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1.
Thermoluminescence dating of potassium feldspar coarse grains from the fossiliferous Holsteinian marine formation of Herzeele, at the southern limit of the North Sea Basin, yields a minimum age of 228 £ 30 ka. A tentative correction for the long-term fading of the TL signal in potassium feldspar is proposed, assuming a lifetime (τ) of 711 ka. This correction would yield a preliminary age estimate of 271 £ 36 ka. These results support a minimum Oxygen Isotope Stage 7 age, but do not preclude, at this stage of our research, a possible correlation with late Oxygen Isotope Stage 9. These results are in agreement, within the error of the method, with independent absolute dates (U-Th and ESR) obtained on in situ marine shells, which range from 225 ka to 348 ka. This study provides evidence of the reliability of TL age estimates on potassium feldspar coarse grains from shallow-marine sediments older than the Last Interglacial.  相似文献   

2.
The grain‐scale spatial arrangement of melt in layer‐parallel leucosomes in two anatectic rocks from two different contact aureoles located in central Maine, USA, is documented and used to constrain the controls on grain‐scale melt localization. The spatial distribution of grain‐scale melt is inferred from microstructural criteria for recognition of mineral pseudomorphs after melt and mineral grains of the solid matrix that hosted the melt. In both rocks, feldspar mimics the grain‐scale distribution of melt, and quartz is the major constituent of the solid matrix. The feldspar pockets consist of individual feldspar grains or aggregates of feldspar grains that show cuspate outlines. They have low average width/length ratios (0.54 and 0.55, respectively), and are interstitial between more rounded and equant (width/length ratios 0.65 for both samples) quartz grains. In two dimensions, the feldspar pockets extend over distances equivalent to multiple quartz grain diameters, possibly forming a connected three‐dimensional intergranular network. Both samples show similar mesoscopic structural elements and in both samples the feldspar pockets have a shape‐preferred orientation. In one sample, feldspar inferred to replace melt is aligned subparallel to the shape‐preferred orientation of quartz, indicating that pre‐ or syn‐anatectic strain controlled the grain‐scale distribution of melt. In the other sample, the preferred orientation of feldspar inferred to replace melt is different from the orientations of all other mesoscopic or microscopic structures in the rock, indicating that differential stress controlled grain‐scale melt localization. This is probably facilitated by conditions of higher differential stress, which may have promoted microfracturing. Grain‐scale melt distribution and inferred melt localization controls give insight into possible grain‐scale deformation mechanisms in melt‐bearing rocks. Application of these results to the interpretation of deep crustal anatectic rocks suggests that grain‐scale melt distribution should be controlled primarily by pre‐ or syn‐anatectic deformation. Feedback relations between melt localization and deformation are to be expected, with important implications for deformation and tectonic evolution of melt‐bearing rocks.  相似文献   

3.
The equilibrium temperatures for coexisting plagioclase and potassium feldspar pairs have been calculated for various textural varieties of feldspar from 3 post-metamorphic granites from the Georgia Piedmont; the Danburg, Siloam, and Stone Mountain plutons. Assuming an intermediate structural state for the feldspars at time of equilibration, crystallization temperatures match those expected from experimental data for quartz monzonite magmas (650 to 780° C). The variations in solidus temperature, recorded in the feldspars, may be used to estimate relative differences in depth of intrusion. Sharp reversals in plagioclase compositional trends may be caused by isothermal decreases in confining pressure associated with upward migration through the crust. In fine grained and slowly cooled intrusions, albite tends to be lost from the alkali feldspar grains, and recrystallizes as separate unzoned grains of oligoclase, thus erasing the previous thermal history. Perthite exsolution and re-equilibration within the alkali feldspar grains appears to continue down to temperatures of 400° C or so, although the zoned plagioclase does not homogenize. The recrystallization associated with changes in structural state may facilitate exsolution within alkali feldspar grains.  相似文献   

4.
Adamellites, granite porphyries, and white granites of the western dome of the Arga-Ynnykh-Khaya Massif (Yakutiya) have been studied with emphasis on the white granites. The main features of its magmatic crystallization and initial transformation have been obtained by investigating the intensity of transformations using stochastic Markov models. The most widespread transformation is albitization. Two varieties exist. First, and most common, is development of a secondary albite crystal between two primary grains of potassium feldspar. Second, secondary albite has developed between primary quartz grains. Both variants have definite probabilities of occurrence. Less commonly, but with a constant probability, a secondary quartz grain forms as an intergrowth between two grains of potassium feldspar.  相似文献   

5.
徐海军  张超  武云  陶明 《地球科学》2016,41(9):1511-1525
文象花岗岩具有特殊文象结构,研究其三维拓扑结构和形成过程有助于了解花岗质岩石的结晶作用.以北京周口店房山岩体和湖北罗田蕙兰山的文象花岗岩为研究对象,综合利用光学显微镜、扫描电镜、电子探针和电子背散射衍射等技术方法,对岩石矿物组成、结晶学取向和拓扑结构进行了系统研究.结果表明:(1) 文象花岗岩的矿物组成与其形成地质环境有关,石英和长石的含量变化范围很大,其中石英含量通常在20%~45%,但是相同地区同期形成的文象花岗岩具有相对稳定的矿物组成;(2) 长石作为寄主矿物通常呈半自形－自形粗大晶体,可以是碱性长石或斜长石,其端元组分以钾长石和钠长石为主,低温下常分解为条纹长石;(3) 石英在长石寄主矿物中规则穿插生长,在三维空间通常呈近似平行板状、长条状/柱状或非连通枝杈状,并只在特定岩石断面形似象形文字;(4) 正交偏光显微镜下,石英可以具有多种消光位,但是通常在一定范围内同时消光;(5) 石英普遍发育道芬双晶,偶见日本双晶;(6) 条纹长石中钾长石与钠长石对应(100)、(010)、(001) 面和[001]轴近似平行;(7) 多数石英颗粒与寄主长石之间具有密切结晶学取向关系,即石英[1123]轴近似平行长石c[001]轴.该研究证实文象花岗岩是石英和长石同时生长的结果,而长石作为寄主矿物影响并控制着石英的成核与生长方向.   相似文献   

6.
钾长石中的铅及其对成矿的贡献   总被引：5，自引：0，他引：5  
在中-酸性岩浆岩、碱性岩浆岩、片麻岩及混合岩等岩石类型中,钾长石是含铅最高的造岩矿物,其铅含量是全岩铅含量的2～10倍,云母类矿物铅含量的3～16倍,石英铅含量的6～32倍.早元古代、太古宙岩石中钾长石含量低,钾长石中的铅占全岩铅含量的比例低于10%,中元古代以来的碱性岩浆岩、富含钾长石的花岗岩和变质岩,钾长石含量增高,其中所含的铅占全岩铅的比例明显增大,钾长石含量达50%～70%的碱性岩,钾长石中的铅约占全岩总铅量的70%～95%.钾长石在后期极易发生水热蚀变,转变为绢云母、方解石、石英等,在这种转变过程中,铅大量析出进入流体相.这种变化能够为后期铅的成矿提供成矿物质.  相似文献   

7.
Microstructural observations of naturally faulted granitic gneisses show that feldspar grains are weaker than quartz grains at temperatures below 325γC and at depths of less than 8–10 km. Feldspar grains sustained most of the deformation by grain-scale faulting, slip along cleavage-controlled fractures and cataclasis. Fracture of feldspar grains within fault zones promoted their alteration to kaolinite. Quartz grains also deformed by fracturing, and often healed to form quartz porphyroclasts and mosaics in a comminuted matrix of feldspar and kaolinite. Syntectonic alteration of the feldspar grains may have weakened the fault zones over time and resulted in foliated textures within the fault zones. This study of naturally deformed rocks confirms published experimental results on the behavior of granitic rocks at low temperatures and pressures and, taken together, these data show that the theology of the upper 10 km of the crust is greatly influenced by cataclastic processes in feldspar.  相似文献   

8.
Myrmekite, as defined here, is the microscopic intergrowth between vermicular quartz and modestly anorthitic plagioclase (calcic albite-oligoclase), intimately associated with potassium feldspar in plutonic rocks of granitic composition. Hypotheses previously invoked in explanation of myrmekite include: (1) direct crystallization; (2) replacement; (3) exsolution. The occurrence of myrmekite in paragneisses and its absence in rocks devold of discrete grains of potassium feldspar challenge those hypotheses based on direct crystallization or replacement. However, several lines of evidence indicate that myrmekite may in fact originate in response to kinetic effects associated with the exsolution of calcic alkali feldspar into discrete potassium feldspar and plagioclase phases. Exsolution of potassium feldspar system projected from [AlSi2O8] involves the exchange CaAlK-1Si-1, in which the AlSi-1 tetrahedral couple is resistant to intracrystalline diffusion. By contrast, diffusion of octahedral K proceeds relatively easily where it remains uncoupled to the tetrahedral exchange. We suggest here that where the ternary feldspar system is open to excess silica, the exchange reaction that produces potassium feldspar in the ternary plane is aided by the net-transfer reaction K+Si=Orthoclase, leaving behind indigenous Si that reports as modal quartz in the evolving plagioclase as the CaAl component is concomitantly incorporated in this same phase. Thus silica is pumped into the reaction volume from a silica reservoir, a process that enhances redistribution of both Si and Al through the exsolving ternary feldspar.  相似文献   

9.
Melt infiltration into quartzite took place due to generation and migration of partial melts within the high‐grade metamorphic rocks of the Big Cottonwood (BC) formation in the Little Cottonwood contact aureole (UT, USA). Melt was produced by muscovite and biotite dehydration melting reactions in the BC formation, which contains pelite and quartzite interlayered on a centimetre to decimetre scale. In the migmatite zone, melt extraction from the pelites resulted in restitic schollen surrounded by K‐feldspar‐enriched quartzite. Melt accumulation occurred in extensional or transpressional domains such as boudin necks, veins and ductile shear zones, during intrusion‐related deformation in the contact aureole. The transition between the quartzofeldspathic segregations and quartzite shows a gradual change in texture. Here, thin K‐feldspar rims surround single, round quartz grains. The textures are interpreted as melt infiltration texture. Pervasive melt infiltration into the quartzite induced widening of the quartz–quartz grain boundaries, and led to progressive isolation of quartz grains. First as clusters of grains, and with increasing infiltration as single quartz grains in the K‐feldspar‐rich matrix of the melt segregation. A 3D–μCT reconstruction showed that melt formed an interconnected network in the quartzites. Despite abundant macroscopic evidence for deformation in the migmatite zone, individual quartz grains found in quartzofeldspathic segregations have a rounded crystal shape and lack quartz crystallographic orientation, as documented with electron backscatter diffraction (EBSD). Water‐rich melts, similar to pegmatitic melts documented in this field study, were able to infiltrate the quartz network and disaggregate grain coherency of the quartzites. The proposed mechanism can serve as a model to explain abundant xenocrysts found in magmatic systems.  相似文献   

10.
Comparison of observed microstructures and deduced physical parameters of natural deformations with those encountered in superplastic metals permits identification of superplastic behaviour in rocks. From microstructural evidence, superplastic flow of feldspar is inferred to have occurred in the deformation of a granite from Miéville, Aiguilles-Rouges massif, Switzerland. The original igneous orthoclase and oligoclase phenocrysts have both recrystallised to fine-grained albite which has deformed by superplastic flow. This change in crystal chemistry was associated with a large negative change in free energy, enabling recrystallised grains of a very small size to be stable under the prevailing conditions, thereby producing the pre-requisite conditions for superplastic behaviour.  相似文献   

11.
Naturally weathered feldspar surfaces in the Jurassic Navajo Sandstone at Black Mesa, Arizona, was characterized with high-resolution transmission and analytical electron microscope (HRTEM-AEM) and field emission gun scanning electron microscope (FEG-SEM). Here, we report the first HRTEM observation of a 10-nm thick amorphous layer on naturally weathered K-feldspar in currently slightly alkaline groundwater. The amorphous layer is probably deficient in K and enriched in Si. In addition to the amorphous layer, the feldspar surfaces are also partially coated with tightly adhered kaolin platelets. Outside of the kaolin coatings, feldspar grains are covered with a continuous 3-5 μm thick layer of authigenic smectite, which also coats quartz and other sediment grains. Authigenic K-feldspar overgrowth and etch pits were also found on feldspar grains. These characteristics of the aged feldspar surfaces accentuate the differences in reactivity between the freshly ground feldspar powders used in laboratory experiments and feldspar grains in natural systems, and may partially contribute to the commonly observed apparent laboratory-field dissolution rate discrepancy. At Black Mesa, feldspars in the Navajo Sandstone are dissolving at ∼105 times slower than laboratory rate at comparable temperature and pH under far from equilibrium condition. The tightly adhered kaolin platelets reduce the feldspar reactive surface area, and the authigenic K-feldspar overgrowth reduces the feldspar reactivity. However, the continuous smectite coating layer does not appear to constitute a diffusion barrier. The exact role of the amorphous layer on feldspar dissolution kinetics depends on the origin of the layer (leached layer versus re-precipitated silica), which is uncertain at present. However, the nanometer thin layer can be detected only with HRTEM, and thus our study raises the possibility of its wide occurrence in geological systems. Rate laws and proposed mechanisms should consider the possibility of this amorphous layer on feldspar surface.  相似文献   

12.
The easternmost part of the Neoproterozoic Araçuaí belt comprises an anatectic domain that involves anatexites (the Carlos Chagas unit), leucogranites and migmatitic granulites that display a well-developed fabric. Microstructural observations support that the deformation occurred in the magmatic to submagmatic state. Structural mapping integrating field and anisotropy of magnetic susceptibility (AMS) revealed a complex, 3D structure. The northern domain displays gently dipping foliations bearing a NW-trending lineation, southward, the lineation trend progressively rotates to EW then SW and the foliation is gently folded. The eastern domain displays E–W and NE–SW trending foliations with moderate to steeply dips bearing a dominantly NS trending lineation. Magnetic mineralogy investigation suggests biotite as the main carrier of the magnetic susceptibility in the anatexites and ferromagnetic minerals in the granulites. Crystallographic preferred orientation (CPO) measurements using the electron backscatter diffraction (EBSD) technique suggest that the magnetic fabric comes from the crystalline anisotropy of biotite and feldspar grains, especially. The delineation of several structural domains with contrasted flow fabric suggests a 3D flow field involving westward thrusting orthogonal to the belt, northwestward orogen-oblique escape tectonics and NS orogen-parallel flow. This complex deformation pattern may be due to interplay of collision-driven and gravity-driven deformations.  相似文献   

13.
非水溶性钾矿制取碳酸钾:副产硅铝胶凝材料   总被引：1，自引：0，他引：1  
白云鄂博稀土-铌-铁矿床上部围岩产富钾板岩, 其K2O平均含量达13.0%, 钾资源储量巨大.矿石的物相组成以微斜长石、黑云母为主, 是一种重要的非水溶性钾矿资源.实验表明, 以碳酸钠为助剂, 经中温烧结, 矿石分解率达98.2%以上.烧结物料中K2O的浸出率约70%, 且在水浸酸化反应过程中, 大部分Fe3+、Ti4+、Mn2+、Mg2+、Ca2+等杂质离子与硅铝质胶体同时沉淀析出, 为制取电子级碳酸钾提供了可能.硅铝质胶体滤渣用于制备矿物聚合材料.采用本项技术开发利用此类非水溶性钾矿资源, 符合节能高效和“清洁生产”的要求, 兼有规模化经济效益和良好的环境效益.   相似文献   

14.
综述了有关长石的碎裂流动、位错蠕变、扩散蠕变、颗粒边界滑动及超塑性流动的特点及有关现象,并指出了长石塑性变形机制研究的重要意义。  相似文献   

15.
Sandstones of early Neoproterozoic Chandarpur Group, Chattisgarh Supergroup, central India display progressive change towards greater textural and mineralogical maturity from base to top of the succession. The clay-silt matrix decreases, sorting of sand grains improves, frequency of rounded grains increases, monocrystalline quartz content increases with concomitant decrease in polycrystalline quartz, feldspar and rock fragments. The trend of variations in different mineralogical and textural attributes, however, exhibits inflections at different stratigraphic levels. The sandstones of the basal Lohardih Formation are alluvial fan deposits, characterized by high matrix and feldspar content, iron-oxide impregnated highly angular grains and poor sorting. Petrographic properties collectively indicate that the sandstones were derived from a weathered granitic crust under a humid climatic condition. Abundance of well rounded grains within the alluvial fan and overlying braided fluvial deposit indicates prolonged wind action during episodes of high aridity. The shallow marine deposit overlying the fluvial deposits in the upper part of the Lohardih Formation exhibits bed-to-bed variation in the frequency of angular grains, feldspar content and overall maturity suggesting environmentally controlled segregation of sediments. The abrupt appearance of coarse-grained immature sandstones with concomitant reappearance of iron-oxide impregnated/altered feldspar grains in the upper part of the shelf deposits of the Chaporadih Formation point to a phase of tectonic uplift that possibly triggered a regression. Continued regression and peneplanation heralded the deposition of supermature medium-grained purple quartzarenite of the upper shoreface Kansapathar Formation in the uppermost part of the Chandarpur succession under a hot desertic climatic condition. The provenance analysis revealed that the Chandarpur clastics were derived from granites and granite-gneisses of a continental block tectonic provenance. Petrographic studies further indicate that high grade metamorphic rocks did not make any perceptible contribution to the Chandarpur system. The Eastern Ghats Granulite Belt apparently did not emerge till the early Neoproterozoic.  相似文献   

16.
非水溶性钾矿制取碳酸钾研究:副产13X型分子筛   总被引：15，自引：0，他引：15  
非水溶性钾矿通常以钾长石为主要矿物相。对其进行粉碎、摇床重选、湿法磁选和化学酸浸除铁等预处理,可制得纯度达75%~95%的钾长石粉体。以碳酸钠为助剂,在760~860 ℃的中温下钾长石发生固相分解反应,生成偏硅酸钠、偏铝酸钾(钠)前体化合物,用于水热合成13X 沸石分子筛。滤液为富含[SiO2(OH)2]2-、Na+、K+的碱性溶液,通入CO2 进行酸化反应,生成SiO3·nH2O胶体沉淀,经250 ℃下煅烧即制成白炭黑。剩余滤液为NaHCO3 KHCO3 H2O三元水盐体系,再经浓缩、分离、纯化、结晶,制取碳酸钠和碳酸钾。由此,原矿中SiO2、Al2O3、K2O 3 种主要组分均可制成高值产品,可达到钾长石资源利用率接近100%、“三废”近于零排放的高效节能、清洁生产的“绿色过程”要求。本项技术实现规模化工业生产,将有利于缓解我国水溶性钾盐资源极缺的矛盾,平衡钾盐市场,提高国民经济可持续发展中钾盐资源的保证程度。  相似文献   

17.
Crystallization of rapakivi is accompanied by a progressive decrease in the alkali content of the high-temperature granitic magma and by the concurrent increase in its iron content, during the crystallization of potassium feldspar which is followed by the crystallization of biotite. The pressure is not high enough to permit a simultaneous crystallization of potassium feldspar, as e. g. of albite. The potassium feldspar-biotite sequence is upset by Mg, in the ovoid varieties of rapakivi in which the potassium feldspar crystallization is arrested, short of its completion, by the crystallization of Mg biotite. – IGR Staff.  相似文献   

18.
ABSTRACT Detrital alkali feldspars currently at burial depths of 3·2–3·5 km in the Upper Jurassic Humber Group of the Fulmar oilfield, UK North Sea, are overgrown and have been partially replaced by authigenic Or‐rich feldspar. Intracrystal microtextures suggest several different provenances for the detrital grains. The overgrowths are uniformly non‐cathodoluminescent and have occasional celsian‐rich zones. Transmission electron microscopy shows that they are composed of a microporous mosaic of subµm‐ to µm‐sized sub‐grains associated with barite, illite and pyrite. The subgrains are somewhat rounded but have an approximate {110} Adularia habit and display a faint modulated microtexture on the nanometre scale. They have triclinic symmetry, but the lattice angles depart only slightly from monoclinic symmetry. These features are characteristic of K‐feldspar precipitated relatively rapidly and at low temperature. Authigenic Or‐rich feldspar has also partially replaced microcline and perthitic albite within the detrital grains, often at a suboptical scale. Although, like diagenetic albitization, replacement by K‐feldspar is probably a very common diagenetic reaction, it has rarely been reported owing to difficulties in imaging the diagnostic textures with the scanning electron microscopy techniques used by most workers. The permeability of the subgrain microtexture may significantly hinder the use of feldspar overgrowths for K/Ar and 40Ar/39Ar dating of diagenesis, and the existence of suboptical, replacive authigenic K‐feldspar within detrital grains may significantly modify the apparent Ar ages of detrital grains. Similar subgrain microtextures in optically featureless quartz overgrowths are also illustrated.  相似文献   

19.
The shallow marine Precambrian Pakhal sandstones (Middle Proterozoic, about 1400 Ma) of the Godavari valley are composed of first-cycle terrigenous grains derived from the crystalline Basement Complex. The sandstones include a large number of rounded and well rounded grains of quartz and feldspar. The rounded grains, without exception, occur in intimate association with angular grains. The sandstones are also characterised by several other types of textural inversions.Relative effectiveness of several processes with regard to the development of roundness of the Pakhal sands has been considered. Simple sedimentary differentiation fails to explain the high degree of roundness of the first-cycle sands. Solutions rich in organic matter cannot be considered effective in rounding Proterozoic sands. Chemical action is inconsistent with the presence of fresh grains of feldspar. Beach processes also fail to explain the presence of large numbers of feldspar grains. The roundness of these sands is best explained by eolian processes, that can effectively round grains of quartz as well as feldspar.The textural inversions of the Pakhal sandstones can be attributed neither to mixing of sands derived from multiple sources nor to mixing of materials coming from different environments. They possibly resulted from mixing of sands with contrasting mechanisms of transport, viz., eolian and aqueous, in a common area of sedimentation.Eolian transport and abrasion processes probably played a more significant role during the time of Pakhal sedimentation than at present, because of the absence of protective vegetation in Proterozoic time.  相似文献   

20.
Sanidine grains (100–600 μm in diameter) were subjected to dissolution at 82°C in aqueous electrolyte solutions of pH ranging from 4 to 8 for 293 or 377 hr. Dissolution equivalent to the removal of silica from the outer 300–900 A of these grains was accomplished. The shallow subsurfaces of feldspar grains were then analyzed for K, Al, and Si by X-ray photoelectron spectroscopy. The results rule out any continuous precipitate layer; if an alkali-depleted subsurface zone (leached layer) was present in the feldspar, the thickness of such a zone approximated by linear increase of alkali concentration with depth was not more than about 17 Å.It is concluded that in the absence of a compact precipitate layer, dissolution of feldspars in the temperature range corresponding to deep diagenesis is controlled by the processes at the feldspar-solution interface and a leached layer more than one feldspar unit cell thick does not form. Whether the same applies at the temperatures of shallow diagenesis and weathering cannot be judged with certainty, but parallels with leached layers on alkali silicate glasses suggest that it does.  相似文献   

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