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1.
Ikizdere Pluton consists of granite, granodiorite, tonalite, monzonite, quartz monzonite containing pinkish colored K-feldspar megacrysts (KFMs). The crystal sizes of the KFMs range from 1 to 4 cm. The lath-shaped megacrysts are uniformly (i.e., randomly) distributed in the host plutonic rocks and have mafic and felsic inclusions whose crystal sizes are smaller than 1 mm. The crystal inclusions are biotite, slightly annitic in composition with XMg[=Fetot/(Fetot+Mg)]=0.50-0.58, amphibole (magnesio-hornblende, XMg[=Mg/(Mg+Fetot)]=0.70-0.79), iron-titanium oxide (low titanium magnetit and ilmenite), plagioclase (Ab75−25An65−35) and as minor quartz. The compositions of the KFMs range from Or95Ab5An0 to Or82Ab17An1. BaO contents of the megacrysts increase from core to rim. The mafic and felsic inclusions are compositionally similar those of the host rocks.The chemical and textural features of K-feldspar are typical for megacrysts that grew as phenocrysts in dynamic granitoidic magma systems. The overgrowth of KFMs and mafic magma injections (magma mixing) may be related to temperature, pressure and compositional fluctuations in the magma chamber. Remnant of earlier formed K-feldspar crystals remain in the felsic magma system, while the mafic injection can decompose some earlier precipitated KFMs. The remnant of K-feldspar remaining after mafic injection are overgrown by rapid diffusion of Ba, K and Na elements in liquid phase, during the later stages of crystallization of the host magma. 相似文献
2.
An antiperthite feldspar (composition of the main part An27.2 Ab69.2Or3.6) has been studied by x-rays and transmission electron microscopy. Complex twinning and exsolution on very fine scale are described for the first time for this compositional range. Evidence is given for a distinct intermediate region between the plagioclase and the potash feldspar. The formation of the crystal probably involves partial replacement, at least two step exsolution, and transformation of monoclinic plagioclase to triclinic plagioclase. 相似文献
3.
Cordierite–quartz and plagioclase–quartz intergrowths in a paragneiss from northern Labrador (the Tasiuyak Gneiss) were studied using SEM, STEM and TEM. The gneiss experienced granulite facies conditions and partial melting during both regional and, subsequently, during contact metamorphism. The microstructures examined all results from the contact metamorphism. Cordierite–quartz intergrowths occur on coarse and fine scales. The former sometimes exist as a ‘geometric’ intergrowth in which the interface between cordierite and quartz appears planar at the resolution of the optical microscope and SEM. The latter exists in several microstructural variants. Plagioclase is present as a minor component of the intergrowth in some examples of both the coarse and fine intergrowth. Grain boundaries in cordierite–quartz intergrowths are occupied by amorphous material or a mixture of amorphous material and chlorite. Cordierite and quartz are terminated by crystal faces in contact with amorphous material. Chlorite is sometimes found on cordierite surfaces and penetrating into cordierite grains along defects. Quartz contains (former) fluid inclusions 10–20 nm in maximum dimension. The presence of planar interfaces between cordierite and the amorphous phase is reminiscent of those between crystals and glass in volcanic rocks, but in the absence of compelling evidence that the amorphous material represents former melt, it is interpreted as a reaction product of cordierite. Plagioclase–quartz intergrowths occur in a number of microstructural variants and are commonly associated with cordierite–quartz intergrowths. The plagioclase–quartz intergrowths display simple, non‐planar interfaces between plagioclase and quartz. Quartz contains (former) fluid inclusions of dimensions similar to those observed in cordierite–quartz intergrowths. The boundary between quartz and enclosing K‐feldspar is cuspate, with quartz cusps penetrating a few tens of nanometres into K‐feldspar, commonly along defects in K‐feldspar and sometimes with very low dihedral angles at their tips. This cuspate microstructure is interpreted as melt pseudomorphs. The plagioclase–quartz intergrowths share some features with myrmekite, but differ in some respects: the composition of the plagioclase (An37Ab62Or1–An38Ab61Or1); the association with cordierite–quartz intergrowths; and microstructures that are atypical of myrmekite (e.g. quartz vermicules shared with cordierite–quartz intergrowths). It is inferred that the plagioclase–quartz intergrowths may have formed from, or in the presence of, melt. Inferred melt‐related microstructures preserved on the nanometre scale suggest that melt on grain boundaries was more pervasive than is evident from light optical and SEM observations. 相似文献
4.
河北武安坦岭多斑斜长斑岩的成因:冻结岩浆房活化机制 总被引：5，自引：3，他引：2
流变学实验表明,当岩浆中晶体体积分数达到约50vol%时,岩浆体实际上处于冻结状态,不再具有整体迁移的能力。但在自然界中仍存在含大量斑晶的浅成火成岩和火山岩。因此,富晶体岩浆的上升过程和侵位机制是近年来地球科学领域关注的热点之一。目前,冻结岩浆房的活化机制主要有二种:升温活化机制和流体活化机制。河北武安坦岭地区新发现的多斑斜长斑岩为揭示冻结岩浆房的活化提供了契机。野外观察和晶体粒度分布(CSD)分析表明,坦岭斜长斑岩中斜长石斑晶高达70vol%,基质为显微晶质结构。斜长石斑晶粒径分布均一,大小约为3.1×1.7mm;显微镜观察和背散射图像揭示,斜长石斑晶具环带结构,由宽广的斜长石核部+宽度可变的条纹长石边部组成,且无熔蚀现象;电子探针成分剖面分析表明,斑晶核部成分为更长石(An_(27)Ab_(71)Or_2),幔部为更长石(An_(13)Ab_(83)Or_4),边部为条纹长石。边部条纹长石的成分有一定变化,从内侧到外侧,主晶钠长石成分由Ab_(53)Or_(47)变为Ab_(99)Or_1,客晶钾长石成分由Ab_(48)Or_(51)变为Ab3Or97。斑晶斜长石核部存在细长条状或斑点状钾长石,且越靠近中心,钾长石斑点的数量越少。这些特点表明,边部条纹长石为交代成因。稀土和微量元素分析则显示,边部条纹长石具弱正Eu异常,相对富集LREE和K、Rb、Ba、Sr等大离子亲石元素,亏损Th、Zr、Nb的特点。CSD相关图解及以上特征表明,斜长石斑晶形成于稳定,封闭的结晶环境,并受到晚期碱交代作用的改造。基质主要由微粒钙质角闪石,条纹长石,石英,钾长石和钠长石组成,含少量自形-半自形磁铁矿和钛铁矿、磷灰石、榍石、金红石和锆石等11种矿物组成。11种矿物相和结构特征暗示基质形成于极端不稳定的结晶环境,与斜长石斑晶形成条件鲜明对照。根据基质的矿物组成,推测形成基质的岩浆具有富含K、Na、Fe、Si和挥发分的特征。这种特征与上述关于条纹长石环边形成条件的判断一致。据此,本文认为:产生斜长石斑晶的岩浆曾经在地壳深部作过长时间滞留,导致了斜长石的稳定结晶,增加了岩浆的粘度和密度,使岩浆处于冻结状态;富碱高铁熔体-流体流的注入大幅降低了岩浆的总粘度,并提高了岩浆的浮力,从而促使冻结岩浆房迅速活化和上升侵位;同时,富碱高铁熔体-流体流强烈交代了先存的斜长石斑晶,使其边部形成条纹长石;这种熔体-流体流则在快速排气,冷却过程中迅速结晶,形成了具有不平衡矿物组合的显微晶质基质。在岩浆侵入体较深部位,富碱高铁熔体-流体经历了很缓慢的固结过程,而相分离产生的流体有可能萃取携带岩浆中的铁质,形成富Fe流体流,后者可能对区内"铁矿浆"型铁矿的形成具有重要的贡献。 相似文献
5.
A high-alumina tholeiitic andesite from the southern portion of the Tweed Shield Volcano in northeastern New South Wales contains abundant megacrysts of plagioclase (Ab50An46Or4) and megacrysts of aluminian bronzite (Ca4Mg71Fe25) and relatively Ca-poor aluminian augite (Ca36Mg41Fe20). The pyroxenes commonly occur as inclusions in the plagioclase megacrysts. Electron probe microanalyses of the pyroxene megacrysts indicate that they differ in character and composition from the groundmass ferromagnesian phases, namely a more Al- and Na-poor augite (Ca41Mg42Fe17) and olivine (Fa53). The bulk composition of the plagioclase megacrysts is slightly more Ab-rich than that of the groundmass plagioclase, but differences in the two compositions are extended by microanalyses of groundmass plagioclases. Evaluation of the megacryst compositions in the light of experimental data and analogous occurrences in alkaline volcanics leads to the interpretation that the megacrysts represent cognate precipitates formed at pressures broadly equivalent to the crust-mantle boundary. More important, they provide strong evidence for the high pressure origin of tholeiitic andesites, customarily interpreted as the products of low pressure fractional crystallization of tholeiitic magma. 相似文献
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
7.
An unusual magmatic three-feldspar syenogabbro occurs 3 m inside the contact of the Klokken gabbro-syenite stock. It contains plagioclase, mesoperthite and cryptoperthite, together with a low temperature symplectite intergrowth. Textural relationships have been investigated by cathodoluminescence, bulk chemistry by microprobe, and exsolution microtextures and intracrystalline boundaries by TEM. The mesoperthite has a bulk composition around Or26Ab52An22, well outside accepted limits of ternary feldspar solid solution. It is a mainly coherent, lamellar intergrowth of sodic andesine and orthoclase with incipient Mtwinning. The cryptoperthite, bulk composition around Or61Ab33An6, is a coherent lamellar intergrowth of orthoclase and sodic low oligoclase. The compositions of the exsolved phases have been estimated. The meso- and cryptoperthite crystals have sharp boundaries with each other. Plagioclase (zoned An 55-35, with 2–3% Or) defines a solidus fractionation path. It behaved as an inert phase during crystallization of the perthites, which grew as homogeneous monoclinic phases in equilibrium on the strain-free ternary solvus, defining a solvus isotherm at ～ 950° C. Both of the monoclinic phases exsolved on reaching the coherent ternary spinodal at temperatures estimated to be close to 800–850° C and 700–830° C respectively. Lamellar periodicities (529±149 nm and 148±18 nm respectively) are considerably finer scale than predicted by coarsening experiments, suggesting that An and/or Al-Si order greatly inhibit coarsening. The symplectite is a coarse, incoherent intergrowth of sodic andesine and nearly pure K-feldspar, probably produced by simultaneous crystallization at <400° C. The new data and literature analyses are used to construct the geometry of the ternary feldspar system. Solvus isotherms implied by existing experimental data approach the Ab apex too closely at high temperature. The critical solution curve becomes slightly more albitic after leaving the binary Ab-Or join, and then turns sharply towards the An-Or join. It intersects the proposed new limit of feldspar solid solution near Or36Ab44An20 at 1,060° C. This probably approximates to the highest temperature on the ternary critical curve at 1 bar. 相似文献
8.
Compositions of immiscible liquids in volcanic rocks 总被引：8，自引：5，他引：8
A. R. Philpotts 《Contributions to Mineralogy and Petrology》1982,80(3):201-218
Immiscible liquids, preserved as chemically distinct, glassy globules, (Si-rich and Fe-rich) occur in many tholeiitic basalts and some alkaline and calcalkaline lavas. The glasses typically form part of a dark mesostasis containing skeletal magnetite crystals. In thick flows, the Si-rich liquid may crystallize to granophyric patches, and the Ferich one to aggregates of hedenbergite, magnetite, and accessory phases. The mesostases containing these immiscible phases constitute from 20% of a primitive olivine tholeiite (MgO=7.5%) to 50% of a highly fractionated quartz tholeiite (MgO=2.8%), but may be less if the rock is oxidized. Abundant ferric iron promotes early crystallization of magnetite and prevents the iron enrichment necessary to reach the immiscibility field; thus, aa flows rarely exhibit immiscibility, whereas the more reduced pahoehoe ones do.Alumina and alkalis are concentrated in the Si-rich liquid, whereas the remainder of the major elements are concentrated in the Fe-rich melt; but the partitioning of Fe, Mg, Ca, and P is less pronounced in alkaline rocks than in tholeiites. Conjugate liquids have compositions of granite and Fe-rich pyroxenite, though the Si-rich melt in alkaline rocks is more syenitic and the Fe-rich one contains considerable normative alkali feldspar. The liquids coexist with plagioclase and augite of, respectively, An50 and Ca34Mg19Fe47 compositions in tholeiites, and An40 and Ca42Mg29Fe29 in alkaline rocks. Immiscibility is not restricted to K-rich residual liquids, but the miscibility gap is narrower for Na-rich compositions. In tholeiitic basalts with 52% SiO2, the Na2O/K2O ratios in conjugate liquids are equal, but at lower silica contents the Si-rich liquid is relatively more sodic, whereas at higher silica contents it is relatively more potassic. This may explain the association of sodic granites with mid ocean ridge basalts.Immiscible liquids are present in sufficient amounts in so many volcanic rocks that magma unmixing should be considered a viable means of differentiation during the late stages of fractionation of common magmas, at least at low pressures. 相似文献
9.
Ordering and exsolution processes in Or-rich alkali feldspar megacrysts from the Eldzhurtinskiy granite (Caucasus) 总被引：3，自引：0，他引：3
G. Witt-Eickschen C. Evangelakakis H. A. Seck H. Kroll A. G. Gurbanov 《Contributions to Mineralogy and Petrology》1996,124(1):71-81
The extremely young (2.5 Ma) I-type Eldzhurtinskiy granite complex (Central Caucasus) is uniform with respect to modal composition,
major and trace element chemistries of bulk rocks and mineral phases. In contrast, it reveals two types of alkali feldspar
megacrysts differing in tetrahedral Al-content (2t1) and exsolution microtextures:
1. Alkali feldspar megacrysts (Or70An2Ab28) from the top of the body consist of ideally coherent intergrowths of fine-scale regular Or- and Ab-rich lamellae. The exsolved
K-feldspar host is monoclinic (2t1=0.7), the exsolved Na-rich phase consists of Albite- and/or Pericline-twinned albite.
2. Megacrysts from greater depths have the same bulk composition, but the exsolved Ab-rich phase occurs in the form of optically
visible, broad lamellae and patches of low albite. In addition, the K-rich host yields a higher degree of (Al, Si) ordering
(2t1=0.8). The evolution of the distinct types of megacrysts reflects differences in the cooling history within the upper and
lower part of the granite body. The occurrence of the coherent lamellae in the megacrysts from the top of the body is attributed
to exsolution under dry conditions during fast cooling, whereas coarsening of lamellae and formation of albite patches in
the megacrysts from the lower part are caused by fluid-feldspar interaction. The transition zone in the body between the two
types of megacrysts is sharp (in a depth interval of 100–200 m) and not related to shear zones.
Received: 12 June 1995 / Accepted: 29 January 1996 相似文献
10.
The electron probe X-ray microanalyzer has been used to determine the compositional variability of the groundmass minerals and glass in 10 specimens from a complete 225-foot section of the prehistoric tholeiitic lava lake of Makaopuhi Crater, Hawaii. The order of beginning of crystallization was: (1) chromite, (2) olivine, (3) augite, (4) plagioclase, (5) pigeonite, (6) iron-titanium oxides and orthopyroxene, (7) alkali feldspar and apatite, and (8) glass.Although the lake is chemically tholeiitic throughout, the occurrence of ferromagnesian minerals is as though there were a gradation from alkali olivine basalt in the upper chill downwards to olivine tholeiite. Groundmass olivine decreases downwards and disappears at about 20 feet. Pigeonite is absent in the uppermost 5±2 feet, then increases in amount down to 20 feet, below which augite and pigeonite coexist in constant 21 proportions. Strong zoning and metastable compositions characterize the pyroxenes of the chilled zones, but these features gradually disappear towards the interior of the lake to give way to equilibrium pyroxenes. Relatively homogeneous poikilitic orthopyroxene ( Ca4Mg70Fe26) occurs in the olivine cumulate zone, having formed partly at the expense of pre-existing olivine, augite, and pigeonite ( Ca8Mg66Fe26). The growth of orthopyroxene is believed to have been facilitated by the slower cooling rate and higher volatile pressure at depth, and by the rise in Mg/Fe ratio of the liquid due to the partial dissolution of settled olivine.Unlike olivine and pyroxene, feldspar is least zoned in the upper and lower chilled regions. The greatest range of compositional zoning in feldspar occurs at 160 to 190 feet, where it extends continuously from Or1.0Ab22An77 to Or64Ab33An3. The feldspar fractionation trend in the An-Ab-Or triangle gradually shifts with depth toward more equilibrium trends, even though the zoning becomes more extreme. The variation with depth in the initial (core) composition of the plagioclase suggests the influence of either slow nucleation and growth (undercooling) or slow diffusion in the liquid, relative to the rate of cooling.Idiomorphic opaque inclusions in olivine phenocrysts are chrome-spinels showing continuous variation from 60 percent chromite to 85 percent ulvospinel and to magnetite-rich spinel. A pre-eruption trend of increasing Al with decreasing Cr can be recognized in chromites from the upper chill. Most of the inclusions show a trend of falling Cr and Al, toward an ulvospinelmagnetite solid solution which is progressively poorer in Usp with depth. This trend was produced by solid state alteration of the chromite inclusions during cooling in the lava lake. Ilmenite (average Ilm91Hm9) coexists with variably oxidized titaniferous magnetite in the basalt groundmass. Estimated oxygen fugacities agree well with other independent determinations in tholeiitic basalt. No sulfide phase has been detected.Fractional crystallization produced a groundmass glass of granitic composition. Average, in percent, is: SiO2, 75.5; Al2O3, 12.5; K2O, 5.7; Na2O, 3.1; CaO, 0.3; MgO, 0.05; total FeO, 1.2; and TiO2, 0.8. Normative Or> Ab. Minor changes in glass composition with depth are consistent with a greater approach towards the granite minimum. Incipient devitrification precluded reliable analysis of glass from the lower half of the section. The SiO2-phase associated with devitrification contains alkalis and Al and is believed to be cristobalite. Needle-like apatite crystals in the groundmass glass are Siand Fe-bearing fluorapatites containing appreciable rare earths (predominantly Ce) and variable Cl.The grain-size and maximum An content of the cores of plagioclase grains were controlled by cooling rate and are at a maximum at the center of the section. The most homogeneous pyroxene (and olivine, Moore and Evans, 1967), most equilibrium pyroxene trends, most abundant alkali feldspar, and most equilibrium feldspar trends are found at 160 to 190 feet, which is appreciably below that part of the lake which was slowest to crystallize. Volatile pressure, increasing with depth, possibly controlled the degree of attainment of equilibrium more than cooling rate.Since they are dependent on cooling history, some of the modal criteria commonly used for recognizing basalt types, such as the absence of Ca-poor pyroxene, presence of groundmass olivine, and the presence of alkali feldspar, should be applied with caution. Petrographic comparison of basalts from one flow, volcano, or province, with another, should recognize the possible variations due to cooling history alone.Publication authorized by the Director, U.S. Geological Survey 相似文献
11.
Two petrologically distinct alkali feldspar syenite bodies （AFS-1 and AFS-2） from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidio-morphic texture and consists of feldspar （Or55Ab43 to Or25Ab71）, ferro-pargasite/ferro-pargasite horn-blende, hastingsite, pyroxene （Wo47, En5, Fs46）, magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene （Wo47-50, En22-39, Fs12e31） set in a groundmass matrix of alkali feldspar （Or99Ab0.77 to Or1.33Ab98）, titanite and magnetite. In comparison to AFS-1, higher elemental concentra-tions of Ba, Sr and PREE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is w1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis. 相似文献
12.
Mohamed M. Hamdy Mohamed A. Abd El-Wahed Ismaeel A. Thabet 《Arabian Journal of Geosciences》2017,10(18):414
Dykes predominate within the Neoproterozoic rocks, especially granites, of Wadi El Redi-Wadi Lahami area in the southern Eastern Desert of Egypt. The dyke swarms form three major suites: from the oldest to the youngest, they are basaltic andesite—Suite 1 (E-W and ENE-WSW), rhyolite—Suite 2 (NE-SW), and andesite—Suite 3 (NNE-SSW, NNW-SSE, and NW-SE). Despite the wide ranges of the dyke compositions, the feldspar and amphibole are usually the essential forming minerals. The plagioclase arrays between Ab0.9An99.10 in the basaltic andesite and Ab98.80An0.70 in the rhyolite, while sanidine ranges from Or44.60Ab49.70 to Or98.40Ab1.60. Amphibole in Suite 1 and 3 (Al2O3, TiO2, Na2O, and K2O are the lowest and those of SiO2 and CaO are the highest) samples are usually magnesio-hornblende, whereas it is edenite and tschermakite in Suite 2 dykes. Despite all parent magmas have calc-alkaline affinity, some elements such as Ni show an erratic behavior against the progressing differentiation from one magma chamber and implying for an assimilation of the country rocks. The high contents of amphibole, the depletion in Ti, and the enrichment in large-ion lithophile elements (such as K, Rb, Ba, Sr, and Ba) compared to the primitive mantle composition are consistent with parent hydrous melts generated due to extension above the subduction zone. The estimated compositions of liquids in equilibrium with amphiboles and the pressures at which they crystallized (4.61–7.8 kbar for the Suite 2 and 1.5–2 kbar for the Suites 1 and 3) are greatly varied. These are indications for a difference in the source regions of the parent magmas of the studied dykes. It is supposed that the Suite 1 and 2 dykes are a conjugate set emplaced due to the NW-SE crustal extension in the Arabian-Nubian shield, whereas the Suite 3 dykes generated due to the rifting along the Red Sea. 相似文献
13.
I.S. Bazhan Shin Ozawa M. Miyahara E. Ohtani K.D. Litasov 《Russian Geology and Geophysics》2017,58(1):12-19
The Novosibirsk H5/6 ordinary chondrite has signs of shock metamorphism, such as dark shock-melt veins (SMVs) crossing the chondrite host rock. The plagioclase composition grains (Ab78An14Or7) with jadeite were found in the host-rock fragments inside the SMVs. Jadeite has an unusual radial-concentric spherulite-like microtexture. The spherulite-like jadeite formed from the molten plagioclase grain under high-pressure and high-temperature conditions during an impact event. The crystallization was accompanied by sodium-potassium differentiation between coexisting jadeite and residual melt. The PT-conditions of jadeite formation were estimated to be 3-14 GPa and 1400-2150 °C. Jadeite crystallization, Na-K differentiation, and the pressure-temperature estimates of jadeite formation in the Novosibirsk chondrite are very close to those in the Chelyabinsk LL5 chondrite. The spherulite-like microtexture and jadeite-glass coexistence, most likely, point to a high cooling rate of the SMVs at the pressure release stage of the metamorphic process. 相似文献
14.
Brajesh Singh Santosh Kumar Masao Ban Kazuo Nakashima 《Journal of Earth System Science》2016,125(7):1329-1352
Felsic magmatism in the southern part of Himachal Higher Himalaya is constituted by Neoproterozoic granite gneiss (GGn), Early Palaeozoic granitoids (EPG) and Tertiary tourmaline-bearing leucogranite (TLg). Magnetic susceptibility values (<3 ×10?3 SI), molar Al2 O 3/(CaO + Na2 O + K 2O) (≥1.1), mineral assemblage (bt–ms–pl–kf–qtz ± tur ± ap), and the presence of normative corundum relate these granitoids to peraluminous S-type, ilmenite series (reduced type) granites formed in a syncollisional tectonic setting. Plagioclase from GGn (An10–An31) and EPG (An15–An33) represents oligoclase to andesine and TLg (An2–An15) represents albite to oligoclase, whereas compositional ranges of K-feldspar are more-or-less similar (Or88 to Or95 in GGn, Or86 to Or97 in EPG and Or87 to Or94 in TLg). Biotites in GGn (Mg/Mg + Fet= 0.34–0.45), EPG (Mg/Mg + Fet= 0.27–0.47), and TLg (Mg/Mg + Fet= 0.25–0.30) are ferribiotites enriched in siderophyllite, which stabilised between FMQ and HM buffers and are characterised by dominant 3Fe\(\rightleftharpoons \)2Al, 3Mg\(\rightleftharpoons \)2Al substitutions typical of peraluminous (S-type), reducing felsic melts. Muscovite in GGn (Mg/Mg + Fet=0.58–0.66), EPG (Mg/Mg + Fet=0.31?0.59), and TLg (Mg/Mg + Fet=0.29–0.42) represent celadonite and paragonite solid solutions, and the tourmaline from EPG and TLg belongs to the schorl-elbaite series, which are characteristics of peraluminous, Li-poor, biotite-tourmaline granites. Geochemical features reveal that the GGn and EPG precursor melts were most likely derived from melting of biotite-rich metapelite and metagraywacke sources, whereas TLg melt appears to have formed from biotite-muscovite rich metapelite and metagraywacke sources. Major and trace elements modelling suggest that the GGn, EPG and TLg parental melts have experienced low degrees (～13, ～17 and ～13%, respectively) of kf–pl–bt fractionation, respectively, subsequent to partial melting. The GGn and EPG melts are the results of a pre-Himalayan, syn-collisional Pan-African felsic magmatic event, whereas the TLg is a magmatic product of Himalayan collision tectonics. 相似文献
15.
Origin of mantle (rapakivi) feldspars: experimental evidence of a dissolution- and diffusion-controlled mechanism 总被引：1，自引：0，他引：1
Experiments designed to simulate the dissolution of alkali feldspar during magma mixing produced plagioclase mantles that are texturally and compositionally similar to those in some hybrid volcanic rocks. In hydrous dacite melt (69% SiO2) at 0.8 GPa, 850°C, orthoclase (Or93) and sanidine (Or30) partially dissolved and were mantled by sodic plagioclase (An25–30). Although plagioclase nucleated epitaxially as a thin shell on the alkali feldspar surface near the time of initial resorption, plagioclase subsequently grew inward —mostly in the form of parallel blades — toaard the receding dissolution surface. Orthoclase dissolved at a rate approximately proportional to the square root of run duration, indicating diffusional control. Plagioclase grew inward within a static boundary zone of melt that formed between the original crystal-dacite interface and the dissolution surface. During orthoclase dissolution, this boundary zone rapidly and simultancously gained Na (by diffusion from dacite) and lost K (by diffusion into dacite); Ca diffused more slowly into this zone, from which non-feldspar species were mostly excluded. Plagioclase was stable where sufficient Ca had diffused in that the boundary zone melt intersected the plagioclase-saturation liquidus. Plagioclase subsequently grew toward the receding dissolution surface as the Ca compositional gradient (and hence the site of plagioclase saturation) stepped inward. Crystallization of plagioclase in the form of parallel blades allowed continued diffusive exchange of melt components between the dissolution surface and the host melt. Bladed growth also served to maintain (at blade tips) proximity of plagioclase to the dissolution surface, thereby apparently preserving (locally) a thin zone of low-variance melt. In natural systems, mantling of alkali feldspar by plagioclase will occur in a similar manner when (a) P, T, or X are changed to induce alkali feldspar dissolution, (b) sufficient Ca is available in the host melt to drive (by diffusion) boundary zone melt compositions to plagioclase saturation, and (c) temperatures are low enough to stabilize sodic plagioclase and to maintain a coherent boundary zone. These reqjirements are satisfied in volcanic systems when alkali feldspar is juxtaposed during mixing with hybrid melts of dacitic composition. Mantled feldspars in some intrusive systems (i.e., rapakivi granites) may form by a similar dissolution- and diffusion-controlled mechanism. Textural evidence of a similar origin may be obscurred in intrusive rocks, however, by products of late-stage magmatic and subsolidus processes. 相似文献
16.
The textural relationships and geochemistry of feldspars from least-altered to sericite-hematite altered and mineralised ~ 1.595 Ga Roxby Downs Granite (RDG) at Olympic Dam, South Australia, were examined. The sample suite is representative of RDG both distal (> 5 km) and proximal (< 1 km) to the hydrothermal breccias of the Olympic Dam Breccia Complex (ODBC), which host Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam. Microscopic observations and quantitative analyses indicate that a range of feldspar reactions have taken place within the RDG hosting the Olympic Dam deposit. An early phase of igneous plagioclase (~ An27–34) is recognised, along with a more abundant, less-calcic plagioclase (~ An12–20) both displaying rapakivi and anti-rapakivi textures with alkali feldspar. Alkali feldspars (~ Or55Ab43An2) record post-magmatic evolution from cryptoperthite to patch perthite. Subsequent patch perthite is overprinted by highly porous, near end-member albite and K-feldspar, while plagioclase undergoes replacement by albite + sericite ± Ba-rich K-feldspar. In sericite-hematite altered and mineralised RDG along the margin of the ODBC, sericite replaces all plagioclase, whereas red-stained, Fe-rich K-feldspar persists. Sulphide-uranium-rare earth element mineralisation is observed in association with hydrothermal feldspars, and increases in abundance with proximity to the orebody. Petrographic observations and whole-rock geochemistry illustrate the transformation of plagioclase and alkali feldspar from igneous to hydrothermal processes, and indicate that hydrothermal albite and K-feldspar formed within the RDG without the need for an external source of alkalis. Feldspar geothermometry indicates a minimum crystallisation temperature of 765 °C at 2.2 kbar for alkali feldspar (pressure estimate obtained using plagioclase-amphibole geobarometry) followed by a range of lower temperature transformations. Late-stage magma mixing/contamination is postulated from supportive temperature and pressure estimates along with feldspar and mafic mineral relationships. 相似文献
17.
Philip M. Orville 《Geochimica et cosmochimica acta》1975,39(8):1091-1105
Scapolite solid solution has been synthesized at 750°C and 4 kbar and is stable relative to plagioclase + calcite + halite over the range of plagioclase compositions from Ab85An15 to Ab70An30, although albite + halite is stable relative to marialite, Na4Al3Si9O24Cl, and anorthite + calcite is stable relative to meionite, Ca4Al6Si6O24CO3. A chloride-free scapolite, mizzonite, has been synthesized at the approximate composition NaCa3Al5Si7O24CO3 (Ab. 2An. CaCO3). In the absence of chloride, a three-phase invariant assemblage, sodic plagioclase (~Ab60An40) + scapolite + calcite is stable relative to plagioclase + calcite over the approximate range of plagioclase composition Ab60An40-Ab35An65 and another three-phase invariant assemblage, calcic plagioclase (~Ab15An85) + scapolite + calcite is stable over the approximate range Ab30An70-An15An85.Unit-cell dimensions and refractive indices have been determined for the scapotite synthesized in these experiments and are compared with values for chemically analyzed natural scapolites.Scapolite must be regarded as a ternary solid solution in which, at a given equivalent An-content, the Cl/CO3 ratio in the large anion site can vary as a function of NaCl and CaCO3 activities. 相似文献
18.
Eight-phase alkali feldspars: low-temperature cryptoperthite, peristerite and multiple replacement reactions in the Klokken intrusion 总被引：1，自引：0，他引：1
Ian Parsons John D. Fitz Gerald Matthew T. Heizler Lynn L. Heizler Tim Ivanic Martin R. Lee 《Contributions to Mineralogy and Petrology》2013,165(5):931-960
Eight feldspar phases have been distinguished within individual alkali feldspar primocrysts in laminated syenite members of the layered syenite series of the Klokken intrusion. The processes leading to the formation of the first four phases have been described previously. The feldspars crystallized as homogeneous sodian sanidine and exsolved by spinodal decomposition, between 750 and 600 °C, depending on bulk composition, to give fully coherent, strain-controlled braid cryptoperthites with sub-μm periodicities. Below ~500 °C, in the microcline field, these underwent a process of partial mutual replacement in a deuteric fluid, producing coarse (up to mm scale), turbid, incoherent patch perthites. We here describe exsolution and replacement processes that occurred after patch perthite formation. Both Or- and Ab-rich patches underwent a new phase of coherent exsolution by volume diffusion. Or-rich patches began to exsolve albite lamellae by coherent nucleation in the range 460–340 °C, depending on patch composition, leading to film perthite with ≤1 μm periodicities. Below ~300 °C, misfit dislocation loops formed, which were subsequently enlarged to nanotunnels. Ab-rich patches (bulk composition ~Ab91Or1An8), in one sample, exsolved giving peristerite, with one strong modulation with a periodicity of ~17 nm and a pervasive tweed microtexture. The Ab-rich patches formed with metastable disorder below the peristerite solvus and intersected the peristerite conditional spinodal at ~450 °C. This is the first time peristerite has been imaged using TEM within any perthite, and the first time peristerite has been found in a relatively rapidly cooled geological environment. The lamellar periodicities of film perthite and peristerite are consistent with experimentally determined diffusion coefficients and a calculated cooling history of the intrusion. All the preceding textures were in places affected by a phase of replacement correlating with regions of extreme optical turbidity. We term this material ultra porous late feldspar (UPLF). It is composed predominantly of regions of microporous very Or-rich feldspar (mean Ab2.5Or97.4An0.1) associated with very pure porous albite (Ab97.0Or1.6An1.4) implying replacement below 170–90 °C, depending on degree of order. In TEM, UPLF has complex, irregular diffraction contrast similar to that previously associated with low-temperature albitization and diagenetic overgrowths. Replacement by UPLF seems to have been piecemeal in character. Ghost-like textural pseudomorphs of both braid and film parents occur. Formation of patch perthite, film perthite and peristerite occurred 104–105 year after emplacement, but there are no microtextural constraints on the age of UPLF formation. 相似文献
19.
The Storm King granite at Bear Mountain, New York contains coarse alkali feldspar mesoperthite. The microstructure of these perthites grades continuously from lamellae to isolated blebs. The K rich phase in all samples has a nerely constant composition (Õr97Ab3), but the Na-rich phase ranges from An3.8 (lamellae) to An21 (blebs). It is suggested that the more calcium-rich feldspars exsolved at a higher temperature and thus experienced a longer time at higher temperature, during which the microstructure became more bleby or spherical in response to minimization of the interfacial energy. Lack of perfect correlation of the microstructure with bulk composition, as well as variation in the Al/Si ordering, may be due to additional factors, of which water activity or concentration is the most likely. 相似文献
20.
R.J. Floran Martin Prinz P.F. Hlava Klaus Keil B. Spettel H. Wänke 《Geochimica et cosmochimica acta》1981,45(12):2385-2391
The Johnstown meteorite is a brecciated orthopyroxenite (diogenite) containing coarsegrained centimeter-sized clasts of cumulate origin that have undergone subsolidus recrystallization. The brecciated portion is dominated by subangular fragments of orthopyroxene (Wo2–3En72–74Fs23–25) in a variably comminuted matrix of the same material. Minor and accessory phases include plagioclase (An82–90Ab10–18Or0–1), diopside (Wo44–45En46–47Fs9–10), olivine (Fo71–72), tridymite, troilite, metallic Ni-Fe (~3% Ni), and chromite (Cm71–80Hc1–8Sp11–19Mt2–4Uv1–3).The clastic component is parental to the brecciated matrix which contains no foreign lithic or mineralogic components. Siderophile trace element studies, however, reveal the presence of meteoritic (chondritic) contamination in the brecciated portion using unbrecciated clasts for indigenous values. Rare earth element abundances show a wide range of values for the light REE in different samples, although all samples exhibit a strong negative Eu anomaly, indicative of earlier plagioclase fractionation. Two pairs of adjacent brecciated and unbrecciated samples from different portions of the meteorite show, respectively, the most enriched and the most depleted light REE patterns. The variability in La content is over a factor of 100. However, in each case the REE pattern for the brecciated portion is very similar to that of the unbrecciated portion. These differences are attributed to sampling of variable amounts of residual, REE-enriched, trapped liquid. The most representative REE pattern for the bulk meteorite has an intermediate composition and was obtained from the largest sample. The data presented here indicate that Johnstown is a monomict breccia, in contrast to several other diogenites which may be considered to be polymict on the basis of their mineral compositions and/or clast populations. 相似文献