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1.
The Taiwan mountain belt, one of the youngest orogenies in the world, is caused by the collision of the Luzon arc with the Eurasian margin, which leads to post-collisional extension and magmatism in the Northern Taiwan Volcanic Zone (NTVZ). The magma chamber process in this region has not previously been elucidated in detail. In this paper, the textural and compositional features of plagioclase phenocrysts in basalt from the Tatun Volcanic Group (TTVG) were studied to restrict the dynamics of magma system. Results show that the magma melts in TTVG are mainly sourced from the underlying MORB-like mantle wedge but influenced by incorporation of subduction components, causing the elevated Sr/Y and Ba/Y ratios in magma melts. The subduction components are mainly transported in the form of sediment melt. The plagioclase phenocrysts in the TTVG volcanic rocks are generally coarsely core-sieved with a clear rim. The An contents in the rims of plagioclase are much lower than those of cores, and elevated FeO concentrations are detected in the plagioclase rims. We propose there exists a double-layer magma chamber in this region. The core of the plagioclase was crystalized in the deeper quiescent magma chamber (~21 km), which was subsequently partially dissolved during the ascent of magma melt under H2O-undersaturated condition, forming the typical coarsely sieved texture and synneusis. When this crystal-rich melt migrates into the shallower chamber, water saturation is reached and more sodic plagioclase formed as the rim of phenocryst. Due to the considerably higher fO2 in the shallow chamber than in the deeper one, and the distribution of Fe between plagioclase and melt positively correlates with fO2, the FeO content in the plagioclase rim elevates in conjunction with increasing fO2.  相似文献   

2.
Samples from several stratigraphic levels in the Miocene volcanicsuccession on Lesbos, Greece, contain complexly zoned clinopyroxeneand feldspar phenocrysts. Electron microprobe analyses havebeen made of zoned clinopyroxenes and feldspars, and their inclusions;and phenocryst mineralogy is compared with host-rock chemicalcomposition. The commonest pyroxene zoning sequence is coresof greenish salite or augite mantled by diopside which, in turn,may be rimmed by augite, with each type of pyroxene in opticaland chemical discontinuity with the next. None of the pyroxeneshas high pressure characteristics. The Ts content of phenocrystcores is correlated with the K-content of the host rock; andthe most diopsidic overgrowths also occur in the most K-richrocks. Augite rims occur only in Al-rich rocks. Most of the features of the pyroxenes can be accounted for bya hypothetical fractional crystallization scheme. However, twoobservations suggest that mixing of small magma batches mayalso be important. The earliest crystallized diopsides thatrim salite are rich in Cr2O3, suggesting precipitation froman unfractionated magma. Anorthoclase (implying advanced crystallizationof feldspars) occurs as cores to feldspar phenocrysts and asinclusions in clinopyroxene cores in several samples.  相似文献   

3.
Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high-temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element(REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end-member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite(An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface(PI), or normal zoning with euhedral form and no resorption surface(P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin-section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in PI, concentrations o f FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre-existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO~T and Sr. However, pre-existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.  相似文献   

4.
The orogenic volcanic rocks of Erromango island are divided into a Quaternary theoleiitic group and an older Pliocene one showing calc-alkalic affinity. The microprobe mineralogy of these lavas agrees with this geochemical distinction and indicates a marked iron enrichment trend in the recent tholeiitic lavas, whereas titanomagnetite occurs as early formed phenocrysts (Al, Cr, Mg-rich) only in Pliocene basalts. Some Ni and Mg-rich olivine phenocrysts (Fo88) and unusual chemically zoned clinopyroxene megacrysts are described from Pliocene basic lavas and interpreted as deep-fractionated minerals. The large compositional range of these latter (diopside core to augite rim, through salite intermediate zone) leads us to discuss the physical conditions and the corresponding cation substitutions prevailing during their formation. Diopside cores crystallized at depths of about 20 km from a relatively ‘primitive’ melt. The formation of salite is considered to be the result of concomitant fractionation of Mg-rich phases and increase of fO2. An early stage of fractionation of Cr-bearing diopside + Ni-bearing olivine would account for the typical Cr and Ni depletion of Erromango lavas.  相似文献   

5.
滇西莴中晚始新世高镁富钾火山岩中单斜辉石斑晶普遍出现正环带结构、反环带结构或韵律环带结构，少量为具绿色核部的单斜辉石(“绿核辉石”)。反环带斑晶和“绿核辉石”的幔部与正环带斑晶的核部具有相似并且相对较窄的成分范围，相对高Mg#（0.83 ~ 0.90）,低TiO2（0.13 % ~ 0.29 %）,Al2O3（0.73 % ~ 1.68 %）和Na2O （0.22 % ~ 0.42 %），为钾质岩浆平衡结晶的产物。反环带斑晶的核部相对低Mg#（0.77 ~ 0.84），但与反环带斑晶的幔部、正环带斑晶的核部均具有相似的Ti/Al比值（0.06 ~ 0.16）；韵律环带结构斑晶的成分变化均在正、反环带斑晶的成分范围之内。莴中高镁富钾火山岩中的这些环带结构单斜辉石斑晶应来源于相似的岩浆体系，反 环带结构表明在岩浆房存在较原始岩浆对较演化岩浆再补给的岩浆混合过程，而韵律环带结构特征揭示曾多次发生这种岩浆混合过程。“绿核辉石”的核部明显低Mg# （0.50 ~ 0.74）,相对富Al2O3（1.66 % ~ 3.63 %）和Na2O（0.87 % ~ 2.17 %），具有明显较低的Ti/Al比值（< 0.05）和较高的AlVI/AlIV比值（0.38 ~ 0.76），为下地壳捕虏晶 来源，证实了在滇西晚始新世富钾岩浆演化过程中存在少量地壳混染作用。  相似文献   

6.
The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999  相似文献   

7.
山东西部莱芜－淄博地区的中生代雪野碳酸岩中发育有大量罕见的反环带云母，云母的电子探针分析表明，云母斑晶的核心为黑云母，过渡带和边缘是金云母，基质中云母为金云母，核心黑云母与过渡带的金云母界限清晰，成分突变以及核心黑云母的溶蚀结构表明，核心黑云母和过渡带与边缘带金云母不是同一岩浆体系结晶的产物，核心黑云母可能是碳酸岩岩浆捕获的外来黑云母，过渡带与边缘带金云母同基质中金云母具有相似的化学成分，从过渡带到边缘带，金云母的Mg(Mg Fe)逐渐降低，反映了碳酸岩岩浆的不断结晶演化过程，TiO2的不断降低，一方面是由于岩浆的分异结晶，另一方面则是岩浆上升期间去气作用导致岩浆内CO2／H2O值降低所致。云母斑晶从过渡带向边缘带Al2O3含量逐渐减少，表明碳酸岩浆自过渡带云母结晶后没有富Al的圈岩物质加入。  相似文献   

8.
Data obtained on lamprophyres from the carbonatite–volcanic unit in the lower horizon of the Tomtor Massif indicate that the rocks and zoned diopside and kaersutite phenocrysts in them are enriched in incompatible elements more significantly than is typical of alkaline ultramafic rocks of the Maymecha–Kotui and Kola provinces. The concentrations of these elements and their indicator ratios in the cores and intermediate zones of the diopside and kaersutite phenocrysts significantly vary, and this suggests that the minerals might have crystallized from different melts. This is consistent with the earlier conclusions, which were derived from studying melt inclusions, that the phenocrysts crystallized from mixing alkaline mafic melts of sodic and potassic types and different Mg–number which were enriched in the carbonatite component. The cores of the diopside phenocrysts started to crystallize from sodic mafic magma in a magmatic chamber, while the intermediate and outermost zones of this mineral crystallized from mixed sodic–potassic mafic melts. The carbonatite component was separated from the sodic mafic melt at high temperature (>1150°C) during diopside core crystallization. The bulk compositions of the alkaline lamprophyres and of the diopside and kaersutite phenocrysts contain lower normalized concentrations of HREE than LREE. This led us to conclude that the parental sodic and potassic mafic melts were derived from an enriched mantle source material under garnet–facies parameters, as is typical of continental rifts. It is noteworthy that the potassic mafic melt was derived at greater depths and lower degrees of melting of the mantle source than the sodic melt. The iron–rich sodic melt from which the cores of the diopside phenocrysts started to crystallize was enriched in V, REE, Y, and volatile components (H2O, CO2, F, Cl, and S). The onset of carbonate–silicate liquid immiscibility was marked by the redistribution of REE and Y into the carbonatite melt. The potassic, more Mg–rich mafic melt from which the intermediate and outermost zones of the diopside phenocrysts crystallized was enriched in Ti, Nb, Zr, and REE and always remained homogeneous when this mineral crystallized.  相似文献   

9.
方沸石响岩是一种罕见的碱性火山岩.采用电子探针、粉晶X射线衍射、扫描电镜、拉曼光谱等研究了青藏高原当雄地区方沸石响岩的主要造岩矿物种属、共生关系和结晶顺序.研究表明, 斑晶由方沸石和长石组成.方沸石为岩浆结晶晚期形成的原生矿物; 长石均发育“次生边”结构, 中央相为斜长石, 边缘相为碱性长石.基质由碱性长石、次透辉石、钛磁铁矿和褐铁矿组成.原生矿物的结晶顺序是: 斑晶长石的中央相→斑晶方沸石+斑晶长石边缘相富钾长石→基质长石→次透辉石→钛磁铁矿和褐铁矿.利用方沸石-熔体平衡估算出方沸石结晶时岩浆的温度和压力条件分别为600~640 ℃和(5~13) ×108 Pa, 考虑到青藏高原当时已形成巨厚地壳, 认为岩浆房存在于地壳深部.   相似文献   

10.
朱永峰 《岩石学报》2012,28(7):2113-2120
在新疆西南天山科桑溶洞地区,新厘定出一套斜长角闪岩-花岗岩地质单元:侵入斜长角闪岩中的新元古代白云母花岗岩(片麻状构造)、以及侵入上述古老岩石单元的早奥陶世花岗岩(块状构造)。片麻状白云母花岗岩中锆石具有热液锆石边、岩浆锆石幔和碎屑锆石核(边-幔-核结构),剔除被热液锆石和碎屑锆石混染的SHRIMP测点,获得岩浆锆石幔的加权平均年龄752.3±5.1Ma(MSWD=0.95),代表岩浆的结晶年龄。块状花岗岩的锆石具有边-核结构,热液锆石边的U-Pb年龄(419.5±5.7Ma)明显偏低。剔除热液锆石和碎屑锆石,获得岩浆锆石的平均U-Pb年龄481.1±4.4Ma(MSWD=0.88),代表花岗岩的结晶年龄。早奥陶世早期,岩浆侵入新元古代片麻状白云母花岗岩中。在晚志留世或者更晚时期,两类花岗岩共同经受了变质热液改造,变质流体交代岩浆锆石,导致锆石溶蚀再生长。  相似文献   

11.
矿物的结构和成分可用来反演岩浆演化过程。为探讨酸性岩浆房内岩浆演化过程,以浙江拔茅火山主体英安岩中的斜长石为例,对其开展了矿物学、地球化学研究。结果显示,斜长石普遍具有核-幔-边结构,暗示其在喷发前处于开放性岩浆房内,并经历了复杂的岩浆过程。对代表性斜长石的电子探针成分及背散射图像分析表明,斜长石核部、幔部及边部分别具有不同的成因,反映了英安岩岩浆从深部聚集并在浅部岩浆房内周期性补给的过程。岩浆的这种周期性补给过程抑制了结晶分异作用对熔体成分的改变,全岩地球化学特征主要反映其源区特征及熔融过程。  相似文献   

12.
Plagioclase phenocrysts from mafic enclaves and plagioclase from its host granite possess a pat-tern of complex zonation .A plagioclase phenocryst can generally be divided into three parts:an oscillatory, locally patchy zoned core (An47-19),a ring with dusty, more calcic plagioclase (An64-20) and a normally zoned rim composed of sodic plagioclase (An22-3.3). Major discontinuities in zoning coincide with resorption surfaces that are overgrown by the more calcic plagioclase.The cores of large plagioclase phenocrysts from mafic enclaves and host granite show similar zoning patterns and similar compositions, indicating their crystallization under the same conditions .Steep normal zoning of the rims of plagioclases both from host granite and mafic enclaves illustrates a drastic decrease in An content which is considered to have resulted from the continuous differentiation of hybrid magma and efficient heat loss because of the upward emplacement of the residual magma.Wide rims of plagioclases from the host granite against the discrete rims of plagioclases from mafic enclaves indicate that differentiation and cooling lasted much longer in the host granite than in the mafic enclaves.  相似文献   

13.
A migmatitic orthogneiss in the Western Segment in the Sveconorwegian Province of the Baltic Shield was dated using the ion-probe U–Pb method on zircon grains, which were also analysed for rare earth elements. Mesosome zircons have 1.605±0.010 Ga magmatic cores, which places the gneiss protolith in the same 1.61–1.59 Ga time bracket as continental arc-related gneisses, abundant in this part of the Sveconorwegian Province. These cores show REE profiles with strong HREE enrichment, positive Ce- and negative Eu-anomalies, typical of magmatic zircon. Migmatite leucosomes are folded and parallel with or slightly discordant to the fabric. They contain a small population of zircon with cores and metamorphic rims, which are interpreted as xenocrysts incorporated in the leucosome during melting of the mesosome. CL-bright metamorphic embayments and rims on xenocrysts reflect 1.01±0.05 Ga Sveconorwegian metamorphic reworking. Ce-anomalies are nearly absent and Eu-anomalies are reduced relative to igneous spots. This is probably a feature of fluid controlled environments where Ce and Eu oxidation states are buffered by the metamorphic fluid. From this and discordant rims from the mesosome we also conclude that the rims formed by reworking of the older zircon where the Pb-loss was also fluid induced. In the leucosome veins, magmatic acicular zircon gives 0.92±0.01 Ga, ascribed to the crystallisation of the veins. They originated by local melting, probably augmented by magma that formed at a deeper level. Widespread granitic and noritic late-Sveconorwegian magmatism close to 0.92 Ga in other parts of the Western Segment has equivalents in the Norwegian sectors of the Sveconorwegian Province. Leucosome formation was therefore part of a regional event related to exhumation of the Sveconorwegian Eastern Segment. We also provide the first unequivocal evidence for ductile deformation related to late-Sveconorwegian magmatism.  相似文献   

14.
Juvenile ejecta from the September and October 1995 eruptionsof Ruapehu volcano, New Zealand, indicate that mixing occurredbetween relatively higher- and lower-temperature (high-T andlow-T) andesitic magmas. Compositional zonations in clinopyroxenephenocrysts provide direct evidence for a pre-eruption crystal–meltmush chamber containing low-T magma, and elucidate the processesof magma mixing and eruption, following the injection of high-Tmagma. Many phenocrysts with Fe-rich cores derived from low-Tmagma have extremely reverse zoned mantles around slightly resorbedcores. Mg-value [100Mg/(Mg + Fe)] increases from 65–70to  相似文献   

15.
The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78–85) correlates positively with the An content of coexisting plagioclase phenocrysts (An85–92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4–5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200 MPa.  相似文献   

16.
While recycling of subducted oceanic crust is widely proposed to be associated with oceanic island, island arc, and subduction-related adakite magmatism, it is less clear whether recycling of subducted continental crust takes place in continental collision belts. A combined study of zircon U–Pb dating, major and minor element geochemistry, and O isotopes in Early Cretaceous post-collisional granitoids from the Dabie orogen in China demonstrates that they may have been generated by partial melting of subducted continental crust. The post-collisional granitoids from the Dabie orogen comprise hornblende-bearing intermediate rocks and hornblende-free granitic rocks. These granitoids are characterized by fractionated REE patterns with low HREE contents and negative HFSE anomalies (Nb, Ta and Ti). Although zircon U–Pb dating gives consistent ages of 120 to 130 Ma for magma crystallization, occurrence of inherited cores is identified by CL imaging and SHRIMP U–Pb dating; some zircon grains yield ages of 739 to 749 Ma and 214 to 249 Ma, in agreement with Neoproterozoic protolith ages of UHP metaigneous rocks and a Triassic tectono-metamorphic event in the Dabie–Sulu orogenic belt, respectively. The granitoids have relatively homogeneous zircon δ18O values from 4.14‰ to 6.11‰ with an average of 5.10‰ ± 0.42‰ (n = 28) similar to normal mantle zircon. Systematically low zircon δ18O values for most of the coeval mafic–ultramafic rocks and intruded country rocks preclude an AFC process of mafic magma or mixing between mafic and felsic magma as potential mechanisms for the petrogenesis of the granitoids. Along with zircon U–Pb ages and element results, it is inferred that the granitic rocks were probably derived from partial melting of intermediate lower crust and the intermediate rocks were generated by amphibole-dehydration melting of mafic rocks in the thickened lower crust, coupled with fractional crystallization during magma emplacement. The post-collisional granitoids in the Dabie orogen are interpreted to originate from recycling of the subducted Yangtze continental crust that was thickened by the Triassic continent–continent collision. Partial melting of orogenic lithospheric keel is suggested to have generated the bimodal igneous rocks with the similar crustal heritage. Crustal thinning by post-collisional detachment postdated the onset of bimodal magmatism that was initiated by a thermal pulse related to mantle superwelling in Early Cretaceous.  相似文献   

17.
This study presents major- and trace-element chemistry of plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens volcano. Despite the considerable variation in textures and composition of plagioclase phenocrysts, distinct segments have been cross-correlated between crystals. The variation of Sr and Ba concentration in the melt, as calculated from the concentration in the phenocrysts using partition coefficients, suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector. In both of these melts Sr and Ba are correlated despite the abundance of plagioclase in the 1980 dacites. We propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower Sr and higher Ba, as documented in the rims of the phenocrysts. Although the cores of the phenocrysts crystallised at relatively shallow depths, they preserve the Sr and Ba of the deep-seated melts as they ascended from a deeper region. Further magma ascent resulted in microlite nucleation, which is responsible for a similar shift to even lower Sr concentration as observed in the groundmass of post-18 May 1980 samples. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

18.
Carbonatites of the Jacupiranga alkaline–carbonatite complex in São Paulo State, Brazil, were used to investigate mineral–fluid interaction in a carbonatite magma chamber because apatite showed a marked discontinuity between primary fluid inclusion-rich cores and fluid inclusion-poor rims. Sylvite and burbankite, apatite, pyrite, chalcopyrite and ilmenite are the common phases occurring as trapped solids within primary fluid inclusions and reflect the general assemblage of the carbonatite. The apatite cores had higher Sr and REE concentrations than apatite rims, due to the presence of fluid inclusions into which these elements partitioned. A positive cerium anomaly was observed in both the core and rim of apatite crystals because oxidised Ce4+ partitioned into the magma. The combined evidence from apatite chemistry, fluid inclusion distribution and fluid composition was used to test the hypotheses that the limit of fluid inclusion occurrence within apatite crystals arises from: (1) generation of a separate fluid phase; (2) utilization of all available fluid during the first stage of crystallization; (3) removal of crystals from fluid-rich magma to fluid-poor magma; (4) an increase in the growth rate of apatite; or (5) escape of the fluids from the rim of the apatite after crystallization. The findings are consistent with fractionation and crystal settling of a carbonatite assemblage in a fluid-stratified magma chamber. Secondary fluid inclusions were trapped during a hydrothermal event that precipitated an assemblage of anhedral crystals: strontianite, carbocernaite, barytocalcite, barite and norsethite, pyrophanite, magnesian siderite and baddeleyite, ancylite-(Ce), monazite-(Ce) and allanite. The Sr- and REE-rich nature of the secondary assemblage, and lack of a positive cerium anomaly indicate that hydrothermal fluids have a similar source to the primary magma and are related to a later carbonatite intrusion.  相似文献   

19.
刘鑫  汤艳杰 《岩石学报》2018,34(11):3315-3326
冀西北姚家庄存在一套晚三叠世的超镁铁岩-正长岩杂岩体,岩体内发育具有环带特征的单斜辉石。辉石的环带有两种:简单环带和复杂环带。简单环带一般为正环带,辉石核部的MgO和Cr2O3含量高,Si O2、Fe O和Na2O含量低;边部的主要氧化物含量与核部刚好相反。简单正环带可以分为两类,其中核边接触带平滑、由核到边化学成分具有渐变特征的正环带辉石可能是岩浆在分离结晶或地壳混染过程中形成。而核边接触带有熔蚀结构、由核到边化学成分突变的正环带辉石可能是早期结晶的辉石颗粒受到晚期低镁岩浆的溶蚀改造而成的。复杂环带具有核-幔-边结构,其中,核部低镁高铁、幔部高镁低铁、边部与核部相似,但其Mg#更低,这些特征暗示了岩浆混合作用的存在,形成辉石核部的母岩浆可能来自富集的岩石圈地幔,幔部高Mg#的特征指示了软流圈地幔物质的贡献,其边部低Mg#的特征则指示了地壳物质的加入。具有韵律环带的复杂辉石是在岩浆多期侵入的过程中形成的。辉石环带的组成特征表明,姚家庄杂岩体是由岩浆多期侵位形成的,后期侵入的岩浆与前期就位的岩浆不断反应,形成了具有多种不同环带特征的辉石,并最终形成了空间上由外到内依次为辉石岩、辉石正长岩和正长岩的环状杂岩体。结合前人的研究成果,推测形成姚家庄岩体的岩浆主要来源于富集的岩石圈地幔,并由少量地壳组分和软流圈物质的贡献。  相似文献   

20.
Serpentinites and talc-carbonate rocks of El Ideid-El Sodmein District (ISD), central Eastern Desert, Egypt, contain variably altered chrome-spinels. Back-scattered electron images and electron microprobe analyses of chrome-spinels and associated silicates are made to evaluate their textural and compositional variations with metamorphism. In most cases the chrome-spinel crystals are concentrically zoned with unaltered cores through transitional zone of ferritchromit to Cr-magnetite toward the rims. In talc-carbonate rocks chrome-spinels are extensively altered to Cr-magnetite. Compared to cores, the metamorphic rims are enriched in Cr# (0.83–1.0 vs. 0.58–0.63 for rims and cores, respectively) and impoverished in Mg# (0.05–0.29 vs. 0.57–0.63), due to Mg–Fe and Al(Cr)–Fe3+ exchange with the surrounding silicates during regional metamorphism rather than serpentinization process. Textural and compositional features of the chrome-spinels suggest transitional greenschist-amphibolite up to lower amphibolite facies metamorphism (at 500–600 °C), which is isofacial with the country rocks. The common preservation of unaltered chrome-spinel cores in the serpentinites, contrary to talc-carbonate rocks, implies that full equilibration has not been attained due to small metamorphic fluid–rock ratio. Microprobe analyses profile across a concentrically zoned grain confirms the presence of two compositional (miscibility?) gaps; one between chrome-spinel core and ferritchromit zone; and another one between ferritchromit zone and Cr-magnetite outer rim.Chrome-spinel cores do not appear to have re-equilibrated completely with the metamorphic spinel rims and surrounding silicates, indicating relic magmatic composition not affected by metamorphism. Core compositions suggest an ophiolitic origin and derivation by high degrees of melting of reduced, depleted harzburgite to dunite mantle peridotites in an oceanic supra-subduction zone (marginal-basin) tectonic environment.  相似文献   

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