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Continuous reactions with amphibole, garnet and plagioclase (1995)

ZINGG, A. J.

Oxford, UK : Blackwell Publishing Ltd

Journal of metamorphic geology 13 (1995), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Abstract End-member, continuous and degenerate reactions are derived for the multisystem with the six components Na2O, CaO, (Mg/Fe)O, Al2O3, SiO2, H2O among the phases plagioclasess, garnetss, amphiboless, cpx, opx, olivine, spinel, quartz and an aqueous fluid. The chemography of this system is degenerate due to the co-linearity 2Opx = Ol + Qtz. This co-linearity has its implications both on reaction space and phase equilibria.From a total of 28 reaction systems, reaction space is derived for nine subsystems (phases in parentheses are absent): Case A1: (Cpx,Ol) (Cpx,Opx) and (Cpx,Qtz), Case A2: (Spl,Ol) (Spl,Opx) and (Spl,Qtz), Case B: (Ol,Opx) (Ol,Qtz) and (Opx,Qtz).In the absence of either cpx or spl (case A), three reactions form an invariant point, either [Cpx] or [Spl], where the co-linear phases olivine, opx and quartz coexist on the transformation line 2Opx = Ol + Qtz. Changing mineral compositions force invariant points to move along the line with the different reaction curves changing their relative position according to Schreinemakers’rules. Zero contours, i.e. the location where (a) phase(s) disappear(s) in reaction space correspond to singular points in phase diagrams. Two types are distinguished; singular points of indispensable and of substitutable phases. In the first case the phase disappears from the entire bundle while in the second it disappears from a single reaction. In the specific case where the substitutable phases are also the co-linear ones, two of the three co-linear phases disappear simultaneously. Two of the three reaction curves coincide.In the system including Cpx and Spl (Case B) three reactions, (Ol,Opx) (Ol,Qtz) and (Opx,Qtz), oppose three invariant points, [Ol], [Opx] and [Qtz]. Invariant points no longer move along the line 2Opx = Ol + Qtz. The coincidence of the zero contours of all three co-linear phases in reaction space-the result of the chemographic degeneracy-causes the respective singular points to coincide in the phase diagrams. This is the location where curves must be rearranged in a bundle to conform Schreinemakers’rules.The reaction Grs1Prp2= 2 Ol + An is fourth order degenerate and part of all nine subsystems (cases A and B). It can be used to relate the different phase diagrams to one another.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1525-1314.1995.tb00230.x

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7-(2'-Deoxy-α-D-ribofuranosyl)hypoxanthine (1996)

Marfurt, J. ; Stulz, E. ; Trafelet, H. U. ; [et al.]

Oxford [u.a.] : International Union of Crystallography (IUCr)

Acta crystallographica 52 (1996), S. 713-716

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ISSN: 1600-5759

Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001

Topics: Chemistry and Pharmacology , Geosciences , Physics

Notes: The α-configured N7 analogue of 2′-deoxyinosine [7-(2′-deoxy-α-D-ribofuranosyl)hypoxanthine, C10H12N4O4] shows the following structural characteristics: (i) the furanose part of the molecule adopts the 2′-endo conformation [pseudorotation phase angle 166.4 (2)°]; (ii) the torsion angle χ (O4′—C1′—N7—C5) is syn [70.9 (1)°] with the base substituent pointing away from the sugar unit; (iii) the nucleobases are not hydrogen bonded to each other; only sugar-base hydrogen bonds between adjacent molecules were detected.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1107/S0108270195012662

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Palaeobotanical and geochronological evidence for the Alpine age of the metamorphism in the Sesia-Zone (1974)

Scheuring, B. ; Ahrendt, H. ; Hunziker, J. C. ; [et al.]

Springer

International journal of earth sciences 63 (1974), S. 305-326

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ISSN: 1437-3262

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Description / Table of Contents: Abstract For a long time the age of the last metamorphism of the Sesia-zone was considered to be Hercynian or older. Basement inclusions in basic volcanics were the main argument for this interpretation. The Trachyandesites — Andesites of the Sesia-zone were regarded as Permian, analogoues to the widespread Permian volcanics of the Southern Alps. Recently, plant remnants have been found in tuffitic interlayers of the Sesia volcanics. These fossils have been described as palaeozoic plants, in contradiction to structual observations and to the numerous radiometric data acquired in the region. To check the age of the fossils a detailed palaeobotanical study was carried out. Our samples contained a very modern flora of definitely Tertiary age. Not one palaeozoic fossil was detected. The Tertiary age found on palaeobotanical evidence proved to be between 29 and 33 m. y. on the basis of total rock K-Ar ages. The last high pressure metamorphism of the Sesia-zone occurred between 90 and 60 m. y. as has been shown by radiometric ages on micas. The external part of the zone was overprinted by the Lepontine phase of metamorphism in greenschist facies 38 m. y. ago.

Abstract: Résumé L'âge du dernier métamorphisme de la zone de Sesia a été pendant longtemps considéré comme hercynien ou plus ancien. Le principal argument à l'appui de cette interprétation consistait dans la présence d'inclusions cristallines du socle dans les trachyandésites et andésites situées dans la bordure interne de la zone de Sesia, ces roches volcaniques étant regardées comme permiennes par analogie avec le volcanisme permien des Alpes méridionales. On a trouvé récemment, dans les tuffites intercalées dans ces roches volcaniques, des fossiles végétaux qui ont été attribués au Carbonifère supérieur. Toutefois cette interprétation s'opposait aux observations structurales et aux nombreux âges radiométriques obtenus dans la zone de Sesia, ce qui a conduit, à une vérification détaillée des arguments paléobotaniques. La flore que nous avons trouvée est une flore très récente, d'âge tertiaire indubitable. On n'a pas trouvé un seul fossile paléozoÏque. Cet âge tertiaire basé sur la paléobotanique a été précisé par des mesures d'âge K-Ar effectuées sur roches totales, qui ont donné 29–33 m. a. La dernière phase métamorphique de haute pression, dans la zone de Sesia-Lanzo, a eu lieu entre 60 et 90 m. a., comme le montrent les âges radiométriques de micas. La partie de cette zone, proche des Alpes, a été soumise ultérieurement à la phase de métamorphisme lépontienne, qui s'est déroulée, il y a 38 m. a., dans un faciès de schistes verts.

Notes: Zusammenfassung Das Alter der letzten Metamorphose der Sesia-Zone wurde lange Zeit für herzynisch oder älter gehalten. Als Argument dienten Grundgebirgseinschlüsse in den Trachyandesiten bis Andesiten der Sesia-Zone. Diese Vulkanite hielt man in Analogie zu den permischen Vulkaniten der Südalpen für permisch. In jüngster Zeit wurden in Tuffiten der Sesia-Vulkanite Pflanzenreste gefunden, die als oberkarbonisch beschrieben wurden. Diese Schlüsse standen in Diskrepanz zu strukturgeologischen Beobachtungen und zu den zahlreichen radiometrischen Altersbestimmungen in der Sesia-Zone und zwangen zur nochmaligen überprüfung der paläobotanischen Evidenzen. Die von uns gefundene Flora weist sehr moderne Züge auf und läßt sich mit Sicherheit ins Tertiär einstufen. Kein einziges paläozoisches Fossil wurde gefunden. Das durch die Pflanzenreste bestätigte tertiäre Alter der Vulkanite konnte mit radiometrischen K-Ar-Gesamtgesteinsaltern auf 29–33 m.y. präzisiert werden. Die letzte Hochdruck-Metamorphose in der Sesia-Zone fand nach K-Ar-Altersbestimmungen an Glimmern zwischen 60 und 90 m. y. statt. Vor ca. 38 m. y. wurden die alpeneinwärts liegenden Partien der Sesia-Zone von der lepontinischen Kristallisationsphase in Grünschieferfazies überprägt.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01820989

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A Nd and Sr isotopic study of the Ivrea zone, Southern Alps, N-Italy (1987)

Voshage, H. ; Hunziker, J. C. ; Hofmann, A. W. ; [et al.]

Springer

Contributions to mineralogy and petrology 97 (1987), S. 31-42

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The Ivrea zone represents a tilted cross section through deep continental crust. Sm-Nd isotopic data for peridotites from Baldissero and Balmuccia and for a suite of gabbros from the mafic formation adjacent to the Balmuccia peridotite provide evidence for an event of partial melting 607±19 Ma ago in an extended mantle source with ɛ 607 Nd =+0.4±0.3. The peridotites are interpreted as the corresponding melt residue, the lower part of the mafic formation as the complementary melts which underwent further differentiation immediately after extraction. The Finero body represents a complex with layers of phlogopite peridotite, hornblende peridotite, and amphibole-rich gabbro. The isotopic signatures fall into two groups: (1) highly radiogenic Nd and low-radiogenic Sr characterize the phlogopite-free, amphibole-rich rocks, whereas (2) low-radiogenic Nd and highly radiogenic Sr is found in ultramafics affected by “phlogopite metasomatism”. Phlogopite metasomatism in the Ivrea zone is dated by a Rb-Sr whole rock isochron yielding 293±13 Ma. It was fed by K-rich fluids which were probably derived from metasediments. The high initial ɛ 293 Nd value of about +7.5 for phlogopite-free samples indicates a high time-integrated Sm/Nd ratio in the Finero protolith 293 Ma ago. Sm-Nd analyses of metapelites from the paragneiss series yield Proterozoic “crustal residence ages” of 1.2 to 1.8 Ga. Internal Sm-Nd isochrons for three garnetiferous rocks show that closure of garnet at temperatures around 600° C or even lower occurred about 250 Ma ago.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00375212

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