surface quenching during each eruptiveepisode. That in turn affected the time available for crystallizationand subsequent re-equilibration of the melt to surface (degassed)conditions. During the explosive phases, the rates of magmaascent were high enough to inhibit crystallization, yieldingcrystal-poor tephra. In contrast, pervasive groundmass crystallizationoccurred in the lava, increasing its yield strength and causinga thick rubbly layer to form during flow emplacement. Lava selvagescollected across the flow-field have strikingly homogeneousglass compositions, demonstrating the high thermal efficiencyof fluid lava transport. Cooling is estimated as 0·3°C/km,showing that rubbly surfaced flows can be as thermally efficientas tube-fed phoehoe lavas. KEY WORDS: lava; crystallization; basalt; cooling rate; pressure; geobarometry; P–T conditions; plagioclase; degassing; Laki, Iceland  相似文献   

12.
The maximum potential temperature of the Archaean mantle is poorly known, and is best constrained by the MgO contents of komatiitic liquids, which are directly related to eruptive temperatures. However, most Archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid. Relatively fresh lavas from the SASKMAR suite, Belingwe Greenstone Belt, Zimbabwe (2.7 Ga) include chills of 25.6 wt.% MgO, and olivines ranging to Fo93.6, implying eruption at around 1520°C. A chill sample from Alexo Township, Ontario (also 2.7 Ga) is 28 wt.% MgO, and associated olivines range to Fo94.1, implying eruption at 1560°C. However, inferences of erupted liquids containing 32–33 wt.% MgO, from lavas in the Barberton Greenstone Belt, South Africa (3.45 Ga) and from the Perseverance Complex, Western Australia (2.7 Ga) may be challenged on the grounds that they contain excess (cumulate) olivine, or were enriched in Mg during alteration or metamorphism. Re-interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt.% MgO corresponding to an eruption temperature of 1580°C. This composition is the highest liquid MgO content of an erupted lava that can be supported with any confidence. The hottest modern magma, on Gorgona Island (0.155 Ga) contained 18–20% MgO and erupted at circa 1400°C.

If 1580°C is taken as the temperature of the most magnesian known eruption, then the source mantle from which the liquids rose would have been at up to 2200°C at pressures of 18 GPa corresponding to a mantle potential temperature of 1900°C. These temperatures are in excess of the mantle temperatures predicted by secular cooling models, and thus komatiites can only be formed in hot rising convective jets in the mantle. This result requires that Archaean mantle jets may have been 300°C hotter than the Archaean ambient mantle temperature. This temperature difference is similar to the 200–300°C temperature difference between present day jets and ambient mantle temperatures. An important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions.  相似文献   


13.
The Mafic and Ultramafic Lavas of the Belingwe Greenstone Belt, Rhodesia   总被引：3，自引：3，他引：3  
The Belingwe Greenstone Belt (2.8 x 109 yrs old) contains a7 km succession of mafic and ultramafic lavas and high-levelintrusions which overlie a thin sedimentary formation, itselfunconformable on a granitic basement. The lavas range in compositionfrom andesites (4 per cent MgO) to peridotitic komatiites (32per cent MgO). The mineralogy and textures of the most magnesianlavas demonstrate that they were extruded in a completely liquidstate. If the source mantle had an MgO content around 40 percent, then partial melts in the range 35 per cent to 55 percent would be required to produce the most magnesian liquidsobserved. Chemical constraints on the petrogenesis of the ultramafic lavasallow estimates of source mantle composition. In particular,if the source had an MgO content around 40 per cent, then theoverall source composition would be similar to that of garnetIherzolite nodules in kimberlites. The calculated REE contentsof the source are close to chondritic. If all the ultramaficlavas were derived from the same source then the variation inliquid composition may have been controlled by orthopyroxeneas well as olivine during partial melting at depth. The evolutionof the less magnesian komatiites, basalts, and andesites canbe explained by lower degrees of partial melting of a commonsource, and by high-level fractionation of parent liquids similarto those extruded as ultramafic lavas. Physical constraints on the origin of the lavas imply derivationfrom a depth of 150 km or more, at temperatures of 1600–2000°C.  相似文献   

14.
Early Proterozoic tholeiitic lavas and sills were emplaced duringthe initial phase of extension of the intra-cratonic CuddapahBasin, southern India. 40Ar–39Ar laser-fusion determinationson phlogopite mica, from the Tadpatri Fm mafic–ultramaficsill complex, constrain the age of the initial phase of extensionand volcanism in the basin at 1·9 Ga. Despite their EarlyProterozoic age, the igneous rocks are unmetamorphosed, undeformedand remarkably fresh. They exhibit a wide range in MgO contents(4–28 wt %) and have undergone varying degrees of accumulationor crystal fractionation. Variable La/Nb ratios (1·2–3·7)and  相似文献   

15.
Widespread penecontemporaneous igneous activity affected NWRussia (the Kola Peninsula and adjoining areas to the SE aroundArkhangelsk) during the Late Devonian (360–380 Ma). Magmatismvaries from tholeiitic basalts, erupted in the axial regionsof former Middle Proterozoic (Riphean) rifts, to strongly alkalinerock-types on and marginal to Archaean cratons. NNE of Arkhangelskkimberlites, olivine lamproites and alkaline picrites were emplaced;all these rock-types are diamondiferous to varying extents.Higher TiO2 (and also total Fe) distinguish predominantly mica-poorEastern Group kimberlites (TiO2 = 2·4–3·1wt %) and spatially associated alkaline picrites (TiO2 = 3·2–3·7wt %) from nearby micaceous Western Group kimberlites (TiO2= 0·8–1·1 wt %). Each rock-type also hasdistinctive rare earth element (REE) patterns, and  相似文献   

16.
BARSDELL  M. 《Journal of Petrology》1988,29(5):927-964
The mineralogy, petrography and geochemistry of a suite of clinopyroxene-richolivine tholenite lavas from Merelava island, Vanuatu are described.Located at the southern end of the Northern Trough back-arcbasin, this suite displays all the characteristics of primitiveisland arc lavas: flat REE patterns, depleted HFSE, enrichmentin K-group elements relative to LREE, highly calcic plagioclase(to An9 3 and Cr-rich spinels (cr-number80) Analysis of groundmasscompositions demonstrates that the variation in MgO within thelava suite (from 13?7 to 4?3% MgO) represents only a small departurefrom a liquid line of descent. Some of the more primitive lavas contain low-Al2O3 clinopyroxenemegacrysts (mg-number = 100Mg/(Mg+Fe2 + and ultramafic xenoliths,the latter ranging from fine-grained, tectonite wehrlites andchnopyroxene-bearing harzburgites, to coarse-grained cumulatewehrlites. The cumulate nodules, megacrysts and phenocrysts are shown tobe co-magmatic, and an empirical compositional relationshipis demonstrated for equilibrium olivine-clinopyroxene pairs,covering the observed fractionation range (mg-numberCpx=0?6375mg-numberO1 + 35?3). On the basis that the most primitive olivine(mg-number 91 7) is close to the liquidus composition, thiscompositional relationship demonstrates that clinopyroxene (mg-number=94,and containing no Fe3+) was also a liquidus phase. Clinopyroxeneswith mg-number>94 are the product of local oxidation duringmixing of primitive, relatively reduced magmas, and more evolved,oxidized magmas. This mixing also gave rise to relatively narrow,reversely zoned, internal rims on many clinopyroxene and olivinephenocrysts, cumulus crystals, and clinopyroxene megacrysts. Fractionation modelling demonstrates that the most differentiatedsample with 19 wt.% Al2O3 can be derived from the most primitivesample with 10?3% Al2O3 by removal of 48% crystals of clinopyroxeneand olivine in the proportions 73:27 Plagioclase is a late crystallizingphase and has an insignificant role in the fractionation process. The parent melt composition (mg-number=77) is deduced from themost primitive olivine composition and the liquid line of descent,and is shown to contain equal amounts of MgO and CaO (137 wt.%),a high CaO/Al2O3 ratio of 1?3 and an unusually low Ni contentof 137 ppm. Data from published high pressure (8–20 kb)experiments on melting of peridotite and pyrolite do not providean explanati in for the large normative diopside component inthis parent melt (38 mol.%), and a hypothesis is proposed wherebyhigh degrees of melting of refractory Iherzolite or harzburgite+acomponent of lower crustal pyroxenite and/or wehrlite takesplace at the base of the crust (5–55 kb). At this depth,and initially under hydrous conditions, high degrees of meltingwould progressively eliminate orthopyroxene and then clinopyroxeneto produce a dunite residue. The liquid produced near the pointof clinopyroxene elimination would be compatible with the highCaO and Sc contents, and high Sc/Ni, Cr/Ni and D1/Hy ratiosof the lavas, and the refractory nature of the phenocrysts.  相似文献   

17.
The fault-bounded Bolívar Ultramafic Complex (BUC) onthe eastern fringes of the Western Cordillera of Colombia wastectonically accreted onto the western coast of South Americain the late Cretaceous–early Tertiary, along with pillowbasalts of the Caribbean–Colombian Oceanic Plateau (CCOP).The complex consists of a lower sequence of ultramafic cumulates,successively overlain by layered and isotropic gabbroic rocks.The gabbros grade into, and are intruded by, mafic pegmatitesthat consist of large magnesiohornblende and plagioclase crystals.These pegmatites yield a weighted mean 40Ar–39Ar step-heatingage of 90·5 ± 0·9 Ma and thus coincidewith the timing of peak CCOP volcanism. The chemistry of theBUC is not consistent with a subduction-related origin. However,the similarity in Sr–Nd–Pb–Hf isotopes betweenthe CCOP and the BUC, in conjunction with their indistinguishableages, suggests that the BUC is an integral part of the plume-derivedCCOP. The parental magmas of the Bolívar complex wereprobably hydrous picrites that underwent 20–30% crystallization.The residual magmas from this fractionation contained  相似文献   

18.
新疆天山地区榆树沟-铜花山蛇绿岩特征和构造背景   总被引：5，自引：3，他引：2  
榆树沟-铜花山蛇绿岩出露于南天山北缘,属哈萨克斯坦-伊犁板块和塔里木板块之间的缝合带。蛇绿岩已被构造肢解,主要由超镁铁岩、堆晶岩、熔岩类组成。超镁铁岩以方辉橄榄岩为主,显示典型的亏损地幔岩特征,橄榄石为富镁型,Fo为90;斜方辉石为顽火辉石,En为90,单斜辉石含量少;副矿物铬尖晶石属低铬型,Cr#值(＝Cr/(Cr+Al)×100)16~28,Mg#值(=Mg/(Mg+Fe)×100)63~75,反映了深海橄榄岩特征。归一化后榆树沟超镁铁岩MgO含量38.84%~44.53%,Al2O3为1.51%~3.63%,CaO为0.42%~5.77%,成分近于大洋二辉橄榄岩;铜花山的超镁铁岩叠加碳酸岩化,LREE强烈富集,可能经历了俯冲洋壳流体改造。熔岩类在榆树沟和铜花山均有较大规模产出,其中榆树沟玄武岩为主,铜花山安山岩和英安岩较多。熔岩多已遭受绿片岩相海底热液蚀变。榆树沟玄武岩的REE含量总体比铜花山的低,稀土配分模式均为轻稀土富集型;玄武岩的微量元素特征表明其源区可能遭受过流体作用影响。铜花山三个高镁火山岩化学成份具有SiO2(32%~36%)含量很低、MgO(20.12%~28.50%)含量高、K2O+Na2O(0.06%~0.46%)含量小于2%的特征,综合分析可归为苦橄岩类。堆晶岩通常以构造小岩块产在超镁铁岩块中或基性熔岩中,以辉长岩为主,有少量橄榄辉石岩等,岩石蚀变、构造变形强烈。堆晶岩的成分接近玄武岩,存在LREE富集和略亏损平坦两种类型。岩石地球化学特征表明榆树沟和铜花山的蛇绿岩形成于MOR构造环境,但具有受俯冲带流体改造特征,并伴生有岛弧火山岩。  相似文献   

19.
The Limahe Ni–Cu sulfide deposit is hosted by a small mafic–ultramafic intrusion (800 × 200 × 300 m) that is temporally associated with the voluminous Permian flood basalts in SW China. The objective of this study is to better understand the origin of the deposit in the context of regional magmatism which is important for the ongoing mineral exploration in the region. The Limahe intrusion is a multiphase intrusion with an ultramafic unit at the base and a mafic unit at the top. The two rock units have intrusive contacts and exhibit similar mantle-normalized trace element patterns and Sr–Nd isotopic compositions but significantly different cumulus mineralogy and major element compositions. The similarities suggest that they are related to a common parental liquid, whereas the differences point to magma differentiation by olivine crystallization at depth. Sulfide mineralization is restricted to the ultramafic unit. The abundances of sulfides in the ultramafic unit generally increase towards the basal contacts with sedimentary footwall. The δ 34S values of sulfide minerals from the Limahe deposit are elevated, ranging from +2.4 to +5.4‰. These values suggest the involvement of external S with elevated δ 34S values. The mantle-normalized platinum-group element (PGE) patterns of bulk sulfide ores are similar to those of picrites associated with flood basalts in the region. The abundances of PGE in the sulfide ores, however, are significantly lower than that of sulfide liquid expected to segregate from undepleted picrite magma. Cr-spinel and olivine are present in the Limahe ultramafic rocks as well as in the picrites. Mantle-normalized trace element patterns of the Limahe intrusion generally resemble those of the picrites. However, negative Nb–Ta anomalies, common features of contamination with the lower or middle crust, are present in the intrusion but absent in the picrites. Sr–Nd isotopes suggest that the Limahe intrusion experienced higher degrees of contamination with the upper crust than did the picrites. The results of this study permit us to suggest that the parental magma of the Limahe intrusion was derived from picritic magma by olivine fractionation and contamination in a staging chamber at mid-crustal levels. Depletion of PGE in the sulfide ores in the Limahe intrusion is likely due to previous sulfide segregation of the parental magmas in the staging chamber. Sulfide mineralization in the Limahe intrusion is related to second-stage sulfide segregation after the fractionated magmas acquired external S from pyrite-bearing country rocks during magma ascent to the Limahe chamber. The abrupt change in mineralogical and chemical compositions between the ultramafic unit and the overlying unit suggests that at least two separate pulses of magma were involved in the development of the Limahe intrusion. We propose that the Limahe intrusion was once a wider part of a dynamic conduit that fed magma to the overlying subvolcanic dykes/sills or lavas. The ultramafic unit formed by the first, relatively more primitive magma, and the mafic unit formed by the second, relatively more fractionated magma. Immiscible sulfide droplets that segregated from the first magma settled down with olivine crystals to form the sulfide-bearing, olivine-rich rocks in the base of the intrusion. The overlying residual liquids were then pushed out of the chamber by the second magma. Critical factors for the formation of an economic Ni–Cu sulfide deposit in such a small intrusion include the dynamic petrologic processes involved and the availability of external sulfur. The Limahe deposit reminds us that small, multiphase, mafic–ultramafic intrusions in the region should not be overlooked for the potential of economic Ni–Cu sulfide deposits.  相似文献   

20.
Within the Zitácuaro–Valle de Bravo (ZVB) regionof the central Mexican Volcanic Belt (MVB), three lava serieshave erupted during the Quaternary: (1) high-K2O basaltic andesitesand andesites; (2) medium-K2O basaltic andesites, andesitesand dacites; (3) high-TiO2 basalts and basaltic andesites. Thedominant feature of the first two groups is the lack of plagioclaseaccompanying the various ferromagnesian phenocrysts (olivine,orthopyroxene, augite, and hornblende) in all but the dacites.This absence of plagioclase in the phenocryst assemblages ofthe high-K2O and medium-K2O intermediate lavas is significantbecause it indicates high water contents during the stage ofphenocryst equilibration. In contrast, the high-TiO2 group ischaracterized by phenocrysts of plagioclase and olivine. Thespatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280–330 km fromthe Middle America Trench (MAT), is characterized by high-K2Omelts that are relatively enriched in fluid-mobile elementsand have the highest 87Sr/86Sr ratios. Medium-K2O basaltic andesiteand andesite lavas are present throughout the transect, butthose closest to the MAT are MgO-rich (3·5–9·4wt %) and have phenocryst assemblages indicative of high magmaticwater contents (3·5–6·5 wt % water) andrelatively low temperatures (950–1000°C). In markedcontrast, the northern section of the ZVB transect (380–480km from the MAT) has high-TiO2, high field strength element(HFSE)-enriched magmas that have comparatively dry (< 1·5wt % magmatic water) and hot (1100–1200°C) phenocrystequilibration conditions. The central section of the ZVB transect(330–380 km from the MAT) is a transition zone and producesmoderately light rare earth element (LREE) and large ion lithophileelement (LILE)-enriched, medium-K2O lavas with phenocryst assemblagesindicative of intermediate (1·5–3·5 wt %)water contents and temperatures. The high-K2O series compositionsare the most enriched in LILE and LREE, with a narrow rangeof radiogenic 87Sr/86Sr from 0·704245 to 0·704507,143Nd/144Nd values ranging from 0·512857 to 0·512927(Nd = 4·27–5·63), and 208Pb/204Pb valuesfrom 38·248 to 38·442, 207Pb/204Pb values from15·563 to 15·585, and 206Pb/204Pb values from18·598 to 18·688. The medium-K2O series compositionsare only moderately enriched in the LILE and LREE, with a broaderrange of 87Sr/86Sr, but similar 143Nd/144Nd and 208Pb/204Pbvalues to those of the high-K2O series. In contrast, the high-TiO2series compositions have little enrichment in LILE or LREE andinstead are enriched in the HFSE and heavy rare earth elements(HREE). The high-TiO2 lavas are isotopically distinct in theirlower and narrower range of 143Nd/144Nd. The isotopic variationsare believed to reflect the upper mantle magma source regionsas the low content of phenocrysts in most lavas precludes significantupper crustal assimilation or magma mixing, other than thatrepresented by the presence of quartz xenocrysts (< 2 vol.%) with rhyolitic glass inclusions, which are found in manyof these lavas. The systematic spatial variation in compositionof the three lava series is a reflection of the underlying subduction-modifiedmantle and its evolution. KEY WORDS: central Mexico; geochemistry; isotopes; Quaternary volcanism; hydrous lavas  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号