The Genesis of Oligoclase in Certain Schists

1933 ◽  
Vol 70 (12) ◽  
pp. 529-541 ◽  
Author(s):  
F. J. Turner
Keyword(s):  
Chemical Composition ◽  
Bulk Composition ◽  
Regional Metamorphism ◽  
Slight Modification ◽  
Igneous Rocks ◽  
Calcic Plagioclase ◽  
Irregular Distribution ◽  
Thermal Metamorphism

The mineralogical changes in green schists and related quartzofelspathic schists of sedimentary origin are discussed, and the following conclusions are reached as to the conditions of formation of oligoclase in these rocks:—(1) Oligoclase normally appears as a product of dynamothermal metamorphism at relatively high grades such as prevail in the zones of almandine and perhaps kyanite. It is accompanied either by deeply-coloured hornblende, hornblende and biotite, or biotite and muscovite, according to the chemical composition of the rocks in which it occurs.(2) Sodic oligoclase containing 10 per cent to 15 per cent of anorthite may occur with pale aluminous hornblende in green schists lying within the more strongly metamorphosed portion of the chlorite zone. The rocks in question are low in potash and have been formed by reconstitution, at a higher grade, of chlorite-epidotealbite-schists containing calcite. This oligoclase-hornblende association is not to be confused with the actinolite-epidote-albite-chlorite assemblage which is formed at any grade within the zone of chlorite, by direct reconstitution of basic igneous rocks without change in bulk composition and in the absence of CO2. A slight modification of Tilley’s subdivision of the green schist facies of Eskola is therefore introduced.(3) A zone of oligoclase representing a grade of metamorphism higher than that attained in the biotite zone, may be recognized for quartzo-felspathic schists of appropriate composition and for many green schists, in areas of progressive regional metamorphism. In the latter case, blue-green hornblende often accompanied by biotite is also present.(4) Oligoclase or more calcic plagioclase and deeply-coloured hornblende form readily during purely thermal metamorphism of only medium grade in the absence of stress. This accounts for the irregular distribution of both these minerals in districts where purely thermal and regional metamorphism have both occurred during the same period of orogeny.

Compositional variation in högbomites from north Connemara, Ireland

1985 ◽  
Vol 49 (354) ◽  
pp. 649-654 ◽  
Author(s):  
N. S. Angus ◽  
R. Middleton
Keyword(s):  
Chemical Composition ◽  
Regional Metamorphism ◽  
The Other ◽  
Partial Substitution ◽  
Compositional Variation ◽  
Physical Parameters ◽  
Chemical Heterogeneity ◽  
High Grade ◽  
Thermal Metamorphism ◽  
Mineral Assemblages

AbstractHögbomite occurs in two contrasting mineral assemblages within the Currywongaun-Dough-ruagh intrusion of north Connemara: a cordierite-rich pelitic xenolith and an orthopyroxenite. In the latter, högbomite and green spinel form blebs within magnetite-ilmenite grains. The högbomite displays significant compositional variation from grain to grain: TiO2 (3.0–6.3%), FeO (21.6–21.3%), MgO (10.0–7.5%), ZnO (3.6–2.4%). This chemical heterogeneity appears to represent variable degrees of partial substitution of Mg and Zn by Ti, in the replacement of spinel by högbomite. By contrast, in the cordierite-hornfels, the högbomite compositions are more notably enriched in iron: TiO2 (4.7–7.0%), FeO (29.6–24.3%), MgO (4.2–6.2%), ZnO (2.7–2.1%). This iron-rich högbomite appears to have formed primarily by interaction between opaque ore and adjacent cordierite, rather than by replacement of spinel.Two high-grade metamorphic episodes appear to be necessary for högbomite growth, one determining chemical composition and the other appropriate physical parameters. In the Connemara occurrences thermal metamorphism and partial melting, coupled with contamination of the surrounding magma, controlled the formation of mineral assemblages rich in Fe, Mg, Al, Ti, and Zn. Emplacement of the intrusion was accompanied by amphibolite facies regional metamorphism and it is to this metamorphic event that the growth of högbomite may be attributed.

The Granitization Process and its Limitations as Exemplified in Certain Parts of New South Wales

1952 ◽  
Vol 89 (1) ◽  
pp. 25-38 ◽  
Author(s):  
Germaine A. Joplin
Keyword(s):  
Wave Front ◽  
New South ◽  
Regional Metamorphism ◽  
New South Wales ◽  
Compressional Wave ◽  
Thermal Metamorphism ◽  
South Wales ◽  
Strong Compression

AbstractIn an attempt to dissect the granitization process in an area of regional metamorphism, it is suggested that granite is forcibly injected during strong compression, that the compressional wave passes upwards and outwards in advance of the magma, and that several changes take place before its arrival. Six stages in this process are recognized in the Cooma and Albury districts of N.S.W.: (1) Regional Metamorphism, (2) Superimposed Thermal Metamorphism, (3) Permeation without Magmatic Addition, (4) Addition from an Attenuated Magma, (5) Formation of a Potassic Wave-Front, and (6) Injection of the Magma as Concordant Intrusions. The conditions favouring granitization are discussed and the question of basic fronts is considered.

Use of pyrite microfabric as a key to tectono-thermal evolution of massive sulphide deposits – an example from Deri, southern Rajasthan, India

1998 ◽  
Vol 62 (2) ◽  
pp. 197-212 ◽  
Author(s):  
Anju Tiwary ◽  
Mihir Deb ◽  
Nigel J. Cook
Keyword(s):  
Thermal Evolution ◽  
Regional Metamorphism ◽  
Massive Sulphide ◽  
Ductile Deformation ◽  
Sulphide Deposit ◽  
Fine Grained ◽  
Thermal Metamorphism ◽  
Sulphide Ores ◽  
History Of ◽  
South Delhi Fold Belt

AbstractPyrite is an ubiquitous constituent of the Proterozoic massive sulphide deposit at Deri, in the South Delhi Fold Belt of southern Rajasthan. Preserved pyrite microfabrics in the Zn-Pb-Cu sulphide ores of Deri reveal a polyphase growth history of the iron sulphide and enable the tectono-thermal evolution of the deposit to be reconstructed.Primary sedimentary features in Deri pyrites are preserved as compositional banding. Regional metamorphism from mid-greenschist to low amphibolite facies is recorded by various microtextures of pyrite. Trails of fine grained pyrite inclusions within hornblende porphyroblasts define S1-schistosity. Pyrite boudins aligned parallel to S1 mark the brittle–ductile transformation of pyrite during the earliest deformation in the region. Isoclinal to tight folds (F1 and F2) in pyrite layers relate to a ductile deformation stage during progressive regional metamorphism. Peak metamorphic conditions around 550°C, an estimation supported by garnet–biotite thermometry, resulted in annealing of pyrite grains, while porphyroblastic growth of pyrite (up to 900 µm) took place along the retrogressive path. Brittle deformation of pyrite and growth of irregular pyritic mass around such fractured porphyroblasts characterize the waning phase of regional metamorphism. A subsequent phase of stress-free, thermal metamorphism is recorded in the decussate and rosette textures of arsenopyrite prisms replacing irregular pyritic mass. Annealing of such patchy pyrite provides information regarding the temperature conditions during this episode of thermal metamorphism which is consistent with the hornblendehornfels facies metamorphism interpreted from magnetite–ilmenite geothermometry (550°C) and sphalerite geobarometry (3.5 kbar). A mild cataclastic deformation during the penultimate phase produced microfaults in twinned arsenopyrite prisms.

scholarly journals Measured and modelled Cloud Condensation Nuclei (CCN) concentration in São Paulo, Brazil: the importance of aerosol size-resolved chemical composition on CCN concentration prediction

2013 ◽  
Vol 13 (12) ◽  
pp. 32353-32389 ◽  
Author(s):  
G. P. Almeida ◽  
J. Brito ◽  
C. A. Morales ◽  
M. F. Andrade ◽  
P. Artaxo
Keyword(s):  
Chemical Composition ◽  
Real Time ◽  
Cloud Condensation Nuclei ◽  
Bulk Composition ◽  
Sao Paulo ◽  
Aerosol Size Distribution ◽  
São Paulo ◽  
Condensation Nuclei ◽  
Aerosol Composition ◽  
The Impact

Abstract. Measurements of cloud condensation nuclei (CCN), aerosol size distribution and non-refractory chemical composition were performed from 16 to 31 October 2012 in the São Paulo Metropolitan Area (SPMA), Brazil. CCN measurements were performed at 0.2%, 0.4%, 0.6%, 0.8% and 1.0% water supersaturation and were subsequently compared with Köhler theory, considering the chemical composition. Real-time chemical composition has been obtained deploying for the first time in SPMA an Aerosol Chemical Ionization Monitor (ACSM). CCN closure analyses were performed considering internal mixture. Average aerosol composition during the studied period yielded 4.81 ± 3.05, 3.26 ± 2.10, 0.30 ± 0.27, 0.52 ± 0.32, 0.37 ± 0.21 and 0.04 ± 0.04 μg m−3 for organics, BC, NH4, SO4, NO3 and Cl, respectively. Particle number concentration was 12 813 ± 5350 cm−3, being a large fraction in the nucleation mode. CCN concentrations were on average 1090 ± 328 cm−3 and 3570 ± 1695 cm−3 at SS = 0.2% and SS = 1.0%, respectively. Results show an increase in aerosol hygroscopicity in the afternoon as a result of aerosol photochemical processing, leading to an enhancement of both organic and inorganic secondary aerosols in the atmosphere, as well as an increase in aerosol average diameter. Considering the bulk composition alone, CCN concentrations were substantially overpredicted (29.6 ± 45.1% at 0.2% supersaturation and 57.3 ± 30.0% at 1.0% supersaturation). Overall, the impact of composition on the calculated NCCN decreases with decreasing supersaturation, partially because using bulk composition introduces less bias for large diameters and lower critical supersaturations. Results suggest that the consideration of only inorganic fraction improves the calculated NCCN. Introducing a size-dependent chemical composition based on filter measurements from previous campaigns has considerably improved simulated values for NCCN (average overprediction error 3.0 ± 33.4% at 0.20% supersaturation and average under prediction error 2.4 ± 20.5% at 1.0% supersaturation). This study provides the first insight on aerosol real-time composition and hygroscopicity on a~site strongly impacted by emissions of a unique vehicular fleet due to the extensive biofuel usage.

Octahedral occupancy and the chemical composition of diagenetic (low-temperature) chlorites

Clay Minerals ◽  
1991 ◽  
Vol 26 (2) ◽  
pp. 149-168 ◽  
Author(s):  
S. Hillier ◽  
B. Velde
Keyword(s):  
Chemical Composition ◽  
Low Temperature ◽  
Bulk Composition ◽  
Compositional Variation ◽  
Low Grade ◽  
Chemical Analyses ◽  
Method Of Calculation ◽  
Sedimentary Sequences ◽  
Wet Chemical ◽  
Potential Use

AbstractThe chemical composition of about 500 diagenetic chlorites, determined by electron microprobe, has been studied in six different sedimentary sequences spanning conditions from early diagenesis to low-grade metamorphism, in the temperature range 40–330°C. The range of Fe/(Fe + Mg) is almost complete and is positively correlated with Al. Five sequences show the same compositional variation. In each, the most siliceous chlorites have the lowest R2+, substantially more octahedral than tetrahedral Al, and the lowest octahedral totals. Conversely, the least siliceous have the highest R2+, nearly equal octahedral and tetrahedral Al, and octahedral totals close to that for an ideal trioctahedral mineral. A dioctahedral substitution Si[]R2−2 (where [] represents a vacant octahedral site) which decreases with temperature, describes this variation. Low octahedral totals are, however, induced by the method of calculation and need not indicate vacancies; for published wet chemical analyses of metamorphic chlorites they may simply indicate oxidation of Fe. Intergrown dioctahedral phyllosilicates may partly account for apparent vacancies in diagenetic chlorites. Nevertheless, the correlation of composition with temperature and similarities to the temperature-related evolution of synthetic chlorites, suggest that diagenetic chlorites are compositionally distinct from, but metastable with respect to, fully trioctahedral metamorphic chlorites. Temperature-related trends are modified by bulk composition, complicating their potential use for low-temperature geothermometry.

Sur la composition chimique moyenne des amphibolites

1953 ◽  
Vol S6-III (1-3) ◽  
pp. 153-173 ◽  
Author(s):  
Pierre Lapadu-Hargues
Keyword(s):  
Chemical Composition ◽  
Sedimentary Rocks ◽  
Point Of View ◽  
Igneous Rocks ◽  
Chemical Analyses ◽  
Average Chemical Composition ◽  
Composition Chimique

Abstract In a majority of cases, amphibolites derived from sedimentary rocks can be distinguished from those derived from dioritic or gabbroic igneous rocks on the basis of average chemical composition. From the purely chemical point of view, it is possible to consider that eclogites result from metamorphism of certain types of heteromorphic gabbros such as the ariegites,and ultimately give rise to amphibolitic rocks. Chemical analyses are included.

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