共查询到20条相似文献，搜索用时 15 毫秒
1.
2.
北部湾涠洲岛及斜阳岛火山岩微量元素和同位素地球化学及其构造意义 总被引：1，自引：1，他引：0
本文在北部湾内一对姊妹火山岛即涠洲岛及斜阳岛火山地质研究基础上,进一步开展火山岩微量元素和Sr-Nd-Pb同位素,以及地幔橄榄岩Re-Os同位素地球化学研究.岛上早晚两期火山岩均为碱性玄武岩,分别属于碱性橄榄玄武岩和碧玄岩.碧玄岩为玻基斑状结构,舍地幔橄榄岩碎块(一般<1cm),表明为地幔岩浆快速喷出地表冷凝而成,岩浆上升过程中极少演化.火山岩微量元素和Sr-Nd-Pb同位素资料表明,涠洲岛及斜阳岛玄武岩与雷琼及北部湾周边、南海海盆玄武岩类似,具有亏损地幔的Sr、Nd同位素组成与Pb同位素显示的EMII富集地幔特征的Dupal异常,表明岩浆并非来自单一地幔源区,不可与OIB或MORB源区简单类比,也非地幔柱成因,而是由两个不同的地球化学组分混合而成.Re-OS同位素特征也指示地幔橄榄岩捕虏体来源于亏损的岩石圈地幔,而非核幔边界.推测涠洲岛及斜阳岛与雷琼及北部湾周边的岩浆可能是由于南海扩张后大陆裂解-软流圈地幔热物质上涌,与上覆薄而年轻的岩石圈地幔相互作用的产物. 相似文献
3.
Detailed chemical and mineralogical data are given for three sequences of basalts and picrite basalts from bore-holes in Western India. The picrite basalts show bulk compositional variation generated by the fractionation of olivine and chromite. Evolved picrite basalt magma appears to have given rise to basalt by the fractionation of olivine+clinopyroxene, despite the presence of abundant plagioclase phenocrysts. It is suggested that a slow settling rate for plagioclase relative to clinopyroxene and olivine is sufficient to account for this feature. The high degree of equilibrium crystallisation which many of the lavas have apparently undergone is interpreted in terms of the mechanism of compensated crystal settling (Cox and Bell, 1972). Experimentally determined atmospheric pressure phase relations are used to model dyke-like magma chambers in some detail. Finally volumetric and age relationships are used to argue that the picrite basalts, despite their porphyritic nature, crystallised from ultramafic liquids containing in some cases at least 16% MgO. 相似文献
4.
Olivine Compositions in Picrite Basalts and the Deccan Volcanic Cycle 总被引：13，自引：0，他引：13
Olivine phenocryst compositions and whole-rock chemical compositionsare used to identify primitive picrite basalts from widely separatedparts of the Deccan flood basalt province. Overall, primitivepicrites constitute a significant volume of rocks within theprovince. Most were probably emplaced along deep faults in theCambay graben and Narmada rift regions. We combine mineral compositiondata on previously described samples from boreholes at Dhandhuka,Wadhwan and Botad with information on new finds of picriticbasalts at Paliad, Anila, Pawagarh, Kawant and Ambadongar tohelp delineate the petrogenesis of these mafic rocks, and wealso examine the nature and probable origin of picrite basaltsfrom other regions of the Deccan, such as the Western Ghats.The combined data suggest that the incidence of high-MgO lavasdecreased with time during the Deccan volcanic cycle. KEY WORDS: Deccan Traps; olivine composition; picrite basalts; volcanic cycle 相似文献
5.
羌塘南部地区大面积出露基性—超基性火成岩。基性岩墙和玄武岩沿龙木错-双湖板块缝合带的南侧呈近东西向分布,大地构造位置位于冈瓦纳大陆北缘,分布范围西起国境线,东到双湖地区,总长约800km,形成时代集中在晚石炭世—早二叠世（284～318Ma）,反映出岩浆的快速侵位-喷溢的过程。基性岩墙和玄武岩主要为碱性系列岩石,少数为亚碱性系列,Ti和Fe的含量均比较高,Mg#比较低,表明它们都是原始岩浆经不同程度分异结晶后的残余熔体。基性岩墙和玄武岩的稀土元素、微量元素、形成环境等均与板内玄武岩或典型的地幔柱成因玄武岩具有相似的特点。另外,苦橄质岩石多与基性岩墙和玄武岩一起产出,主要为单斜辉石橄榄岩和二辉橄榄岩,矿物成分贫挥发性组分,主量、稀土和微量元素的特点反映出其很可能代表了地幔柱原始岩浆结晶的产物而非岩浆捕获体,而且其源区很可能为石榴子石二辉橄榄岩。由此,初步认为羌塘南部基性岩墙和玄武岩为地幔柱的头部减压熔融的产物,而苦橄质岩石为地幔柱尾柱通道熔融的产物。 相似文献
6.
羌塘南部地区大面积出露基性—超基性火成岩。基性岩墙和玄武岩沿龙木错-双湖板块缝合带的南侧呈近东西向分布,大地构造位置位于冈瓦纳大陆北缘,分布范围西起国境线,东到双湖地区,总长约800km,形成时代集中在晚石炭世—早二叠世(284~318Ma),反映出岩浆的快速侵位-喷溢的过程。基性岩墙和玄武岩主要为碱性系列岩石,少数为亚碱性系列,Ti和Fe的含量均比较高,Mg#比较低,表明它们都是原始岩浆经不同程度分异结晶后的残余熔体。基性岩墙和玄武岩的稀土元素、微量元素、形成环境等均与板内玄武岩或典型的地幔柱成因玄武岩具有相似的特点。另外,苦橄质岩石多与基性岩墙和玄武岩一起产出,主要为单斜辉石橄榄岩和二辉橄榄岩,矿物成分贫挥发性组分,主量、稀土和微量元素的特点反映出其很可能代表了地幔柱原始岩浆结晶的产物而非岩浆捕获体,而且其源区很可能为石榴子石二辉橄榄岩。由此,初步认为羌塘南部基性岩墙和玄武岩为地幔柱的头部减压熔融的产物,而苦橄质岩石为地幔柱尾柱通道熔融的产物。 相似文献
7.
The Lilloise Intrusion, East Greenland: Fractionation of a Hydrous Alkali Picritic Magma 总被引：2，自引：1，他引：1
The Lilloise is an 8 km4 km layered mafic intrusion which cutsthe plateau basalts of the East Greenland Tertiary province.Lilloise was intruded at 50 Ma, 4–5 Ma after cessationof the voluminous tholeiitic magmatism which accompanied riftingof the East Greenland continental margin. Lilloise is unusualamong layered intrusions in the province because it had a hydrousalkali picrite parent magma and generated a late-stage effluxof magmatic water from the intrusion into the aureole rocks.The three major subdivisions of the layered rocks are: olivine-clinopyroxene,olivine-clinopyroxene-plagioclase and plagioclase-amphibolecumulates. Massive subsidence of the intrusion before completesolidification resulted in deformation of the internal layeringand downturn of the bedding in the surrounding basalts. A strikingfeature of the intrusion is the injection of the layered rocksby a plexus of magmatic sheets which formed at the time of subsidence.The composition of these sheets is representative of the fractionationtrend of the intrusion and ranges from hawaiite to mildly saturatedquartz trachyte. The fractionation trend is successfully explainedby extraction of cumulus minerals of the layered rocks froma parent magma represented by alkali picrite dykes of a contemporaneousregional dyke swarm. Saturated to mildly over-saturated syenitesare a major component of the East Greenland province and theLilloise intrusion is illustrative of an important magmatictrend towards such compositions at this stage in the openingof the North Atlantic. KEY WORDS: Lilloise intrusion; East Greenland; alkali picrite magma; layered intrusion; magmatic differentiation
*Corraponding author 相似文献
8.
琼北火山活动分期与全新世岩浆演化 总被引：12，自引：9，他引：12
琼北火山活动始于始新世以来，早第三纪火山岩以夹层隐伏于不同时期的地层中。本文新提供的琼北火山岩K-AT年龄和砂岩捕虏体热释光年龄，从年代学上确定了琼北存在全新世火山喷发活动。琼北地表火山岩从早到晚分为6期：蓬莱期(中新世)、金牛岭期(上新世)、多文岭期(早更新世)、东英期(中更新世)、道堂期(晚更新世)、雷虎岭期(全新世)。雷虎岭期又分为早晚两个亚期：雷虎岭亚期和马鞍岭亚期，它们分别属于橄榄拉斑玄武岩和石英拉斑玄武岩。火山岩稀土元素、微量元素和Sr，Nd，Pb同位素地球化学特征揭示，岩浆来自亏损地幔(DMM)和富集岩石圈或亏损地幔(DMM)与俯冲洋壳两种不同的地球化学组分混合源区。橄榄拉斑玄武岩是相对原始的地幔岩浆。橄榄拉斑玄武岩岩浆经约10％橄榄石分离结晶可以形成石英拉斑玄武岩浆。岩浆上升过程中未受明显的陆壳混染。 相似文献
9.
Six crystalline mixtures, picrite, olivine-rich tholeiite, nepheline basanite, alkali picrite, olivine-rich basanite, and olivine-rich alkali basalt were recrystallized at pressures to 40 kb, and the phase equilibria and sequences of phases in natural basaltic and peridotitic rocks were investigated.The picrite was recrystallized along the solidus to the assemblages (1) olivine+orthopyroxene+ clinopyroxene +plagioclase+spinel below 13 kb, (2) olivine+orthopyroxene+clinopyroxene+spinel between 13 kb and 18 kb, (3) olivine+orthopyroxene+clinopyroxene+ garnet+spinel between 18 kb and 26 kb, and (4) olivine+clinopyroxene+garnet above 26 kb. The solidus temperature at 1 atm is slightly below 1,100° and rises to 1,320° at 20 kb and 1,570° at 40 kb. Olivine is the primary phase crystallizing from the melt at all pressures to 40 kb.The olivine-rich tholeiite was recrystallized along the solidus into the assemblages (1) olivine+ clinopyroxene+plagioclase+spinel below 13 kb, (2) clinopyroxene+orthopyroxene+ spinel between 13 kb and 18 kb, (3) clinopyroxene+garnet+spinel above 18 kb. The solidus temperature is slightly below 1,100° at 1 atm, 1,370° at 20 kb, and 1,590° at 40 kb. The primary phase is olivine below 20 kb but is orthopyroxene at 40 kb.In the nepheline basanite, olivine is the primary phase below 14 kb, but clinopyroxene is the first phase to appear above 14 kb. In the alkali-picrite the primary phase is olivine to 40 kb. In the olivine-rich basanite, olivine is the primary phase below 35 kb and garnet is the primary phase above 35 kb. In the olivine-rich alkali basalt the primary phase is olivine below 20 kb and is garnet at 40 kb.Mineral assemblages in a granite-basalt-peridotite join are summarized according to reported experimental data on natural rocks. The solidus of mafic rock is approximately given by T=12.5 P
Kb+1,050°. With increasing pressure along the solidus, olivine disappears by reaction with plagioclase at 9 kb in mafic rocks and plagioclase disappears by reaction with olivine at 13 kb in ultramafic rocks. Plagioclase disappears at around 22 kb in mafic rocks, but it persists to higher pressure in acidic rocks. Garnet appears at somewhat above 18 kb in acidic rocks, at 17 kb in mafic rocks, and at 22 kb in ultramafic rocks.The subsolidus equilibrium curves of the reactions are extrapolated according to equilibrium curves of related reactions in simple systems. The pyroxene-hornfels and sanidinite facies is the lowest pressure mineral facies. The pyroxene-granulite facies is an intermediate low pressure mineral facies in which olivine and plagioclase are incompatible and garnet is absent in mafic rocks. The low pressure boundary is at 7.5 kb at 750° C and at 9.5 kb at 1,150° C. The high pressure boundary is 8.0 kb at 750° C and 15.0 kb at 1,150° C. The garnet-granulite facies is an intermediate high pressure facies and is characterized by coexisting garnet and plagioclase in mafic rocks. The upper boundary is at 10.3 kb at 750° C and 18.0 kb at 1,150° C. The eclogite facies is the highest pressure mineral facies, in which jadeite-rich clinopyroxene is stable.Compositions of minerals in natural rocks of the granulite facies and the eclogite facies are considered. Clinopyroxenes in the granulite-facies rocks have smaller jadeite-Tschermak's molecule ratios and higher amounts of Tschermak's molecule than clinopyroxenes in the eclogite-facies rocks. The distribution coefficients of Mg between orthopyroxene and clinopyroxene are normally in the range of 0.5–0.6 in metamorphic rocks in the granulite facies. The distribution coefficients of Mg between garnet and clinopyroxene suggest increasing crystallization temperature of the rocks in the following order: eclogite in glaucophane schist, eclogite and granulite in gneissic terrain, garnet peridotite, and peridotite nodules in kimberlite.Temperatures near the bottom of the crust in orogenic zones characterized by kyanitesillimanite metamorpbism are estimated from the mineral assemblages of metamorphic rocks in Precambrian shields to be about 700° C at 7 kb and 800° C at 9 kb, although heat-flow data suggest that the bottom of Precambrian shield areas is about 400° C and the eclogite facies is stable.The composition of liquid which is in equilibrium with peridotite is estimated to be close to tholeiite basalt at the surface pressure and to be picrite at around 30 kb. The liquid composition becomes poorer in normative olivine with decreasing pressure and temperature.During crystallization at high pressure, olivine and orthopyroxene react with liquid to form clinopyroxene, and a discontinuous reaction series, olivine orthopyroxene clinopyroxene is suggested. By fractional crystallization of pyroxenes the liquid will become poorer in SiO2. Therefore, if liquid formed by partial melting of peridotite in the mantle slowly rises maintaining equilibrium with the surrounding peridotite, the liquid will become poorer in MgO by crystallization of olivine, and tholeiite basalt magma will arrive at the surface. On the other hand, if the liquid undergoes fractional crystallization in the mantle, the liquid may change in composition to alkali-basalt magma and alkali-basalt volcanism may be seen at a late stage of volcanic activity.Publication No. 681, Institute of Geophysics and Planetary Physics, University of California, Los Angeles. 相似文献
10.
Gough Island: Evaluation of a fractional crystallization model 总被引：3，自引：0，他引：3
Gough Island is composed of an alkaline olivine basalt-trachyte series. A fractional crystallization model for the development of these rocks has been evaluated by correlating the geochemical trends of major and trace elements. Beginning with an alkali olivine basalt parent the major element abundances were used to determine the varying proportions of crystallizing minerals required to generate the various residual liquids. A least-squares computer model was used for this calculation. The modal proportions of cumulative minerals and trace element distribution coefficients were used to predict the trace element abundances in each rock type.Three significant trace element trends are observed in Gough Island rocks: (1) increasing rare earth (RE) abundance and relative light RE enrichment with increasing major element differentiation, (2) marked Eu, Sr, and Ba depletions in late stage trachytes, (3) Cr and M enrichment in picrite basalt.The trace element abundances predicted by the fractional crystallization model are in good agreement with these observed trends. A fractional crystallization process involving olivine, pyroxene, feldspar, and apatite accounts for all the significant major and trace element trends observed in Gough Island rocks. 相似文献
11.
Petrological and geochemical investigations have been conducted on the little studied Neogene basaltic rocks of the Madeiran Islands. The Madeiran suite of minor intrusives and lavas consists of parental, unusually soda rich, alkali olivine basalts with hawaiite, mugearite and essexite derivatives. Olivine and clinopyroxene are dominant phenocryst and cumulus nodule phases. Low pressure fractionation of the parental magma by precipitation of these minerals gave rise to the hawaiitic trend. That olivine settling precedes clinopyroxene in the fractionation process can be deduced from Ca and Ni variations in the analysed rocks and phenocryst separates. Late stage feldspar flotation in a hawaiitic derivative liquid led to extrusive mugearites and an intrusive essexite.Low K/Rb ratios in the Madeiran basalts (ave. 325) point to the influence of phlogopite rather than hornblende in the mantle melting zone. The primitive alkali olivine basalt magma is thought to have arisen by partial melting following water release from small amounts of phlogopite (no more than 1%) at mantle depths around 100 km. A deep level of magma generation is consistent with the low values of heat flow recorded in ocean basins. Many other oceanic alkali basalt provinces remote from ridge systems may have arisen in a similar way. 相似文献
12.
岩浆的分离结晶作用和地壳同化混染作用是造成硫饱和的重要因素。本文以金平-Song Da地区二叠纪低钛苦橄岩为原生岩浆,使用MELTS程序模拟了岩浆在分离结晶和围岩同化混染作用的控制下达到硫饱和,发生硫化物熔体的熔离。模拟结果表明,低钛苦橄质岩浆从源区上升到浅部岩浆房的过程中发生了约10%的橄榄石的分离结晶,形成高镁的玄武质岩浆。高镁玄武质岩浆在浅部岩浆房内同化混染>18%的围岩,并经历约27%硅酸盐矿物的分离结晶后达到硫饱和。熔离的硫化物熔体在岩浆通道内聚集形成了白马寨铜镍硫化物矿。经历硫化物熔体熔离后的残余岩浆喷出地表形成了金平地区亏损Ni和Cu并具有强烈地壳混染特征的低钛玄武岩。 相似文献
13.
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
14.
Paul T. Robinson 《Contributions to Mineralogy and Petrology》1969,22(4):349-360
In north-central Oregon numerous small flows of alkali-olivine basalt occur in the Oligocene to early Miocene John Day Formation. Chemically, these basalts are characterized by relatively low SiO2 and K2O and very high TiO2 and iron. Fifteen analysed specimens (44 to 48 percent SiO2) have an average of 3.6 percent TiO2 and 15 percent total iron. The average composition of the Oregon basalts compares closely with the average hawaiite of the Hawaiian Islands, differing only in having slightly higher iron and slightly lower SiO2 and total alkalis. Closely associated flows of trachyandesite and quartz latite are chemically related to the basalts and probably formed by differentiation of an alkali-olivine basalt magma.Typical basalt specimens have 10 to 15 percent of modal olivine, interstitial alkali feldspar, and abundant clay minerals and chlorophaeite. Textures are subophitic or intersertal and phenocrysts are rare. Plagioclase laths are slightly zoned and range in composition from An68 to An44. Purplish-brown titaniferous augite is the only pyroxene, and ilmenite is the dominant opaque mineral.Distinct differences in composition and age, and the lack of transitional varieties indicate that these basalts are unrelated to the younger Columbia River basalts. They presumably represent a separate parent magma of alkalic affinity that was generated independently within the mantle. 相似文献
15.
Primary basalts and magma genesis 总被引：1，自引：0，他引：1
R. N. Thompson 《Contributions to Mineralogy and Petrology》1974,45(4):317-341
Three Eocene lavas from Skye, NW Scotland, have been subjected to anhydrous experimental studies within their melting ranges at pressures up to 30 kb. Two of these, an olivine-phyric magnesian alkali basalt and a near-aphyric Mg-poor transitional basalt, appear to show four-phase points on their liquidi at high pressures which are thought to have genetic significance. From experimental and mineralogical evidence, the magnesian basalt is postulated to be a primary magma, erupted without significant compositional change from its genesis by slight partial melting of a relatively Fe-rich spinel lherzolite upper mantle at about 60 km depth. The liquid seems to have had a reaction relationship with Ca-poor pyroxene (pigeonite) in the residual lherzolite. Partial crystallization of batches of this magma, delayed during its ascent at depths of about 40 km, is thought to have given rise to the Mg-poor basaltic liquids. The third lava studied experimentally, a sparsely olivine-phyric hawaiite, does not have olivine on the liquidus in any part of its anhydrous P-T diagram and therefore cannot have been derived under anhydrous conditions from olivine-saturated sources. The mineralogy and chemistry of the lavas are used to support an hypothesis that the hawaiites are products of partial crystallization of pockets of basalt magma at depths approximating to the crust/ mantle boundary beneath Skye, with
rising to sufficient values to make the residual liquids comparatively rich in normative feldspar. Finally, the genesis of all other Skye Eocene lavas is reviewed in the light of the new experimental data. 相似文献
16.
The investigation of rocks, minerals, and melt inclusions showed that porphyritic alkaline picrites and meimechites crystallized from different parental magmas. At a similar ultrabasic composition, the alkaline picrite melts were enriched in K2O relative to Na2O, and contained up to 0.12–0.13 wt % F and less Cr, Ni, and H2O (only 0.01–0.16 wt % H2O, versus 0.6–1.6 wt % in the meimechite melts) compared with the meimechite magmas. The crystallization of alkaline picrite melts occurred under stable conditions at relatively low temperatures without abrupt changes: olivine and clinopyroxene crystallized at 1340–1285 and 1230–1200°C, respectively, as compared with 1600–1450 and 1230–1200°C in the meimechites. The alkaline picrite melts evolved toward melanephelinite, nephelinite, tephrite, and trachydolerite; whereas the meimechite magmas gave rise to subalkaline picritic rocks. The partitioning of vanadium between olivine and melt suggests that the meimechite magma crystallized under more oxidizing conditions compared with the alkaline picrite melts: the KDV values for the meimechite melts (0.011–0.016) were three times lower than those for the alkaline picrite melts (0.045–0.052). The parental magmas of the alkaline picrites and meimechites were enriched in trace elements relative to mantle levels by factors of tens to hundreds. The alkaline picrite magma showed lower LILE and LREE contents compared with the meimechite magma. The magmas had also different indicator ratios of incompatible elements, including those immobile in aqueous fluids. It was concluded that the meimechite and alkaline picrite melts were derived from different mantle sources. The former were generated at lower degrees of melting of an undepleted mantle source, and the meimechite melts were produced by high-degree melting of a probably lherzolite-harzburgite source. 相似文献
17.
通过对个旧地区印支期火山岩的地质、岩石地球化学、微量元素和稀土元素等研究表明,该期火山岩的岩石类型为杏仁状斑状纤闪石化玄武岩,属于硅不饱和、富碱、高钛,镁、铁偏高,铝偏低,具有大陆碱性橄榄玄武岩的组分特征.该玄武岩为陆内拉张环境下基性程度高、分异程度低的幔源岩浆分异的产物,形成于大陆板内裂谷环境,并与拉裂作用有关. 相似文献
18.
Petrology of the Western Reykjanes Peninsula, Iceland 总被引：3，自引：3，他引：3
The active tholeiitic volcanic zone of the Reykjanes Peninsulaconsists of five volcanic fissure swarms, the two westernmostof which are the subject of this petrological study. The recent(less than 12,000 years) extrusives of the swarms group morphologicallyand petrographically into small picrite basalt lava shields,large olivine tholeiite lava shields and tholeiite fissure lavas;formed in that chronological succession. The picrite basalts exhibit a primitive mineralogy with chromite,olivine (Fo 89) and plagioclase (An 90) as phenocrysts and mayrepresent a primary liquid from the mantle. Simultaneous crystallizationof olivine, plagioclase and augite to form glomerocrysts inthe fissure lavas indicate low pressure cotectic crystallizationconditions. Twenty-eight new major element chemical analyses of the lavasare presented. They are generally characterized by a low contentof alkalies and high CaO. The lavas constitute two main suites,a lava shield suite and a fissure lava suite. There is a positivecorrelation between the volume of individual lavas and the contentof incompatible elements of the lavas within each group. Likewisethere is an overall chemical trend through time demonstrated,for example, by a rise in K2O from about 0.02 per cent to 0.24per cent during the last, approximately, 12,000 years. There is an apparent chemical zoning within each volcanic swarmsuch that the most evolved and youngest lavas are found in thecentral axial area of the swarm. This central area is also characterizedby graben subsidence, high magnetic anomalies and high temperaturethermal areas, all indicative of shallow magma reservoir(s).In spite of indications of fractional crystallization in theevolution of the olivine tholeiites and tholeiites, some otherprocesses must be sought to explain the volume chemistry relations.Cyclic volcanic activity is tentatively suggested to explainthe observed regular temporal variations within the swarm, eachcycle starting with the formation of picrite basalts. 相似文献
19.
Albert Günter Herrmann 《Contributions to Mineralogy and Petrology》1968,17(4):275-314
Several types of basaltic and related rocks from NW Germany have been analysed for 14 lanthanides and yttrium. Alkali olivine basalts (13 samples) are the most common products of the late Tertiary volcanism in Northern Hessia and Lower Saxony. One basalt intermediate in composition between alkali olivine basalts and tholeiitic basalts has been investigated (intermediate basalt) beside 3 samples of the tholeiitic type. Several rare effusive rock species occur in this area. The number of samples is indicated in brackets: nepheline basanite (1), olivine nephelinites (5), peridotite inclusions (2) from one of the above mentioned alkali olivine basalts. Trachytes (3) and phonolite (1) from the Westerwald area, also Tertiary in age. Three nepheline leucite tephrites from the Eifel area, Pleistocene in age, and pyroxenes from the Recent Stromboli (Italy) have been included in this investigation.The lanthanides and yttrium are analysed after chemical preconcentration controlled by the use of spikes. La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb have been determined with good accuracy and precision by X-ray fluorescence, Eu, Tb, Ho, Tm and Lu by optical emission spectrography.Following earlier suggestions the distribution pattern of the lanthanides in basalts has been compared with that of chondrites. Tholeiitic basalts of the area under investigation show only a slight deviation from the relative distribution of the lanthanides in chondrites. The latter contain a twentieth of the absolute concentration in tholeiitic basalts. All other effusive rocks of this volcanic province have higher Y and La-Lu abundances and increasing ratios of La-Eu/Y, Gd-Lu (in brackets) in the following sequence: intermediate basalt (3.7); alkali olivine basalts (7.6); nepheline leucite tephrites (8.8); nepheline basanite (9.1); olivine nephelinites (10.2); phonolite (11.1); trachytes (11.6). The highest concentration of yttrium and of the lanthanides is observed in olivine nephelinites (up to 860 ppm Y, La-Lu).The observed increase in absolute concentration of the lanthanides and in relative accumulation of the light lanthanide elements from chondrites to tholeiitic basalts, to intermediate basalt, to alkali olivine basalts and to nepheline basanite makes a genetic interrelation in this sequence of rock types probable. Chondrites resemble garnet peridotites as potential main constituents of the upper earth's mantle. The pattern of the distribution of the lanthanides confirms a hypothesis that some tholeiitic basalts represent the most primitive of all basaltic magmas. Several models on the origin of both tholeiitic and alkali olivine basalts from potential source rocks or melts in the mantle have been checked with the data on the abundances of the lanthanides. There is still a lack of information on rare earths distribution in abundant rock forming minerals to completely exclude crystal fractionation under different pressures in the mantle as the origin of the different tholeiitic and alkali olivine basalt magmas. Alkali and gas accumulation (including the light lanthanides) in the upper parts of deep seated magma reservoirs should be considered as a potential source of the different alkali basalts. This is a process which has been observed by Richter and Moore (1966) in Hawaiian lava pools.The concentration of all and accumulation of the light lanthanides in the olivine nephelinites of our area is much too high to be explained by assuming an assimilation of sedimentary carbonate rocks in alkali olivine basalt melts. 相似文献
20.
Ultramafic inclusions in basaltic rocks from Hawaii 总被引：4，自引：0，他引：4
Dr. Richard W. White 《Contributions to Mineralogy and Petrology》1966,12(3):245-314
Ultramafic inclusions and the enclosing basaltic rocks were collected from a number of localities in the Hawaiian Islands; these and other specimens were studied by standard pétrographic techniques and with an electron microprobe. Emphasis was on determination of mineral assemblages, mineral compositions, and variations in composition. Sixty-eight inclusions and thirteen basaltic rocks are described, with partial chemical analyses (Ti, Al, Cr, Fe, Mn, Ni, Mg, Ca, Na, K) of olivines, orthopyroxenes, clinopyroxenes, and some feldspars and other minerals. Inclusions range from dunite to anorthosite, and basaltic hosts range from olivine nephelinite to olivine tholeiite. The inclusions are separable into three categories, which correlate with three groups basaltic hosts: Lherzolite inclusions are relatively poor in Fe, and the component minerals have limited ranges of composition. In Hawaii, lherzolite inclusions occur preferentially in extremely undersaturated hosts (olivine nephelinite, nepheline basanite, and ankaratrite). Other varieties of inclusions (dunite, wehrlite, feldspathic peridotite, pyroxenite) are relatively rich in Fe, and the component minerals have wider ranges of composition. These inclusions, together with gabbro, occur preferentially in hosts which are but moderately undersaturated (alkaline olivine basalt, hawaiite, and ankaramite). The sparse inclusions in nearly-saturated basalt (olivine tholeiite) are petrographically distinct from those in the other two categories. These correlations suggest that the inclusions and the enclosing basaltic rocks are genetically related. As the three suites of inclusions differ chemically, mineralogically, physically, and texturally, more than one origin is probable. 相似文献