XII.—Petrology and Petrogenesis of some Garnetiferous Peridotites

1963 ◽  
Vol 65 (12) ◽  
pp. 251-314 ◽  
Author(s):  
M. J. O'Hara ◽  
F. H. Stewart
Keyword(s):  
South Africa ◽  
Crystalline Rocks ◽  
Chemical Analyses ◽  
X Ray ◽  
Metamorphic History ◽  
Igneous Origin ◽  
Mineral Assemblages ◽  
History Of ◽  
Reliable Interpretation ◽  
Recent Experimental Work

SynopsisGarnetiferous peridotite masses which occur among gneisses in the Tafjord district of Norway are cold intrusions emplaced as crystalline rocks late in the metamorphic history of the region. No relationship other than similarity of mineral facies can be established between the peridotites and the eclogite bodies in the surrounding gneiss. Chemical analyses of twelve olivines, fourteen orthopyroxenes, nine chrome diopsides, nine garnets, three amphiboles and two rocks are presented, representing material from the Tafjord and Almklovdalen districts of Norway, a garnetiferous peridotite mass near Bellinzona, Switzerland, and the garnet-peridotite inclusions in the kimberlite pipes of South Africa. Optical and X-ray data for the analyzed and some unanalyzed assemblages are also presented. The mineral assemblages of these rocks are compared with each other, and with data from the peridotites of layered tholeiitic intrusions, alpine-type peridotites and the peridotite nodules in basalts. The garnet-peridotites of Norway, Switzerland and South Africa are believed to be little altered fragments of the mantle, whereas the peridotite nodules in basalts and the alpine-type peridotites are believed to be of igneous origin. Examination of the distribution of cations between the coexisting phases suggests that there are too many variables to permit a reliable interpretation of the results.The orthopyroxenes from the garnetiferous peridotites are not rich in A1203, contrary to expectations based upon recent experimental work.

A retrogressive sapphirine-cordierite-talc paragenesis in a spinel-orthopyroxenite from southern Karnataka, India

1993 ◽  
Vol 57 (387) ◽  
pp. 273-288 ◽  
Author(s):  
C. R. L. Friend ◽  
A. S. Janardhan ◽  
N. Shadakshara Swamy
Keyword(s):  
Dharwar Craton ◽  
Amphibolite Facies ◽  
Coarse Grained ◽  
Cooling History ◽  
Metamorphic History ◽  
Gneiss Complex ◽  
Igneous Origin ◽  
Mineral Assemblages ◽  
History Of ◽  
Granoblastic Texture

AbstractWithin amphibolite facies Peninsular gneisses in the south of the Dharwar craton, units of Sargur supracrustal rocks contain ultrabasic enclaves. One of these enclaves is an orthopyroxenite which comprises bronzite, spinel and minor phlogopite preserving coarse-grained, relic textures of probable igneous origin. After incorporation into the gneisses the enclave evolved through several distinct stages, elucidation of which allow an assessment of its metamorphic history.Firstly, deformation during closed system, anhydrous recrystallisation caused the coarse-grained textures to be partially overprinted by similar mineral assemblages but with a granoblastic texture. Secondly, open system hydration caused retrogression of the bronzite to alumino-gedrite at the margins of the enclave. Subsequently, the penetration of these fluids along grain boundaries caused reactions between spinel and bronzite to produce reaction pockets carrying assemblages of peraluminous sapphirine associated with cordierite and talc. The differences in the mineral assemblages in each pocket coupled with slight variations in their chemistry, suggest that equilibrium did not develop over the outcrop. Because sapphirine + magnesite is present in some pockets, it is evident that CO2 was also a component of the fluid.Phase relations from the MASH portion of the FMASH system, to which the chemistry of the reaction pockets approximates, suggest that the hydrous metamorphism causing the changes depended upon the assemblage enstatite + spinel + vapour which exists at PT conditions above the position of I16, ∼760°C at 3 kbar and below I21 at ∼765°C at 5.6 kbar (Seifert, 1974, 1975), where sapphirine is replaced by kornerupine. The suggested path of reaction occurred between I18 and I21. Subsequent reactions related to I20 cause the formation of cordierite. Talc formation has to be modelled in a different reaction grid.The metamorphism recorded by these reactions is thus at a maximum of amphibolite facies and is interpreted to have formed during the uplift and cooling history of the gneiss complex when hydrous fluids were free to migrate. Given the complex high-grade metamorphic history of this part of the Dharwar craton this event is likely to be late Archaean or Palaeoproterozoic in age.

scholarly journals Controls of metamorphic grade in shear belts

1979 ◽  
Vol 89 ◽  
pp. 47-62
Author(s):  
J Grocott
Keyword(s):  
Vertical Displacement ◽  
Metamorphic Grade ◽  
Perfect Equilibrium ◽  
Horizontal Gradients ◽  
Metamorphic History ◽  
Mineral Assemblages ◽  
History Of ◽  
Set Up ◽  
The Relationship ◽  
Shear Belt

The relationship between metamorphic grade and deformation is examined for shear belts occurring in Precambrian shields. A particular case, where isotherms have fallen continuously relative to an originally horizontal datum surface, for some time prior to the initiation of the shear belt and throughout its life, is examined in detail. Such behaviour of isotherms appears to be common. A type of syn-tectonic metamorphic boundary occurring within shear belts and not coinciding with a strain gradient is discussed in detail. Such boundaries are called active facies boundaries, as mineral assemblages on each side tend to maintain perfect equilibrium with metamorphic conditions during deformation. The orientation of active facies boundaries depends on the vertical displacement rate. In ductile thrust zones horizontal gradients in metamorphic conditions can be set up, and folIowing erosion once active facies boundaries may be exposed. The metamorphic history of rocks in such zones will vary vertically, and, under certain circumstances, laterally. A model is set up to predict these variations, and is applied to the northem boundary of the Ikertoq shear belt, western Greenland.

Metamorphic history of the Hemlo gold deposit from Al2SiO5 mineral assemblages, with implications for the timing of mineralization

1999 ◽  
Vol 36 (1) ◽  
pp. 33-46 ◽  
Author(s):  
Wayne G Powell ◽  
David RM Pattison ◽  
Paul Johnston
Keyword(s):  
Gold Deposit ◽  
Quartz Veins ◽  
Metamorphic History ◽  
Fluid Infiltration ◽  
Mineral Assemblages ◽  
Metamorphic Assemblage ◽  
The Matrix ◽  
History Of ◽  
T Path ◽  
Late Growth

Textural relations between Al2SiO5 phases, and deformation fabrics, provide constraints on the metamorphic history of the Hemlo gold deposit. Kyanite in the deposit is most common within and on the margins of boudinaged quartz ± realgar veins, and less commonly as rotated porphyroblasts within the matrix of schistose rocks. Kyanite predates the main (D2) schistosity. Sillimanite postdates kyanite, occurring irregularly as discrete knots and foliae that run parallel to, but sometimes cut across, the principal (D2) foliation, indicating that sillimanite postdates the D2 foliation. We regard kyanite to be part of the peak metamorphic assemblage, with sillimanite representing a partial later overprint most likely related to fluid infiltration. Rare andalusite occurs in two associations: as late-stage, clean, idioblastic crystals; and as large, fractured grains, locally overprinted by sillimanite, in boudinaged quartz-realgar veins. We suggest two possible origins for this second form of andalusite, one involving generally late growth, the second involving early growth prior to the development of peak metamorphic kyanite. Although not unambiguous, we prefer the second scenario. P-T conditions from petrogenetic grid constraints, and new geothermobarometric estimates, indicate 6-7 GPa, 600-650°C for the peak kyanite grade metamorphism (ca. 2677 Ma?), and 4-5 GPa, 600°C for the later sillimanite overprint (ca. 2672 Ma?). We see these two events as part of an evolving P-T path in a single metamorphic event. In our early andalusite scenario, the andalusite may have formed from pyrophyllite breakdown at 2-4 GPa, 450°C, possibly associated with emplacement of the regional suite of granodiorite plutons (ca. 2686 Ma). Late andalusite formed sporadically on the retrograde path. The occurrence of deformed andalusite and aligned kyanite in and on the margins of boudinaged auriferous realgar-stibnite-quartz veins provides evidence in support of a premetamorphic mineralization event.

A comparative X-ray diffraction, Mössbauer and NMR spectroscopic study of the vermiculites from Béni Bousera, Morocco and Palabora, Republic of South Africa

Clay Minerals ◽  
2002 ◽  
Vol 37 (2) ◽  
pp. 367-376 ◽  
Author(s):  
R. Badreddine ◽  
D. Vandormael ◽  
A. -M. Fransolet ◽  
G. J . Long ◽  
W. E. E. Stone ◽  
...  
Keyword(s):  
South Africa ◽  
Nuclear Magnetic Resonance ◽  
Nmr Spectra ◽  
Magic Angle Spinning ◽  
Magic Angle ◽  
Chemical Analyses ◽  
X Ray Diffraction ◽  
Tetrahedral Sheet ◽  
X Ray ◽  
Angle Spinning

AbstractFive vermiculite samples collected from Béni Bousera, Morocco and four from Palabora, South Africa were investigated by X-ray diffraction, chemical analysis, 57Fe Mössbauer spectroscopy, and 27Al magic angle spinning nuclear magnetic resonance. The X-ray diffraction studies indicate that all vermiculites have very similar crystallographic parameters. The chemical analyses and the NMR spectra indicate that the Béni Bousera vermiculites contain Al3+ cations in both octahedral and tetrahedral sheets and the Palabora vermiculites contain Al3+ in the tetrahedral sheet. The Mössbauer spectra indicate that the Béni Bousera vermiculites contain more Fe2+ cations than the Palabora vermiculites and do not contain tetrahedral Fe3+ cations. The different cation compositions and distribution in the two sets of vermiculites may result from different parent minerals, i.e. chlorite in the case of Béni Bousera and phlogopite in the case of Palabora, and different genetic processes, i.e. weathering in Béni Bousera and hydrothermal alteration in Palabora.

scholarly journals Kaolinization processes in the phyolitic rocks of Kefalos, Kos island, Aegean Sea, Greece

2001 ◽  
Vol 34 (3) ◽  
pp. 867 ◽  
Author(s):  
D. PAPOULIS ◽  
P. TSOLIS - KATAGAS
Keyword(s):  
Electron Microscopy ◽  
Aegean Sea ◽  
Chemical Analyses ◽  
Bulk Rock ◽  
Secondary Mineral ◽  
X Ray Diffraction ◽  
X Ray ◽  
Mineral Assemblages ◽  
Analytical Scanning Electron Microscopy ◽  
Scanning Electron

Kaolin occurrences in the South Kefalos peninsula, Kos island, Aegean sea, Greece, are products of hydrothermal alteration of rocks of rhyolitic composition. The chemical, mineralogical and textural characteristics of kaolin occurrences were investigated by X-Ray diffraction (XRD), Analytical scanning electron microscopy (SEM) and bulk rock chemical analyses. Attempts have been made to correlate the primary with secondary mineral assemblages and to establish the factors controlling the stepwise character of the transition from the fresh rhyolitic to kaolinized rocks. The kaolinization processes follows two main alteration pathways: I) Kfeldspar —» Mixed-layer kaolinite-smectite —» kaolinite —> dickite. II) Na-plagioclase —> kaolinite —> dickite

scholarly journals Robert Millner Shackleton. 30 December 1909 – 3 May 2001

2004 ◽  
Vol 50 ◽  
pp. 285-297
Author(s):  
John Frederick Dewey ◽  
Bernard Elgey Leake
Keyword(s):  
South Africa ◽  
Imperial College ◽  
Chemical Analyses ◽  
Geological Society ◽  
North Wales ◽  
History Of ◽  
Silver Medal ◽  
The Cost ◽  
The Antarctic ◽  
Published Account

Robert Millner Shackleton, who died peacefully in his sleep on 3 May 2001, was born on 30 December 1909 in Purley, Surrey, the son of John Millner Shackleton (an electrical engineer of Irish derivation who, at one time, worked for the Post Office telephones) and Agnes Mitford Shackleton (née Abraham). He was distantly related to the Antarctic explorer Sir Ernest Shackleton and was educated at the Quaker school of Sidcot, which profoundly influenced his subsequent life and career. He entered Liverpool University in January 1927 and graduated with a first–class honours BSc in geology in July 1930 under P. G. H. Boswell FRS, the first George Herdman Professor of Geology. He was only the fourth student in the history of the department to achieve a First. Shackleton's first visit to Africa was as an undergraduate in July to September 1929 to attend the 15th International Congress in Pretoria, South Africa. He always remembered Boswell's help and how he had persuaded him into going and even shared a cabin on the Union Castle ship to South Africa with him to reduce the cost at a time when most professors would not have done so. He saw the Karroo, the Kimberley diamond mine, the Witwatersrand mines, the Bushveld, Rhodesia, and the Drakensberg. This visit to Africa was to be the foundation of his love of Africa, its people and its geology. Shackleton went on to complete a PhD at Liverpool in December 1933 on the Moel Hebog area of North Wales, between Tremadoc and Nantlle, although some of the work was done while at Imperial College, London (IC), where he was Beit Research Fellow from 1932 to 1934, largely facilitated by Boswell, who was also an IC man and had moved back there to the Chair in 1930. The Moel Hebog mapping included examining some cliff faces never scaled by any geologist or, indeed, anyone before; it was part of a systematic re–survey of North Wales encouraged by Boswell, and followed the surveys of Snowdonia by David and Howell Williams. The Moel Hebog mapping was superb and, with his other field achievements, led to his receiving the Silver Medal of the Liverpool Geological Society in 1957. Shackleton was one of several Liverpool students, including one of us (B.E.L.), who from the 1920s onwards did part of their PhD work at IC. He had a petrological training, being taught silicate analysis by A. W. Groves at IC, but the petrological and palaeogeographic interpretation of his PhD area was hindered by the fact that ignimbrites had not yet been recognized and only a few chemical analyses could be completed. The published account (7) è did not appear until 1959 and then only because of the encouragement and devoted help given by Dr J. C. Harper.

scholarly journals Blueschist mylonitic zones accommodating syn-subduction exhumation of deeply buried continental crust: the example of the Rocca Canavese Thrust Sheets Unit (Sesia–Lanzo Zone, Italian Western Alps)

2021 ◽  
Vol 114 (1) ◽  
Author(s):  
Manuel Roda ◽  
Michele Zucali ◽  
Luca Corti ◽  
Roberto Visalli ◽  
Gaetano Ortolano ◽  
...  
Keyword(s):  
Microstructural Analysis ◽  
Oceanic Lithosphere ◽  
Western Alps ◽  
White Mica ◽  
X Ray ◽  
Metamorphic History ◽  
Thrust Sheets ◽  
History Of ◽  
Tectonic Contact ◽  
Elemental Maps

AbstractThe Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of the Sesia–Lanzo Zone (SLZ), bounded by the Periadriatic (Canavese) Lineament that separates the Alpine subduction complex from the Southalpine domain. The RCTU is limited to the south by the Lanzo Massif (LM) and to the east by the Eclogitic Micaschists Complex (EMC). Particularly the tectonic contact area of the RCTU, adjacent to the neighbouring SLZ and the LM is characterised by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ) that was active under blueschist-to greenschist-facies conditions. Despite the dominant mylonitic structure, some rocks (garnet-bearing gneiss, garnet-free gneiss and orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. The scarcity of superposed structures and the small size of relicts impose a detailed microstructural analysis supported by chemical investigation to reconstruct the tectono-metamorphic history of the MZ. Therefore, we integrated the classical meso- and microstructural analysis approach with a novel quantitative technique based on the Quantitative X-Ray Map Analyzer (Q-XRMA), used to classify rock-forming minerals starting from an array of X-ray elemental maps, both at whole thin section and micro-domain scale, as well as to calibrate the maps for pixel-based chemical analysis and end-member component maps, relevant for a more robust conventional geothermobarometer application as well for calculating reliable PT pseudosections. Pre-Alpine relicts are garnet and white mica porphyroclasts in the garnet-bearing gneiss and biotite and K-feldspar porphyroclasts in garnet-free gneiss and orthogneiss, respectively, providing no PT constraints. The Alpine evolution of the MZ rocks, has been subdivided in three deformation and metamorphic stages. The first Alpine structural and metamorphic equilibration stage (D1 event) occurred at a pressure of ca. 1.25–1.4 GPa and at a temperature of ca. 420–510 °C, i.e. under blueschist-facies conditions. The D2 event, characterised by a mylonitic foliation that is pervasive in the MZ, occurred at ca. 0.95–1.1 GPa and ca. 380–500 °C, i.e. under epidote-blueschist-facies conditions. The D2 PT conditions in the MZ rocks are similar to those predicted for the blocks that constitute the RCTU mélange, and they overlap with the exhumation paths of the EMC and LM units. Therefore, the RCTU, EMC and LM rocks became coupled together during the D2 event. This coupling occurred during the exhumation of the different tectono-metamorphic units belonging to both continental and oceanic lithosphere and under a relatively cold thermal regime, typical for an active oceanic subduction zone, pre-dating Alpine continental collision.

X-Ray Fluorescence of Rock Samples as Applied to Geological Problems

1968 ◽  
Vol 22 (5) ◽  
pp. 420-422 ◽  
Author(s):  
Richard A. Laidley
Keyword(s):  
Volcanic Rocks ◽  
Analytical Techniques ◽  
Compositional Variation ◽  
Chemical Analyses ◽  
Geologic History ◽  
X Ray ◽  
History Of ◽  
Precision And Accuracy ◽  
Rock Specimens

Three methods of sample preparation of rock specimens have been evaluated for precision and accuracy. A fusion technique was found to give the best precision. Standard deviations (1 σ) using this method and expressed as percent of the amount present for several elements are: Al 0.83, Si 0.64, K 0.43, Ca 0.43, Mn 0.48, Fe 0.24. Three recent studies of compositional variation were performed on volcanic rocks. Analyses of samples from the Hopi Buttes, Arizona, on obsidian flows from Newberry Caldera, Oregon, and on igneous rocks collected from grids or linear traverses give data which, in each case, are useful in making significant interpretations about the geologic history of the rocks concerned. These examples illustrate the rapidity and high quality of quantitative chemical analyses which can be obtained by application of x-ray fluorescence analytical techniques.

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