ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00555.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00556.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractSpecimen thickness is the main experimental factor controlling the results of illite crystallinity (IC) or crystallite size measurements on sedimentation slides. Different values obtained from thick and thin preparations are due to grain‐size gradation effects, which may exclude larger and higher ordered grains from contributing to the diffraction. Orientation effects control the measured peak intensity. The change from poor particle orientation in thick slides to high orientation in very thin slides is marked by an increase in specimen density, diminishing non‐basal reflections, and by a strong increase in peak intensity. A plateau with constant peak breadths is observed if thin slides of well ordered, platy illites are used. A similar plateau can be recognized for thick preparations of specimens from less ordered materials, but not from well ordered ones. Therefore, it is suggested that IC is determined on very thin sedimentation slides with a thickness of 0.25 mg/cm2or less. Ultrasonic and H2O2treatments enhance the degree of particle orientation by destruction of grain aggregates and organic compounds, leading to smaller peak breadths and higher intensit

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00557.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractIn well NJ‐15 of the Nesjavellir geothermal field, Iceland, the transition of discrete smectite into discrete chlorite has been studied from drill cuttings recovered at depths of less than 1714 m and over a continuous range of temperatures between 60 and 300° C. At temperatures below 180° C, the clay fractions contain mixtures of di‐ and trioctahedral smectites, whose layer charge increases with depth. Between 200 and 240° C, discrete smectites have transformed into smectite‐rich, randomly interstratified chlorite and smecite (R0 C/S). Because the abundance of chlorite interlayers in this C/S is generally<20%, its presence can be detected only by electron microprobe techniques and not by X‐ray diffraction. Between 245 and 265° C, both regularly (R1) and randomly interstratified C/S are the predominant layer silicates. Discrete chlorite first appears at approximately 270° C and coexists with minor amounts ofR0 C/S at higher temperatures.R0 andR1 C/S form a nearly complete compositional series between trioctahedral saponite and discrete chlorite end‐members. The interlayer cation and Si content of smectites and C/S decrease with increasing temperature. The Mg/(Mg + Fe) content of smectite, C/S, and chlorite is unrelated to temperature. The percentage of chlorite in C/S, as determined by electron microprobe analyses, increases continuously with increasing temperature, except for occurrences of smectite‐rich C/S in fresh basaltic dykes which have not thermally equilibrated with the higher gra

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00558.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractFive basalt samples from the Point Sal ophiolite, California, were examined using HRTEM and AEM in order to compare observations with interpretations of XRD patterns and microprobe analyses. XRD data from ethylene‐glycol‐saturated samples indicate the following percentages of chlorite in mixed‐layer chlorite–smectite identified for each specimen: (i) L2036 ± 50%, (ii) L2035 ± 70 and 20%, (iii) 1A‐13 ± 70%, (iv) 1B‐42 ± 70%, and (v) 1B‐55 = 100%. Detailed electron microprobe analyses show that ‘chlorite’analyses with high Si, K, Na and Ca contents are the result of interlayering with smectite‐like layers. The Fe/(Fe + Mg) ratios of mixed‐layer phyllosilicates from Point Sal samples are influenced by the bulk rock composition, not by the percentage of chlorite nor the structure of the phyllosilicate.Measurements of lattice‐fringe images indicate that both smectite and chlorite layers are present in the Point Sal samples in abundances similar to those predicted with XRD techniques and that regular alternation of chlorite and smectite occurs at the unit‐cell scale. Both 10‐ and 14‐Å layers were recorded with HRTEM and interpreted to be smectite and chlorite, respectively. Regular alternation of chlorite and smectite (24‐Å periodicity) occurs in upper lava samples L2036 and 1A‐13, and lower lava sample 1B‐42 for as many as seven alternations per crystallite with local layer mistakes. Sample L2035 shows disordered alternation of chlorite and smectite, with juxtaposition of smectite‐like layers, suggesting that randomly interlayered chlorite (<0.5)–smectite exists. Images of lower lava sample 1B‐55 show predominantly 14‐Å layers. Units of 24 Å tend to cluster in what may otherwise appear to be disordered mixtures, suggesting the existence of a corr

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00559.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractMafic phyllosilicates in metabasites affected by low‐grade regional metamorphism from Wales and eastern North Greenland show variations in their structure and chemistry. These variations are related to four mineral zones in these metabasites, which are recognized on the presence/absence of various key calc‐silicate minerals and also actinolite. Zones 1 and 2 equate with the zeolite facies, zone 3 with the prehnite–pumpellyite facies (or prehnite–actinolite facies in rocks with appropriate bulk rock composition) and zone 4 with the greenschist facies. Whilst variations in Fe/(Fe + Mg) in chlorite correlate closely with Fe/(Fe + Mg) ratios in the whole‐rock, other chemical variations are clearly unrelated to whole‐rock compositions. Contents of Alivare seen to increase systematically in samples from zone 1 through to zone 4, which relate to an increase in temperature. Calibration of alteration temperatures, calculated using the chlorite geothermometer (based on Alivcontents) developed for meta‐andesites in the Los Azufres geothermal system (Mexico), againstxvalues (an estimate of the proportion of chlorite to swelling component in the mafic phyllosilicates) shows a decrease in the swelling component in passing from zone 1 to zone 4, i.e. with an increase in temperature. Calculated temperatures compare favourably with published stability estimates for the various key calc‐silicates and actinolite. These data indicate that the chlorite geothermometer, although developed for meta‐andesites from a hydrothermal system, does show a correlation with temperatures estimated from calc‐silicate assemblages in metabasites affected by low‐grade metamorphism develope

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00560.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractFairly strong (r= 0.75–0.85) positive linear correlations were found between crystallinity indices (peak widths) measured on the first two basal reflections of chlorite and those of illite–muscovite in<2‐μm fractions of a representative shale–slate–phyllite series from Palaeozoic and Mesozoic formations of northeast Hungary. The metamorphic grade ranges from late or deep diagenesis through anchizone to epizone conditions. Chlorite crystallinity values measured on air‐dried and ethylene‐glycol‐solvated samples suggest that the effects of expandable interlayers are negligable, especially in the higher grade (∼temperature) part of the series. However, the greater scattering of crystallinity values for the chlorite 001 reflection compared to those of the 002 reflection may be related to the effects of minor amounts of interlayered and/or discrete smectite and/or vermiculite. With increasing metamorphic grade and advancing equilibrium recrystallization, the chlorite compositions in different samples become more homogenous. No correlation exists between crystallinity and changes in chlorite composition as estimated from the intensity ratios of basal reflections. Hence an increase of domain size and a decrease of lattice distortion with increasing grade (∼temperature) may be decisive factors affecting chlorite crystallinity.Chlorite crystallinity can be applied as a reliable regional, statistical technique complementary with, or instead of, the illite crystallinity method. The illite and chlorite crystallinity scales used here are related to Kübler's epi‐, anchi‐ and diagenetic zones and correlated with coal rank, conodont colour alteration and mineral facies data. As the effects of the detrital white mica can be observed even in the<2‐μm fractions of anchizonal metapelites, the anchizone boundaries determined solely on the base of ‘fixed’illite crystallinity values may vary with amounts of detrital and newly formed muscovite–illite. Hence a complex approach utilizing more than one method for determination of grade is preferred for petrogenetic purposes, even if relationships between crystallinity scales, coal rank and mineral facies also vary strongly in different

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00561.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractIllite crystallinity (IC) and other indicators of the grade of very‐low‐grade metamorphism associated with the appearance of various stages of slaty cleavage in phyllosilicate‐rich rocks have been compiled from a wide variety of terranes. IC values have been converted to a Kübler‐equivalent standard scale, but the diverse characterizations of the cleavage fabrics in published descriptions do not always allow an unequivocal identification of equivalent stages of cleavage development.Nevertheless, there exists a distinct relationship between grade and the appearance of various stages of cleavage development.(1) Indications of incipient slaty cleavage, such as S0–S1 pencil structure, appearance of primary (S1) crenulation cleavage and of closely‐spaced cleavage without parallel fabric in the microlithons, is associated with a wide range of mostly medium‐ and high‐grade diagenetic IC values.(2) The appearance of smooth cleavage with a strong parallel fabric in the microlithons and/or quartz–mica ‘beards’and the chlorite–mica stacks shortened at a high angle to (001), and of irregular cleavage in sandy beds is associated with a much narrower range of predominantly low‐ and medium‐grade anchimetamorphic grades (rarely high‐grade diagenetic). The first appearance of these stages of cleavage development with higher grades can often be related to post‐kinematic magmatic heating, polymetamorphism (pre‐cleavage metamorphism), or ‘static’recrystallization without cleavage formation, for example in low‐strain zones.There exists a relationship between finite strain, fabric and metamorphic grade in mudstones and slates; in coarser clastic rocks the same finite strain–fabric relationship occurs at appreciably higher grades. A relationship between finite strain in carbonate rocks and IC in the nearby rocks has been reported from the Helvetic zone of the Swiss Alps.The earlier stages of cleavage formation are associated with little improvement in IC; the narrow range of IC associated with smooth cleavage is concluded to represent recrystallization and grain

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00562.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractRegional metamorphism in the external Variscides of southwest England varied from diagenetic level to greenschist facies. There is a fundamental difference in the metamorphic character between the northern and southern regions of the area. In the north, M1 metamorphism is of a sedimentary burial character associated with high heat flow, whilst to the south it is related to tectonic burial during thrust thickening processes, with lower geothermal gradients. This pattern appears to be related to the character of basin development and its subsequent tectonic evolution. The northern region has features that accord with a diastathermal (extensional) origin for the very low‐grade metamorphism whilst in the southern region the very low‐grade metamorphism is linked to thrusting as a consequence of Variscan compression. The Tintagel High‐Strain Zone presents an anomaly in this regional pattern where an M2 metamorphic phase is attributed to localized D2 thrust stacking along the southern margin of the Culm Basin.There is no extensive overprint of the regional metamorphic pattern by the contact aureoles surrounding the granite plutons of the region. However, there is a noticeable coincidence between the areas of regional epizone grade and the extent of the geophysically defined subsurface limit of the granite batholith (excluding the North Devon area). This link is attributed largely to the late‐stage structural up‐doming of the higher grade areas over the roof of the

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00563.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractWhite mica crystallinity studies have been carried out on 90 samples of mudrocks, six of spotted slate, and five of accretionary lapilli tuff from the area around the Berwyn Hills, North Wales. Strain was measured for some of the spotted slate and tuff samples. The metamorphic grade increases from southeast to northwest, with values of the Kübler index varying from 0.64 to 0.20Δ2θ. Metamorphic zonal boundaries follow the strike of bedding and cleavage, but crystallinity values increase into stratigraphically younger rocks on the northwest side of the Berwyn Dome. This effect is attributed mainly to a rapid increase in the thickness of synmetamorphic overburden to the northwest, comprising exposed Silurian turbidites and inferred Lower Devonian non‐marine sediments. Strain variations have a more local influence on crystallinity, and lateral variations in the contemporary geothermal gradient cannot be ruled out. However, only with unrealistically high gradients would the need for a thick Lower Devonian component to the overburden be removed. This reasoning implies that the metamorphic peak was coeval with the Acadian (late Caledonian) event, rather than with an early diastathermal e

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1991.tb00564.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY