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Dehydration veins in diagenetic and very-low-grade metamorphic rocks: features of the crustal seismogenic zone and their significance to mineral facies (1993)

COOMBS, D. S.

Oxford, UK : Blackwell Publishing Ltd

Journal of metamorphic geology 11 (1993), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Abstract Declining temperatures during decay of a hydrothermal system, or during uplift and erosion, tend to result in veins involving progressive hydration reactions, e.g. veins with laumontite cutting prehnitepumpellyite facies rocks, and stilbite veins cutting laumontite veins.In contrast, examples are described of analcime replacement of heulandite along fractures in heulanditized vitric tuff, of replacement of analcime by albite along fractures in quartz-analcime rock, of joint-controlled replacement of heulandite in tuff by laumontite + quartz + (Na, K)-feldspars, of replacement of laumontite by prehnite + quartz along fractures in alumontitized vitric tuff, and of laumontitebearing feldspathic sandstones cut by vein assemblages of quartz and prehnite ° Calcite. The vein mineral assemblage, sometimes with pumpellyite and/or epidote in the prehnite-bearing veins, tends to spread as a zone of dehydration into the adjacent country rock. Except perhaps for albite replacement of analcime, and for laumontite replacement of heulandite, these open-system reactions involve cation activity ratios in the fluid. All involve dehydration. They are favoured by an increase in temperature, and except under certain situations where P-T equilibrium curves have negative slopes, are favoured by a fall in PH2O. Evidence indicates that in at least some cases the triggering mechanism was a drop in PH2O; this may be a widespread phenomenon associated with brittle fracture in the seismogenic upper crust. This may cause fluid pressure to drop from values approaching lithostatic to nearer hydrostatic, and equilibrium may be displaced to yield a less hydrous assemblage that appears as a dehydration vein and vein verge. The dehydration vein assemblage may be diagnostic of a higher grade mineral facies and adds to the mineral complexity attributable to varying permeabilities and fluid pressures in upper crustal strata. Mineral facies are likely to be more uniformly distributed in higher grade rocks from below the brittle-ductile transition zone.Reactions involving complex solid solutions are inappropriate as facies boundaries.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1525-1314.1993.tb00156.x

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Reactions leading to the formation and breakdown of stilpnomelane in the Otago Schist, New Zealand (2000)

Li, G. ; Essene, E. J. ; Peacor, D. R. ; [et al.]

Oxford, UK : Blackwell Science Inc

Journal of metamorphic geology 18 (2000), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Semi-pelitic rocks ranging in grade from the prehnite–pumpellyite to the greenschist facies from south-eastern Otago, New Zealand, have been investigated in order to evaluate the reactions leading to formation and breakdown of stilpnomelane. Detrital grains of mica and chlorite along with fine-grained authigenic illite and chlorite occur in lower-grade rocks with compactional fabric parallel to bedding. At higher grades, detrital grains have undergone dissolution, and metamorphic phyllosilicates have crystallized with preferred orientation (sub)parallel to bedding, leading to slaty cleavage. Stilpnomelane is found in metapelites of the pumpellyite–actinolite facies and the chlorite zone of the greenschist facies, but only rarely in the biotite zone of the greenschist facies. Illite or phengite is ubiquitous, whereas chlorite occurs only rarely with stilpnomelane upgrade of the pumpellyite-out isograd. Chemical and textural relationships suggest that stilpnomelane formed from chlorite, phengite, quartz, K-feldspar and iron oxides. Stilpnomelane was produced by grain-boundary replacement of chlorite and by precipitation from solution, overprinting earlier textures. Some relict 14 Å chlorite layers are observed by TEM to be in the process of transforming to 12 Å stilpnomelane layers. The AEM analyses show that Fe is strongly partitioned over Mg into stilpnomelane relative to chlorite (KD≈2.5) and into chlorite relative to phengite (KD≈1.9). Modified A′FM diagrams, projected from the measured phengite composition rather than from ideal KAl3Si3O10(OH)2, are used to elucidate reactions among chlorite, stilpnomelane, phengite and biotite. In addition to pressure, temperature and bulk rock composition, the stilpnomelane-in isograd is controlled by variations in K, Fe3+/Fe2+, O/OH and H2O contents, and the locus of the isograd is expected to vary in rocks of different oxidation states and permeabilities. Biotite, quartz and less phengitic muscovite form from stilpnomelane, chlorite and phengite in the biotite zone. Projection of bulk rock compositions from phengite, NaAlO2, SiO2 and H2O reveals that they lie close to the polyhedra defined by the A′FM minerals and albite. Other extended A′FM diagrams, such as one projected from phengite, NaAlO2, CaAl2O4, SiO2 and H2O, may prove useful in the evaluation of other low-grade assemblages.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1525-1314.2000.00263.x

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Porphyroblastic manganaxinite metapelagites with incipient garnet in prehnite–pumpellyite facies, near Meyers Pass, Torlesse Terrane, New Zealand (1996)

COOMBS, D. S. ; KAWACHI, Y. ; FORD, P. B.

Oxford : Blackwell Science Ltd

Journal of metamorphic geology 14 (1996), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Moderately manganiferous siliceous pelagites near Meyers Pass, Torlesse Terrane, South Canterbury, New Zealand, have been metamorphosed in the prehnite–pumpellyite facies. A conodont colour index measurement suggests T max in the range 190–300 °C. Porphyroblastic manganaxinite, manganoan pumpellyite, manganoan chlorite and trace spessartine-rich garnet and sphalerite have formed in an extremely fine-grained quartz–albite–berthierine–phengite–titanite groundmass. Porphyroblastic manganaxinite semischists and schists are distinctive rocks in prehnite–pumpellyite to lower-grade greenschist and blueschist facies of New Zealand and Japan. Mn in the manganoan pumpellyites substitutes for Ca in W sites. Total Fe/(Fe+Mg) ratios in chlorite are dependent on oxidation state, being ≤0.22 in red hematitic hemipelagites, and ≥0.61 in low-f O2 grey metapelagites. In the low-f O2 metapelagites, manganoan berthierine with little or no chlorite is inferred in the groundmass and iron-rich chlorite occurs as porphyroblasts and veinlets, whereas in the red rocks, Mg-rich chlorite occurs both in groundmasses and veinlets. Variably high Si in the manganoan chlorites correlates with evidence for contaminant phases. The Mn content of chlorite contributing to garnet growth is dependent on metamorphic grade; incipient spessartine indicates a saturation value of 6–8% MnO in chlorite in low-f O2 rocks at Meyers Pass. Lower MnO contents are recorded for otherwise analogous rocks with increasing metamorphic grade, but at a given grade coexisting chlorite and garnet are richer in Mn where f O2 is high. Manganaxinite and manganoan pumpellyite also contributed to reactions forming grossular–spessartine solid solutions. Formation of garnet in siliceous pelagites is dependent on both Mn and Ca content. The spessartine component increases with grade into the greenschist facies. Partial recrystallization of berthierine to chlorite and the growth of porphyroblastic patches of other minerals was facilitated by brittle fracture and access of fluids to an otherwise impermeable matrix; to this extent the very low-grade metamorphism was episodic.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1525-1314.1996.05783.x

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4

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Palæomagnetic Studies of Cenozoic Volcanic Rocks in New Zealand (1959)

COOMBS, D. S. ; HATHERTON, T.

[s.l.] : Nature Publishing Group

Nature 184 (1959), S. 883-884

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] POSTULATED reversals of the geomagnetic field1 Jr offer a potential tool for the chronological subdivision and correlation of lavas within a restricted volcanic province2"4. On a broader scale they provide potential world-wide datum planes of virtually instantaneous time significance so far as most ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/184883a0

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5

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Pitcairn Island—another Pacific hot spot? (1974)

Duncan, R. A. ; McDougall, I. ; Carter, R. M. ; [et al.]

[s.l.] : Nature Publishing Group

Nature 251 (1974), S. 679-682

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] The volcanic Pitcairn Island, which has surface lavas with apparent ages between 0.45 and 0.93 Myr, may be the youngest member of another NW-SE trending volcanic lineament in the Pacific ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/251679a0

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6

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Pumice from the South Sandwich Eruption of March 1962 reaches New Zealand (1966)

COOMBS, D. S. ; LANDIS, C. A.

[s.l.] : Nature Publishing Group

Nature 209 (1966), S. 289-290

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] On January 24, 1964, H.M.N.Z.S. Pukaki collected pea- and marble-sized pieces of pumice floating in abundance south of Macquarie Island at 60 S., 160 E., and these, together with a larger block carrying barnacles, were forwarded to the University of Otago. During the southern autumn and winter of ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/209289b0

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7

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Clinopyroxenes in the tawhiroko tholeiitic dolerite at Moeraki, north-eastern Otago, New Zealand (1973)

Nakamura, Yasuo ; Coombs, D. S.

Springer

Contributions to mineralogy and petrology 42 (1973), S. 213-228

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Clinopyroxenes from various lithologic units in a single tholeiitic dolerite intrusion 50 m thick were analysed with the electron microprobe. Microphenocrysts of augite in the chilled marginal rock have the least scattered Ca∶Mg∶Fe ratios, around Ca38Mg50Fe12, and lowest Ti/Al ratios, less than 1/4, of the augites examined; they crystallized prior to emplacement. The augites which crystallized in situ are more Fe-rich and have higher Ti/Al ratios, close to 1/2. Pigeonite started crystallizing later than augite and formed rims to augite crystals. Continuous zoning from augite to subcalcic augite as was previously reported by Benson (1944) for the same dolerite is not observed. Subcaloic ferroaugites, however, link augite and ferropigeonite in the outermost margins of pyroxenes in the later segregations. The small scale differentiation in situ gave rise to globules, veins, and pegmatoids, in which sector-zoning of augite is well developed. The {100} sectors are enriched in MgSiO3, FeSiO3, and MnSiO3 components relative to the {010} and the {110} sectors, and are accordingly depleted in other components. The protosite theory of sector-zoning formation proposed by Nakamura (1973) is further developed and the environment which favours the development of sector-zoning is discussed. The formation of augite rims around quartz-rich xenoliths and the compositional characteristics of the augites concerned are due to local high silica activities. Comments are made concerning some lunar pyroxenes in the light of the present study.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00371586

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8

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Andradite and andradite-grossular solid solutions in very low-grade regionally metamorphosed rocks in Southern New Zealand (1977)

Coombs, D. S. ; Kawachi, Y. ; Houghton, B. F. ; [et al.]

Springer

Contributions to mineralogy and petrology 63 (1977), S. 229-246

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Almost pure andradite and intermediate members of the andradite-grossular series (gros40–49, and 47–54, py0–3, alm0–3, spess0–2, hydrogarnet0–3), often framboidal in habit, are widespread in metabasites including lavas, minor intrusions, and volcanic sandstones and breccias metamorphosed under prehnite-pumpellyite and pumpellyite-actinolite facies conditions, possibly extending into the zeolite facies. Coexisting phases include iron-rich epidotes (100 Fe*/Fe*+Al=22–34), pumpellyite, prehnite, actinolite, and chlorite, electron microprobe analyses of which are given, as well as quartz, albite, and calcite. Zoisite (100 Fe*/Fe*+Al=1–5) and iron-poor epidote (100 Fe*/Fe*+Al=11–18) occur in 2 rocks in pseudomorphs after plagioclase together with more iron-rich epidote, but not in close association with the garnets. Coexisting pumpellyite is iron-rich (FeO* 9–14%) in the prehnite-pumpellyite facies and iron-poor (FeO* 5%) in the pumpellyiteactinolite facies. Chlorites and actinolites vary widely and sympathetically in FeO/MgO+FeO ratio. Andradite is also described from a stilpnomelane-actinolite-hematite-bearing andradite quartzite of the pumpellyite-actinolite facies. Conditions of formation involved temperatures of 300 to 400 ° or less, at pressures up to a few kilobars. A wide range of oxygen fugacities is possible, but $$\mu _{CO_2 } $$ in the fluid phase was low. Grandite and chlorite are incompatible in the pumpellyite-actinolite and greenschist facies in the presence of quartz but the 2 minerals occur together in some pumpellyite-actinolite facies assemblages as a result of incomplete reaction and/or local deficiency in silica. In the greenschist facies the association is replaced by epidote-actinolite±hematite and sodic amphibole. Whereas at medium to high grades of metamorphism andradite and grandite are characteristic of skarns irrespective of $$\mu _{CO_2 } $$ , at very low grades they are found in mafic volcanic rocks and volcanogenic sediments as well as in certain cherty rocks of unusual composition, rodingites, and serpentinites, where $$\mu _{CO_2 } $$ was very low.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00375574

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An occurrence of ardennite in quartz veins in piemontite schist, Western Otago, New Zealand (1993)

Coombs, D. S. ; Kawachi, Y. ; Reay, A.

Springer

Mineralogy and petrology 48 (1993), S. 295-308

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ISSN: 1438-1168

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Description / Table of Contents: Zusammenfassung Ardennit mit der Zusammensetzung (Mn2+ 3.488Ca0.509Ba0.002)Σ=4(Mg0.916Fe3+ 0.165Mn3+ 0.099Cu0.033Ni0.009Zn0.006 Ti0.008Al4.764)Σ=6(As5+ 0.823V5+ 0.022P0.005B0.069Al0.042Si5.039)Σ=6O21.81(OH)6.17 tritt in Crack-seal-Quarzgängen in Quarz-Albit-Piemontit-Spessartin-Phengit-Hämatit-Chlorit-Rutil-Turmalin ± Calcit-Schiefern der Haast Schiefer-Gruppe nahe der Arrow Junction, West-Otago, Neuseeland, auf. Die Proportionen von Mn2+/Mn3+ hängen von der Kalkulation und der Genauigkeit der Analyse ab. Bor wird zum ersten Mal im Ardennit bestimmt. Andere Eigenschaften sind:α = 1.734(3),β = 1.735(3),γ = 1.751(3), 2Vz = 30(2)°; a = 8.721(1), b = 5.816(1), c = 18.545(3) Å, V = 940.7(2) Å3. Assoziierte Mineralphasen sind Spessartin, Hämatit, Piemontit, der ≈ 0.7% SrO und 0.06% PbO enthält und Phengit. Spät gebildete Gangmineralien, wie Chlorit, Albit und Mn-Calcit, sind unter geringer oxidierenden Bedingungen entstanden. Digenit mit etwas Covellin tritt in kleinen Mengen zusammen mit Mn-Calcit und Quartz in einem querschlägigen Gang auf, Chalcopyrit und gediegenes Kupfer kommen in anderen späten Gängen vor. Arsen und andere Komponenten des Ardennites and der assoziierten Minerale können von hochoxidierten, Fe-Mn-Oxid- und Hydroxyd-führenden, Sireichen, pelagischen Sedimenten hergeleitet werden, die das Ausgangsgestein für den Piemontit darstellen. Die Gänge wurden in einem relativ frühen Stadium, nach dem Metamorphosehöhepunkt, innerhalb der Chloritzone der Grünschiefer-Fazies, unter ungefähr 4.5 kbar und 390°C, gebildet.

Notes: Summary Ardennite of complex composition: (Mn2+ 3.488Ca0.509Ba0.002)Σ=4(Mg0.916916 Fe3+ 0.165 Mn3+ 0.099Cu0.033Ni0.009Zn0.006 Ti0.008Al4.764)Σ=6(As5+ 0.823V5+ 0.022P0.005B0.069Al0.042Si5.039)Σ=6O21.81(OH)6.17 occurs in crack-seal quartz veins in quartz-albite-piemontite-spessartine-phengitehematite-chlorite-rutile-tourmaline ± calcite schist of the Haast Schist Group near Arrow Junction, western Otago, New Zealand. The Mn2+/Mn3+-ratio is sensitive to calculations and to accuracy of analyses. Boron is detected in ardennite for the first time. Other properties includeα = 1.734(3),β = 1.735(3),γ = 1.751(3), 2VZ = 30(2)°;a = 8.721(1),b = 5.816(1),c = 18.545(3) Å,V = 940.7(2) Å3. Associated mineral phases are spessartine, hematite, piemontite containing ≈0.7% SrO and ≈0.06% PbO, and phengite. Later-stage vein minerals comprise chlorite, albite, and manganoan calcite which were deposited under less highly oxidizing conditions. Digenite with minor intergrown covellite occurs in small amount with manganoan calcite and quartz in a cross-cutting late-stage veina chalcopyrite and native copper occur in other late-stage veins. Arsenic and other components of the ardennite and associated minerals are derived from highly oxidized ferromanganese oxide- and hydroxide-bearing siliceous pelagic sediments that formed the protolith for the piemontite schist. The veins formed at a relatively early stage after metamorphism peaked in the chlorite zone of the greenschist facies under conditions that have been estimated at about 4.5 kbar, 390 °C.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01163105

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