Variation in the Composition of Plagioclase and Epidote in some Metamorphic Rocks near Bancroft, Ontario

1973 ◽  
Vol 10 (6) ◽  
pp. 852-868 ◽  
Author(s):  
Ermanno R. Rambaldi
Keyword(s):  
Chemical Reactions ◽  
Metamorphic Grade ◽  
Chemical Data ◽  
Metamorphic Rocks ◽  
General Increase ◽  
Al Content ◽  
Metamorphic Terrain ◽  
Anorthite Content ◽  
Normal Zoning

A Precambrian metamorphic terrain near Bancroft displays a variation in metamorphic grade, as indicated by the presence of the following metamorphic zones: garnet, staurolite–biotite, kyanite–biotite, sillimanite, and sillimanite–garnet–biotite.In rocks that contain plagioclase and epidote, there is a general increase in the anorthite content of plagioclase and a decrease in the Al content of epidote with increasing metamorphic grade. Plagioclase crystals are commonly zoned, becoming richer in anorthite toward the rims, and epidote crystals may also be zoned, becoming enriched in iron toward the rims. Various chemical reactions are proposed to account for these changes.In rocks that do not contain epidote, plagioclase is typically albitic in composition, regardless of metamorphic grade.In rocks containing microcline, some albite has evidently exsolved from the microcline, resulting in the development of sodic rims about plagioclase crystals and normal zoning in these crystals.The distribution of albite between plagioclase and microcline is in agreement with the results from other metamorphic terrains of similar grade. Observed irregularities in this distribution may be attributed to exsolution of albite from microcline.Mineralogical and chemical data from the study area provide an opportunity to propose various chemical reactions, some of which may closely approach those that have taken place within the rocks.

Variation in the Composition of Muscovite and Biotite in some Metamorphic Rocks near Bancroft, Ontario

1973 ◽  
Vol 10 (6) ◽  
pp. 869-880 ◽  
Author(s):  
Ermanno R. Rambaldi
Keyword(s):  
Chemical Composition ◽  
Chemical Equilibrium ◽  
Element Distribution ◽  
Metamorphic Grade ◽  
Chemical Data ◽  
Metamorphic Rocks ◽  
Exchange Equilibrium ◽  
Metamorphic Terrain ◽  
Fe Content ◽  
Compositional Study

A study of mica composition in a metamorphic terrain which displays a variation in metamorphic grade from the garnet to the sillimanite–garnet–biotite zone, has revealed that muscovite and phengite occur over the whole range of metamorphic conditions that were encountered. The distribution of Mn, Li, Ti, Na, Fe, and Mg between these minerals and coexisting biotite shows many regularities which are interpreted in relation to exchange equilibrium. Two trends of element distribution are suggested, corresponding to the association phengite–biotite and muscovite–biotite. The chemical composition of the mica minerals is not correlative with metamorphic grade.A compositional study of phengite has revealed that the Fe2+/Fe3+ ratio in this mineral is higher in rocks that contain epidote than in rocks that are free of epidote.The distribution of Na and K between muscovite and plagioclase is affected by the Fe content of muscovite and the An content of plagioclase.Mineralogical and chemical data from the study area provide an indication that a generalized chemical equilibrium was established in the rocks during metamorphism.

The texture and composition of tourmaline in metasediments of the Sinai, Egypt: Implications for the tectono-metamorphic evolution of the Pan-African basement

2007 ◽  
Vol 71 (1) ◽  
pp. 17-40 ◽  
Author(s):  
M. M. Abu El-Enen ◽  
M. Okrusch
Keyword(s):  
Regional Metamorphism ◽  
Metamorphic Grade ◽  
Crystalline Basement ◽  
Amphibolite Facies ◽  
Metamorphic Rocks ◽  
Pan African ◽  
Chemical Signatures ◽  
Mineral Assemblages ◽  
The Matrix ◽  
Metamorphic Facies

AbstractAccessory tourmaline in metasediments from the Sinai crystalline basement exhibits textural and chemical signatures that relate to the evolution of regional metamorphism and deformation during the Pan-African orogeny and testifies to different P-T path segments. Tourmaline inclusions in various porphyroblasts were formed during the prograde phase of metamorphism; acicular to prismatic crystals in the matrix, oriented sub-parallel to, and enveloped by, the main foliation crystallized syntectonically under prograde and peak metamorphic conditions; tourmaline cross-cutting the main foliation may have formed just after the peak or during the retrograde phase of metamorphism. Some of the cores in tourmaline crystals, showing different colours, are interpreted as former detrital grains. The abundance of tourmaline decreases with increasing peak metamorphic conditions. The tourmaline investigated belongs to the schorl-dravitess group, generally with XMg of 0.42–0.73 and XCa = Ca/(Ca+Na+K+□) of 0.02–0.24, typical of tourmalines in metapelites and metapsammites; whereas detrital cores have been derived from various sources, including former tourmaline-quartz and pre-existing high-metamorphic rocks. Tourmaline of the Sinai metasediments was formed during metamorphism of the sedimentary precursors, essentially in a closed system, where clay minerals and organic matter, together with detrital tourmaline, served as the source of boron. Although a metamorphic facies should be defined by characteristic mineral assemblages present in metamorphic rocks, tourmaline chemistry is a good monitor of P-T conditions in the metapelites and semi-metapelites investigated, showing an increase in XMg with increasing metamorphic grade, where XturMg = 0.60 distinguishes between greenschist and lower-amphibolite facies, while XturMg = 0.65 could distinguish lower- from middle- to upper-amphibolite facies. The results of tourmaline-biotite geothermometry compare well with our former temperature estimates using conventional geothermometry and phase-diagram modelling.

Zoisite-clinozoisite relations in low- to medium-grade high-pressure metamorphic rocks and their implications

1980 ◽  
Vol 43 (332) ◽  
pp. 1005-1013 ◽  
Author(s):  
Masaki Enami ◽  
Shohei Banno
Keyword(s):  
Phase Diagram ◽  
High Pressure ◽  
Electron Probe ◽  
Metamorphic Grade ◽  
Metamorphic Rocks ◽  
Grade Metamorphism ◽  
Electron Probe Microanalyser ◽  
Schematic Phase Diagram ◽  
Increasing Temperature ◽  
Host Rocks

SummaryCoexisting zoisite and clinozoisite in seventeen specimens from six localities in Japan have been studied with the electron-probe microanalyser. Zoisite and clinozoisite are commonly zoned, but compositional gaps between them are systematic. Referring to the metamorphic grade of the host rocks, a temporary and schematic phase-diagram for the system Ca2Al3Si3O12-(OH)-Ca2Al2Fe3+Si3O12(OH) is presented. With increasing temperature, in the range of low- to medium-grade metamorphism, the compositional gap between the two epidote-group minerals shifts towards higher Fe3+ compositions.

scholarly journals Thermodynamic modelling of carbonate-silicate rocks from the Sakar unit, Sakar-Strandzha zone, SE Bulgaria

2021 ◽  
Vol 82 (3) ◽  
pp. 76-78
Author(s):  
Tzvetomila Vladinova ◽  
Milena Georgieva
Keyword(s):  
Metamorphic Grade ◽  
Thermodynamic Modelling ◽  
Greenschist Facies ◽  
Amphibolite Facies ◽  
General Increase ◽  
Metasedimentary Rocks ◽  
Town Area ◽  
East West ◽  
Silicate Rocks

The P-T evolution of carbonate-bearing metasedimentary rocks from the Sakar unit (Sakar-Strandzha Zone, SE Bulgaria) has been obtained using Perple_X modelling and conventional geothermometry. The metamorphic conditions vary from greenschist facies (250–350 °C/2–4 kbar) in the Klokotnitsa village area to amphibolite facies (550–650 °C/4.5–6.5 kbar) in the Topolovgrad town area, confirming a general increase of the metamorphic grade at east-west direction.

Geochemical evidence for loss of Na and K from Moinian calc-silicate pods during prograde metamorphism

1980 ◽  
Vol 117 (3) ◽  
pp. 267-275 ◽  
Author(s):  
P. W. G. Tanner ◽  
R. G. Miller
Keyword(s):  
Major Element ◽  
Spatial Relationship ◽  
Metamorphic Grade ◽  
Diffraction Measurement ◽  
X Ray Diffraction ◽  
Prograde Metamorphism ◽  
X Ray ◽  
Late Precambrian ◽  
Anorthite Content ◽  
Silicate Rocks

SummaryA total of 111 major element analyses have been made of impure calcareous rocks from the Late Precambrian Moinian sequence in NW Scotland. Samples were collected along a 36 km traverse from three stratigraphic horizons which are repeated by folding and vary in metamorphic grade from lower to upper amphibolite facies. Anorthite content of the plagioclase, which increases progressively from albite to anorthite, was assessed by X-ray diffraction measurement of 2θ (131–11); this value is used as a ‘metamorphic index’ for rocks with a CaO/Al2O3 (wt. %) ration of 0.3–0.7 and Na2O/Al2O3, < 0.35. Na2O/Al2O3 and K2O/Al2O3 ratios plotted against this index, together with the spatial relationship between metamorphic/chemical isograds and stratigraphic boundaries show that both Na (0.24–4.82% Na2O) and K (0.01–3.28% K2O) are lost from the calc-silicate rocks during prograde metamorphism. These chemical changes result in a variation of the stable mineralogy with increasing metamorphic grade and take place through the medium of the intergranular fluid.

The crystal chemistry of the gedrite-group amphiboles. II. Stereochemistry and chemical relations

2008 ◽  
Vol 72 (3) ◽  
pp. 731-745 ◽  
Author(s):  
F. C. Hawthorne ◽  
M. Schindler ◽  
Y. Abdu ◽  
E. Sokolova ◽  
B. W. Evans ◽  
...  
Keyword(s):  
Maximum Degree ◽  
Detailed Examination ◽  
Bond Valence ◽  
Linear Functions ◽  
Metamorphic Rocks ◽  
Sum Rule ◽  
Al Content ◽  
Bond Topology ◽  
The Mean ◽  
The Individual

AbstractThe general formula of the amphiboles of this series may be written as NaxMg2(Mg(5-y)Aly,)(Si(8-z)Alz)O22(OH)2, where Mg = Mg + Fe2+ + Mn2+ and Al = Al + Fe3+ + Ti. The individual <T–O> distances are linear functions of their [4]Al content, and the [4]Al content is strongly ordered in the following way: T1B > T1A » T2B » T2A. The <M1-O>, <M2-O> and <M3–O> distances are linear functions of the mean ionic radius of their constituent cations. End-member compositions may be written as follows: A☐Mg2Mg5Si8O22(OH)2O22(OH)2; A☐Mg2(Mg3Al2)(Si6Al2)O22(OH)2; ANaMg2Mg5(Si7Al) These compositions define a plane in xyz space across which the data of Schindler et al. (2008), measured on amphiboles from amphibolites, follow a tightly constrained trajectory. Anthophyllite–gedrite amphiboles equilibrated under significantly different P-T conditions (e.g. igneous rocks, contact-metamorphic rocks) follow trends that diverge from this trajectory, with greater Na and [4]Al contents and relatively smaller [6]Al contents. Detailed examination of the local bond topology involving the A and M2 sites indicates that the maximum degree of bond-valence compensation will occur for incorporation of ANa and M2Al in the ratio 4:10, and hence 2.5 ANa = M2Al in these amphiboles. This relation closely fits the data of Schindler et al. (2008), suggesting that the variation in chemical composition in anthophyllite–gedrite amphiboles is strongly constrained by the anion bond-valence requirements of the Pnma amphibole structure. We further suggest that different compositional trends for ortho-amphiboles equilibrated under different P-T conditions are the result of the valence-sum rule operating with (different) bond-lengths characteristic of these P-T conditions.

The geometric and kinematic development of the Emma Lake thrust stack, Grenville Front, southwestern Labrador

1991 ◽  
Vol 28 (1) ◽  
pp. 136-144 ◽  
Author(s):  
Dennis Brown ◽  
Jeroen van Gool ◽  
Tom Calon ◽  
Toby Rivers
Keyword(s):  
Microstructural Analysis ◽  
Regular Sequence ◽  
Metamorphic Grade ◽  
Greenschist Facies ◽  
Metamorphic Rocks ◽  
Duplex Structure ◽  
Structural Geometry ◽  
Thrust Belts ◽  
Fold And Thrust Belts

The Emma Lake thrust stack is a duplex structure consisting of basement-cored fold nappes with thrust-through overturned limbs developed in greenschist-facies metamorphic rocks. Geometric relationships between fold nappes and thrust surfaces indicate both regular-sequence and out-of-sequence thrusting occurred during development of the thrust stack. Microstructural analysis shows that regular-sequence thrusts developed under ductile conditions, whereas out-of-sequence thrusts developed in a brittle–ductile field.The structural geometry and metamorphic grade of the Emma Lake thrust stack indicate that it developed under ductile conditions at upper midcrustal levels. The geometry of the Emma Lake thrust stack suggests that structural styles common to higher level fold and thrust belts may be traced into deeper levels of the crust.

Metamorphic rocks of the 1985 Tibet Geotraverse, Lhasa to Golmud

1988 ◽  
Vol 327 (1594) ◽  
pp. 203-213 ◽  
Keyword(s):  
Tibetan Plateau ◽  
Regional Metamorphism ◽  
Metamorphic Grade ◽  
Metamorphic Rocks ◽  
The Tibetan Plateau ◽  
Crustal Anatexis ◽  
Prograde Metamorphism ◽  
Lhasa Terrane ◽  
The North ◽  
Peak Metamorphism

Two examples of uplifted basement have been studied in the Lhasa Terrane of the Tibetan Plateau. The Nyainqentanglha orthogneisses are bounded by staurolite-garnet schists to the north which record prograde metamorphism at 5 .0 ± 1.3 kbar, 610 ± 70 °C. Garnet sillimanite xenoliths within the orthogneiss suggest that peak temperatures reached at least 700 ± 70 °C at 5.1 ± 2 .5 kbar. These P / T fields reflect high T /low P metamorphism during Eocene subduction, and indicate that the syntectonic Nyainqentanglha orthogneiss was emplaced at depths greater than 10 km. Sillimanite-bearing assemblages from the Amdo gneisses in the northern Lhasa Terrane provide evidence of crustal anatexis at temperatures > 680 °C. This event is poorly constrained in time but is probably Cambrian or earlier. Within the Kunlun Terrane, biotite and garnet isograds north of the Xidatan Fault indicate an increase in metamorphic grade from north to south, reaching peak metamorphism at 470 ± 30 °C, 4 .3 ± 1.5 kbar synchronous with the emplacement of the Triassic batholith. Regional metamorphism was followed by uplift of at least 2 km before emplacement of post-tectonic, early Jurassic granites.

Amphibolitic rocks from the Precambrian of Grand Canyon: mineral chemistry and phase petrology

1978 ◽  
Vol 42 (322) ◽  
pp. 199-207 ◽  
Author(s):  
Malcolm D. Clark
Keyword(s):  
Significant Proportion ◽  
Grand Canyon ◽  
Mineral Chemistry ◽  
Metamorphic Grade ◽  
Metamorphic Rocks ◽  
Mafic Complex ◽  
Assem Blages ◽  
Second Period ◽  
Pale Green ◽  
Phase Relationships

SummaryThe amphibolites and mafic schists that occur in the Precambrian metamorphic rocks of the Grand Canyon are divided into five major groups: anthophyllite and cordierite-anthophyllite rocks, early amphibolites, Granite Park mafic complex, hornblende-beating dykes, and tremolite-bearing dykes. Many types of amphibole occur in these rocks. Microprobe analyses identify gedrite, anthophyllite, cummingtonite, and grunerite, as well as three groups of calcic amphiboles. These last range in composition from colourless tremo-litic or actinolitic amphiboles, through pale-green horn-blende, to strongly pleochroic green-brown hornblende, which contains a significant proportion of the tscherma-kite and pargasite endmembers. Phase relationships between the coexisting amphiboles and other minerals are presented for two regional metamorphic events. Assem-blages containing chlorite, garnet, and hornblende were formed during the first event; from the absence of staurolite, but the presence of almandine garnet and oligoclase-andesine, it is concluded that the metamorphic grade was between the upper greenschist and the lower amphibolite facies. The second period of metamorphism produced rocks of the staurolite and sillimanite zones, within which three main ‘subfacies’ can be distinguished on the basis of phase relationships in the mafic schists.

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