首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
Whole-rock Nd and Sr isotopic compositions of the mafic-ultramafic complex near Finero demonstrate that the magma was derived from a depleted, perhaps MORB-type mantle reservoir. The Sm-Nd data for the Amphibole Peridotite unit can be interpreted as an isochron with an apparent age of 533 ± 20 Ma, which is consistent with a 207Pb/206Pb evaporation age of 549 ± 12 Ma of a single zircon grain from the Internal Gabbro unit. However, the interpretation of these apparent ages remains open to question. We therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 Ma ago, the latter being the most likely age of emplacement of the much larger magma body near Balmuccia (Val Sesia). The implication of the older emplacement age (if correct) would be that the igneous complex may be related to the numerous amphibolite units, which are intercalated with the metapelites of the overlying Kinzigite Formation, and together with them may constitute an accretionary complex. In this case, the mafic-ultramafic complex itself might also be part of such an accretionary complex (as has been proposed for the Balmuccia peridotite).

Internal Sm-Nd isochrons involving grt, cpx, plag and amph from the Internal Gabbro unit yield concordant ages of 231 ± 23, 226 ± 7, 223 ± 10, 214 ± 17, and 203 ± 13 Ma. These results confirm published evidence for a separate, regional heating event about 215 ± 15 Ma ago.

Initial Nd(533) values average +6.3 ± 0.4 for six samples of the Amphibole Peridotite unit and +6.0 ± 1.2 for ten samples of the External Gabbro unit. 87Sr/86Sr ratios require little or no age correction and range from 0.7026 to 0.7047 (with two outliers at 0.7053 and 0.7071). Strong correlations between 87Sr/86Sr and K2O and weaker correlations between initial Nd and K2O imply a comparatively minor (≤ 10%) contamination of the External Gabbro magma by crustal material and a later alteration by a crustal or seawater-derived fluid. These results contrast sharply with the isotopic composition (negative Nd and high 87Sr/86Sr values) of the associated mantle rocks, the Phlogopite Peridotite unit, which has been pervasively metasomatized by crustal fluids. This type of metasomatism and its isotopic signature are never seen in the magmatic complex. This evidence rules out any direct genetic relationship between the igneous complex and the mantle peridotite. The crust-mantle interaction is the opposite of that seen at Balmuccia, where the mantle peridotite is essentially ‘pristine’ and the magmatic body has been extensively contaminated by assimilation of crustal rocks.  相似文献   


2.
The Balmuccia alpine lherzolite massif is a fragment of subcontinental lithospheric mantle emplaced into the lower crust 251 Ma ago during the final, extensional phase of the Hercynian orogeny. The Balmuccia massif consists largely of lherzolite, with subordinate harzburgite and dunite, and an array of dike rocks formed in the mantle before crustal emplacement. Dike rocks include websterite and bronzitite of the Cr-diopside suite, spinel clinopyroxenite and spinel-poor websterite of the Al-augite suite, gabbro and gabbronorite of the late gabbro suite, and hornblendite of the hydrous vein suite. The dike rocks display consistent intrusive relationships with one another, such that Cr-diopside suite dikes are always older than dikes and veins of the Al-augite suite, followed by dikes of the late gabbro suite and veins of the hydrous vein suite. Phlogopite (phl) veinlets that formed during interaction with the adjacent crust are the youngest event. There are at least three generations of Cr-diopside suite dikes, as shown by crosscutting relations. Dikes of the Al-augite suite form a polybaric fractionation series from spinel clinopyroxenite to websterite and feldspathic websterite, which crystallized from aluminous alkaline magmas at relatively high pressures. The late gabbro suite of dikes intruded at lower pressures, where plagioclase saturation occurred before significant mafic phase fractionation. Hornblendite veins have distinct compositional and isotopic characteristics, which show that they are not related to either the Al-augite suite or to the late gabbro dike suite. Cr-diopside suite dikes have Nd and Sr isotopic compositions similar to those of the host lherzolite and within the range of compositions defined by ocean–island basalts. The Al-augite dikes and the hornblendite veins have Sr and Nd isotopic compositions similar to those of Cr-diopside suite lherzolite and websterite. The late gabbro dikes have Nd and Sr isotopic compositions similar to mid-ocean ridge basalt (MORB) asthenosphere. Lead isotopic compositions for all of the samples fall in the present-day MORB field on the 208Pb/204Pb vs. 206Pb/204Pb diagram but are displaced above this field on the 207Pb/204Pb vs. 206Pb/204Pb diagram. There is overlap in the data between the Cr-diopside suite and the Al-augite and hydrous vein suites, with the exception that the Cr-diopside websterite dikes have more radiogenic Pb than any of the other samples. In Pb–Pb space as well, the late gabbro suite has the least radiogenic isotopic compositions, reflecting a change in magma source region during uplift. These data show that tectonic thinning of subcontinental lithospheric mantle during extension caused a change in the source regions of mantle-derived magmas from an ocean island basalt (OIB)-like lithosphere to the underlying MORB asthenosphere. They also demonstrate that the upper mantle acquires its heterogeneous isotopic character through several different processes, including in situ radiogenic growth, addition of asthenospheric melts, dike-wall rock ionic exchange, redistribution of the lithospheric dike and vein materials by melting, and in the late stages of emplacement, assimilation of crustal materials.  相似文献   

3.
Trace element analyses have been obtained employing RNAA andINAA techniques for 23 bulk-rock specimens and for five pairsof mechanically separated opx and cpx from Western Alpine peridotites.Investigated rocks include 5 garnet lherzolites from Alpe Arami,and spinel (+plagioclase) lherzolites from Finero (2), Balmuccia(7), Baldissero (6) and Lanzo (3). Three pyroxene pairs wereanalysed from Balmuccia and two from Baldissero. All rocks exhibit marked LREE depletions relative to chondriticabundances except for the two Finero samples which appear tobe HREE depleted. Separated minerals also show LREE depletionsand HREE enrichments relative to chondrites. However, intermediaterare earths are markedly depleted in opx whereas they are enrichedin coexisting cpx. Higher overall concentrations and patternssimilar to those of the bulk rocks indicate that REE distributionsin lherzolites are dominated by clinopyroxene chemistry. Incontrast, both opx and cpx appear to contribute equally to the3d transition element geochemistry of the investigated peridotites. Most of the investigated rocks show the effects of early partialmelting of a pre-existing mantle source material characterizedby ‘chondritic’ REE fractionation and by a 3d transitionelement composition near the estimated values of Jagoutz etal. (1979). The melting process probably developed in a closed system (equilibriummelting) and at temperatures which, for the spinel peridotiteprotolith, seemingly were compatible with estimates of Presnallel al. (1979) for the ‘melting at the cusp’ process(T = 1200–1250 °C). In some cases the residual rocksunderwent a further contamination event. This is particularlyevident for the Lanzo peridotites, but possibly also for singleBaldissero and Balmuccia specimens. During ascent to the surface, the rocks underwent subsolidusannealing which occurred at temperatures around 900–1000°C under more or less closed system conditions.  相似文献   

4.
Phlogopite-bearing lithologies are the main constituent of the Phlogopite-Peridotite unit of the Finero sequence and the result of pervasive migration of metasomatizing melts/fluids. Conversely, the presence of phlogopite within the associated Finero Mafic Complex, a mafic-ultramafic pluton intruded into the metamorphic basement of the Adria plate, is mentioned in literature as rare. Recent detailed fieldwork has evidenced the presence of two distinct phlogopite-rich ultramafic lithologies within the Amphibole-Peridotite unit of the Finero Mafic Complex, where phlogopite is always associated with amphibole. Field and petrographic features of these occurrences, as well as major- and trace-element mineral chemistry, are here presented to i) place constraints on the nature of the parent melt from which they have been generated and ii) to address their relationship with the other lithologies of the Finero Complex. We find that these rocks were formed by late melt migrations along shear zones under high-T conditions. The geochemical affinity of these lithologies is different to the tholeiitic-transitional affinity reported in literature for the Finero Mafic Complex. The enrichment in LREE, Th, U and Sr of the associated amphibole possibly suggests that these phlogopite-bearing lithologies are genetically related to the metasomatic events that have affected the Finero mantle massif.  相似文献   

5.
The Kalka Intrusion, central Australia has a 5000 m-thick layered sequence comprising Pyroxenite, Norite and Anorthosite Zones; an Olivine Gabbro Zone is enclosed by, and has a facies relationship with, the Norite Zone. Contamination is evidenced by high initial 87Sr/86Sr ratios ( 0.708) in the lower four-fifths of the intrusion, and resulted in pyroxenite rather than peridotite as a basal crystal accumulate. After an early phase of erratic buildup in contamination due to assimilation of ragged granulite wall-rock, armouring of the walls and mixing produced an homogeneous Norite Zone (threshold) magma body crystallising opx-cpx-plag. Within the succeeding 3500 m of section plagioclase An values have a general decline up sequence (An74-60-feldspar threshold) with superimposed short term digressions to more calcic compositions. Initial 87Sr/86Sr ratios also fall very gradually (0.7081-0.7078 — isotopic threshold) with transient fluctuations to distinctly lower values. Maxima in plagioclase An contents and 87Sr/ 86Sr minima may be correlated with the spasmodic appearance of olivine and pronounced lithological variation. Such features are explained by the ponding of fresh bodies of uncontaminated magma on the floor of the chamber; these formed a hybrid with threshold magma that temporarily dominated crystallisation thereby perturbing plagioclase and isotopic compositions; eventually, mixing of the hybrid into the overlying threshold magma returned crystallisation to its initial state. The facies-controlled Olivine Gabbro Zone is the physical expression of ponded basal hybrid magmas. The onset of the Anorthosite Zone is marked by a pronounced decline in 87Sr/86Sr ratios to around 0.705 registering a major influx of new magma. In this instance the new magma dominated the system and a change to plagioclase as the dominant liquidus phase indicates a drastic change in input magma composition. The evolution of Kalka was determined by an interplay of crystal fractionation, fresh magma pulses, and magma mixing, with the latter effects producing both increases and decreases in plagioclase An contents; the complete process can only be revealed by combined mineral composition and isotopic data.  相似文献   

6.
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   

7.
腾冲马鞍山、打鹰山、黑空山火山岩浆来源与演化   总被引：10，自引：6，他引：4  
赵勇伟  樊祺诚 《岩石学报》2010,26(4):1133-1140
本文对马鞍山、打鹰山、黑空山火山岩主微量和Sr、Nd、Pd同位素地球化学研究认为,腾冲火山岩浆源区具有MORB与富集地幔混合之特征,推测为新特提斯俯冲洋壳重新熔融,导致腾冲地区的高钾钙碱性岩浆的火山活动,解释了腾冲在新生代大陆板内构造环境背景下出现岛弧或活动大陆边缘火山岩地球化学特征的现象。马鞍山、打鹰山和黑空山火山高钾钙碱性岩浆经历了岩浆房阶段辉石、钛铁矿的结晶分离作用和岩浆上升过程中斜长石的结晶分离作用,导致岩浆成分从中基性向中酸性演化,火山岩从玄武质粗安岩→粗安岩→粗面质英安岩演化。  相似文献   

8.
The Sausfjellet pluton is made up of two intrusive units emplaced into high-grade metamorphic rocks of the Helgeland Nappe Complex of the Uppermost Allochthon in the Norwegian Caledonides. The eastern part of the pluton intruded marble and less voluminous calc-silicate and pelitic rocks. The western half is hosted predominantly by semi-pelitic migmatite with intercalated marble. Remelting of the migmatite during pluton emplacement occurred in a thermal aureole as much as 1000 m wide. The early gabbroic unit forms the southeastern part of the body; it consists of hornblende-bearing to hornblende-rich gabbro and diorite which is thought to have crystallized from an H2O-rich andesitic parental magma. The younger dioritic unit underlies the central and western parts of the pluton, as well as a zone as much as 200 m wide that separates the rest of the pluton from its host rocks (herein the “annular zone”). The interior or central zone of the dioritic unit is pyroxene diorite that is locally interlayered with anorthosite. The western and annular zones are, by comparison, mineralogically heterogeneous. They range from diorite to quartz monzonite and from biotite-bearing two- and three-pyroxene assemblages to biotite–hornblende assemblages. Neither rock type nor mafic assemblage is correlated with position in the pluton or proximity to a contact. Stoped blocks of a distinctive coarse-grained diorite, as well as pyroxene-rich calc-silicates, are present in the gabbroic unit and the central zone of the dioritic unit. The few stoped blocks observed in the western zone of the dioritic unit are predominantly quartz-rich gneiss. Chemical variation in the central zone of the dioritic unit is interpreted to result from accumulation of pyroxenes+plagioclase from an H2O-poor andesitic parent. These rocks have approximately constant δ18O of +6.6±0.2‰ and lack evidence of in situ assimilation. Heterogeneities in the western and annular zones of the dioritic unit are reflected in variable, anomalously enriched incompatible element contents and in δ18O, which ranges from +6.7‰ to +8.6‰. Petrologic models indicate that the magma parental to the central zone could also be parental to the western and annular zones. If so, evolution of the western and annular zone magma was by crystal accumulation and assimilation of metapelitic host rocks. As much as 20% of the mass of the western and annular zones can be ascribed to assimilated material, which apparently entered the magma by stoping. Therefore, the asymmetrical zoning of the pluton is due to differences in host rock compositions and the relative ability of the magma to assimilate its host rocks.  相似文献   

9.
Volcanic rocks of the Latir volcanic field evolved in an open system by crystal fractionation, magma mixing, and crustal assimilation. Early high-SiO2 rhyolites (28.5 Ma) fractionated from intermediate compositionmagmas that did not reach the surface. Most precaldera lavas have intermediate-compositions, from olivine basaltic-andesite (53% SiO2) to quartz latite (67% SiO2). The precaldera intermediate-composition lavas have anomalously high Ni and MgO contents and reversely zoned hornblende and augite phenocrysts, indicating mixing between primitive basalts and fractionated magmas. Isotopic data indicate that all of the intermediate-composition rocks studied contain large crustal components, although xenocrysts are found only in one unit. Inception of alkaline magmatism (alkalic dacite to high-SiO2 peralkaline rhyolite) correlates with, initiation of regional extension approximately 26 Ma ago. The Questa caldera formed 26.5 Ma ago upon eruption of the >500 km3 high-SiO2 peralkaline Amalia Tuff. Phenocryst compositions preserved in the cogenetic peralkaline granite suggest that the Amalia Tuff magma initially formed from a trace element-enriched, high-alkali metaluminous magma; isotopic data suggest that the parental magmas contain a large crustal component. Degassing of water- and halogen-rich alkali basalts may have provided sufficient volatile transport of alkalis and other elements into the overlying silicic magma chamber to drive the Amalia Tuff magma to peralkaline compositions. Trace element variations within the Amalia Tuff itself may be explained solely by 75% crystal fractionation of the observed phenocrysts. Crystal settling, however, is inconsistent with mineralogical variations in the tuff, and crystallization is thought to have occurred at a level below that tapped by the eruption. Spatially associated Miocene (15-11 Ma) lavas did not assimilate large amounts of crust or mix with primitive basaltic magmas. Both mixing and crustal assimilation processes appear to require development of relatively large magma chambers in the crust that are sustained by large basalt fluxes from the mantle. The lack of extensive crustal contamination and mixing in the Miocene lavas may be related to a decreased basalt flux or initiation of blockfaulting that prevented pooling of basaltic magma in the crust.  相似文献   

10.
  相似文献   

11.
The Uitkomst complex in eastern Transvaal, South Africa, is a mineralized, layered ultrabasic to basic intrusion of Bushveld complex age (2.05–2.06 Ga) that intruded into the sedimentary rocks of the Lower Transvaal Supergroup. The complex is situated 20 km north of Badplaas. It is elongated in a northwesterly direction and is exposed over a total distance of 9 km. The intrusion is interpreted to have an anvil-shaped cross-section with a true thickness of approximately 800 m and is enveloped by metamorphosed and, in places, brecciated country rocks. Post-Bushveld diabase intrusions caused considerable vertical dilation of teh complex.The complex consists of six lithological units (from bottom to top): Basal Gabbro, Lower Harzburgite, Chromitiferous Harzburgite, Main Harzburgite, Pyroxenite and Gabbronorite. The Basal Gabbro Unit, developed at the base of the intrusion and showing a narrow chilled margin of 0.2 to 1.5 m against the floor rocks, has an average thickness of 6 m and grades upwards into the sulphide-rich and xenolith-bearing sequence of the Lower Harzburgite Unit. The latter unit averages 50 m in thickness and is gradationally overlain by the chromite-rich harzburgite of the Chromitiferous Harzburgite Unit (average thickness 60 m). Following on from the Chromitiferous Harzburgite Unit is the 330 m thick Main Harzburgite Unit. The Pyroxenite and Gabbronorite Units (total combined thickness of 310 m) form the uppermost formations of the intrusion. The three lower lithological units, Basal Gabbro to Chromitiferous Harzburgite, are highly altered by late magmatic, hydrothermal processes causing widespread serpentinization, steatitization, saussuritization and uralitization.Field relations, petrography and mineral and whole rock chemistry suggest the following sequence of events, The original emplacement of magma took place from northwest to southeast. The intrusion was bounded between two major fracture zones that gave rise to an elongated body, which acted as a conduit for later magma heaves. The first magma pulses, forming the chilled margin of the intrusion, show chemical affinities to a micropyroxenite described from the Bushveld complex. The Lower Harzburgite and Chromitiferous Harzburgite Units, judged from the abundance of xenoliths, originated by crystal settling from a contaminated basic magma. The Main Harzburgite crystallized from a magma of constant, probably also basic, composition, which flowed through the conduit after formation of the lower three lithological units. At a late stage of emplacement, after replenishment in the conduit came to a standstill, closed system conditions developed in the upper part of the complex, resulting in a magma fractionation trend of increasing incompatible elements contents towards the top of the intrusion.The mineralization in the lower three rock units and at the base was most probably caused by a segregating sulphide liquid forced to precipitate by the oxidative degassing of dolomite. Sulphur isotope ratios indicate various degrees of contamination of the magma by the enveloping sedimentary rocks, which provided the necessary amounts of S to reach S saturation.  相似文献   

12.
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   

13.
Chromite separates within the Peridotite Zone of the 2,700 Ma-old Stillwater Complex are characterized by low 187Re/188Os (0.009 to 1.74) and relatively high Os concentrations (8.74 to 78.2 ppb). Their calculated initial Os isotopic compositions likely reflect the compositions of the magmas from which they crystallized. The chromites show variable initial Os isotopic compositions (%Os of +2.0 to +16.4) over the vertical extent of the Peridotite Zone, implicating at least two sources of Os. Both the range of %Os and values of %Os decrease upsection. These variations in %Os were caused by mixing of variable proportions of two magmas having different Os isotopic compositions. One of the magmatic components was a more primitive magma with a nearly chondritic Os isotopic composition. The other magma had a radiogenic Os isotopic composition as a result of assimilation of crust, perhaps of sedimentary rocks beneath the Stillwater Complex. The gradual decrease in the initial %Os values of the chromite layers with increasing stratigraphic height implies a decreasing relative contribution from the contaminated magma throughout the growth of the Peridotite Zone. Small variations in %Os between different chromite occurrences within the H multicyclic unit reflects the petrologic requirement that chromite layers crystallize from slightly different proportions of the magmas, compared to chromite from olivine- and orthopyroxene-rich layers.  相似文献   

14.
成中梅  路凤香 《现代地质》1997,11(2):149-156
摘　 要　 阐述了火山岩晶体粒度分布的基本原理和研究方法‚分析了老山沟火山岩斜长石、 辉石、橄榄石、铁钛氧化物的晶体粒度分布。前3种矿物在中间粒度处发生了有意义的弯折‚ 原因是岩浆上升时过冷度增大。估算了岩浆房中斜长石、橄榄石的成核速率、结晶时间及上 升过程中处于结晶带的时间。研究表明‚在岩浆的演化过程中‚晶体的分离和累积作用及岩 浆的混合和对围岩的同化作用可以忽略。  相似文献   

15.
The Balmuccia peridotite massif in the central Ivrea Zone constitutes an upper mantle slice which has been tectonically emplaced into the crust. It represents the residue from partial melting of undepleted mantle material and varies in composition from lherzolite to harzburgite and subordinate dunite. Dikes of websterite and gabbroic pods within the peridotite can be subdivided into an older Crdiopside suite and a younger Al-augite suite. Nd isotopic data on whole rocks of these lithotypes in combination with independent observations suggest that the dikes formed during a Hercynian event about 270 Ma ago. The rocks of the Cr-diopside dikes, in particular, display isotopic signatures similar to those of the lherzolite and represent fractionates from partial melts derived from the lherzolite wall rock. The Sm-Nd data of the pyroxenites and gabbros of the Al-augite suite, in contrast, scatter widely and suggest that partial melting of lherzolite was triggered or at least accompanied by introduction of fluids and/or liquid phases. These fluids or liquids carried exotic isotopic components from elsewhere in the crust-mantle complex, and deposited them within the rocks by metasomatic reactions. Two distinct types of metasomatism must have operated not only within the Balmuccia body, but also in the complex of Finero: The first type of metasomatism introduced mantle-derived volatiles and is responsible for formation of amphibole. The other type has a crustal source and led to formation of phlogopite, which occurs mainly within mantle rocks of Finero, but occasionally, within the Balmuccia body also.  相似文献   

16.
地质过程中,氯可以影响很多元素的性质。本文利用前人的实验数据,模拟计算不同压力和初始含水量等条件下,氯在岩浆和共存富水流体相中浓度随岩浆结晶分异的变化。结果显示,在岩浆演化之初,氯在各种压力下都基本留在岩浆中,表现出高度不相容元素的特点。对于结晶分异晚期的岩浆体系,氯的行为受压力的影响较大。在低压下,氯在岩浆/富水流体之间的分配系数较高,水是否达到饱和对氯在岩浆中的含量变化影响不大。随着压力的升高,该系数迅速降低。在中等压力下,岩浆中水含量一旦达到饱和,大量的氯进入流体相,形成高盐度的流体,氯在岩浆中的含量即迅速降低。随着岩浆分异的进行,共存的流体中的氯含量也随之降低。在更高的压力下,水无法达到饱和,氯始终表现为高度不相容的特点。氯的上述性质可以很好地解释它在很多结晶分异程度较低的火山岩玻璃(如MORB和OIB)中表现为高度不相容的特点,也可以帮助理解氯在侵入岩和喷出岩中性质的差别,以及“花岗岩大岩体不成矿,小岩体成大矿”的现象。  相似文献   

17.
P. Peltonen 《Lithos》1995,34(4):253-274
Ultramafic rocks occur within polydeformed high grade metasedimentary rocks throughout the Vammala Nickel Belt (Fennoscandian Shield) and have been subdivided into cumulate-textured intrusive bodies and high-Mg (picritic) eruptive units.

The crystallisation sequence olivine(+cotectic chromite)-orthopyroxene-clinopyroxene-amphibole, the decreased stability field of olivine and fractionation of relatively Al-rich pyroxenes and chromite all suggest that the cumulate-textured intrusions crystallised from a relatively hydrous parental magma at moderate crustal pressures. The parental magma was enriched in Th, LILE and LREE, being similar to modern tholeiitic arc basalts. Trace element ratios suggest that Svecofennian turbidites were the ultimate source of this enriched component. Although the mechanism of contamination is inconsistent with simultaneous assimilation and fractional crystallisation in situ, this can be explained by a model in which magma flowed through subvertical conduits and assimilated adjacent crust.

Characteristics of metamorphism, deformation and subsolidus equilibration and age determinations all suggest that emplacement of cumulate-textured bodies coincided with the peak of ongoing regional metamorphism and deformation. The cumulate-textured intrusions are interpreted as representing middle crustal expressions of early Proterozoic Svecofennian arc magmatism. This model also predicts that the same magmatism resulted in the emplacement of more evolved arc plutonic complexes at higher crustal levels (that have since been eroded), and the formation of large ultramafic cumulate complexes in the lower crust and at the crust-mantle boundary region, and may thus assist in interpreting deep seismic and gravimetric anomalies within Svecofennian domain.

In contrast, the metapicritic supracrustal rocks have trace element signatures similar to transitional MORB, and cannot represent magmas complementary to the cumulate-textured intrusions. The metapicrites represent supracrustal formations which are clearly older than the synorogenic cumulate-textured intrusions, and their spatial association is coincidental only.  相似文献   


18.
The results of experimental studies and examination of variations in major elements, trace elements and Sr isotopes indicate that fractionation, assimilation and magma mixing combined to produce the lavas at Medicine Lake Highland. Some characteristics of the compositional differences among the members of the calc-alkalic association (basalt-andesite-dacite-rhyolite) can be produced by fractional crystallization, and a fractionation model reproduces the major element trends. Other variations are inconsistent with a fractionation origin. Elevated incompatible element abundances (K and Rb) observed in lavas intermediate between basalt and rhyolite can be produced through assimilation of a crustal component. An accompanying increase in 87Sr/86Sr from ∼ 0.07030 in basalt to ∼0.7040 in rhyolite is also consistent with crustal assimilation. The compatible trace element contents (Ni and Sr) of intermediate lavas can not be produced by fractional crystallization, and suggest a magma-mixing origin for some lavas. Unusual phenocryst assemblages and textural criteria in these lavas provide additional evidence for magma mixing. A phase diagram constructed from the low pressure melting experiments identifies a distributary reaction point, where olivine+augite react to pigeonite. Parental basalts reach this point at low pressures and undergo iron-enrichment at constant SiO2 content. The resulting liquid line of descent is characteristic of the tholeiitic trend. Calc-alkalic differentiation trends circumvent the distributary reaction point by three processes: fractionation at elevated pH2O, assimilation and magma mixing.  相似文献   

19.
M.E.P. Gomes  A.M.R. Neiva 《Lithos》2005,81(1-4):235-254
Deformed Hercynian peraluminous granitoids ranging from tonalite to granite crop out in the Rebordelo–Agrochão area, northern Portugal and some of them contain tonalitic and granodioritic enclaves. Variation diagrams of major and trace elements of the rocks, biotites and sphenes show fractionation trends. The most- and the least-deformed samples of granite and their biotites also define fractionation trends. There is decrease in all rare earth element (REE) contents and increase in the Eu anomaly in REE patterns from the most- to the least-deformed samples of granite. All the granitoids define a whole-rock Rb–Sr errorchron. A whole-rock Rb–Sr isochron for the least-deformed samples of granite yields an age of 357±9 Ma and an initial 87Sr/86Sr ratio of 0.7087±0.0007. Geochemical modelling suggests that the tonalitic magma evolved by AFC (fractional crystallization of magnesiohornblende, plagioclase, quartz, biotite and ilmenite, and assimilation of metasediments) to originate tonalitic and granodioritic enclaves, granodiorite and granite. δ18O values support this mechanism. The tonalite is hybrid and derived by interaction of a mantle-derived magma and crustal materials.  相似文献   

20.
HOOPER  P. R. 《Journal of Petrology》1988,29(5):1097-1118
Variable isotope and incompatible element ratios require multiplesources for the Columbia River Basalt Group (CRBG) as a whole,but smaller, stratigraphically coherent, groups of flows withinthe CRBG show much smaller variations in these ratios and appearto be derived from a single source. One such group of flows,the American Bar (AB) subgroup of the Imnaha Basalt, is examinedin detail. Sixty-nine samples from the six major AB flows havebeen reanalyzed for 23 major and trace elements in order tofurther constrain the processes responsible for the variationsbetween their compositions. It is shown that the compositional differences between the ABflows include the compatible behavior of Sr, which requiresthe participation of plagioclase, and can be accommodated ina model which fractionates a crystal assemblage of plagioclase,clinopyroxene, and orthopyroxene (16:10:4). Apparent differencesin the degree of fractionation between major elements ({smalltilde}30%) and trace elements ({small tilde}50%) are mainlyexplicable in terms of recharge accompanying the crystal fractionation(RFC), but some ambiguity in the enrichment factors for Y, P,and Zr (but not for Nb) remains unexplained and permits thepossibility of some crustal assimilation. The fractionation assemblage (plagioclase+augite?orthopyroxene)differs from the observed phenocryst suite (plagioclase+olivine+augite)and, in the absence of critical experimental data, is assumedto represent a higher pressure assemblage stable near the crust/mantleboundary. The physical model which has been developed to explainthe AB eruption entails a large magma reservoir in the lowercrust immediately above the crust/mantle boundary in which fractionationand recharge accompanied periodic eruption, probably withoutsignificant crustal assimilation either within the magma reservoiror during the rapid expulsion of magma to the surface. Chemical,isotopic, and mineralogic differences between coherent flowsubgroups (Rock Creek, American Bar, and the lower flows ofGrande Ronde Basalt, for example) are abrupt. They include changesin isotope and incompatible element ratios as well as changesin SiO2 and K concentrations which require magma derivationfrom a different combination of sources.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

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