Isograds and mineral assemblages in the eastern axial zone, Montagne Noire (France): implications for temperature gradients and P–T history

1982 ◽  
Vol 19 (1) ◽  
pp. 129-143 ◽  
Author(s):  
Peter H. Thompson ◽  
Jean-Pierre Bard
Keyword(s):  
Metamorphic Grade ◽  
Temperature Gradients ◽  
Low Grade ◽  
Reaction Sequence ◽  
Axial Zone ◽  
Rock Composition ◽  
Mineral Assemblages ◽  
Montagne Noire ◽  
Sequence Study

Detailed petrography across a metapelitic sequence in the eastern axial zone of the Montagne Noire, France, is the basis for a sequence of isograds marking the first appearance of biotite–cordierite, staurolite, andalusite, and sillimanite. The juxtaposition of low-grade biotite-free rocks against medium-grade rocks at the gently dipping biotite–cordierite isograd is attributed to tectonic telescoping of the metamorphic sequence. Study of mineral assemblages with respect to an AFM reaction sequence indicates the staurolite isograd is related to changes in rock composition, and complex assemblages in the sillimanite zone may be the result of unstable persistence of minerals formed when metamorphic grade was lower. These assemblages are interpreted to contain a record of part of the P–T history during which pressure decreased as temperature increased. P–T profiles show that temperature gradients of 200–300 °C/km suggested by previous workers are not required to explain the isograd pattern; gradients of 37 °C/km or less are sufficient.

scholarly journals Clay diagenesis and low-grade metamorphism of Tithonian and Berriasian sediments in the Cameros Basin (Spain)

Clay Minerals ◽  
2001 ◽  
Vol 36 (3) ◽  
pp. 325-333 ◽  
Author(s):  
J. F. Barrenechea ◽  
M. Rodas ◽  
M. Frey ◽  
J. Alonso-Azcárate ◽  
J. R. Mas
Keyword(s):  
Clay Mineral ◽  
Mixed Layer ◽  
Sedimentary Facies ◽  
Metamorphic Grade ◽  
Metamorphic Event ◽  
Low Grade ◽  
Grade Metamorphism ◽  
Mineral Assemblages ◽  
Cameros Basin ◽  
Southern Border

AbstractThe clay mineral assemblages of the Tithonian and Berriasian sediments (Tera and Oncala Groups) in the eastern part of the Cameros basin are investigated at seven localities. The lowest-grade assemblage, located on the southern border of the basin, contains calcite + quartz + hematite + kaolinite + mixed-layer illite-smectite (R = 1, 65 85% illite layers) + discrete illite (IC = 0.5 0.65Δ°2θ). Systematic increases in the illite and chlorite crystallinities suggest increasing metamorphic grade from the northwest part of the basin to the southeast. This trend does not follow the pattern previously described for the overlying late Berriasian–early Aptian sediments (Urbión and Enciso Groups), which exhibit a higher metamorphic grade. This may result from local variations in sedimentary facies, as well as the circulation of hot migratory fluids. Tertiary compression occurring long after the main metamorphic event is considered to be responsible for the enhanced illite and chlorite crystallinities measured in the SE extreme of the basin.

Phase equilibria and mixed parageneses of metabasites in lowgrade metamorphism

1985 ◽  
Vol 49 (352) ◽  
pp. 321-333 ◽  
Author(s):  
J. G. Liou ◽  
S. Maruyama ◽  
M. Cho
Keyword(s):  
Phase Equilibria ◽  
Model System ◽  
Metamorphic Grade ◽  
Low Grade ◽  
Qualitative And Quantitative ◽  
Basaltic Rocks ◽  
Mineral Assemblages ◽  
The Common ◽  
Invariant Points ◽  
Model Phase

AbstractThe system Na2O-CaO-MgO Al2O3-SiO2-H2O is proposed to model phase equilibria and mineral parageneses for low-temperature metamorphism of basaltic rocks. Univariant reactions marking the transitions between various sub-greenschist facies are identified and some have been experimentally determined. The introduction of Fe2O3 into the model system at fixed FeO/MgO ratio creates continuous reactions for facies boundaries and discontinuous reactions for invariant points of the model system. Both qualitative and quantitative effects on P-T displacement and phase compositions are discussed. The XFe3+ isopleths for epidote were plotted to exemplify the transition from the zeolite through prehnite-pumpellyite to prehnite-actinolite facies. T-XFe3+ relations were established for continuous and discontinuous reactions relating such facies transitions. Because of the common occurrence of two or three Ca-Al hydrosilicates in low-grade metabasites, an isobaric Al-Ca-Fe3+ projection from chlorite may be used to illustrate mineral assemblages and compositions of the coexisting Ca-Al silicates in the presence of quartz, albite, and chlorite. Reported occurrences in several classic burial metamorphic terrains and ocean-floor metabasites in ophiolites are described. Only the composition of a mineral from a buffered assemblage can constrain the intensive properties for metamorphism; previously reported compositional trends for pumpellyite and epidote with increasing metamorphic grade are oversimplified.

scholarly journals Paleotemperature investigation of the Variscan southern external domain: the case of the Montagne Noire (France)

2020 ◽  
Author(s):  
Clément Montmartin ◽  
Michel Faure ◽  
Hugues Raimbourg
Keyword(s):  
Thermal Structure ◽  
Foreland Basin ◽  
Carbonaceous Material ◽  
Low Grade ◽  
Illite Crystallinity ◽  
Axial Zone ◽  
Massif Central ◽  
External Domain ◽  
Montagne Noire ◽  
Oriented Parallel

The Montagne Noire located in the southern part of the French Massif Central represents the northern part of the South-Variscan Foreland. It is subdivided into three parts. The granite-migmatite Axial Zone dome is surrounded by non- or weakly metamorphosed Paleozoic sedimentary series. Both northern and southern flanks of the Montagne Noire dome are deformed by km-scale, south to southeast facing recumbent folds and thrusts sheets. The Raman Spectroscopy of Carbonaceous Material (RSCM) method, carried out in the low-grade metamorphic rocks of the southern flank of the Montagne Noire, yielded temperatures comprised between 400°C near the dome, and 230°C in the southern domain. Three Raman geothermometers were used to cover this temperature range. RSCM temperatures comply qualitatively with previous estimates based on illite crystallinity, conodont colour alteration, and fluid inclusions carried out in the same area, which document a metamorphic temperature increase towards the dome. The isotherms cut across the different nappe contacts and are oriented parallel to the southern margin of the Axial Zone. This temperature distribution supports the idea that the thermal structure was acquired during the Axial Zone dome emplacement. The thermal structure acquired during the recumbent folds emplacement and burial of the sedimentary series is totally overprinted by the doming. In addition, in a domain relatively remote from the Axial Zone dome, the RSCM measurements yielded significantly higher temperatures than illite crystallinity. This discrepancy points to a higher sensitivity of RSCM to short-lived thermal events than illite crystallinity, possibly because of more efficient kinetics of the carbonization reaction. On the other hand, high RSCM temperatures analysed far from the Axial Zone, between 300°C and 360°C, could be explained by the presence of granitic plutons under the foreland basin.

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.

Mineral Assemblages in Diagenetic and Low-Grade Metamorphic Iron-Formation

1973 ◽  
Vol 68 (7) ◽  
pp. 1063-1074 ◽  
Author(s):  
Bevan M. French
Keyword(s):  
Iron Formation ◽  
Low Grade ◽  
Mineral Assemblages

scholarly journals [MnO|SiO2,Al2O3,FeO,MgO] balanced log-ratio in chlorites: a tool for chemo-stratigraphic mapping and proxy for the depositional environment

Clay Minerals ◽  
2018 ◽  
Vol 53 (3) ◽  
pp. 351-375
Author(s):  
Annette Süssenberger ◽  
Solveig Pospiech ◽  
Susanne Theodora Schmidt
Keyword(s):  
Depositional Environment ◽  
Regional Metamorphism ◽  
Upper Jurassic ◽  
Contact Metamorphism ◽  
Low Grade ◽  
Rock Composition ◽  
Magallanes Basin ◽  
Global Sea Level ◽  
Rock Analysis ◽  
Log Ratio

ABSTRACTThe [MnO|SiO2,Al2O3,FeO,MgO] balanced ratio (i.e. the isometric log-ratio of the MnO concentration relative to the concentration of SiO2, Al2O3, FeO and MgO) of chlorite and of whole-rock composition is an effective discriminant between Mesozoic stratigraphic formations in the Magallanes Basin (Chile). The MnO content in chlorite is only controlled by the host rock chemistry and is dependent on the geological environment. The MnO content in chlorite remains unchanged at low-grade metamorphic conditions. Single-grain chlorite analysis (n = 1042, electron microprobe) and whole-rock analysis (n = 40, X-ray fluorescence) were used to discriminate stratigraphic formations and to decipher differences in the depositional environment in the Magallanes Basin. The samples are from one Upper Jurassic and three Cretaceous sedimentary units that were affected either by low-grade regional metamorphism or by Miocene contact metamorphism. The highest [MnO|SiO2,Al2O3,FeO,MgO] values are recorded in the upper Zapata Formation. The Punta Barrosa, Cerro Toro and Tobífera Formations show slightly lower [MnO|SiO2,Al2O3,FeO,MgO] values. Elevated [MnO|SiO2,Al2O3,FeO,MgO] values at the transition between Zapata and Punta Barrosa Formations record an oxygenated shallow marine environment that can be linked to the closure of the Rocas Verdes Basin and the onset of fold-and-thrust belt formation. Decreasing [MnO|SiO2,Al2O3,FeO,MgO] values from the Punta Barrosa towards the Cerro Toro Formation indicate gradually increasing water depths during the Upper Cretaceous that correlate well with the global sea level.

The stilpnomelane group of minerals

1938 ◽  
Vol 25 (163) ◽  
pp. 172-206 ◽  
Author(s):  
C. Osborne Hutton
Keyword(s):  
New Zealand ◽  
Analytical Data ◽  
Low Grade ◽  
Preliminary Work ◽  
Mineral Assemblages

Work which has been in progress during the last few years on the low-grade regionally metamorphosed rocks of western Otago, New Zealand, has shown that members of the stilpnomelane group, hydrous iron- (or manganese-) rich silicates, are exceedingly widespread in rocks with the most varied mineral assemblages. In many cases the mineral in question is in sufficient quantity to rank as an essential constituent and may even be the most abundant one present. Hence in view of the importance of this mineral in the rocks of this region, it was considered that exact analytical data were necessary, and preliminary work in this connexion was carried out by Dr. F. J. Turner and the writer in 1934, but at that time, owing to the difficulties of separation, the pure minerals could not be obtained for analysis.

Clay mineral evidence for low-grade Caledonian and Variscan metamorphism in south-western Dyfed, south Wales

1980 ◽  
Vol 43 (331) ◽  
pp. 857-863 ◽  
Author(s):  
D. Robinson ◽  
R. A. Nicholls ◽  
L. J. Thomas
Keyword(s):  
Lower Cambrian ◽  
Volatile Matter ◽  
Low Grade ◽  
X Ray Diffraction ◽  
Illite Crystallinity ◽  
X Ray ◽  
South Wales ◽  
Mineral Assemblages ◽  
Variscan Metamorphism ◽  
Pelitic Rocks

SummaryIllite crystallinity determinations on Palaeozoic pelitic rocks, whose stratigraphic range runs from Lower Cambrian to Westphalian, indicate that anchimetamorphism has affected both the Lower and Upper Palaeozoic sequences. Two metamorphic episodes are in evidence, with the earlier, Caledonian, being of slightly higher grade. The higher anchizone crystallinity values are recorded from the Fishguard area in which mineral assemblages of the prehnite-pumpellyite facies have recently been recognized in basic igneous rocks. The later metamorphic episode has affected rocks to the south of the Variscan front. Here crystallinity values are low anchizone but some straddle the boundary with the diagenetic state. The Pembroke coalfield lies in this southern area and has coals largely of anthracite rank with volatile matter contents of between 10.1 and 5% Grade of metamorphism as indicated by crystallinity and by coal rank data from the Pembroke coalfield shows anomalous results to that described from the main South Wales coalfield. A neo-formed 2M illite from the Variscan spaced cleavage is described with analytical and X-ray diffraction data.

Varying Trends in the Metamorphism of Dolerites

1951 ◽  
Vol 88 (1) ◽  
pp. 25-35 ◽  
Author(s):  
John Sutton ◽  
Janet Watson
Keyword(s):  
Regional Metamorphism ◽  
Metamorphic Grade ◽  
Low Grade ◽  
Basic Rocks ◽  
High Metamorphic Grade

AbstractThe metamorphism of Pre-Cambrian dolerites in the Northwest Highlands is described and compared with that of basic rocks in the Southwest Highlands, Banffshire and other regions. The first products of metamorphism are not the same in every area. It is suggested that the trend of regional metamorphism differs according to the environment in which it took place, and that a sequence of changes established in one area cannot be used as a standard by which to judge the changes in other areas. In particular, rocks of high metamorphic grade have not invariably passed through stages characterized by low-grade minerals.

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