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1.
Rodrigues Island is composed of a differentiated series of transitional-mildly alkaline olivine basalts. The lavas contain phenocrysts of olivine (Fo88–68)±plagioclase (An73–50), together with a megacryst suite involving olivine, plagioclase, kaersutite, clinopyroxene, apatite, magnetite and hercynite-rich spinels. Troctolitic-anorthositic gabbro xenoliths are widely dispersed throughout the lavas and are probably derived from the upper parts of an underlying layered complex: the megacrysts may originate from coarse, easily disaggregated differentiates near the top of this body.Modelling of major and trace element data suggests that the majority of chemical variation in the lavas results from up to 45% fractionation of olivine, clinopyroxene, plagioclase and magnetite at low pressures, in the ratio 2035396. The clinopyroxene-rich nature of this extract assemblage is significantly different to that of the xenoliths, and suggests that clinopyroxene-rich gabbros and/or ultrabasic rocks may lie at greater depth.Sr and Nd isotopic data (87Sr/86Sr 0.70357–070406,143Nd/144Nd 0.51283–0.51289) indicate a mantle source with relative LREE depletion, and emphasise an unusual degree of uniformity in Indian Ocean island sources. A small group of lavas with strong HREE enrichment suggest a garnet-poor source for these, while high overall Al2O3/ CaO ratios imply high clinopyroxene/garnet ratios in refractory residua.  相似文献   

2.
Volcanic rocks in the Middle–Lower Yangtze River Valley (MLYRV) constitute a bimodal magmatic suite, with a significant compositional gap (between 50% and 63% SiO2) between the mafic and felsic members. The suite is characterized by a relatively wide spectrum of rock types, including basalts, trachytes, and rhyolites. The basaltic rocks have low-to-moderate SiO2 contents of 46.00–50.01%, whereas the trachytes and rhyolites possess SiO2 contents in the range of 63.08–77.61%. Rocks of the bimodal suite show moderate enrichment of LILEs, negative Nb, Ta, and Ti anomalies, and are significantly enriched in LREEs. The basalts were most likely generated by parental mafic magmas derived from enriched lithospheric mantle with minor assimilation of crustal materials involving coeval crystal fractionation during magma evolution. The results of energy-constrained assimilation and fractional crystallization simulations demonstrate that the felsic magma was produced by the mixing of 5–20% lower crustal anatectic melts with an evolved mafic magma (～48% SiO2) and accompanied by extensive clinopyroxene, plagioclase, biotite, and Fe–Ti oxide fractionation. Our model for the genesis of felsic rocks in bimodal suites is different from the traditional models of crustal melting and fractional crystallization or assimilation–fractional crystallization of basaltic liquids.  相似文献   

3.
Iron isotope and major- and minor-element compositions of coexisting olivine, clinopyroxene, and orthopyroxene from eight spinel peridotite mantle xenoliths; olivine, magnetite, amphibole, and biotite from four andesitic volcanic rocks; and garnet and clinopyroxene from seven garnet peridotite and eclogites have been measured to evaluate if inter-mineral Fe isotope fractionation occurs in high-temperature igneous and metamorphic minerals and if isotopic fractionation is related to equilibrium Fe isotope partitioning or a result of open-system behavior. There is no measurable fractionation between silicate minerals and magnetite in andesitic volcanic rocks, nor between olivine and orthopyroxene in spinel peridotite mantle xenoliths. There are some inter-mineral differences (up to 0.2‰ in 56Fe/54Fe) in the Fe isotope composition of coexisting olivine and clinopyroxene in spinel peridotites. The Fe isotope fractionation observed between clinopyroxene and olivine appears to be a result of open-system behavior based on a positive correlation between the Δ56Feclinopyroxene-olivine fractionation and the δ56Fe value of clinopyroxene and olivine. There is also a significant difference in the isotopic compositions of garnet and clinopyroxene in garnet peridotites and eclogites, where the average Δ56Feclinopyroxene-garnet fractionation is +0.32 ± 0.07‰ for six of the seven samples. The one sample that has a lower Δ56Feclinopyroxene-garnet fractionation of 0.08‰ has a low Ca content in garnet, which may reflect some crystal chemical control on Fe isotope fractionation. The Fe isotope variability in mantle-derived minerals is interpreted to reflect subduction of isotopically variable oceanic crust, followed by transport through metasomatic fluids. Isotopic variability in the mantle might also occur during crystal fractionation of basaltic magmas within the mantle if garnet is a liquidus phase. The isotopic variations in the mantle are apparently homogenized during melting processes, producing homogenous Fe isotope compositions during crust formation.  相似文献   

4.
Felsic alkalic rocks are a minor component of many ocean island volcanic suites, and include trachyte and phonolite as well as various types of alkaline and peralkaline rhyolite. However, there is considerable debate on the nature of their formation; for example, are they formed by partial melting of anomalous mantle or the final products of fractional crystallization of mafic magmas. The phonolites and foidal phonolites on Rarotonga were formed by low pressure crystal fractionation of two chemically distinct parental magmas. Low silica and high silica mafic magmas produced a basanite-foidal phonolite series and an alkali basalt-phonolite series, respectively. The foidal phonolite composition evolved from the low silica mafic magmas by approximately 60% fractionation of titanaugite + leucite + nepheline + magnetite + apatite. Fractionation continued with the crystallization of aegirine-augite + nepheline + kaersutite + magnetite + apatite. The phonolites formed from the alkali basalts by approximately 40% fractionation of kaersutite + titanaugite + Fe-Ti oxide + plagioclase + apatite and continued to evolve further by fractionation of anorthoclase + nepheline + aegerine-augite + Fe-Ti oxides. As the magmas fractionated in both suites, their overall viscosities (solid + liquid) increased until a point was reached whereby viscosity inhibited the eruption of magmas with compositions intermediate between the mafic rocks and the felsic rocks. However, the magmas continued to fractionate under static conditions with the residual fluid becoming foidal phonolitic in the low silica suite or phonolitic in the high silica suite. These phonolitic liquids, as a result of an increase in volatiles and enrichment of alkalis over aluminum, would actually have a lower viscosity than the intermediate liquids. This decrease in viscosity and the switch from a magma chamber being predominantly a liquid with suspended solids to a solid crystalline network with an interstitial liquid enabled phonolitic liquids to migrate, pool, and eventually erupt on the surface.  相似文献   

5.
Ach'Uaine Hybrid appinites represent a rare example of lamprophyric magmas that were demonstrably exactly contemporaneous with felsic differentiates, preserved within a suite of minor, hypabyssal intrusions emplaced at the end of the Caledonian orogeny in northern Scotland. Numerous small stocks, bosses and dykes show outcrop-scale relationships characteristic of mingling between lamprophyric and syenitic magmas, and are commonly cut by sharp-sided granite veins. The mafic rocks are characterised by Ni and Cr abundances and MgO sufficiently high to signal derivation from a mantle source within which radiogenic 87Sr/86Sr and nonradiogenic 143Nd/144Nd ratios require significant time-integrated incompatible element enrichment. This is manifest in high Ba, Sr and light REE abundances and incompatible element ratios in the derived magmas directly comparable with those of high Ba-Sr granitoids and related rocks. Quantitative major element, trace element, radiogenic and stable isotope modelling is consistent with early fractionation of clinopyroxene and biotite, accompanied by minor crustal assimilation, having driven the evolving lamprophyric magma to cogenetic syenite. Subsequent derivation of granite required a major change to feldspar-dominated crystal fractionation with continued, still minor contamination. The elemental and isotopic characteristics of the granitic terminus are so similar to high Ba-Sr granitoids both locally and worldwide, that these too may have had large mantle components and represent significant juvenile additions to the crust. Received: 26 September 1995 / Accepted: 5 June 1996  相似文献   

6.
辽河盆地沙三期火山-侵入岩地球化学与岩石成因   总被引：8，自引：0，他引：8  
辽河盆地沙三期火山-侵入岩为一套偏碱性的双峰式岩系，其基性端元为碱性玄武岩，中偏碱性端元为粗面质熔岩和侵入岩。碱性玄武岩富集高场强元素(如Nb、Th、Zr、Hf、V等)和轻稀土、Sr、Ba等大离子亲石元素，而亏损Rb和K，具有与板内碱性玄武岩和洋岛玄武岩类似的特征。粗面质岩石显示与基性端元相似的地球化学特征，其不相容元素含量总体上高于碱性玄武岩，但具强烈的Sr和Eu亏损。矿物学、岩石学及地球化学证据表明，玄武质岩石是软流圈地幔低程度部分熔融的产物，并经历了橄榄石和辉石的分离结晶作用，其源区可能有金云母和石榴石残留。玄武质岩浆上升到较浅部位后进一步发生橄榄石、辉石、斜长石和磁铁矿等的分离结晶作用而形成粗面质岩浆，地壳物质混染作用不显著。  相似文献   

7.
Mineral chemistry, major and trace elements, 40Ar/39Ar age and Sr–Nd–Pb isotopic data are presented for the Late Cretaceous Hamsilos volcanic rocks in the Central Pontides, Turkey. The Hamsilos volcanic rocks mainly consist of basalt, andesite and associated pyroclastics (volcanic breccia, vitric tuff and crystal tuff). They display shoshonitic and high-K calc-alkaline affinities. The shoshonitic rocks contain plagioclase, clinopyroxene, alkali feldspar, phlogopite, analcime, sanidine, olivine, apatite and titanomagnetite, whereas the high-K calc-alkaline rocks contain plagioclase, clinopyroxene, orthopyroxene, magnetite / titanomagnetite in microgranular porphyritic, hyalo-microlitic porphyritic and glomeroporphyritic matrix. Mineral chemistry data reveal that the pressure condition of the clinopyroxene crystallisation for the shoshonitic rocks are between 1.4 and 6.3 kbar corresponds to 6–18-km depth and the high-K calc-alkaline rocks are between 5 and 12 km. 40Ar/39Ar age data changing between 72 ± .5 Ma and 79.0 ± .3 Ma (Campanian) were determined for the Late Cretaceous Hamsilos volcanic rocks, contemporaneous with the subduction of the Neo-Tethyan Ocean beneath the Pontides. The studied volcanic rocks were enriched in the large-ion lithophile and light rare earth element contents, with pronounced depletion in the contents of high-field-strength elements. Chondrite-normalised rare earth element patterns (LaN/LuN = 6–17) show low to medium enrichment, indicating similar sources of the rock suite. Initial 87Sr/86Sr values vary between .70615 and .70796, whereas initial 143Nd/144Nd values change between .51228 and .51249. Initial 206Pb/204Pb values vary between 18.001 and 18.349, 207Pb/204Pb values between 15.611 and 15.629 and 208Pb/204Pb values between 37.839 and 38.427. The main solidification processes involved in the evolution of the volcanic rocks consist of fractional crystallisation, with minor amounts of crustal contamination ± magma mixing. According to geochemical evidence, the shoshonitic melts in the Hamsilos volcanic rocks were possibly derived from the low degree of partial melting of a subcontinental lithospheric mantle (SCLM), while the high-K calc-alkaline melts were derived from relatively high degree of partial melting of SCLM that was enriched by fluids and/or sediments from a subduction of oceanic crust.  相似文献   

8.
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   

9.
Certain petrological features of oceanic volcanic and plutonic rocks are not completely consistent with previously proposed models of crystal fractionation or magma mixing. For example, Sr is often higher in the differentiated basalts of a suite of aphyric rocks than in the relatively primitive basalts even though the differentiated basalts have apparently been produced by crystallization of large amounts of plagioclase with olivine and clinopyroxene. Additionally, oceanic basalts and gabbroic rocks often contain plagioclase crystals in excess of the appropriate cotectic proportions. Certain differentiated oceanic basaltic glasses and aphyric rocks crystallize plagioclase as the liquidus mineral, which would seem inconsistent with the strongly cotectic nature of the olivine + plagioclase + liquid surface.It is proposed here that plagioclase in mid-ocean ridge magma chambers separates from the basaltic liquid that it crystallizes in at a slower rate than does co-crystallizing olivine or pyroxene. Magma mixing in which a portion of the plagioclase remains suspended in the liquid during crystallization results in much more complex liquid lines of descent in mixed magmas and appears to resolve the apparent discrepancies noted above.  相似文献   

10.
Hawaiite-type lavas were erupted in three cycles (3.7, 1.2, and 0.3 M.y.) at Crater Flat, Nevada. The compositions of all three cycles, considered together, form a straddling alkalic series as defined by Miyashiro, in which the less evolved basalts plot near the normative olivine-diopside divide and the more evolved basalts project into hypersthene or nepheline fields. Fractionation modeling based on the oldest cycle allows the removal of olivine, clinopyroxene, and amphibole to arrive at the more evolved hawaiite compositions. In general, fractionation of phlogo-pite or feldspar is limited by the fractionation modeling and by Eu/REE relations. In detail, all hawaiites within one cycle (3.7 M.y.) need not be derived from a single parent magma. Varied parentage is more evident between cycles although all cycles are consistently of hawaiite composition. Basalts of the youngest two cycles are generally enriched in trace elements. Superimposed on this enrichment is a lack of Rb variation, leading to Rb/Sr ratios far lower than required to generate the high 87Sr/86Sr ratio (0.707) typical of basalts in this region. The very low Rb/Sr ratios limit processes that may lead to trace-element enrichment during magma evolution (cyclic recharge of a fractionating magma chamber). Decreased fractions of mantle melting leaving phlogopite in the residuum or an earlier event of metasomatic transport from phlogopite-bearing mantle rocks into a phlogopite-absent mantle assemblage might explain the observed trace-element enrichment with low Rb/Sr.  相似文献   

11.
The Early Palaeozoic East Krkonoše Complex (EKC) situated in the central West Sudetes, NE Bohemian Massif, is a volcano‐sedimentary suite containing abundant mafic and felsic volcanics metamorphosed to greenschist facies. The trace element distribution patterns and Nd isotope signatures (ENd500 = + 3.1 to + 6.6) of the metabasites (metabasalts) indicate that they may be related to a rising mantle diapir associated with intracontinental rifting. At the early stage, limited melting of an upwelling asthenosphere produced alkali basalts and enriched tholeiites which compositionally resemble oceanic island basalts. A later stage of rifting with larger degrees of melting at shallower depths generated tholeiitic basalts with E‐MORB to N‐MORB characteristics. The values of (87Sr/86Sr)i = 0.706 and ENd500 = − 5 ±1 of the porphyroids (metarhyolites) as well as the lack of rocks with intermediate compositions suggest that the felsic rocks were formed by a partial melting event of continental crust triggered by mantle melts. The geochemistry of the EKC bimodal metavolcanics and their association with abundant terrigenous metasediments suggest that the felsic–mafic volcanic suite was generated during intracontinental rifting. This process, widespread in Western and Central Europe during the Early Palaeozoic, is evidence of large‐scale fragmentation of the northern margin of the Gondwana supercontinent. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

12.
Tholeiitic basalts in various stages of alteration were dredged from Late Cretaceous volcanic rocks (60 -67 Ma) in the Hebrides Terrace seamount area in the Atlantic Ocean. These rocks are extrusive olivine basalts, including high- and low-Al basalts. High-Al basalts are depleted in MgO, CaO, Cr,Sc, V, St, Zr and enriched in TiO2, Na2O, Nb, Rb as compared with low-A1 basalts. Petrography and bulk-rock composition (major, trace and rare-earth elements) data defined clear tholeiitic suites displaying possible liquid lines of descent related to different degrees of crystal fractionation and partial melting.Isotopic dating of dredged samples gave the guyot an age of 60 - 67 Ma, in support of the assumption that it was formed during the Late Cretaceous.  相似文献   

13.
Most Hawaiian basaltic shield volcanoes are capped by moderately to strongly evolved alkalic lavas (MgO<4.5 wt.%). On Mauna Kea Volcano the cap is dominantly composed of hawaiite with minor mugearite. Although these lavas contain dunite and gabbroic xenoliths, they are nearly aphyric with rare olivine and plagioclase phenocrysts and xenocrysts. The hawaiites are nearly homogeneous in radiogenic isotope ratios (Sr, Nd, Pb) and they define coherent major and trace element abundance trends. These compositional trends are consistent with segregation of a plagioclase-rich cumulate containing significant clinopyroxene and Fe-Ti oxides plus minor olivine. Elements which are usually highly incompatible, e.g., Rb, Ba, Nb, are only moderately incompatible within the hawaiite suite because these elements are incorporated into feldspar (Rb, Ba) and oxides (Nb). However, in the most evolved lavas abundances of the most incompatible elements (P, La, Ce, Th) exceed (by 5–10%) the maximum enrichments expected from models based on major elements. Apparently, the crystal fractionation process was more complex than simple, closed system fractionation. The large amounts of clinopyroxene in the fractionating assemblage and the presence of dense dunite xenoliths with CO2 inclusions formed at minimum pressures of 2 kb are consistent with fractionation occurring at moderate depths. Crystal segregation along conduit or magma chamber walls is a possible mechanism for explaining compositional variations within these alkalic cap lavas.  相似文献   

14.
Tertiary volcanism in the İkizce region at the western edge of the eastern Pontides paleo-magmatic arc is represented by basaltic and andesitic rocks associated with sediments deposited in a shallow basin environment. The basaltic rocks contain plagioclase (An58–80), olivine (Fo82–84), clinopyroxene (Wo44–48En35–42Fs7–17), hornblende (Mg# = 0.68–0.76) phenocrysts, and magnetite microcrysts, whereas the andesitic rocks include plagioclase (An25–61), clinopyroxene (Wo46–49En38–43Fs11–13), hornblende (Mg# = 0.48–0.81), biotite (Mg# = 0.48–0.60) phenocrysts, titanomagnetite, apatite, and zircon microcrysts.Geochemical data indicate magmatic evolution from tholeiitic-alkaline transitional to calc-alkaline characteristics with medium-K contents. The geochemical variation in the rocks can be explained by fractionation of common mineral phases such as clinopyroxene, olivine, hornblende, plagioclase, magnetite, and apatite. The trace elements’ distributions of the volcanic rocks show similarities to those of E-Type MORB, have a shape that is typical of rocks from subduction-related tectonic setting with enrichment in LILE and to a lesser extent in LREE, but depletion in HFSE. The rocks evolved from a parental magma derived from an enriched source formed by subduction induced metasomatism of basaltic rocks, the latter formed through clinopyroxene ± olivine controlled fractionation in a high level magma chamber. The andesitic rocks developed through hornblende ± plagioclase controlled fractionation in shallow level magma chamber(s).  相似文献   

15.
Late Carboniferous (300–290 Ma) calc-alkaline basalts, andesites, and rhyolites typical of volcanic arc settings occur in the intermontane Saar-Nahe basin (SW Germany) within the Variscan orogenic belt. The volcanic rock suite was emplaced under a regime of tensional tectonics during orogenic collapse and its origin has been explained by melting of mantle and crust in the course of limited lithospheric rifting. We report major, trace and rare-earth-element data (REE), and Nd-Pb-Sr-O isotope ratios for a representative sample suite, which are fully consistent with an origin closely related to plate subduction. Major and trace element data define continuous melt differentiation trends from a precursor basaltic magma involving fractional crystallization of olivine, pyroxene, plagioclase, and magnetite typical of magma evolution in a volcanic arc. This finding precludes an origin of the andesitic compositions by mixing of mafic and felsic melts as can be expected in anorogenic settings. The mafic samples have high Mg numbers (Mg# = 65–73), and high Cr (up to 330 ppm) and Ni (up to 200 ppm) contents indicating derivation from a primitive parental melt that was formed in equilibrium with mantle peridotite. We interpret the geochemical characteristics of the near-primary basalts as reflecting their mantle source. The volcanic rocks are characterized by enrichment in the large ion lithophile elements (LILE), negative Nb and Ti, and positive Pb anomalies relative to the neighboring REE, suggesting melting of a subduction-modified mantle. Initial Nd values of −0.7 to −4.6, Pb, and 87Sr/86Sr(t) isotope ratios for mafic and felsic volcanics are similar and indicate partial melting of an isotopically heterogeneous and enriched mantle reservoir. The enrichment in incompatible trace elements and radiogenic isotopes of a precursor depleted mantle may be attributed to addition of an old sedimentary component. The geochemical characteristics of the Saar-Nahe volcanic rocks are distinct from typical post-collisional rock suites and they may be interpreted as geochemical evidence for ongoing plate subduction at the margin of the Variscan orogenic belt not obvious from the regional geologic context. Received: 3 August 1998 / Accepted: 2 January 1999  相似文献   

16.
Cenozoic（Miocene to Pleistocene） basaltic rocks in Jiangsu province of eastern China include olivine tholeiite and alkali basalt.We present major,trace element and Sr-Nd isotopic data as well as Ar-Ar dating of these basalts to discuss the petrogenesis of the basalts and identify the geological processes beneath the study area.On the basis of chemical compisitions and Ar-Ar dating of Cenonoic basaltic rocks from Jiangsu province,we suggest that these basalts may belong to the same magmatic system.The alkali basalts found in Jiangsu province have higherΣFeO,MgO,CaO,Na2O, TiO2 and P2O5 and incompatible elements,but lower Al2O3 and compatible elements contents than olivine tholeiite which may be caused by fractional crystallization of olivine,pyroxene and minor plagioclase.In Jiangsu basaltic rocks the incompatible elements increase with decreasing MgO/ΣFeO ratios.The primitive mantle-normalized incompatible elements and chondrite-normalized REE patterns of basaltic rocks found in Jiangsu province are similar to those of OIB.Partial loss of the mantle lithosphere accompanied by rising of asthenospheric mantle may accelerate the generation of the basaltic magma.The 143Nd/144Nd vs.87Sr/86Sr plot indicates a mixing of a depleted asthenospheric mantle source and an EMI component in the study area.According to Shaw’s equation,the basalts from Jiangsu province may be formed by l%-5%partial melting of a depleted asthenospheric mantle source.On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen（1982）,we suggest that basalts from Jiangsu province may belong to a magmatic system with JF-2 as the primitive magma which has undergone fractional crystallization and evolved progressively to produce other types of basalts.  相似文献   

17.
The Quaternary alkaline volcanic field of Southern Turkey is characterized by intra-continental plate-type magmatic products, exposed to the north of the ?skenderun Gulf along a NE-SW trending East Anatolian Fault, to the west of its intersection with the N–S trending Dead Sea Fault zone. The ?skenderun Gulf alkaline rocks are mostly silica-undersaturated with normative nepheline and olivine and are mostly classified as basanites and alkaline basalts with their low-silica contents ranging between 43 and 48?wt.% SiO2. They display Ocean Island Basalt (OIB)–type trace element patterns characterized by enrichments in large-ion-lithophile elements (LILE) and light rare earth element (LREE), and have (La/Yb)N?=?8.8–17.7 and (Hf/Sm)N?=?0.9–1.6 similar to those of basaltic rocks found in intraplate suites. The basanitic rocks have limited variations Sr-Nd isotopic ratios (87Sr/86Sr?=?0.70307–0.70324, 143Nd/144Nd?=?0.512918–0.521947), whereas the alkali basalts display more evolved Sr-Nd isotopic ratios (87Sr/86Sr?=?0.70346-0.70365, 143Nd/144Nd?=?0.512887–0.521896). The ?skenderun Gulf alkaline rocks also display limited Pb isotopic variations with 206Pb/204Pb?=?18.75–19.09 207Pb/204Pb?=?15.61–15.66 and208Pb/204Pb?=?38.65–39.02, indicating that they originated from an enriched lithospheric mantle source. Calculated fractionation vectors indicate that clinopyroxene and olivine are the main fractionating mineral phases. Similarly, based on Sr-Nd isotopic ratios, the assimilation and fractional crystallization (AFC) modeling shows that the alkali basalts were affected by AFC processes (r?=?0.2) and were slightly contaminated by the upper crustal material.The high TiO2 contents, enrichments in Ba and Nb, and depletions in Rb can likely be explained by the existence of amphibole in the mantle source, which might, in turn, indicate that the source mantle has been affected by metasomatic processes. The modeling based on relative abundances of trace elements suggests involvement of amphibole-bearing peridotite as the source material. ?skenderun Gulf alkaline rocks can thus be interpreted as the products of variable extent of mixing between melts from both amphibole-bearing peridotite and dry peridotite.  相似文献   

18.
位于中国东南部的三水盆地、珠江口盆地、雷琼半岛和北部湾地区广泛分布新生代火山岩。火山岩的形成时间具有从内陆向沿海变新的特点,早第三纪三水和珠江口盆地火山岩具有由玄武岩与粗面岩-流纹岩构成的双峰式特点。其中玄武岩和粗面岩的微量元素和稀土元素的配分形式相似,富集大离子亲石元素并且有相似的εNd(T)同位素组成(2.34～6.4),说明它们来自相同的地幔源区,为同源岩浆演化的产物。玄武岩和粗面岩经历了不同的结晶分异过程,其中玄武岩在较深部岩浆房中经历橄榄石和单斜辉石为主的分离结晶作用,而粗面岩则是在浅部岩浆房中由玄武岩浆分异形成的过渡性岩浆再经过强烈的钾长石和斜长石、以及磷灰石的结晶分异形成的。晚第三纪珠江口盆地和北部湾火山岩、雷琼半岛第四纪火山岩则由碱性和拉斑玄武岩构成。这些火山岩的形成时间和地球化学和同位素特征表明它们经历了连续的软流圈地幔上涌和部分熔融过程,受控于自晚中生代以来的地幔柱构造。南海的形成是地幔柱活动引起的地幔上涌和大陆裂解作用的结果。  相似文献   

19.
Tholeiites accompanying a majority of alkali basalts are restricted to the highly productive central part of the CECV plume activity in Vogelsberg and Hessian Depression. They mainly occur as quartz tholeiites which according to experiments of partial melting and material balances are products of olivine tholeiitic primary melts. The differentiation from olivine to quartz tholeiitic melts took place in lower crustal magma chambers where olivine tholeiitic melt intruded due to a density comparable with that of the country rocks. The fractionation due to separation of olivine and some clinopyroxene caused contamination of tholeiite magmas by tonalitic partial melts from the wall rocks of the magma chambers. The latter process is indicated by relatively high Rb, K and Pb and low Nb concentrations and by Nd, Sr and Pb isotopes. Contaminating crustal melts, which roughly attained a proportion of 10%, contained very low 143Nd/144Nd ratios from a Nd/Sm fractionation as old as 2.6 Ga. This is the first evidence from mafic rocks of this high age in the lower crust beneath Central Europe. Modelling with incompatible elements allows to recognize olivine tholeiites as products of about 1% partial melting of plume rocks consisting of 35% primitive and 65% depleted mantle materials. The production of tholeiites other than alkali basalts is restricted to the highest plume activity and the largest fraction of MORB type source rocks. Received: 10 December 1999 / Accepted: 23 June 2000  相似文献   

20.
Chemical analyses of over seventy lavas or dykes containing spinel lherzolite inclusions of high pressure mineralogy, show that most host magmas are of alkali olivine basalt or basanite composition with relatively rare olivine nephelinites, and olivine melilitites. The 100 Mg/Mg+Fe++ ratios of host magmas display a strong maximum at about Mg70 consistent with partial melting of source peridotite with olivine of Fo88–90. In contrast to these primary magmas, there occur some host magmas with 100 Mg/Mg+Fe++<60 and with chemical compositions resembling those of classical hawaiite, mugearite, and nepheline benmoreite magmas. It is inferred that these magmas have been produced by crystal fractionation, within the upper mantle, of parental basanites or alkali olivine basalts. The presence of kaersutitic hornblende xenocrysts accompanying the lherzolite inclusions, and the nature of the chemical variation between associated basanites and nepheline benmoreites suggests that crystal fractionation has been dominated by kaersutitic hornblende, together with olivine and, in some cases, probably clinopyroxene. The mantle-derived nepheline benmoreite magmas also show similarities to some plutonic nepheline syenites.  相似文献   

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