Isabella Memmi

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Hemin Koyi

Uppsala University

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Università di Bologna

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University of Salamanca

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Università degli Studi "La Sapienza" di Roma

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Hungarian Academy of Sciences

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Adam Mickiewicz University in Poznań

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Vrije Universiteit Amsterdam

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Mehrstufige metamorphe Reequilibration eklogitischer Gesteine aus dem hercynischen Basement NE Sardiniens (Italien

Mineralogy and Petrology, 1998

Eclogitic rocks are hosted within gneisses and migmatites of the Hercynian basement of NE Sardini... more Eclogitic rocks are hosted within gneisses and migmatites of the Hercynian basement of NE Sardinia. They are characterized by two compositional layers: garnet-pyroxene rich-layers and amphibole-plagioclase layers. The former contain structural, mineralogical and compositional relics of eclogite facies re-equilibration. Four stages of evolution have been identified: an eclogite stage, a granulite stage and a retrograde amphibolite to greenschist stage. A possible pre(?)-eclogite stage is documented by inclusions of euhedral tschermakitic amphibole + zoisite within the core of garnet. This early stage was followed by an increase in pressure under which the eclogite climax developed (T up to 700 °C, P =13-15 kbar), as documented by omphacite inclusions towards the rim of garnet. Characteristic mineral reactions after the eclogitic stage are: omphacite → diopside + plagioclase (symplectite) and garnet → orthopyroxene + plagioclase. These reactions testify the presence of a granulite stage during which the peak of metamorphism was reached (T up to 870'C, P ≈ 10 kbar). The orthopyroxene → cummingtonite + quartz and garnet + diopside → hornblende + plagioclase (kelyphite) transformations indicate extensive amphibolite retrogression (T = 550650 °C, P = 3-7 kbar). Finally, actinolite and chlorite developed (greenschist stage) at falling temperature and pressure (T = 300-400 °C, P < 2-3 kbar). Eklogitische Gesteine kommen in Gneisen und Migmatiten des hereynischen Basements in NE Sardinien vor. Sie sind durch einen modalen Zweitagenbau von einerseits Granat-Pyroxen andererseits Amphibol-Plagioklas charakterisiert. In ersteren sind Relikte einer strukturellen, mineralogischen und mineralchemischen Reequilibration unter eklogitfaziellen Bedingungen enthalten. Vier Entwicklungstadien sind unterscheidbar: ein Eklogit-, ein Granulit-, sowie ein retrogrades Amphibolit- und Grünschieferstadium. Ein mögliches prä(?)-eklogitisches Stadium ist durch die Einschlüsse von tschermakitischem Amphibol + Zoisit in Granatkernen angedeutet. Auf dieses Frühstadium folgte, dokumentiert durch Omphaziteinschlüsse im Granatrandbereich, Druckzunahme und es wurden eklogitfazielle Bedingungen (T bis zu 700 °C, P = 13-15 kbar) erreicht. Charakteristische Mineralreaktionen nach der Eklogitbildung sind Omphazit → Diopsid + Plagioklas (Symplektite) und Granat → Orthoyroxen + Plagioklas. Diese Reaktionen belegen das Vorliegen eines Granulitstadiums, während dem der Metamorphosehöhepunkt (T bis 870°C, P ≈ 10 kbar) erreicht wurde. Die Umwandlungen von Orthopyroxen → Cummingtonit + Quarz und Granat + Diopsid → Hornblende + Plagioklas (Kelyphit) belegen eine intensive retrograde amphibolitfazielle Überprägung (T = 550-650°C, P = 3-7 kbar). Aktinolith und Chlorit (Grünschieferstadium) bildeten sich bei weiter fallenden Temperaturen und Drucken (T = 300-400 °C, P < 2-3 kbar).

Li, B-rich Rhaetian metabauxite, Tuscany, Italy: reworking of older bauxites and igneous rocks

Chemical Geology, 1998

Li, B-rich metabauxite layers occur in two outcrops of talus metabreccias intercalated between No... more Li, B-rich metabauxite layers occur in two outcrops of talus metabreccias intercalated between Norian and Liassic marbles in the Alpi Apuane, Tuscany, Italy. Field evidence, petrographical, geochemical, and paleogeographic, paleoclimatologic considerations suggest that bauxitic material was derived from a multistage reworking of Late Carboniferous–early Autunian primary deposits now completely removed. The identified lithotypes are: cookeite-rich metabauxite, cookeite-bearing metabreccia, chloritoid schist

Ca distribution between almandine-rich garnet and plagioclase in pelitic and psammitic schists from the metamorphic basement of North-Eastern Sardinia

Contributions To Mineralogy and Petrology, 1982

Garnet and plagioclase pairs from fourteen selected samples, from garnet to sillimanite zones, co... more Garnet and plagioclase pairs from fourteen selected samples, from garnet to sillimanite zones, collected along a NS traverse throughout the metamorphic basement of NE Sardinia, have been analyzed by microprobe. Beyond the garnet isograd, plagioclase has albitic composition and the garnet (a Ca-rich almandine) shows Ca/ Ca+Mg+Fe ratios of about 0.35–0.30, fairly constant from core to rim. Towards the North, still in the garnet zone, when on the large albitic core of plagioclase a thin and discontinuous oligoclasic rim (An22–An18) formed, we observe in the garnet edge an abrupt decrease of the Ca/Ca+Mg+Fe ratio (0.27–0.16). In the staurolite and sillimanite zones garnet does not show significant Ca-zoning and it is characterized by low Ca content (Ca/Ca+Mg+Fe<0.1); the coexisting plagioclase has oligoclasic (An16–An27) composition. The chemical data and the microstructural evidence on growth time indicate that the garnet and plagioclase had a strong mutual interference in determining the relative Ca distribution. The most relevant reactions are discussed and, in particular, the antipathetical Ca-zoning, recorded by garnet and plagioclase in the garnet zone, is considered as the evidence of temperature increase during growth of the two minerals. It is also suggested that the sharp variation of Ca content at the garnet edge was controlled by the discontinuous nature of plagioclase solid solution in the “peristeritic” range. The order of appearance of garnet and oligoclase in the basement of NE Sardinia is also discussed in comparison with other well known metamorphic sequence (Vermont, New Zealand and Dalradian). It is concluded that the different order of appearance is controlled other than the different nature of the calcic phases in the lower grade zones also by the $X_{CO_2 }$ in the fluid phase.

Sudoite, a rock-forming mineral in Verrucano of the Northern Apennines (Italy) and the sudoite-chloritoid-pyrophyllite assemblage in prograde metamorphism

Contributions To Mineralogy and Petrology, 1989

Sudoite, the di-trioctahedral chlorite with ideal composition (Mg2Al)(Al2)(Si3Al)O10(OH)8 is a wi... more Sudoite, the di-trioctahedral chlorite with ideal composition (Mg2Al)(Al2)(Si3Al)O10(OH)8 is a widespread rock-forming mineral in meta-siltstones and psammites of the Verrucano sequence of the Northern Apennines. Sub-ellipsoidal aggregates, probably derived from muscovite clasts, consisting of sudoite, pyrophyllite and muscovite, are common; sudoite may also occur as thin blades in the rock matrix. The co-existence of sudoite, Ferich chloritoid and pyrophyllite, reported here for the first time, has been observed in specimens from the M. Argentario and Monticiano-Roccastrada areas. This three-phase assemblage, diagnostic of a specific metamorphic facies, may be a tool for detailed zonation of low-grade terranes.

Isabella Memmi

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