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1.
Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have206pb/204Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher206Pb/204Pb, such as that expressed in lavas from St. Paul Island.  相似文献   

2.
Approximately 160 Ma old basaltic lavas obtained from ODP Site 801 in the Pigafetta Basin represent the first Jurassic oceanic crust recovered in the Pacific Ocean and the oldest in situ oceanic crust discovered anywhere. The basement consists of an upper alkali olivine basalt sequence and a lower tholeiitic sequence separated by a yellow Fe-rich hydrothermal sedimentary deposit. The aphyric and sparsely plagiodase-olivine±spinel phyric tholeiites exhibit depleted, open–system fractionated characteristics with trace element abundances and Pb–Nd isotopic compositions similar to normal mid-ocean ridge basalts (N-MORB). The aphyric alkali basalts, although showing some overlap in isotopic composition with MORB, exhibit strong similarities in terms of incompatible element abundances to ocean island basalts (OIB). They could represent either OIB-type off-axis volcanism or an alkalic event possibly associated with the waning stages of spreading axis volcanism in the Pigafetta Basin. All lavas have undergone low-grade anoxic smectite–carbonate alteration, although flows underlying the Fe-rich sediments have suffered hydrothermal alteration and fracturing.  相似文献   

3.
The Koloa Volcanic Suite of Kauai, Hawaii   总被引：3，自引：3，他引：3  
The Koloa lavas of the post-erosional volcanic suite of Kauaivary in composition from melilitite to alkali olivine basalt.87Sr/86Sr varies from {small tilde}0.7030 to 0.7033, the rangebeing similar to that of other Hawaiian post-erosional suites.The chemical composition within single lava flows is variableand forms major-element trends that are different from the mainKoloa trend. The main trend can be related to increased degreesof partial melting during decreasing pressure. The combinedevidence from Sr and Nd isotopes and rare earth elements (REEs)could suggest that an ocean island basalt (OIB) plume was infiltratedby a carbonate-rich melt with a high concentration of Sr andREEs derived from the mid-ocean ridge basalt (MORB) source.However, the Pb-isotope systematics of the Hawaiian islandsdo not suggest that a MORB source was involved during the generationof the posterosional lavas. Instead, we suggest that the Hawaiiantholeiitic and nephelinitic magma suites are derived from azoned plume with a low 87Sr/86Sr core and a high 87Sr/86Sr concentricmargin. The trends of the single flows and their variation inisotope ratios suggest that they formed from magmas accumulatedmainly from the central core of the zoned plume.  相似文献   

4.
Active spreading ridges in the North Fiji Basin range from well-developed stable ridges where largescale mantle upwelling is in progress to proto-ridges where spreading is incipient. South of 17°S, where the central ridge of the North Fiji Basin has a bathymetric profile normally expected of a fast-spreading, steadystate mid-ocean ridge, basalts are evolved N-type MORBs. North of 17°S, where the central ridge is propagating northward into old North Fiji Basin crust and spreading is in the initial stages, two types of basalt have been recovered: N-type MORBs from this northern arm of the central ridge are believed to be samples of older North Fiji Basin crust; basalts with transitional alkalic chemistry (up to 0.5% Ne in the Norm) and characterized by strong relative enrichments in Rb, Ba, K, Nb, La, Ce, Sr, P, Zr, and Ti are believed to be associated with incipient rifting. Among the latter group are compositions that are intermediate between transitional alkalic types and MORBs and these are geochemically similar to the back-arc basin (BABB) magma type defined by Sinton and Fryer (1987) from a study of Mariana back arc basin basalts. Dredges along the South Pandora Ridge, a transform zone characterized by short spreading segments, are dominated by basalts that are enriched in large-ion lithophile and high field strength minor and trace elements and compositions range from types resembling ocean island tholeiites to transitional alkalic varieties. Basalts from Rotuma are regarded as alkalic end-members of the South Pandora Ridge magmatic spectrum. In areas of the North Fiji Basin where relatively fast spreading must be accompanied by largescale asthenospheric upwelling, depleted (N-type) MORBs dominate, whereas in areas of slow mantle upwelling, or where some other tectonic effect (e.g. a transform fault) causes a transient thermal disturbance within the lithosphere or upper asthenosphere, enriched (alkalic) magmas either dominate or make a significant and noticeable contribution to the overall chemical characteristics of basalts being erupted. The MORBs have a depleted asthenospheric source, and the alkalic component is believed to derive from an enriched lithospheric or shallow asthenospheric source. The BABB magma type may simply be part of the spectrum of mixed magmas that can occur in the transitional tectonic settings represented by the early development of most back-arc basins.  相似文献   

5.
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6.
GANDY  M. K. 《Journal of Petrology》1975,16(1):189-211
The calc-alkaline lava sequence of the eastern Sidlaw Hillsforms a small part of an extensive volcanic province of LowerOld Red Sandstone (Devonian) age in Scotland and N. England.The Sidlaw lavas ranging from olivine basalt to dacite are allporphyritic with combinations of olivine, plagioclase, clinopyroxene,orthopyroxene, and opaque oxide pheno-crysts. Chemically, thelavas are slightly more alkalic than modern calc-alkaline lavas.There is considerable variation in the ‘incompatible elements’.The differentiation of the lavas can be accounted for by fractionationof olivine+plagioclase+minor ore from a chemically variable,immediately parental magma at low pressure (c. 1 kb PH2O). Itis suggested that fractionation of variable amounts of olivineand clinopyroxene from an olivine tholeiite at moderate PH2Ocould give rise to this chemically variable, high alumina, immediatelyparental magma.  相似文献   

7.
A convergent margin magma series with characteristic low Nband Ta abundances and enrichments in alkalis and alkaline earthsis intercalated with typical intraplate alkalic basalts in aback-arc setting, 200–250 km above the Wadati-Benioffzone on the North Island, New Zealand. These two contrastingmagma types, together with late-stage K-rich maflc lavas, wereerupted over a short time period (1{dot}60–2{dot}74 Ma)and constitute the Alexandra Volcanics. Field relationshipsindicate that these diverse magma types were contemporaneous,and thus their mantle source regions coexisted, in a singletectonic environment. The convergent margin magma series forms a linear chain of stratovolcanoesaligned at right angles to the present subduction zone. Closed-systempolybaric fractional crystallization models can explain theevolution from ankaramites to transitional olivine basalts toolivine tholeiites to high-Al basalts to medium- and high-Kandesites. The most primitive lavas have geochemical (high LIL/LREEand LIL/HFS element ratios) and Sr, Nd, and Pb isotopic compositionstypical of convergent margin magmas. Calculated source compositionssuggest that three components are involved: a MORB component,a component derived from subducted oceanic crust, and a contributionfrom subducted sediments. The alkalic basalts occur as dispersed monogenetic volcanoesand are intercalated with the larger convergent margin stratovolcanocs.These basalts are enriched in LILE, LREE, Nb, and Ta, and havelow Ba/Nb and Ba/La ratios, all of which are characteristicof ocean island (intraplate) basalts (OIBs). Their relativelyhigh Nd (+5{dot}5 and low 87Sr/86Sr(0{dot}703l–0{dot}7036)are also typical of OIBs. These alkalic magmas were derivedfrom the underlying continental lithospheric mantle that hasbeen enriched by upward-migrating silica-undersaturated melts,probably including volatiles, from the low- velocity zone. Asubducted slab component is not required to account for theirincompatible element enriched character. The K-rich mafic lavas, basanites, and absarokites are volumetricallyminor and cap the largest of the stratovolcanoes, Pirongia.The basanites have geochemical and isotopic compositions whichsuggest they are mixtures of multiple source components, includingthe alkalic and convergent margin region.  相似文献   

8.
Quaternary lavas from the NE Japan arc show geochemical evidenceof mixing between mantle-derived basalts and crustal melts atthe magmatic front, whereas significant crustal signals arenot detected in the rear-arc lavas. The along-arc chemical variationsin lavas from the magmatic front are attributable almost entirelyto geochemical variations in the crustal melts that were mixedwith a common mantle-derived basalt. The mantle-derived basaltshave slightly enriched Sr–Pb and depleted Nd isotopiccompositions relative to the rear-arc lavas, but the variationis less pronounced if crustal contributions are eliminated.Therefore, the source mantle compositions and slab-derived fluxesare relatively uniform, both across and along the arc. Despitethis, incompatible element concentrations are significantlyhigher in the rear-arc basalts. We examine an open-system, fluid-fluxedmelting model, assuming that depleted mid-ocean ridge basalt(MORB)-source mantle melted by the addition of fluids derivedfrom subducted oceanic crust (MORB) and sediment (SED) hybridsat mixing proportions of 7% and 3% SED in the frontal- and rear-arcsources, respectively. The results reproduce the chemical variationsfound across the NE Japan arc with the conditions: 0·2%fluid flux with degree of melting F = 3% at 2 GPa in the garnetperidotite field for the rear arc, and 0·7% fluid fluxwith F = 20% at 1 GPa in the spinel peridotite field beneaththe magmatic front. The chemical process operating in the mantlewedge requires: (1) various SED–MORB hybrid slab fluidsources; (2) variable amounts of fluid; (3) a common depletedmantle source; (4) different melting parameters to explain across-arcchemical variations. KEY WORDS: arc magma; crustal melt; depleted mantle; NE Japan; Quaternary; slab fluid  相似文献   

9.
CLAGUE  DAVID A. 《Journal of Petrology》1988,29(6):1161-1186
Ultramafic xenoliths were recovered in four alkalic lava flowsfrom Loihi Seamount at depths between 2200 and 1400m. No xenolithbearing flows were sampled near the summit despite a concentrateddredge program. The flows, three of alkalic basalt and one ofbasanite, contain common olivine megacrysts and small xenolithsof dunite, rarer harzburgite, and a single wehrlite. Olivinemegacrysts as large as 8 mm are Fo84–88 6 and containmagnesiochromite inclusions with 1?1–3?5 wt.% TiO2 Dunitecontains Fo83 5–88?5 olivine, magnesiochromite with l?5–6?9wt.% TiO2 (avg. 3?2 wt.%), and extremely rare chrome-rich diopside.The wehrlite contains euhedral Fo85 9 olivine and magnesiochromitewith 1?9–4?7 wt.% TiO2 poikilitically enclosed in chrome-richdiopside (Wo45 4En48 0Fs6?6).Most of the olivine megacrysts,dunite, and the wehrlite are cumulates of Loihi alkalic lavasthat accumulated in a magma storage zone located at least 16kmbelow sea level. The rarity of dunite related to tholeiiticmagmas supports the interpretation that the alkalic lavas atLoihi generally predate the tholeiitic lavas. The harzburgitexenoliths have cataclastic textures and contain Fo89 5–926 olivine, enstatite (Wo2 0–2?7En90?0–88 7Fe8?0–8?6),Cr-rich endiopside (Wo43 4–44 5En52 0–50 0Fs4 6–45), and translucent red-brown magnesiochromite. The harzburgitexenoliths, which have 2-pyroxene temperatures of 1066 ? 35?C,originated in the uppermost mantle in a region of high strainrate, probably near the boundary between the mantle and theoverlying ocean crust. The presence of upper mantle xenolithsindicates that the magma storage zone is located below the baseof the ocean crust within the uppermost mantle.  相似文献   

10.
Eighty samples of submarine basaltic lava were sampled from an 8 km segment of the floor and walls of the inner rift valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (project Famous). The samples were collected from outcrops and talus slopes by the three submersibles: Alvin, Archimede, and Cyana at water depths of about 2600 meters.The early formed mineral content of the pillow lavas' glassy margins enables classification of the rocks into 5 types: (1) olivine basalt, (2) picritic basalt, (3) plagioclase-olivine-pyroxene basalt, (4) aphyric basalt, and (5) plagioclase-rich basalt. Chemical and mineralogical study indicates that at least 4 types are directly interrelated and that types (1) and (2) are higher-temperature, primitive lavas, and types (3) and (4) are lower-temperature, differentiated lavas derived from the primitive ones by crystal-liquid differentiation. The plagioclase-rich basalts also have a chemical composition of their glass comparable to that of the most differentiated basalts (types 3 and 4) but they differ in their greater amount of early formed plagioclase (12–35%).In general, the mineralogical variation across the rift valley shows an assymetrical distribution of the major basalt types. Despite the mineralogical diversity of the early formed crystals, the chemistry of the basalt glasses indicates a symmetrical and a gradual compositional change across the rift valley. Based primarily on their chemistry, the rock types 1 and 2 occupy an axial zone 1.1 km wide and make up the central volcanic hills. Differentiated lavas (types 3, 4) occupy the margins and walls of the inner rift valley and also occur near the center of the rift valley between the central hills.FeO/MgO ratios of olivine and coexisting melt indicate that the average temperature of eruption was 40 ° C higher for the primitive melts (types 1 and 2). Aside from major elements trends, the higher temperature character of the primitive basalts is shown by their common content of chrome spinel.The thickness of manganese oxide and palagonite on glassy lava provide an estimate of age. In a general fashion the relative age of the various volcanic events follow the compositional zoning observed in the explored area. Most of the youngest samples are olivine basalt of the axial hills. Most older samples occur in the margins of the rift valley (West and N.E. part of explored area) but are significantly younger than the spreading age of the crust on which they are erupted. Intermediate lava types occur mainly east of the rift valley axis and in other areas where plagioclase—olivine—pyroxene basalt and aphyric basalt are present.The above relations indicate that the diverse lava types were erupted from a shallow, zoned magma chamber from fissures distributed over the width of the inner rift valley and elongate parallel to it. Differentiation was accomplished by cooling and crystallization of plagioclase, olivine, and clinopyroxene toward the margins of the chamber. The centrally located hills were built by the piling up of frequent eruption of mainly primitive lavas which also are the youngest flows. In contrast smaller and less frequent eruptions of more differentiated lavas were exposed on both sides of the rift valley axis.Contribution n ° 480 du Départment Scientifique, Centre Océanologique de Bretagne  相似文献   

11.
Origin of the Differentiated and Hybrid Lavas of Kilauea Volcano, Hawaii   总被引：1，自引：6，他引：1  
Kilauea Volcano has erupted lava from its summit caldera andfrom two rift zones that extend from the summit towards theeast and south-west. Lavas erupted from the summit of the volcanodiffer from each other principally in their content of olivineand define lines of ‘olivine control’ on magnesiavariation diagrams. Lavas erupted on the rift zones may be similarin composition to the summit lavas or may be differentiatedby processes that involve minerals other than olivine. All ofthe differentiated lavas have less than 6·8 per centMgO and plot off the extension of olivine control lines forthe summit lavas. Prehistoric vents (before A.D. 1750) fromwhich differentiated lavas have been erupted are found on theeast rift zone and in the western Koae fault zone adjacent tothe south-west rift zone; historic vents for differentiatedlavas are confined to the east rift zone. Twenty-one new analysesare presented for several of the east rift differentiates andfor the newly discovered differentiates adjacent to the south-westrift zone. The differentiates have MgO as low as 3·9per cent and SiO2 as high as 56 per cent; both extremes arefound in the prehistoric lavas adjacent to the south-west rift. Detailed petrochemical studies suggest the following conclusions:
  1. Thechemical composition of magma erupted at Kilauea summitvarieswith the date of eruption. Lavas erupted before 1750,duringthe eighteenth and nineteenth centuries, and in the twentiethcentury form groups that can be distinguished chemically. Ona lesser scale, each Kilauea summit eruption in the twentiethcentury has a chemistry that is distinctive with respect tothe chemistry of every other summit eruption.
  2. During lateprehistoric time pockets of differentiated magmawere formedwithin the rift zones by separation of the liquidremainingafter partial crystallization of bodies of summitmagma. Thisprocess presumably is still going on within theeast rift zone,but the more recently separated liquids havenot yet been eruptedto the surface. The relative time at whichthese differentiatedmagmas were produced can be estimated fromcalculations basedon their chemical compositions, which showthat the differentiatescould lie on the liquid line of descentfor Kilauea summit magmaof prehistoric composition but noton any liquid line of descentfor younger summit magmas.
  3. Lava from some eruptions, notablythe early part of the 1955eruption on the lower east rift,has the composition of theliquid fraction as it is generatedwithin the rift. Lava compositionsof other eruptions, includingthose of the later lavas of 1955,are best explained by mixingof magma supplied from a centralreservoir beneath Kilauea summitwith the differentiated liquidin the rift. Lava from each summiteruption is unique chemically,so it is possible to recognizeits presence or absence as componentsof mixing in such mixedlavas. It appears that summit magmaof composition characteristicof the 1952 and 1961 Halemaumaueruptions contributed to thecomposition of the mixed lavasproduced in the latter part ofthe 1955 eruption. Summit magmaof 1961 composition is alonesufficient to explain the compositionof mixed lavas eruptedin 1960 and 1961. In rift lavas eruptedfrom 1962 to 1965, thecomposition of lava erupted in Halemaumauin 1967, in additionto the 1961 composition, is a componentof mixing, and it isthe dominant summit component in the compositionof the two1965 eruptions. The proportion of summit magma todifferentiatedmagma needed to explain the composition of lavaserupted onthe upper east rift increases from 1961 to 1965;this increaseindicates that the differentiated magma was beingdiluted andused up by repeated flooding of this part of therift zone bymagma supplied from the central reservoir.
  4. The fact that componentsof ‘summit composition’appear in rift eruptionsbefore they appear undiluted in Halemaumausuggests that thecentral reservoir is vertically zoned. Rifteruptions are fedfrom lower levels where younger magma is available,and summiteruptions are fed from the relatively older magmaabove. Thechemical distinction between lava of successive summiteruptionsimplies that significant convective mixing of magmadoes nottake place throughout the central reservoir.
  5. The unique anduniform composition of lava of each successivesummit eruptionalso suggests that summit eruptions end whenall of the magmaof one composition has been erupted. The magmaerupted fromthe upper levels of the reservoir during one cycleis continuallyreplaced from below by younger magma of differentcomposition.In order for eruption to be renewed in Halemaumau,new magmafrom the mantle must be held in storage at intermediatelevelsbefore it attains an ‘eruptive state’.
  6. The hypothesispresented in 2–4 above permits qualitativepredictionsconcerning future lava compositions. The compositionof thenext lava to be erupted in Halemaumau is expected tobe distinctfrom that of the 1967 eruption, and this compositionwill presumablybe identified in rift eruptions occurring between1967 and thetime of its appearance in Halemaumau.
  7. Differentiates of prehistoricage also were apparently formedin the same way as those ofhistoric age, but the mixing cannotbe described quantitativelybecause of poor control on the stratigraphyand the compositionsof erupted lavas. One lava in the Koaegroup, that from YellowCone, appears to be a mixture of a picriticmagma (12 per centMgO) with a differentiated liquid with lessthan 2·5per cent MgO and nearly 60 per cent SiO2.
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12.
西昆仑库地蛇绿岩地质、地球化学及其成因研究   总被引：16，自引：0，他引：16       下载免费PDF全文
西昆仑库地蛇绿混杂岩由方辉橄榄岩和纯橄榄岩等地幔变质橄榄岩、豆荚状铬铁矿、堆晶橄榄岩、堆晶辉石岩和辉长岩、辉绿岩墙、块状和枕状玄武岩等组成。强亏损方辉橄榄岩为主的地幔岩组合,二辉石的低Al含量和铬尖晶石的高Cr#,以及岩石的富Mg、Ni和贫Al、Ca特征一致表明地幔橄榄岩类是经较高程度部分熔融后的地幔残余,与消减带之上蛇绿岩中的同类岩石相近。岩石富Rb、Ba、U、Th、LREE,说明地幔残余岩石受到了来自消减带的洋壳重熔组分的混染。堆晶岩以辉石岩和辉长岩为主,可能属PPG系列,指示岩浆是在消减带环境和含水条件下熔融的。辉长岩为低Ti蛇绿岩型,代表洋内弧后盆地早期环境或弧前环境。辉绿岩和玄武岩为洋中脊拉斑玄武岩和岛弧拉斑玄武岩的过渡类型;玄武岩和辉绿岩相比富Ba、Th、LREE,贫Ta,指示玄武岩较辉绿岩更多地受到来自消减带洋壳重熔组分的影响。库地蛇绿岩形成时的古构造环境是消减带之上的弧间或弧后盆地。  相似文献   

13.
The Alpine peridotite massif of Lanzo (Italy) contains three generations of basic dikes (gabbros and basalts). The older gabbros are plagioclase-rich mantle segregates while the younger gabbro dikes are cumulates very similar in chemical composition to recent oceanic gabbros and gabbros from ophiolitic complexes. They both were derived from the N-type mid-ocean ridge basalt (MORB) magmas which were progressively more depleted in incompatible elements and were probably generated during a dynamic melting of a rising mantle diapir. The basaltic dikes are the N-type MORB and closely resemble the Alpine-Apennine ophiolitic basalts. They were derived from a different upper mantle source than the parental magmas of the gabbros. The source of the basalts was less depleted in light REE. The presence of basic magmas with N-type MORB affinities in the Lanzo massif is consistent with the close genetic relationship between the Alpine peridotite body and the ophiolites of the Liguro-Piemontese basin.  相似文献   

14.
The postshield and posterosional stages of Haleakala Volcano contain intercalated alkalic basalt and evolved alkalic lavas. Isotopic and incompatible element abundance ratios in the Haleakala postshield basalts changed systematically with time, providing evidence for significant temporal changes in the mantle components contributing to the magmatic sources. Specifically, a depleted, i.e. low87Sr/86Sr and high143Nd/144Nd, mantle component is more abundant in younger lavas. However, as magma-production rates decreased during the postshield and posterosional stages, basaltic melts in magma reservoirs cooled and fractionated, leading to evolved residual melts such as hawaiite. Because primary basalt compositions changed with time, the evolved Haleakala lavas formed from a range of parental compositions. However, basalts and evolved lavas of similar age and isotopic ratios (Sr and Nd) have major and trace element contents that are consistent with a crystal-fractionation model. Although alkalic basalt and hawaiite are the dominant lavas of the postshield stages of both Haleakala and Mauna Kea volcanoes, there are important differences between their lavas. For example, compositional differences between the hawaiite suites at Haleakala and Mauna Kea indicate that, on average, the evolved lavas at Haleakala formed at lower pressures. Also, at Haleakala basalts are intercalated with hawaiites, whereas at Mauna Kea basalts and hawaiites are separated by a sharp boundary. These differences probably reflect a higher magma supply rate to the Haleakala volcano.  相似文献   

15.
张越  陈隽璐  孙吉明  唐卓  李平  白建科 《地质通报》2019,38(9):1431-1442
阿尔曼太蛇绿岩带位于新疆东准噶尔地区,蛇绿岩中变质橄榄岩、堆晶岩、基性火山岩较发育,层序组合虽受构造破坏,但从总体来看仍是一套组合比较完整的蛇绿岩,岩石变形变质强烈,普遍发生绿泥石化、绿帘石化。蛇绿岩中基性熔岩可分为3种类型,即洋岛玄武岩(OIB)、洋中脊玄武岩(MORB)和岛弧玄武岩(IAT)。其中洋岛玄武岩不属于蛇绿岩成分,是后期卷入蛇绿岩带随其他组分一同构造就位而成;基性熔岩主量和微量元素特征揭示岩浆源于亏损的地幔源区,且存在消减组分加入的交代作用,表明其成因与俯冲作用有关。结合岩石地球化学特征和构造环境判别图解,基性熔岩显示出IAT和MORB兼具并呈现过渡的特点,推断该蛇绿岩的形成与岛弧相关,其形成可能介于洋脊到海沟之间的偏海沟区域。  相似文献   

16.
Soret separation of mid-ocean ridge basalt magma   总被引：8，自引：0，他引：8  
Chemical differentiation of an initially homogeneous mid-ocean ridge basalt (MORB) liquid has been experimentally observed in a temperature gradient above the liquidus. The magnitude of this effect in producing differences in composition is comparable to that of crystal-liquid fractionation for a given temperature difference. However the chemical changes produced by the two processes, Soret and crystal-liquid fractionation, are different. Soret separations resemble those observed in a third process — silicate liquid immiscibility. This similarity is a reflection of the fact that the Soret-separable components are the same network-former/network-modifier structural components which segregate during silicate liquid immiscibility.The differences between Soret and crystal-liquid separations allow the recognition of Soret processes as anomalies in MORB suites which are not compatible with normal crystal fractionation processes. The common occurrence of primary, cumulus, magnesian orthopyroxene in MOR gabbros and the absence of such orthopyroxene as phenocrysts in the coeval erupted MORBS is one such anomaly. The peculiar covariation of plagioclase and olivine compositions in some 3-phase olivine-plagioclase-clinopyroxene MOR gabbros, when compared with normal crystal fractionation results, is another anomaly which may be understood in terms of Soret processes operating in conjunction with normal crystal fractionation. These Soret processes must operate through some sort of convective thermal boundary layer at the margin of a MOR magma chamber to allow Soret diffusive exchange to occur before the temperature contrasts which drive it are dissipated. These driving temperature contrasts are also maintained in part by the periodic replenishment of hot, fresh magma into the MOR magma chamber.  相似文献   

17.
The Heathcote Greenstone Belt is composed mainly of Lower Cambrian metavolcanic rocks and is one of three outcropping belts of the apparent basement to the Lachlan Fold Belt in SE Australia. The greenstones may be assigned to two broad magma series. A younger tholeiitic series with mid‐ocean ridge basalt (MORB) affinities has intruded through, and been erupted upon low‐Ti, intermediate SiO2 lavas. The latter were originally boninites (both clinoenstatite‐phyric and more fractionated orthopyroxene‐phyric varieties) and plagioclase‐phyric, low‐Ti andesites. They have partially re‐equilibrated to the lower greenschist facies and outcrop mainly in the central segment of the Heathcote Greenstone Belt, where deeper stratigraphic levels are exposed. Tholeiitic lavas and sills metamorphosed to the prehnite‐pumpellyite facies dominate the northern and southern segments. As the association boninite/low‐Ti lavas/MORB is known only from modern West Pacific‐type settings involving island arcs and backarc basins, the early history of the Lachlan Fold Belt is inferred to have taken place in a similar setting.  相似文献   

18.
Glassy pillow basalts with unusual geochemical characteristics for mid-ocean ridge basalt (MORB) have been dredge sampled from the Southwest Indian Ridge between 12 and 15°E during Leg ANT IV/4 of the F.S. POLARSTERN. Lavas from 4 of 6 dredges are moderately nepheline normative, highly K-enriched (0.5–1.77 wt% K2O) alkali basalts and hawaiites. Mg-numbers indicate that many of the lavas are fairly primitive (Mg No.=63–67), yet show extreme enrichment in incompatible elements; e.g. Nb (24–60 ppm), Ba (170–470 ppm) and Sr (258–460 ppm). Incompatible-element ratios such as Zr/Nb (3–5) and Y/Nb (0.46–1.1) are extremely low even for E-type (enriched) MORB, whereas (La/Yb)n ratios are particularly high (3.4–7.8). 87Sr/86Sr (0.70290–0.70368), 143Nd/144Nd (0.51302–0.51284) and 206Pb/204Pb (18.708–19.564) isotopic ratios further indicate the geochemically enriched nature of these lavas, which range from the compositional field for depleted N-type (normal) MORB towards the composition of Bouvet Island lavas. Mutually correlated incompatible-element and Sr-, Nd- and Pb-isotopic ratios allow a fairly well constrained model to be developed for the petrogenesis of these unusually alkalic mid-ocean ridge lavas. The alkalic nature and degree of enrichment in incompatible elements is ascribed to particularly low degrees of partial melting (3–5 wt%), at greater than usual depth, of a source region that has experienced prior geochemical enrichment (by veining) related to the upwelling Bouvet mantle plume. To account for the observed compositional variations, a model is proposed whereby mixing between partial melts derived from these geochemically enriched silicate veins, and an incipient to low percentage (±2%) melt from the surrounding geochemically depleted suboceanic asthenosphere occurs as a consequence of increasing degree of melting with adiabatic upwelling. Eruption of these alkalic lavas in this spreading ridge environment is attributed to a temporary hiatus in tholeiitic volcanism and associated spreading along this section of the Southwest Indian Ridge, related to readjustment of spreading direction to a more stable plate geometry.  相似文献   

19.
A systematic search for evidence of high-temperature hydrousalteration within the gabbros of the Samail ophiolite (Oman)shows that most of the gabbros have been affected by successivestages of alteration, starting above 975°C and ending below500°C. Sr and O isotopic analyses provide constraints onthe nature and origin of the fluids associated with these alterationevents. Massive gabbros, dykes and veins and their associatedminerals depart from mid-ocean ridge basalt (MORB)-source magmasignatures (87Sr/86Sr >0·7032 and depleted  相似文献   

20.
P. Thy 《Lithos》1991,26(3-4):253-269
The south-eastern propagating rift zone of Iceland shows a progression from tholeiitic, to transitional, and mildly alkalic basalts going toward the front of the propagator. A petrogenetic model has been formulated based on low and high pressure melting experiments. The evolution of the magmas behind the front of the propagating rift is dominated by near surface processes (e.g., Katla and Hekla volcanic systems). Compared with the one atmosphere liquid line of descent, the Vestmannaeyjar lavas, erupted at the front of the propagating rift, have systematically higher Al2O3 and Na2O contents consistent with the experimentally determined effects of high pressure, Labradoritic plagioclase megacrysts, which occur in the early phase of the Surtsey eruption at the front of the propagator, are consistent with the predicted effect of high pressure on plagioclase composition. On the other hand, augites similar to those of the high pressure experiments are unknown among the phenocryst and xenolith assemblages of the Vestmannaeyjar lavas (and other Icelandic lavas). Petrographic evidence points toward a high water content in the evolved lavas of the Vestmannaeyjar. A relatively high water activity and high pressure in the magma chambers at the front of the propagator could have caused a significant suppression of the liquids temperature, in particular for plagioclase. Seismic and magnetotelluric evidence suggest that magma chambers behind the propagating front occur at the depth equivalent to 2–3 kbar of pressure. At the front, magma chambers have been located by geophysical evidence at significantly greater depths equivalent to 3–8 kbar. The lavas erupted at the front of the propagator are located to the alkalic side of the thermal divide and, therefore, can be expected to evolve toward nepheline saturation under slightly hydrous conditions. The most evolved of these lavas are of ferrobasaltic compositions and may be saturated with augite. The lavas erupted behind the front are located to the transitional side of the divide and evolve toward quartz saturation under essentially anhydrous conditions. In contrast to the Vestmannaeyjar lavas, the lavas behind the propagator often contain augite as a phenocryst. It is concluded that the chemical variation observed along the south-eastern propagating rift of Iceland is an effect of primary chemical features and that water plays a significant role on the liquid lines of descent at the front of the propagator.  相似文献   

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