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1.
A worldwide data set of major element and sulfur analyses of undegassed lavas, pumices, and melt inclusions from 14 volcanic locations was selected to investigate the compositional effects on sulfur solubility in magmas. We utilized analyses on calc-alkaline, alkaline, and tholeiitic rocks, with a range of 3400 ppm S variation. There is a strong correlation between chemical composition and the sulfur concentration: the less silicic and the more alkaline the rocks are, the more dissolved sulfur they can carry. Also, sulfur concentration is higher in rocks that represent less polymerized melts. Elemental correlations between FeO and S, well-defined for tholeiites, do not hold for alkaline melts. The compositional effects are at least as important as the better-known pressure, temperature, and f(O2) dependencies.  相似文献   

2.
《International Geology Review》2012,54(11):1297-1312
ABSTRACT

Early Palaeozoic magmatic records and tectonic reconstructions along the northern margin of Gondwana are still pending problems. In this paper, Late Silurian Dawazi and Dazhonghe volcanics in SW Yunnan Province (China) were studied. The Dazhonghe volcanics (419 Ma) have variable chemical compositions with SiO2 ranging between 49.8 and 79.5 wt.%, whereas, the Dawazi volcanics (417–429 Ma) form a bimodal volcanic suite consisting mainly of silicic rocks and subordinate basaltic rocks with a SiO2 content gap of ca. 15 wt.%. The Dazhonghe volcanics display calc-alkaline elemental compositions with enrichment in light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs) (e.g. Nb, Ta and Ti) and positive εNd(t) values (+ 4.0 to + 5.5). The Dawazi basaltic rocks are calc-alkaline, depleted in HFSEs, enriched in large ion lithophile elements (LILEs) (e.g. Cs, Rb, U and K), and have high εNd(t) values of ?1.7 to + 5.4. The Dawazi silicic rocks have high Na2O/K2O ratios and positive εNd(t) values of + 2.4 to + 5.0, which are equivalents of calc-alkaline I-type granites. The Dazhonghe volcanics are dominated by fractional crystallization (FC) from a calc-alkaline primary magma which originated from an enriched mantle source metasomatized by subduction-related, sediment-derived fluid. The Dawazi basaltic rocks were derived from partial melting of an enriched mantle source metasomatized by subducted oceanic sediment/slab-derived fluids; the Dawazi silicic rocks originated from partial melting of the juvenile mafic lower crust with extensive FC. Both the Dazhonghe and Dawazi volcanics were generated in a continental back-arc extension setting. Combined with previous geological observations, a Late Silurian Prototethyan arc and back-arc extension system is proposed along the northern margin of Gondwana in SW China.  相似文献   

3.
The Rainy Lake area in northern Minnesota and southwestern, Ontario is a Late Archean (2.7 Ga) granite-greenstone belt within the Wabigoon subprovince of the Canadian Shield. In Minnesota the rocks include mafic and felsic volcanic rocks, volcaniclastic, chemical sedimentary rocks, and graywacke that are intrucded by coeval gabbro, tonalite, and granodiorite. New data presented here focus on the geochemistry and petrology of the Minnesota part of the Rainy Lake area. Igneous rocks in the area are bimodal. The mafic rocks are made up of three distinct suites: (1) low-TiO2 tholeiite and gabbro that have slightly evolved Mg-numbers (63–49) and relatively flat rare-earth element (REE) patterns that range from 20–8 x chondrites (Ce/YbN=0.8–1.5); (2) high-TiO2 tholeiite with evolved Mg-numbers (46–29) and high total REE abundances that range from 70–40 x chondrites (Ce/YbN=1.8–3.3), and (3) calc-alkaline basaltic andesite and geochemically similar monzodiorite and lamprophyre with primitive Mg-numbers (79–63), enriched light rare-earth elements (LREE) and depleted heavy rare-earth elements (HREE). These three suites are not related by partial melting of a similar source or by fractional crystallization of a common parental magma; they resulted from melting of heterogeneous Archean mantle. The felsic rocks are made up of two distinct suites: (1)low-Al2O3 tholeiitic rhyolite, and (2) high-Al2O3 calc-alkaline dacite and rhyolite and consanguineous tonalite. The tholeiitic felsic rocks are high in Y, Zr, Nb, and total REE that are unfractionated and have pronounced negative Eu anomalies. The calcalkaline felsic rocks are depleted in Y, Zr, and Nb, and the REE that are highly fractionated with high LREE and depleted HREE, and display moderate negative Eu anomalies. Both suites of felsic rocks were generated by partial melting of crustal material. The most reasonable modern analog for the paleotectonic setting is an immature island arc. The bimodal volcanic rocks are intercalated with sedimentary rocks and have been intruded by pre- and syntectonic granitoid rocks. However, the geochemistry of the mafic rocks does not correlate fully with that of mafic rocks in modern are evvironments. The low-TiO2 tholeiite is similar to both N-type mid-ocean-ridge basalt (MORB) and low-K tholeiite from immature marginal basins. The calc-alkaline basaltic andesite is like that of low-K calc-alkaline mafic volcanic rocks from oceanic volcanic arcs; however, the high-TiO2 tholeiite is most similar to modern E-type MORB, which occurs in oceanic rifts. The conundrum may be explained by: (1) rifting of a pre-existing immature arc system to produce the bimodal volcanic rocks and high-TiO2 tholeiite; (2) variable enrichment of a previously depleted Archean mantle, to produce both the low- and high-TiO2 tholeiite and the calc-alkaline basaltic andesite, and/or (3) enrichment of the parental rocks of the high-TiO2 tholeiite by crustal contamination.  相似文献   

4.
Quartz-rich xenoliths in lavas and pyroclastic rocks from VulcanoIsland, part of the Aeolian arc, Italy, contain silicic meltinclusions with high SiO2 (73–80 wt %) and K2O (3–6wt %) contents. Two types of inclusions can be distinguishedbased on their time of entrapment and incompatible trace element(ITE) concentrations. One type (late, ITE-enriched inclusions)has trace element characteristics that resemble those of themetamorphic rocks of the Calabro-Peloritano basement of theadjacent mainland. Other inclusions (early, ITE-depleted) havevariable Ba, Rb, Sr and Cs, and low Nb, Zr and rare earth element(REE) contents. Their REE patterns are unfractionated, witha marked positive Eu anomaly. Geochemical modelling suggeststhat the ITE-depleted inclusions cannot be derived from equilibriummelting of Calabro-Peloritano metamorphic rocks. ITE-enrichedinclusions can be modelled by large degrees (>80%) of meltingof basement gneisses and schists, leaving a quartz-rich residuerepresented by the quartz-rich xenoliths. Glass inclusions inquartz-rich xenoliths represent potential contaminants of Aeolianarc magmas. Interaction between calc-alkaline magmas and crustalanatectic melts with a composition similar to the analysed inclusionsmay generate significant enrichment in potassium in the magmas.However, ITE contents of the melt inclusions are comparablewith or lower than those of Vulcano calc-alkaline and potassicrocks. This precludes the possibility that potassic magmas inthe Aeolian arc may originate from calc-alkaline parents throughdifferent degrees of incorporation of crustal melts. KEY WORDS: melt inclusions; crustal anatexis; magma assimilation; xenoliths; Vulcano Island  相似文献   

5.
Characteristics and tectonic setting of the shoshonite rock association   总被引：6，自引：0，他引：6  
Gregg W. Morrison 《Lithos》1980,13(1):97-108
A review of the major occurrences of shoshonitic rocks suggests there is a group that is near-silica saturated, K-rich and has low iron enrichment that cannot be unambigously classified as part of calc-alkaline or alkali-basalt associations. This group is here referred to as the shoshonite rock association. The shoshonite rock association is characterised by: hypersthene-olivine normative basalts, low iron enrichment, high Na2O + K2O, high content of light ion lithophile elements, high but variable Al2O3, high Fe2O3/FeO and low TiO2. Mineralogical characteristics include: coexisting plagioclase and sanidine in the groundmass, K-feldspar rims on plagioclase phenocrysts, plagioclase An50?85 Ab40?15 Or10?0 and low TiO2 content and lack of iron enrichment in clinopyroxene. Shoshonitic rocks on continental margins are younger, stratigraphically higher and more distant from the oceanic trench than the high-K calc-alkaline or calc-alkaline suites, but there is a complete gradation between the suites. A similar zonation occurs in some island arcs. In other island arcs there is no spatial zonation of the suites but successively more K-rich lavas are produced above an ever steepening subduction zone. Steepening leads to ‘failure’ or flipping of the subduction zone and uplift and block faulting within the arc. Shoshonitic rocks are most commonly associated with this phase of island are development.  相似文献   

6.
The fluid inclusions in samples of quartz, apatite, epidote, diopside, beryl and phenakite from Alpine veins in gneisses, amphibolites and mica schists from the western Tauern Window were analysed by microthermometrical, chemical and neutron activation methods. The inclusions of the eclogites contain a high density CO2 phase without optically detectable H2. In the Greiner Schieferserie the fluid inclusions show high CO2/H2O ratios and low salt contents. In the Zentralgneis area inclusions with low CO2/H2O ratios and high salt contents are typical. In the calcareous mica schists of the lower Schieferhülle, in the eastern part of the investigated area, generally no CO2 could be detected in the inclusions. These inclusions contain aqueous solutions showing a low salt content. The only CO2 bearing inclusions observed here were in the graphite-rich rocks of the so-called Habachzungen and in the eclogites from south of the Großvenediger. Trapping pressures estimated from the fluid inclusions are up to 7.5 kbar in the eclogites, but in general the pressures are between 2 and 4 kbar. These pressure data are in good agreement with the pressure data of mineral equilibria. The chemically analysed elements in the fluid inclusions are Na, K, Cs, Mg, Ca, Mn, As, Cl and Br. From the K/Na ratios temperatures between 435 and 490°C can be deduced. The very low Cl/Br ratios (<110) suggest that the dissolved elements came from the country rocks. The alkali/chlorine ratios (～1) indicate that the positive loadings of the cations are balanced by Cl.  相似文献   

7.
Late Permian-Early Triassic (P2-T1) volcanic rocks distributed on the eastern side of ocean-ridge and oceanic-island basalts in the Nan-Uttaradit zone were analyzed from aspects of petrographic characteristics, rock assemblage, REE, trace elements, geotectonic setting, etc., indicating that those volcanic rocks possess the characteristic features of island-arc volcanic rocks. The volcanic rock assemblage is basalt-basaltic andesite-andesite. The volcanic rocks are sub-alkaline, dominated by calc-alkaline series, with tholeiite series coming next. The chemical composition of the volcanic rocks is characterized by low TiO2 and K2O and high Al2O3 and Na2O. Their REE patterns are of the flat, weak LREE-enrichment right-inclined type. The trace elements are characterized by the enrichment of large cation elements such as K, Rb and Ba, common enrichment of U and Th, and depletion of Nb, Ta, Zr and Hf. The petrochemical plot falls within the field of volcanic rocks, in consistency with the plot of island-arc volcanic rocks in the Jinsha River zone of China. This island-arc volcanic zone, together with the ocean-ridge/oceanic island type volcanic rocks in the Nan-Uttaradit zone, constitutes the ocean-ridge volcanic rock-island-arc magmatic rock zones which are distributed in pairs, indicating that the oceanic crust of the Nan-Uttaradit zone once was of eastward subduction. This work is of great significance in exploring the evolution of paleo-Tethys in the Nan-Uttaradit zone.  相似文献   

8.
Sixteen selected samples from the Upper Cretaceous volcanic belt of the Eastern Pontids have been analysed for major elements, Rb, Sr and Zr. On the basis of the K2O versus SiO2 distribution, two groups of rocks have been distinguished, one with calc-alkaline affinity and a second group with shoshonitic character. The calc-alkaline rocks have porphyritic texture with clinopyroxene, plagioclase and orthopyroxene as phenocryst and in the groundmass. The orthopyroxene is lacking in the shoshonites where plagioclase, clinopyroxene and, in the more evolved terms, amphibole and biotite are the main phenocryst minerals. The shoshonitic rocks have higher K2ONa2O ratio, K2O, P2O5 and Rb, contents with respect to the calc-alkaline samples. The TiO2 content is invariably low, never exceeding approximately 1%. The occurrence of volcanic rocks ranging in composition from calc-alkaline to shoshonitic in the Upper Cretaceous volcanic belt of the Eastern Pontids suggests that the Upper Cretaceous volcanic cycle reached its mature stage before the onset of the Eocene calc-alkaline volcanism which is believed to be neither genetically nor tectonically related with the Upper Cretaceous volcanism.  相似文献   

9.
In east-central Finland, Archaean terrains present three main lithologic units: a) gneissic basement, emplaced from 2.86 G.a. to 2.62 G.a., b) greenstone belt (2.65 G.a.) and c) calc-alkaline magmatism (2.50 G.a. to 2.40 G.a). Twenty three rocks of the calc-alkaline suite have been chosen for geochronologic and Rb-Sr isotopic studies. These rocks are subdivided into three groups: 1) acid volcanics from Luoma, 2) augen gneiss from Arola, and 3) post kinematik pink leucogranite from Arola. The 2.50±0.10 G.a. age of the Luoma volcanics indicates that they represent the upper part of a greenstone belt composed of a single sequence of volcanic rocks. The ages, initial 87Sr/86Sr (ISr) and major element compositions of the augen gneisses of Arola and Suomussalmi indicate that these rocks are the plutonic equivalents of the Luoma acid volcanics. The Arola pink leucogranite marks the terminal phase of Archaean magmatic activity (from 2.86 G.a. to 2.41 G.a.). This was followed by at least 0.40 G.a. of quiescence. The ISr and major element compositions suggest that the genesis of the calc-alkaline magmatic rocks involved crustal materials, but all their geochemical features cannot be explained without the participation of mafic greenstone belt materials. The first crustal components had low I and low K2O/ Na2O ratios while the younger ones (calc-alkaline magmas) had medium to high ISr and high K2O/Na2O ratios. Thus the petrogenetic processes have changed with time from ensimatic to ensialic, implying major reworking of preexisting crustal materials. This evolution leads to the accretion of the continental crust from the mantle.  相似文献   

10.
本文对青藏高原羌北-昌都地块阿布日阿加措地区的晚二叠世那益雄组火山岩进行了年代学和地球化学研究。该火山岩主要由玄武岩、安山玄武岩、安山岩、流纹岩、凝灰岩组成,具有碱性玄武岩到酸性熔岩的特征。锆石U-Pb年代学研究表明该火山岩的形成时代为251. 1±4. 8～249. 6±1. 3 Ma之间。地球化学分析结果表明,该地区的流纹岩具有高的Si O2(74. 85%～77. 55%)和Na2O+K2O(5. 40%～6. 61%)含量,较低的MgO、K2O和Ca O含量,Al2O3含量低且稳定,里特曼指数平均为1. 15,小于3. 3。安山岩Si O2含量55. 13%～56. 28%,Na2O+K2O含量4. 13%～6. 15%,里特曼指数平均为2. 20,小于3. 3,属于钙碱性安山岩。碱性玄武岩Si O2含量51. 49%,Na2O+K2O含量6. 34%,里特曼指数为4. 73,属于碱性系列。稀土元素配分曲线为富集LREE的右倾型。另外,富集大离子亲石元素(LILE) Th、U,亏损高场强元素(HFSE) Nb、Ta等特征,均说明羌北-昌都地块阿布日阿加措地区的火山岩形成于陆缘岛弧环境。  相似文献   

11.
The Rio Itapicuru greenstone terrain of north-central Bahia State consists of belts of supracrustal rocks surrounding granitic plutons and domes. The basal supracrustal rocks are predominantly massive metabasalts with minor amounts of intercalated chemical sedimentary rocks and mafic tuffs. They are overlain by a middle unit of intermediate to acid pyroclastic rocks, lavas, and volcaniclastic sediments, and an upper unit of greywackes, sandstones and conglomerates.A geochemical study of major and trace elements of the volcanic rocks indicates the existence of a chemical discontinuity between the basaltic and the acid to intermediate members. The basalts are typical tholeiites with Ti, Zr, Sr, Y and Nb contents analogous to those of modern ocean-floor tholeiites or, alternatively, low-K tholeiites of primitive island arcs. In contrast, compositional variations of the hornblende-bearing andesites and dacites fall along indisputably calc-alkaline trends of low FeO and TiO2 contents which decrease with increasing differentiation. The lithostratigraphic and chemical variations within lavas of the Rio Itapicuru greenstone are comparable to those described from the Western Australian greenstone belts. Only in greenstone belts of the Canadian type do thick calc-alkaline sequences containing abudant basaltic andesites overlie conformably and transitionally the underlying tholeiitic basalts. Elsewhere the calc-alkaline sequences, if present, do not contain basaltic andesites and are chemically unrelated to the underlying basalts.  相似文献   

12.
Süphan is a 4,050 m high Pleistocene-age stratovolcano in eastern Anatolia, Turkey, with eruptive products consisting of transitional calc-alkaline to mildly alkaline basalts through trachyandesites and trachytes to rhyolites. We investigate the relative contributions of fractional crystallization and magma mixing to compositional diversity at Süphan using a combination of petrology, geothermometry, and melt inclusion analysis. Although major element chemistry shows near-continuous variation from basalt to rhyolite, mineral chemistry and textures indicate that magma mixing played an important role. Intermediate magmas show a wide range of pyroxene, olivine, and plagioclase compositions that are intermediate between those of basalts and rhyolites. Mineral thermometry of the same rocks yields a range of temperatures bracketed by rhyolite (~750°C) and basalt (~1,100°C). The linear chemical trends shown for most major and trace elements are attributed to mixing processes, rather than to liquid lines of descent from a basaltic parent. In contrast, glassy melt inclusions, hosted by a wide range of phenocryst types, display curved trends for most major elements, suggestive of fractional crystallization. Comparison of these trends to experimental data from basalts and trachyandesites of similar composition to those at Süphan indicates that melt inclusions approximate true liquid lines of descent from a common hydrous parent at pressures of ~500 MPa. Thus, the erupted magmas are cogenetic, but were generated at depths below the shallow, pre-eruptive magma storage region. We infer that chemical differentiation of a mantle-derived basalt occurred in the mid- to lower crust beneath Süphan. A variety of more and less evolved melts with ≥55 wt% SiO2 then ascended to shallow level where they interacted. The presence of glomerocrysts in many lavas suggests that cogenetic plutonic rocks were implicated in the interaction process. Blending of diverse, but cogenetic, minerals, and melts served to obscure the true liquid lines of descent in bulk rocks. The fact that chemical variation in melt inclusions preserves deep-seated chemical differentiation indicates that inclusions were trapped in phenocrysts prior to shallow-level blending. Groundmass glasses evolved after mixing and display trends that are distinct from those of melt inclusions.  相似文献   

13.
Chemical data on late cenozoic lavas (83 new analyses) from southern of Peru indicate a zonal arrangement of lavas types according to their distance from the trench. The nearest belong to a calc-alkaline suite and the farthest are shoshonites rocks. The calc-alkaline rocks show progressive increase in K2O content and in K2O/Na2O ratio in northeastern direction. The shoshonitic rocks are more potassic and titaniferous, and the K2O/Na2O ratio is approximatively constant. The potash content of the two series correlate with the depth of the inclined seismic zone beneath the volcanoes, but this correlation is less well established than in island arcs.  相似文献   

14.
The Mt. Erciyes stratovolcano was built up in an intraplate tectonic environment as a consequence of Eurasian and Afro-Arabian continental collision. However, the volcanic products generally exhibit a calc-alkaline character; minor amounts of tholeiitic basalts are also present. Tholeiitic basalts show high Fe2O3, MgO, CaO, low K2O, and depleted Ba, Nb, and especially Rb (2.3-5.97 ppm) contents, low 87Sr/86Sr (0.703344-0.703964), and high 143Nd/144Nd (0.512920-0.512780) isotopic ratios. These compositional features show that they were derived from a depleted asthenospheric mantle source, possibly a MORB-like source component. In contrast, calc-alkaline basaltic rocks exhibit relatively high large-ion-lithophile and high-field-strength elements, high 87Sr/86Sr (0.704591-0.70507) and low 143Nd/144Nd (0.51272-0.512394) isotopic ratios.

The bulk-rock chemistry of the tholeiitic basalts reflects the chemical composition of the extracted source component. Furthermore, trace-element concentrations may be calculated from an accepted mantle source component (starting composition) for different degrees of partial melting. These calculations also provide a sensitive approach to the origin of tholeiitic basalts. Modeled trace-element compositions of tholeiitic basalts are calculated from a primitive mantle composition. Calculated trace-element compositions imply that tholeiitic basalts are derived by minor fractional melting (1-1.5 %), in the absence of assimilation or deep-crustal melting. The calc-alkaline basalts were subsequently produced from initially tholeiitic basalts by the way of an AFC (assimilation-fractional crystallization) process, with a crustal assimilation of 10-15 %.

The geochemical data, partial melting, and AFC modeling all indicate that basaltic products have a complex evolutionary history involving partial melting from a MORB-like mantle source. The assimilation and fractional crystallization processes are considered as providing an example for the chemical evolution of basaltic products, from tholeiitic to calc-alkaline, in an intraplate environment.  相似文献   

15.
Summary The low-pressure emplacement of a quartz diorite body in the metapelitic rocks of the Gennargentu Igneous Complex (Sardinia, Italy) produced a contact metamorphic aureole and resulted in migmatisation of part of the aureole through partial melting. The leucosome, formed by dehydration melting involving biotite, is characterised by granophyric intergrowth and abundant magnetite crystals. A large portion of the high temperature contact aureole shows petrographic features that are intermediate between quartz diorite and migmatite s.s. (i.e. hybrid rocks). A fluid inclusion study has been performed on quartz crystals from the quartz diorite and related contact aureole rocks, i.e. migmatite sensu stricto (s.s.) and hybrid rocks. Three types of fluid inclusions have been identified: I) monophase V inclusions, II) L + V, either L-rich or V-rich aqueous saline inclusions and III) multiphase V + L + S inclusions. Microthermometric data characterised the trapped fluid as a complex aqueous system varying from H2O–NaCl–CaCl2 in the quartz diorite to H2O–NaCl–CaCl2–FeCl2 in the migmatite and hybrid rocks. Fluid salinities range from high saline fluids (50 wt% NaCl eq.) to almost pure aqueous fluid. Liquid-vapour homogenisation temperatures range from 100 to over 400 °C with an average peak around 300 °C. Temperatures of melting of daughter minerals are between 300 and 500 °C. Highly saline liquid- and vapour-rich inclusions coexist with melt inclusions and have been interpreted as brine exsolved from the crystallising magma. Fluid inclusion data indicate the formation of fluid of high iron activity during the low-pressure partial melting and a fluid mixing process in the hybrid rocks.  相似文献   

16.
对长岭凹陷深层天然气藏储层——营城组火山岩中发育的流体包裹体进行了详细研究,结果表明在火山岩发育的石英、方解石细网脉中均存在较多的碳质流体包裹体,单个包裹体激光拉曼光谱分析结果表明其主要为CO2及CH4两种类型的碳质包裹体。其中方解石细网脉体中发育有原生及次生CH4包裹体,而含CO2包裹体多以原生包裹体产于石英细网脉中。很多含CO2包裹体的石英细脉中发现了含CH4包裹体的方解石脉体的角砾,这就表明石英细脉形成晚于方解石细脉。营城组火山岩储层中CO2及CH4包裹体的产状特征研究表明,松辽盆地深层天然气藏的形成系火山岩成岩后CO2及CH4等气体不同期次充注的结果,CH4气的充注时间早于CO2气,火山岩中发育的原生孔隙及次生裂隙为上述气体的充注和聚集提供了重要通道。  相似文献   

17.
在华北克拉通中部的山西云中山地区,新太古代花岗闪长质片麻岩中存在一些超镁铁质岩-镁铁质岩块及由斜长角闪岩、角闪变粒岩、石英岩和石榴夕线黑云片岩等岩石类型构成的变质表壳岩残片,其中的超镁铁质-镁铁质岩、斜长角闪岩和角闪变粒岩构成一套高镁火成岩组合。超镁铁质岩已变质为橄榄绿泥阳起片岩等岩石类型,呈变余斑状结构,橄榄石斑晶仍有保存;岩石SiO_2含量为39.22%～44.99%,Al_2O_3为8.82%～13.47%,Mg O为19.24%～22.13%,Na_2O+K_2O=0.71%～1.11%,CaO为5.75%～8.42%;Al_2O_3/TiO_2=14.8～17.4,CaO/Al_2O_3=0.60～0.84;化学成分上与科马提岩有一定的相似性。与之紧密伴生的斜长角闪岩也具有高镁特征,Mg O含量为11.28%～15.09%,铝、硅和碱质均偏低,具正铕异常,显示堆晶辉长岩的特征。非高镁斜长角闪岩有相对高的铝、硅和碱质,其原岩应为钙碱性玄武岩。角闪变粒岩样品的SiO_2含量为54.21%～55.71%,Al_2O_3为14.24%～15.49%,Mg O为6.26%～8.28%,Fe OT/Mg O=1.11～1.58,高钠低钾,Na_2O+K_2O=3.7%～4.78%,Na_2O/K_2O=5.15%～13.13,Mg#=53.0～61.5,属于高镁安山岩。由超镁铁质质岩-斜长角闪岩-角闪变粒岩构成的变质高镁火山岩组合具有钙碱性系列趋势。超镁铁质岩稀土元素含量总量较低,具有轻稀土富集和重稀土亏损的稀土型式;斜长角闪岩与超镁铁质岩比较,除富集大离子亲石元素和Cr、Ni明显较低外,具有相似的微量元素图谱形态。三种岩石类型在微量元素蛛网图上均显示出Ta、Nb、Ti负异常和Pb正异常。野外产状和岩石地球化学特征表明超镁铁质岩和高镁斜长角闪岩属于阿拉斯加型杂岩体,角闪变粒岩属于赞岐岩质高镁安山岩。在Zr/Nb-Nb/Th和Nb/Y-Zr/Y构造环境判别图解上显示出与俯冲相关的演化趋势,在Hf-Th-Ta、Nb/La-(La/Sm)N和Th/Yb-Nb/Yb图解上也落在岛弧钙碱性岩石区域。以上特征表明高镁火成岩组合形成于与板块俯冲相关的岛弧构造背景。野外地质关系和锆石U-Pb年龄限定高镁火成岩组合形成时代在～2.5Ga。云中山地区阿拉斯加型镁铁质-超镁铁质杂岩与赞岐岩质高镁安山岩共生,表明该地区存在新太古代的板块俯冲作用,为太古宙存在板块构造机制提供了新证据。  相似文献   

18.
Abstract Fluid evolution paths in the COHN system can be calculated for metamorphic rocks if there are relevant data regarding the mineral assemblages present, and regarding the oxidation and nitrodation states throughout the entire P-T loop. The compositions of fluid inclusions observed in granulitic rocks from Rogaland (south-west Norway) are compared with theoretical fluid compositions and molar volumes. The fluid parameters are calculated using a P-T path based on mineral assemblages, which are represented by rocks within the pigeonite-in isograd and by rocks near the orthopyroxene-in isograd surrounding an intrusive anorthosite massif. The oxygen and nitrogen fugacities are assumed to be buffered by the coexisting Fe-Ti oxides and Cr-carlsbergite, respectively. Many features of the natural fluid inclusions, including (1) the occurrence of CO2-N2-rich graphite-absent fluid inclusions near peak M2 metamorphic conditions (927° C and 400 MPa), (2) the non-existence of intermediate ternary CO2-CH4-N2 compositions and (3) the low-molar-volume CO2-rich fluid inclusions (36–42 cm3 mol?1), are reproduced in the calculated fluid system. The observed CO2-CH4-rich inclusions with minor N2 (5 mol%) should also include a large proportion of H2O according to the calculations. The absence of H2O from these natural high-molar-volume CO2-CH4-rich inclusions and the occurrence of natural CH4-N2-rich inclusions are both assumed to result from preferential leakage of H2O. This has been previously experimentally demonstrated for H2O-CO2-rich fluid inclusions, and has also been theoretically predicted. Fluid-deficient conditions may explain the relatively high molar volumes, but cannot be used to explain the occurrence of CH4-N2-rich inclusions and the absence of H2O.  相似文献   

19.
在龙江盆地地区,龙江组由中—中酸性火山岩、火山碎屑岩和火山碎屑沉积岩组成。岩石地球化学显示为钙碱性岛弧火山岩系列,火山熔岩主体具有高SiO_2、Na_2O、Al_2O_3、Sr,低MgO、Y、Yb,和高场强元素Ta、Nb明显亏损等特征,与典型埃达克岩地球化学特征吻合,Sr和Sr/Y值显示为俯冲型埃达克岩。地球化学相关图解等表明岩浆在早期演化过程阶段可能发生了陆壳混染,经历了斜长石、辉石、铁钛氧化物和磷灰石的分离结晶作用。锆石LAICP-MSU-Pb谐和年龄为(125.9±1.5)Ma,与在该组中发现的化石资料完全一致,代表了火山岩的形成年龄。从东北地区火山岩的定年结果及其空间展布来看,龙江组火山岩的形成既与环太平洋构造体系有关,又与蒙古—鄂霍茨克构造带相联系。  相似文献   

20.
《Precambrian Research》1986,34(1):37-68
The early Proterozoic supracrustal rocks of the Salida area of central Colorado consist of strongly bimodal sequences of volcanogenic rocks. The mafic rocks — basalts, basaltic volcaniclastics, and related gabbro sheets — are distinctly tholeiitic, display a strong iron-enrichment trend, and typically contain less than 50% SiO2. The felsites are rhyolites to dacites and contain more than 70% SiO2.Major and trace element modeling show that the mafic rocks underwent two stages of crystal fractionation, the first involving olivine and plagioclase, the second involving plagioclase and clinopyroxene. Fractionation occurred within individual injections as they rose toward the surface rather than in a single magma chamber at depth. Field relations and major element data support the derivation of the felsic rocks from a magma generated by anatexis of sialic crust. However, the low Sr and high heavy REE concentrations in these rocks are not compatible with a partial melting model and suggest that the felsic volcanic rocks could have been derived by extensive fractional crystallization of the mafic magma.Normalized trace element abundances and trace element ratios of the mafic rocks are most like continental flood basalts such as the Columbia River basalts. They also display some similarity to immature back-arc basin tholeiites developed on continental crust, such as those of the Sarmiento complex. The felsic rocks have strong chemical affinities to within-plate rhyolites rather than calc-alkaline rhyolites from orogenic areas. The chemical data, as well as the petrographic, stratigraphic, and regional field data all indicate that the early Proterozoic supracrustal rocks of the Salida area developed along a continental margin, probably within an immature back-arc basin underlain by sialic crust. Remnants of the arc system of similar age may lie to the south in northern New Mexico and southwestern Colorado.  相似文献   

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