Epidiorite–limestone contact relations at Burawai, Hazara District, West Pakistan

1965 ◽  
Vol 34 (268) ◽  
pp. 82-91
Keyword(s):  
Regional Metamorphism ◽  
Thermal Metamorphism ◽  
Mineral Assemblages

SummarySpecial mineral assemblages in calcareous rocks 0–6 ft from the contact of an epidiorite sill are interpreted as the result of polymetamorphism–regional metamorphism superimposed on an earlier thermal metamorphism in which metasomatism, especially migration of potassium, was important.

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.

Further Data on the Petrology of the Pelitic Hornfelses of the Carn Chuinneag–Inchbae Region, Ross-shire, with Special Reference to the Status of Almandine

1954 ◽  
Vol 91 (6) ◽  
pp. 445-462 ◽  
Author(s):  
R. I. Harker
Keyword(s):  
Special Reference ◽  
Regional Metamorphism ◽  
The Other ◽  
Contact Metamorphism ◽  
Chemical Data ◽  
Chemical Analyses ◽  
Thermal Metamorphism ◽  
Massive Structure ◽  
Mineral Assemblages ◽  
The Status

AbstractThe pelitic hornfelses surrounding the Carn Chuinneag–Inchbae complex have, with the other rocks in the area, been subjected to the regional metamorphism which affected the Moine Series as a whole. The minerals of the original hornfelses have reacted to form assemblages which are typical of regional rather than purely thermal metamorphism although the massive structure of the hornfelses is well preserved.By studying the changes which took place in the hornfelses during the regional metamorphism it has been possible to deduce the most likely mineral assemblages indigenous to the original hornfelses.The results of the chemical analyses of some of the pelitic hornfelses are presented and chemical data on some of the garnets are given. It is considered unlikely that all the garnets in the hornfelses, which are essentially almandines, formed as a result of the earlier contact metamorphism: some at least probably occur as a result of the later regional metamorphism.

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.

Metamorphism of the Settler Schist, southwest of Yale, British Columbia

1976 ◽  
Vol 13 (3) ◽  
pp. 405-421 ◽  
Author(s):  
Lee C. Pigage
Keyword(s):  
British Columbia ◽  
Linear Regression ◽  
Microprobe Analysis ◽  
Regional Metamorphism ◽  
Fluid Phase ◽  
Contact Metamorphism ◽  
Major Constituent ◽  
Minor Components ◽  
Mineral Assemblages ◽  
Regression Techniques

Pelitic metasediments immediately southwest of Yale, British Columbia contain mineral assemblages characteristic of staurolite through sillimanite zones of the Barrovian facies series. Microprobe analyses of major constituent phases in the pelites are combined with linear regression techniques to formulate probable kyanite- and sillimanite-forming reactions.A zone some 3 km wide contains the assemblage staurolite–kyanite–garnet–biotite–muscovite–quartz–plagioclase-ilmenite-rutile, which is univariant in AFM projection. Within precision limits of microprobe analysis, this assemblage is also univariant using linear regression techniques. Univariant relations are possible if [Formula: see text] with the composition of the fluid phase being buffered by the progressing reaction. This assemblage emphasizes the need for precise analyses when using the regression method, since minor components are often within permissible error limits rather than being balanced.Pelitic and calc-silicate assemblages from the metasediments restrict estimates of pressure–temperature conditions during regional metamorphism to 6–8 kbar and 550–770 °C. Pseudomorphs after andalusite indicate that contact metamorphism preceded regional upgrading of the pelites.

Nature of the Quetico–Wabigoon boundary in the de Courcey – Smiley Lakes area, northwestern Ontario

1976 ◽  
Vol 13 (6) ◽  
pp. 737-748 ◽  
Author(s):  
Manfred M. Kehlenbeck
Keyword(s):  
Regional Metamorphism ◽  
Sedimentary Rocks ◽  
Metamorphic Grade ◽  
Boundary Zone ◽  
Present Level ◽  
Metasedimentary Rocks ◽  
The North ◽  
Northwestern Ontario ◽  
Multiple Phase ◽  
Mineral Assemblages

In the de Courcey – Smiley Lakes Area, the boundary between the Quetico and Wabigoon Belts is expressed by a sequence of pelitic to semi-pelitic schists and gneisses. At the present level of erosion, these metasedimentary rocks are in contact with granodioritic gneisses, granites, and pegmatites, which are exposed to the south.To the north of this area, regional metamorphism of volcanic and sedimentary rocks has resulted in greenschist facies assemblages, which characterize the Wabigoon Belt in general. In the boundary zone, the metamorphic grade increases southward toward de Courcey and Smiley Lakes.Formation of three distinct foliation surfaces was accompanied by syn-tectonic as well as post-tectonic recrystallization, producing polymetamorphic schists.In the boundary zone, mineral assemblages comprising andalusile, sillimanite, cordierite, garnet. biotite, and muscovite form a facies series of the Abukuma type.The boundary between the Quetico and Wabigoon Belts in this area is a complex zone in which rocks of both belts have been reconstituted by multiple-phase metamorphism and partial melting.

Polythermal metamorphism of limestones at Kilchoan, Ardnamurchan

1965 ◽  
Vol 34 (268) ◽  
pp. 1-15 ◽  
Author(s):  
S. O. Agrell
Keyword(s):  
Vapour Phase ◽  
High Ratio ◽  
Thermal Metamorphism ◽  
Metamorphic History ◽  
Mineral Assemblages

SummaryJurassic limestones occurring in a screen between two ring dykes are shown to have undergone two periods of thermal metamorphism; the first characterized by a high ratio of CO2:H2O and the second by a high ratio of H2O:CO2 in the vapour phase. The first metamorphism produced minerals characteristic of Bowen's decarbonation series. Some of these minerals became unstable in the second metamorphism and (OH, F) bearing minerals were produced. A stage of fracture-controlled replacement veins with mineral assemblages which indicate a varying CO2:H2O ratio in the vapour and a general falling temperature sequence terminate the metamorphic history. Three new minerals, rustumite (Ca3Si2O7.Caterminate (OH)2), dellaite (Ca12Si6O22(OH)4), and kilchoanite (Ca3Si2O7), of which the last has previously been described, are characteristic of the second episode of metamorphism.

XX.—The Metamorphism of the Deeside Limestone, Aberdeenshire

1933 ◽  
Vol 57 (2) ◽  
pp. 557-592 ◽  
Author(s):  
Arthur G. Hutchison
Keyword(s):  
Regional Metamorphism ◽  
High Grade ◽  
Thermal Metamorphism

The limestone to be described occurs in two separate areas (fig. 1)—one in Lower Deeside around Banchory, another in Middle Deeside around Aboyne. The whole of the Banchory and a large part of the Aboyne outcrops provide limestone types in a high grade of regional metamorphism (the associated schists contain sillimanite). In the latter outcrop the limestone has undergone thermal metamorphism at the contacts with Newer Granite intrusions. Many of the resulting hornfelses have suffered hydrothermal alterations with development of prehnite and zeolites. Newer Granite pegmatites, intruded at the time of hornfelsing, share in this hornfelsing and later hydrothermal modifications. In addition, they exercise exopneumatolytic and exohydrothermal metamorphism. Quite local metamorphisms take place at hornblende-schist and Older Granite contacts.

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.

Petrochemistry of metamorphosed pillows, and the geochemical status of the amphibolites (Proterozoic) from the Sirohi district, Rajasthan, India

1984 ◽  
Vol 121 (5) ◽  
pp. 465-473 ◽  
Author(s):  
P. K. Bhattacharyya ◽  
A. D. Mukherjee
Keyword(s):  
Regional Metamorphism ◽  
Element Distribution ◽  
Island Arcs ◽  
Oceanic Sediments ◽  
Crystalline Core ◽  
Mineral Assemblages ◽  
Trace Element Contents ◽  
Closed Systems ◽  
Low K ◽  
Middle Proterozoic

AbstractRelic pillows in the middle Proterozoic amphibolites, occurring in the Sirohi Road–Abu Road tract of Rajasthan, India exhibit contrasted mineral assemblages from core to rim – mimetic after the crystalline core, the zone of incipient crystallization, and the rim of the original pillows. The major element distribution pattern across the pillows indicates exchange of Na–Al for Ca (Mg, Fe) in an inner reaction zone, surrounding the core and in the inner margin of the rim, and Fe–Al exchange for Ca–Si at the outer margin of the rim.Despite such exchanges around the rims, these pillows have retained their initial geochemical characteristics internally and thus have largely acted as closed systems during post-emplacement metamorphism. Mineral parageneses indicate that the contrasted mineral assemblages could evolve from domainal characters of the co-existing fluids, the compositions of which were only buffered by the reacting minerals during regional metamorphism.The major, minor and trace element contents of the pillows and of amphibolites of diverse petrographic character in the region further establish that the pillow interiors and the massive amphibolites were least modified during metamorphism(s), and represent oceanic tholeiites. Their average 2300 ppm K, 4.5 ppm Rb, 150 ppm Sr, along with the K/Rb and K/Sr ratios of 510 and 15 respectively resemble that of the low K-tholeiites, occurring nearest to the trenches in modern island arcs. On the other hand, the higher values of 17300 ppm K, 4.9 ppm Rb, and 210 ppm Sr of the banded and the schistose amphibolites indicate that they were contaminated in various magnitudes by oceanic sediments.

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