Pre‐Alpine eclogites in the Pennine Basement Complex of the Eastern Alps

Droop, G. T. R.

doi:10.1111/j.1525-1314.1983.tb00261.x

Cited by 17 publications

(7 citation statements)

References 22 publications

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“…diopside-oligoclase intergrowths as very fine-grained vermicular symplectite. Similar intergrowths were described in retrograded eclogites of Norway (Mysen, 1972;Dunn & Medaris, 1989), in pre-Alpine eclogites of the Eastern Alps (Droop, 1983) and in VendBe, France (the 'kelyphitoid' of Bribre, 1920, quoted in Godard, 1988. During a later stage of retrogression, symplectitic diopside is replaced by hornblende and a kelyphite rim of emerald green hornblende, plagioclase f clinozoisite develops around garnet.…”

Section: Petrography Eclogitesupporting

confidence: 65%

Petrology of very‐high‐pressure eclogitic rocks from the Brossasco‐Isasca Complex, Dora‐Maira Massif, Italian Western Alps

Kiénast

Lombardo

Biino

et al. 1991

Journal Metamorphic Geology

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Petrographical and mineral chemical data are given for the eclogites which occur in the garnet‐kyanite micaschists of the Penninic Dora‐Maira Massif between Brossasco, Isasca and Martiniana (Italian Western Alps) and for a sodic whiteschist associated with the pyrope‐coesite whiteschists of Martiniana. The Brossasco‐Isasca (BI) eclogites are fine grained, foliated and often mica‐rich rocks with a strong preferred orientation of omphacite crystals and white micas. Porphyroblasts of hornblende are common in some varieties, whilst zoisite and kyanite occur occasionally in pale green varieties associated with leucocratic layers with quartz, jadeite and garnet. These features differentiate the BI eclogites from the eclogites that occur in other continental units of the Western Alps, which all belong to type C. Garnet, sodic pyroxene and glaucophane are the major minerals in the sodic whiteschist. Sodic pyroxene in the eclogites is an omphacite often close to Jd50Di50, with very little acmite and virtually no AlIV, and impure jadeite in the leucocratic layers and in the sodic whiteschist. Garnet is almandine with 20–30 mol. % for each of the pyrope and grossular components in the eclogites and a pyrope‐rich variety in the sodic whiteschist. White mica is a variably substituted phengite, and paragonite apparently only occurs as a replacement product of kyanite. Amphibole is hornblende in the eclogites, but the most magnesian glaucophane yet described in the sodic whiteschist. Quartz pseudomorphs of coesite were found occasionally in a few pyroxenes and garnets. The P‐T conditions during the VHP event are constrained in the eclogites by reactions which define a field ranging from 27–28 kbar to 35 kbar and from 680 to 750° C. These temperatures are consistent with the results of garnet‐pyroxene and garnet‐phengite geothermometry which suggest that the eclogites may have equilibrated at around 700° C. In the sodic whiteschist pressures ranging from 29 to 35 kbar can be deduced from the stability of the jadeite‐pyrope garnet‐glaucophane compatibility. As in the eclogites water activity must have been low. Such conditions are close to the P‐T values estimated for the early Alpine recrystallization of the pyrope‐coesite rock and, like petrographical and mineralogical features, set aside the BI eclogites from the other eclogites of the Western Alps, instead indicating a close similarity to some of the eclogite bodies occurring in the Adula nappe of the Central Alps. An important corollary is that glaucophane stability, at least in Na‐ and Mg‐rich compositions and under very high pressures, may extend up to 700° C, in agreement with the HT stability limit suggested by experimental studies.

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Section: Petrography Eclogitesupporting

confidence: 65%

Petrology of very‐high‐pressure eclogitic rocks from the Brossasco‐Isasca Complex, Dora‐Maira Massif, Italian Western Alps

Kiénast

Lombardo

Biino

et al. 1991

Journal Metamorphic Geology

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“…Zoisite occurs sometimes as inclusions within garnet.The textures present, the diopside-plagioclase symplectites and garnets rimmed by plagioclase coronas, are typical of those occurring in many eclogites (e.g Mysen and Griffin, 1973;. DeWit and Strong, 1975;Droop, 1983). They are consistent with the operation of a multivariant eclogite amphibolite reaction of the form: garnet + omphacite + quartz + H20 --+ hornblende + plagioclase.Chemical mineralogy.…”

supporting

confidence: 55%

Eclogitic amphibolites from the Grampian Moines

Baker

1986

Mineral. mag.

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A B S TR A C T. Amphibolites from the Grampian Moines of Scotland contain clinopyroxene-plagioclase symplectites and plagiodase-hornblende coronas around garnet. The clinopyroxene-plagioclase symplectites are interpreted as former omphacites. It is inferred that an eclogitic garnetomphacite quartz assemblage once existed, but has since undergone partial reaction to an amphibolite facies hornblende-plagioclase assemblage. An early highpressure metamorphic event occurred in parts of the Moine.

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“…Neverthe1ess, Ihe amphibole's composilion (high A1 v 1 and Na COn¡Cnls), Is consislent with crystallization under high pressure conditions (see Ungareui el (1/., bolite-facies event must be responsible for the absence of characteristic zonation in garnets and consequent homogeneity (see also Yardley, 1977;1983). Furthermore, the Na/(Na + Ca) vs. Al/(Si + Al) ratios of amphiboles indicate pressure between the trends of high and medium pressure amphiboles (closer to the high pressure trend), according to Laird and Albee (1981) discrimination diagrams.…”

Section: Resulls and Discussionmentioning

confidence: 99%

“…Varied assemblages characterized as xenoliths (garnet bearing peridotites, garnet clinopyroxenites, eclogites «sensu lato», and similar mafic rocks) hosted in various lithologic environments and subjected to varied retrograde processes, have been reported in literature (e.g., Arculus and Smith, 1979;Sculze and Helmstaedt, 1979;Droop, 1983;Baker, 1986;Dodge et al, 1988;Mukhopadhyay, 1991).…”

Section: Introductionmentioning

confidence: 99%

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Kassoli-Fournaraki¹

1995

Estud. geol.

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A small lensoidal garnetiferous body, consisting of garnet + amphibole + epidote + quartz, and occuring within the gneisses, amphibolites and migmatites of the eastern part of the Chalkidiki main body, is studied in terms of petrography, mineralogy and metamorphic conditions. Garnet corresponds to almandine rich almandine-grossular solid solution and displays negligible core-rim compositional variations. Garnet grains exhibit alteration to amphibole (mainly ferropargasitic hornblende and ferropargasite) and epidote. The chemical composition of garnet and amphibole imply conditions of upper amphibolite facies. It is considered that the studied garnetiferous lens may represent a retroeclogite, lacking any relic eclogitic assemblage, which completely recrystallized and equilibrated extensively under the prevalent high-T amphibolite facies. The slight enrichment of epidote in Ce, La, Y, and consequent zoning are attributed to compositional variations of the fluid phase during the amphibolitization stage. RESUMENSe estudia la petrografía, mineralogía y condiciones metamórficas de un pequeño cuerpo lenticular, constituido por granate + anfidol + epidota + cuarzo, que aparece incluido en los gneises, anfibolitas y migmatitas del sector oriental de Chalkidiki. El granate es rico en almandino y muestra una pequeña variación composicional centro-borde. Los cristales de granate están parcialmente alterados a anfidol (hornblenda ferropargasí-tica y ferropargasita) y epidota. La composición química de granates y anfíboles, implican condiciones de la facies de las anfibolitas. Se considera que estos cuerpos granatífe-ros representan retroeclogitas que han recristalizado y se han reequilibrado completamente bajo las nuevas condiciones metamórficas. El ligero enriquecimiento en La, Ce e Y de la epidota y la consecuente zonación se atribuyen a las variaciones de composición de la fase fluida durante el estadio de anfibolitización.

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Pre‐Alpine eclogites in the Pennine Basement Complex of the Eastern Alps

Cited by 17 publications

References 22 publications

Petrology of very‐high‐pressure eclogitic rocks from the Brossasco‐Isasca Complex, Dora‐Maira Massif, Italian Western Alps

Petrology of very‐high‐pressure eclogitic rocks from the Brossasco‐Isasca Complex, Dora‐Maira Massif, Italian Western Alps

Eclogitic amphibolites from the Grampian Moines

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