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1.
陈博  朱永峰 《岩石学报》2010,26(8):2287-2298
克拉玛依蛇绿混杂岩带百口泉剖面由尖晶石蛇纹岩、辉长岩、玄武岩和硅质岩组成。岩相学研究表明,百口泉辉长岩分为堆晶岩(具堆晶结构)和辉长岩(具辉长结构)两类,且均经历了低-中级变质改造。微量元素地球化学显示其岩浆起源于亏损地幔,强烈的Sr异常和Eu异常表明强烈的斜长石堆晶过程。根据其稀土配分模式推测其源区为尖晶石相的地幔橄榄岩。结合野外地质关系和地球化学特征,白碱滩尖晶石二辉橄榄岩能够代表其源区成分,利用微量元素模拟其岩浆演化过程显示:尖晶石二辉橄榄岩发生2.5%部分熔融所形成的熔体,通过10%～20%分离结晶可以形成堆晶辉长岩,经过80%～90%分离结晶则可以形成具辉长结构的辉长岩。因此,蛇绿混杂岩中零星分布的堆晶岩和辉长岩团块是同源岩浆演化的产物。对分离结晶过程中Nb元素地球化学行为的研究表明,岩浆的结晶分异能够导致辉长岩明显亏损Nb。  相似文献   

2.
Xenolithic inclusions in calc-alkaline andesite from Mt. Moffettvolcano, Adak Island, Aleutian arc, reveal a nearly continuousrecord of crystallization of basaltic magmas in the crust, andpossibly upper mantle, of the arc. The record is more detailedand continuous than that obtained from study of calc-alkalinevolcanic rocks in the arc. Cumulate xenoliths form a progressiveseries in modal mineralogy from ultramafic, hornblende-bearingolivine clinopyroxenite to both hornblende-bearing and hornblende-freegabbros. The cumulate hornblende gabbro xenoliths are typicalof those found in island arc andesites worldwide. Xenolithicinclusions without cumulate textures, here termed compositexenoliths, are characterized by forsteritic olivine, zoned Cr-diopsideand hornblende, and are interpreted to have resulted from reactionand chilling upon magma mixing at depth. The olivine and clinopyroxene in both cumulate and compositexenoliths show the largest and the most complete variation trendsfor Ni, Cr, and FeO/MgO ratio yet reported in igneous xenolithsfrom island arc volcanic rocks. Variation of Ni in olivine indicatesthat the parent magmas for the xenoliths had minimum MgO contentsof 9 wt. per cent. Variation of Cr in clinopyroxene indicatesthat the magmas were basaltic rather than picritic, probablyin equilibrium with spinel lherzolite at near Moho depths. Successiveinjections of batches of primary melt into a magma chamber fractionatingolivine and clinopyroxene can reproduce observed compatibleelement depletion trends. A steady-state process of cotecticcrystallization in a magma chamber continually replenished withbasaltic magma is a possible mechanism for producing large accumulationsof olivine and clinopyroxene, suggesting that Alaskan-type ultramaficcomplexes are related to hydrous basaltic magmas in island arcs.This steady-state open-system crystallization process can alsoyield the abundant high-alumina basalt type in the Aleutianarc. Continued crystallization of high-alumina basalt in lowercrustal magma chambers, recorded in a mineralogically coherentseries of pyroxenite to hornblende gabbro xenoliths, can yieldbasaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found atMt Moffett, evidence that the arc crust is igneous in origin,with the lower crust formed of gabbro crystallized from mantle-derivedmelts. Ultramafic cumulates may reside in both the lower crustor upper mantle beneath the arc. A model is proposed wherebythe cumulate crystallization products of hydrous, mantle beneaththe arc. A model is define the upper mantle and lower crustof the arc over time.The net composition added to the crustof the arc is that of high-alumina basalt.  相似文献   

3.
Analytical expressions for the variation in D La and D Yb with increasing liquid SiO2 for olivine, plagioclase, augite, hornblende, orthopyroxene, magnetite and ilmenite (Brophy in Contrib Mineral Petrol 2008, online first) have been combined with numerical models of hydrous partial melting, of mid-ocean ridge (MOR) cumulate gabbro melting, and fractional crystallization of slightly hydrous mid-ocean ridge basalt (MORB) magma to assess a melting versus fractionation origin for oceanic plagiogranite. For felsic magmas (>63 wt.% SiO2) the modeling predicts the following. MOR cumulate gabbro melting should yield constant or decreasing La and constant Yb abundances with increasing liquid SiO2. The overall abundances should be similar to those in associated mafic magmas. MORB fractional crystallization should yield steadily increasing La and Yb abundances with increasing SiO2 with overall abundances significantly higher than those in associated mafic magmas. Application to natural occurrences of oceanic plagiogranite indicate that both MOR cumulate gabbro melting and MORB fractionation are responsible. Application of the model results to Icelandic rhyolites strongly support a fractional crystallization rather than a crustal melting origin.  相似文献   

4.
Quaternary basalts, andesites and dacites from the Abu monogenetic volcano group, SW Japan, (composed of more than 40 monogenetic volcanoes) show two distinct chemical trends especially on the FeO*/MgO vs SiO2 diagram. One trend is characterized by FeO*/MgO-enrichment with a slight increase in SiO2 content (Fe-type trend), whereas the other shows a marked SiO2-enrichment with relatively constant FeO*/MgO ratios (Si-type trend). The Fe-type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma. Rocks of the Si-type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and/or fine-grained basaltic inclusions. They are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz, sodic plagioclase and hornblende phenocrysts. Petrographic variation as well as chemical variation from basalt to dacite of the Si-type trend is accounted for by various mixing ratios of basalt and dacite magmas. Pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing. Olivine and spinel, and quartz, sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas, respectively, before the mixing. Within a lava flow, the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow, and the number of resorbed phenocrysts varies inversely, suggesting zonation in the magma chamber.The mode of mixing changes depending on the mixing ratio. In the mafic mixture, basalt and dacite magmas can mix in the liquid state (liquid-liquid mixing). In the silicic mixture, on the other hand, the basalt magma was quenched and formed inclusions (liquid-solid mixing). During mixing, the disaggregated basalt magma and the host dacite magma soon reached thermal equilibrium. Compositional homogenization of the mixed magma can occur only when the equilibrium temperature is sufficiently above the solidus of the basalt magma. The Si-type trend is chemically and petrographically similar to the calc-alkalic trend. Therefore, a calc-alkalic trend which is distinguished from a fractional crystallization trend (e.g. Fe-type trend) may be a product of magma mixing.  相似文献   

5.
We report the occurrence of orthopyroxene gabbro from the Phenai Mata Igneous Complex (containing thoeliitic and alkaline rocks) that occur within Deccan Traps. The P-T calculations based on two pyroxene thermometry vary from 8.5±1.0 kbar and 963±39 °C. These gabbroic rocks exhibit high Mg# (0.67 to 0.71). But their primary magma signature can be negated due to their high SiO2 (> 50 wt %), low Ni (32–35 ppm) and Cr (105–182 ppm) contents. Further, simple fractional crystallization was not responsible for the modification of the magma. Modeling carried out using trace element concentrations revealed that concurrent assimilation and fractional crystallization (AFC) was responsible for the genesis of these rocks. Small pods of magma could have accumulated in the crustal portions and concurrent assimilation and fractional crystallization have taken place in the generation of gabbro and orthopyroxene gabbro in the present study area.  相似文献   

6.
At Medicine Lake volcano, California, andesite of the Holocene Burnt Lava flow has been produced by fractional crystallization of parental high alumina basalt (HAB) accompanied by assimilation of granitic crustal material. Burnt Lava contains inclusions of quenched HAB liquid, a potential parent magma of the andesite, highly melted granitic crustal xenoliths, and xenocryst assemblages which provide a record of the fractional crystallization and crustal assimilation process. Samples of granitic crustal material occur as xenoliths in other Holocene and Pleistocene lavas, and these xenoliths are used to constrain geochemical models of the assimilation process.A large amount of assimilation accompanied fractional crystallization to produce the contaminated Burnt lava andesites. Models which assume that assimilation and fractionation occurred simultaneously estimate the ratio of assimilation to fractional crystallization (R) to be >1 and best fits to all geochemical data are at an R value of 1.35 at F=0.68. Petrologic evidence, however, indicates that the assimilation process did not involve continuous addition of granitic crust as fractionation occurred. Instead, heat and mass transfer were separated in space and time. During the assimilation process, HAB magma underwent large amounts of fractional crystallization which was not accompanied by significant amounts of assimilation. This fractionation process supplied heat to melt granitic crust. The models proposed to explain the contamination process involve fractionation, replenishment by parental HAB, and mixing of evolved and parental magmas with melted granitic crust.  相似文献   

7.
The Big Jim complex is a concentrically zoned ultramafic to felsic plutonic complex which intruded the pelitic Chiwaukum schist. Most of the major plutonic rock types (from websterite through hornblendite, gabbronorite, hornblende gabbro and diorite, to granodiorite) enclose harzburgite and metaperidotite xenoliths similar to foliated metaperidotite lenses included in the Chiwaukum schist. The larger xenoliths preserve tectonite fabrics. All have Mg#'s (mole fraction MgO/(MgO+FeO*)) from 0.90 to 0.89, the same as those of Chiwaukum metaperidotites, and distinctly different from undeformed Big Jim dunite (Mg#'s 0.84 to 0.82) and websterite (0.82 to 0.78). Contact relations indicate widespread, stepwise replacement of harzburgite by pyroxenite, hornblendite, gabbro and diorite. Thermodynamic modelling using an expanded regular solution model for silicate liquids (Ghiorso 1985; Ghiorso and Carmichael 1985) predicts that reaction between olivine (Fo90) and a liquid with the composition of Big Jim diorite +1.5 wt% H2O, at 1,100° C and 3 kb, would produce websterite (Mg#'s 0.75 to 0.81) and dunite (0.79 to 0.82). This process is exothermic and results in a negative change in volume, since it increases total solid mass. Under conditions of decreasing temperature, modelled crystal fractionation with assimilation of olivine reproduces important features of the chemical variation observed in the Big Jim complex where crystal fractionation alone fails. The Big Jim complex has affinities with other ultramafic to felsic plutonic complexes such as the Bear Mountain complex (Snoke et al. 1981, 1982) and the Emigrant Gap complex (James 1971). The latter have wehrlite and clinopyroxenite, rather than websterite, but both have concentric zoning, with olivine-bearing rock types surrounded by successively more felsic pyroxenite, gabbro and diorite. In general, concentrically zoned complexes of this type may form where magma reacts with mantle-derived wall rock or ultramafic cumulates. Assimilation of peridotite in fractionating magma may be important in subduction-related magmatic arcs.  相似文献   

8.
Postcollapse lavas of the Infiernito caldera grade stratigraphically upward from nearly aphyric, high-silica rhyolite (76% SiO2) to highly prophyritic trachyte (62% SiO2). Plagioclase, clinopyroxene, orthopyroxene, magnetite, ilmenite, and apatite occur as phenocrysts throughout the sequence. Sanidine, biotite, and zircon are present in rocks with more than about 67% SiO2. Major and trace elements show continuous variations from 62 to 76% SiO2. Modeling supports fractional crystallization of the observed phenocrysts as the dominant process in generating the chemical variation.Temperatures calculated from coexisting feldspars, pyroxenes, and Fe-Ti oxides agree and indicate crystallization from slightly more than 1100° C in the most mafic trachyte to 800° C in high-silica rhyolite. The compositional zonation probably arose through crystallization against the chilled margin of the magma chamber and consequent rise of more evolved and therefore less dense liquid.Mineral compositions vary regularly with rock composition, but also suggest minor mixing and assimilation of wall rock or fluids derived from wall rock. Mixing between liquids of slightly different compositions is indicated by different compositions of individual pyroxene phenocrysts in single samples. Liquid-solid mixing is indicated by mineral compositions of glomerocrysts and some phenocrysts that apparently crystallized in generally more evolved liquids at lower temperature and higher oxygen fugacity than represented by the rocks in which they now reside. Glomerocrysts probably crystallized against the chilled margin of the magma chamber and were torn from the wall as the liquid rose during progressive stages of eruption. Assimilation is indicated by rise of oxygen fugacity relative to a buffer from more mafic to more silicic rocks.Calculation of density and viscosity from the compositional and mineralogical data indicates that the magma chamber was stably stratified; lower temperature but more evolved, thus less dense, rhyolite overlay higher temperature, less evolved, and therefore more dense, progressively more mafic liquids. The continuity in rock and mineral compositions and calculated temperature, viscosity, and density indicate that compositional gradation in the magma chamber was smoothly continuous; any compositional gaps must have been no greater than about 2% SiO2.  相似文献   

9.
Two picrite flows from the SW rift zone of Mauna Loa containxenoliths of dunite, harzburgite, lherzolite, plagioclase-bearinglherzolite and harzburgite, troctolite, gabbro, olivine gabbro,and gabbronorite. Textures and olivine compositions precludea mantle source for the xenoliths, and rare earth element concentrationsof xenoliths and clinopyroxene indicate that the xenolith sourceis not old oceanic crust, but rather a Hawaiian, tholeiitic-stagemagma. Pyroxene compositions, phase assemblages and texturalrelationships in xenoliths indicate at least two different crystallizationsequences. Calculations using the pMELTS algorithm show thatthe two sequences result from crystallization of primitive MaunaLoa magmas at 6 kbar and 2 kbar. Independent calculations ofolivine Ni–Fo compositional variability in the plagioclase-bearingxenoliths over these crystallization sequences are consistentwith observed olivine compositional variability. Two parentsof similar bulk composition, but which vary in Ni content, arenecessary to explain the olivine compositional variability inthe dunite and plagioclase-free peridotitic xenoliths. Xenolithsprobably crystallized in a small magma storage area beneaththe rift zone, rather than the large sub-caldera magma reservoir.Primitive, picritic magmas are introduced to isolated rift zonestorage areas during periods of high magma flux. Subsequenteruptions reoccupy these areas, and entrain and transport xenolithsto the surface. KEY WORDS: xenolith; Hawaii; volcano plumbing; mineral composition; picrite  相似文献   

10.
The mechanisms and the timescales of magmatic evolution were investigated for historical lavas from the Askja central volcano in the Dyngjufjöll volcanic massif, Iceland, using major and trace element and Sr, Nd, and Pb isotopic data, as well as 238U-230Th-226Ra systematics. Lavas from the volcano show marked compositional variation from magnesian basalt through ferrobasalt to rhyolite. In the magnesian basalt-ferrobasalt suite (5-10 wt% MgO), consisting of lavas older than 1875 A.D., 87Sr/86Sr increases systematically with increasing SiO2 content; this suite is suggested to have evolved in a magma chamber located at ∼600 MPa through assimilation and fractional crystallization. On the other hand, in the ferrobasalt-rhyolite suite (1-5 wt% MgO), including 1875 A.D. basalt and rhyolite and 20th century lavas, 87Sr/86Sr tends to decrease slightly with increasing SiO2 content. It is suggested that a relatively large magma chamber occupied by ferrobasalt magma was present at ∼100 MPa beneath the Öskjuvatn caldera, and that icelandite and rhyolite magmas were produced by extraction of the less and more evolved interstitial melt, respectively, from the mushy boundary layer along the margin of the ferrobasalt magma chamber, followed by accumulation of the melt to form separate magma bodies. Ferrobasalt and icelandite lavas in the ferrobasalt-rhyolite suite have a significant radioactive disequilibrium in terms of (226Ra/230Th), and its systematic decrease with magmatic evolution is considered to reflect aging, along with assimilation and fractional crystallization processes. Using a mass-balance model in which simultaneous fractional crystallization, crustal assimilation, and radioactive decay are taken into account, the timescale for the generation of icelandite magma from ferrobasalt was constrained to be <∼3 kyr which is largely dependent on Ra crystal-melt partition coefficients we used.  相似文献   

11.
The rhyolite of Little Glass Mountain (73–74% SiO2) is a single eruptive unit that contains inclusions of quenched andesite liquid (54–61% SiO2) and partially crystalline cumulate hornblende gabbro (53–55% SiO2). Based on previous studies, the quenched andesite inclusions and host rhyolite lava are related to one another through fractional crystallization and represent an example of a fractionation-generated composition gap. The hornblende gabbros represent the cumulate residue associated with the rhyolite-producing and composition gap-forming fractionation event. This study combines textural (Nomarski Differential Interference Contrast, NDIC, imaging), major element (An content) and trace element (Mg, Fe, Sr, K, Ti, Ba) data on the style of zonation of plagioclase crystals from representative andesite and gabbro inclusions, to assess the physical environment in which the fractionation event and composition gap formation took place. The andesite inclusions (54–61% SiO2) are sparsely phyric with phenocrysts of plagioclase, augite and Fe-oxide±olivine, +/–orthopyroxene, +/–hornblende set within a glassy to crystalline matrix. The gabbro cumulates (53–55% SiO2) consist of an interconnected framework of plagioclase, augite, olivine, orthopyroxene, hornblende and Fe-oxide along with highly vesicular interstitial glass (70–74% SiO2). The gabbros record a two-stage crystallization history of plagioclase+olivine+augite (Stage I) followed by plagioclase+orthopyroxene+ hornblende+Fe-oxide (Stage II). Texturally, the plagioclase crystals in the andesite inclusions are characterized by complex, fine-scale oscillatory zonation and abundant dissolution surfaces. Compositionally (An content) the crystals are essentially unzoned from core-to-rim. These features indicate growth within a dynamic (convecting?), reservoir of andesite magma. In contrast, the plagioclase crystals in the gabbros are texturally smooth and featureless with strong normal zonation from An74 at the core to around An30. K, and Ba abundances increase and Mg abundances decrease steadily towards the rim. Ti, Fe, and Sr abundances increase and then decrease towards the rim. The trace element variations are fully consistent with the two-stage crystallization sequence inferred from the gabbro mineralogy. These results indicate progressive closed-system in situ crystallization in a quiescent magmatic boundary layer environment located along the margins of the andesite magma body. The fractional crystallization that generated the host rhyolite lava is one of inward solidification of a crystallizing boundary layer followed by melt extraction and accumulation of highly evolved interstitial liquid. This mechanism explains the formation of the composition gap between parental andesite and rhyolite magma compositions.  相似文献   

12.
Glass Mountain consists of a 1 km3, compositionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust. Mixing of magmas produced by fractional crystallization of andesite and crustal melting generated the rhyolite of Glass Mountain. Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H2O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5 km3) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled from the cumulate residues are preserved as magmatic inclusions and indicate that this fractionation process occurred at two distinct depths. The presence and composition of amphibole in magmatic inclusions preserve evidence for crystallization of the andesite at pressures of at least 200 MPa (6 km depth) under near H2O-saturated conditions. Mineralogical evidence preserved in olivine-plagioclase and olivine-plagioclase-high-Ca clinopyroxene-bearing magmatic inclusions indicates that crystallization under near H2O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence indicate that the andesite underwent fractional crystallization to form the differentiated melts but had no chemical interaction with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the crust. Received: 26 March 1996 / Accepted: 14 November 1996  相似文献   

13.
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   

14.
To constrain the amount and rate of crustal contamination that is possible in basaltic and jotunitic magma, and to gain an insight into the physical and thermal processes of assimilation in crustal magma chambers, we have modelled published Sr and Nd isotopic data from three layered intrusions. Well-exposed sequences of cumulates with no evidence of magma recharge provide direct records of concurrent assimilation and fractional crystallization (AFC). The key to the modelling is that F, the mass fraction of magma remaining in the chamber, can be estimated from the thicknesses of the studied cumulate sequences. This allows AFC model curves to be fitted to the isotopic data by varying r, the ratio of the rate of mass assimilated to the rate of mass crystallized. The results of modelling show that r is nearly constant in 800 to 2000 m thick sequences of cumulates displaying up-section decreases in anorthite content of plagioclase, increases in whole-rock Sr0 (initial 87Sr/86Sr) and decreases in whole-rock εNd0 (initial εNd). The r-values of the layered sequences range from 0.12 in the Fongen–Hyllingen Intrusion, over 0.20 in the Bjerkreim–Sokndal Intrusion, to 0.27 in the Hasvik Intrusion. The total amount of assimilation, the bulk crust/magma ratio, reaches values of 0.08, 0.19 and 0.28 at the level of the most contaminated samples after 60% to 80% crystallisation, whereas the instantaneous crust/magma ratio of the most contaminated magmas were respectively 0.14, 0.46, and 0.70, for the three intrusions.Innumerable country rock xenoliths occur in the three layered intrusions and played a crucial role in the assimilation process. The xenoliths spalled off the roofs of the magma chambers during magma emplacement and their initial temperature and composition relate to r in the intrusions. In the Hasvik Intrusion (r = 0.27), the initial temperature of the country rocks was 450 °C and the xenoliths were fusible metasediments and therefore produced a high fraction of partial melt that could be assimilated. In the Bjerkreim–Sokndal Intrusion (r = 0.20), the country rocks were initially at temperatures of 640–880 °C but included both refractory massif-type anorthosite and fusible gneisses. In the Fongen–Hyllingen Intrusion (r = 0.12), the country rocks were cooler (300 °C) and the xenoliths include refractory metabasalt (dominant) and fusible metapelite. We argue that the refractory metabasalt and anorthosite xenoliths acted mainly as heat sinks, resulting in reduced r-values in Fongen–Hyllingen and Bjerkreim–Sokndal Intrusions.Heating of refractory and fusible xenoliths, and melting of fusible xenoliths absorbed sensible and latent heat of the magma. Energy-balanced modelling shows that up to 75% of the heat available was absorbed by xenoliths within the magma chambers, promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the envelope of country rocks alone. The high r-values reflect the amount of heat absorbed by heating and melting country rock within the magma chambers themselves, and their constancy reflects the ready availability of fusible xenoliths.  相似文献   

15.
This paper presents new petrographic observations and geochemical and microprobe analyses for the Laomiaojishan, Xiaotongguanshan, and Tianebaodanshan intrusions in the Tongguanshan mineral district, East China. The plutons vary in composition from quartz monzonitic diorite to pyroxene monzonitic diorite, and contain gabbroic to dioritic xenoliths. The Xiaotongguanshan intrusion yields a SHRIMP zircon U–Pb age of 139.5±2.9 Ma, indicating Late Jurassic to Early Cretaceous magmatism in the Lower Yangtze River Valley. Relative to host rocks, the gabbro and diorite xenoliths are low in SiO2 (52.03–54.61 wt‐%), Al2O3 (12.87–14.43 wt‐%), and total alkalis (Na2O+K2O; 5.26–6.30 wt‐%), but high in MgO (5.41–11.66 wt‐%); the host rocks have high SiO2 (59.97–64.44 wt‐%), Al2O3 (16.43–17.59 wt‐%), and total alkalis (6.67–8.25 wt‐%), but are low in MgO (1.52–2.50 wt‐%). Concentrations of rare earth elements (REEs) in the xenoliths (165.70–190.40 ppm) are similar to those in the host rocks (166.12–185.95 ppm), although the ratio of light REEs to heavy REEs in the xenoliths (3.39–4.27) is lower than that in the host plutons (4.86–5.94). All of the analysed rocks show similar REE patterns, although the xenoliths display marked positive Eu anomalies and the host rocks show slightly negative Eu anomalies. Values of epsilon Nd (t) ranges from ?4.9 to ?9.9 in the gabbro xenoliths and from ?11.4 to ?11.9 in the host intrusives. Initial 87Sr/86Sr ratios are 0.7064–0.7073 in the xenoliths and 0.7072–0.7084 in the quartz monzonitic diorite host rocks. Crystallization temperatures of hornblende and plagioclase in the gabbro xenoliths, diorite xenoliths, and host rocks are 816, 773–790, and 664–725°C, respectively, based on an amphibole–plagioclase geothermometer. The pressures recorded by these phases indicate that they formed at depths of 26, 12–15, and 3–4 km, respectively, based on an aluminum‐in‐hornblende geobarometer. The petrological and geochemical features of the analysed intrusions and xenoliths are consistent with their derivation from basic to intermediate‐acidic magmas that possibly formed via a series of complex interactions between underplated, mantle‐derived basaltic magma and varying amounts of middle‐ to lower‐crustal material, followed by assimilation–fractional crystallization.  相似文献   

16.
Chemical interaction between tholeiitic magmas of the East Greenland Tertiary macrodike complex and anatectic melts of the Precambrian basement produced a wide range of hybrid magmas. Field evidence indicates that, although coexisting magmas were stirred, mechanical mixing only occurred to a limited extent before segregation of magmas into a stratified system. The initial 87Sr/86Sr and 143Nd/144Nd isotope ratios for hybrid compositions fall between those of the mafic and felsic end-members. However, the covariation of these isotope ratios differs from that expected of bulk mixing. Major- and trace-element distributions in hybrid magmas are also inconsistent with simple mixing, as well as with fractional crystallization coupled with bulk assimilation (AFC) involving reasonable end-members of the macrodike-crust system. Rather, the chemical and isotopic modification of mafic and felsic magmas of the macrodike complex appears to have been controlled fundamentally by interdiffusion of silicate liquid species during mingling and buoyant roofward segregation of crust-derived granophyres. The relationships among juxtaposed hybrid magmas of the Miki Fjord macrodike are shown to be consistent with expectations of selective diffusional exchange based on available experimental interdiffusion data for silicate liquids. Comparison between these hybrid compositions and rocks from the felsic series of the Vandfaldsdalen macrodike suggest that the latter compositions were affected by a similar opensystem process operating presumably during the transient development of the felsic cap. Once hybrid magmas ponded at the roof of the intrusion they effectively were isolated from further exchange.  相似文献   

17.
The Cordillera del Paine pluton in the southernmost Andes of Chile represents a deeply dissected magma chamber where mafic magma intruded into crystallizing granitic magma. Throughout much of the 10x15 km pluton, there is a sharp and continuous boundary at a remarkably constant elevation of 1,100 m that separates granitic rocks (Cordillera del Paine or CP granite: 69–77% SiO2) which make up the upper levels of the pluton from mafic and comingled rocks (Paine Mafic Complex or PMC: 45–60% SiO2) which dominate the lower exposures of the pluton. Chilled, crenulate, disrupted contacts of mafic rock against granite demonstrate that partly crystallized granite was intruded by mafic magma which solidified prior to complete crystallization of the granitic magma. The boundary at 1,100 m was a large and stable density contrast between the denser, hotter mafic magma and cooler granitic magma. The granitic magma was more solidified near the margins of the chamber when mafic intrusion occurred, and the PMC is less disrupted by granites there. Near the pluton margins, the PMC grades upward irregularly from cumulate gabbros to monzodiorites. Mafic magma differentiated largely by fractional crystallization as indicated by the presence of cumulate rocks and by the low levels of compatible elements in most PMC rocks. The compositional gap between the PMC and CP granite indicates that mixing (blending) of granitic magma into the mafic magma was less important, although it is apparent from mineral assemblages in mafic rocks. Granitic magma may have incorporated small amounts of mafic liquid that had evolved to >60% SiO2 by crystallization. Mixing was inhibited by the extent of crystallization of the granite, and by the thermal contrast and the stable density contrast between the magmas. PMC gabbros display disequilibrium mineral assemblages including early formed zoned olivine (with orthopyroxene coronas), clinopyroxene, calcic plagioclase and paragasite and later-formed amphibole, sodic plagioclase, mica and quartz. The early formed gabbroic minerals (and their coronas) are very similar to phenocrysts in late basaltic dikes that cut the upper levels of the CP granite. The inferred parental magmas of both dikes and gabbros were very similar to subalkaline basalts of the Patagonian Plateau that erupted at about the same time, 35 km to the east. Mafic and silicic magmas at Cordillera del Paine are consanguineous, as demonstrated by alkalinity and trace-element ratios. However, the contemporaneity of mafic and silicic magmas precludes a parent-daughter relationship. The granitic magma most likely was derived by differentiation of mafic magmas that were similar to those that later intruded it. Or, the granitic magma may have been contaminated by mafic magmas similar to the PMC magmas before its shallow emplacement. Mixing would be favored at deeper levels when the cooling rate was lower and the granitic magma was less solidified.  相似文献   

18.
许王  董永胜  张修政  邓明荣  张乐 《地质通报》2014,33(11):1673-1681
香桃湖变质堆晶辉长岩是由原"猫耳山岩组"解体出来的早古生代基性岩,其对于研究区域上龙木错—双湖—澜沧江缝合带的构造演化具有重要意义。岩相学研究表明,该堆晶辉长岩经历了高级角闪岩相变质,并遭受了绿片岩相退变质作用的改造;地球化学研究显示,其Si O2含量为44.7%~46.2%,Mg O为7.05%~8.48%,Mg#值为51.02~53.90,相容元素Cr含量为168×10-6~288×10-6、Ni为60×10-6~103×10-6,远低于原生玄武质岩浆,反映香桃湖变质堆晶辉长岩经历了一定程度镁铁质矿物的分离结晶作用;部分样品微量和稀土元素显示出OIB的特征,另一部分显示E-MORB的特征。因此,该变质堆晶辉长岩的原岩应形成于受洋中脊中轴型地幔热柱叠加的洋中脊,其可能为红脊山地区早古生代的大洋残片。  相似文献   

19.
 The Aurora volcanic field, located along the northeastern margin of Mono Lake in the Western Great Basin, has erupted a diverse suite of high-K and shoshonitic lava types, with 48 to 76 wt% SiO2, over the last 3.6 million years. There is no correlation between the age and composition of the lavas. Three-quarters of the volcanic field consists of evolved (<4 wt% MgO) basaltic andesite and andesite lava cones and flows, the majority of which contain sparse, euhedral phenocrysts that are normally zoned; there is no evidence of mixed, hybrid magmas. The average eruption rate over this time period was ∼200 m3/km2/year, which is typical of continental arcs and an order of magnitude lower than that for the slow-spreading mid-Atlantic ridge. All of the Aurora lavas display a trace-element signature common to subduction-related magmas, as exemplified by Ba/Nb ratios between 52 and 151. Pre-eruptive water contents ranged from 1.5 wt% in plagioclase-rich two-pyroxene andesites to ∼6 wt% in a single hornblende lamprophyre and several biotite-hornblende andesites. Calculated oxygen fugacities fall within –0.4 and +2.4 log units of the Ni-NiO buffer. The Aurora potassic suite follows a classic, calc-alkaline trend in a plot of FeOT/MgO vs SiO2 and displays linear decreasing trends in FeOT and TiO2 with SiO2 content, suggesting a prominent role for Fe-Ti oxides during differentiation. However, development of the calc-alkaline trend through fractional crystallization of titanomagnetite would have caused the residual liquid to become so depleted in ferric iron that its oxygen fugacity would have fallen several log units below that of the Ni-NiO buffer. Nor can fractionation of hornblende be invoked, since it has the same effect as titanomagnetite in depleting the residual liquid in ferric iron, together with a thermal stability limit that is lower than the eruption temperatures of several andesites (∼1040–1080°C; derived from two-pyroxene thermometry). Unless some progressive oxidation process occurs, fractionation of titanomagnetite or hornblende cannot explain a calc-alkaline trend in which all erupted lavas have oxygen fugacites ≥ the Ni-NiO buffer. In contrast to fractional crystallization, closed-system equilibrium crystallization will produce residual liquids with an oxygen fugacity that is similar to that of the initial melt. However, the eruption of nearly aphryic lavas argues against tapping from a magma chamber during equilibrium crystallization, a process that requires crystals to remain in contact with the liquid. A preferred model involves the accumulation of basaltic magmas at the mantle-crust interface, which solidify and are later remelted during repeated intrusion of basalt. As an end-member case, closed-system equilibrium crystallization of a basalt, followed by equilibrium partial melting of the gabbro will produce a calc-alkaline evolved liquid (namely, high SiO2 and low FeOT/MgO) with a relative f O 2 (corrected for the effect of changing temperature) that is similar to that of the initial basalt. Differentiation of the Aurora magmas by repeated partial melting of previous underplates in the lower crust rather than by crystal fractionation in large, stable magma chambers is consistent with the low eruption rate at the Aurora volcanic field. Received: 7 July 1995 / Accepted: 19 April 1996  相似文献   

20.
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   

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