Pumpellyite in low-grade metamorphic rocks from Ligurian and Lucanian Apennines,Maritime Alps and Calabria (Italy) |
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| Authors: | Luciano Cortesogno Gabriella Lucchetti Piera Spadea |
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| Institution: | (1) Istituto di Petrografia, Università di Genova, I-16132 Genova, Italy;(2) Istituto di Mineralogia, Università di Genova, I-16132 Genova, Italy;(3) Istituto di Scienze della Terra, Università di Udine, I-33100 Udine, Italy |
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| Abstract: | In the Ligurian Apennines and the Brianzonese area of the Maritime Alps (Northern Italy), and in the Lucanian Apennines and Calabria (Southern Italy), pumpellyite formed during the Alpine metamorphism is widespread in terranes of Late Paleozoic to Miocene age, particularly in mafic ophiolites (Tethyan ophiolites of Jurassic-Early Cretaceous age).Pumpellyite developed under variable metamorphic conditions, which include zeolite, prehnite-pumpellyite, pumpellyite-actinolite, lawsonite-albite and blueschist facies. Pumpellyite from rocks belonging to all these facies was studied in 30 specimens of different chemistry, derived mostly from ophiolitic basalt and gabbro, but also from gneiss, amphibolite, and greywacke protoliths.Microprobe data give evidence of strong compositional variations of pumpellyite. The ranges are extreme for Al and Fetot (Al2O3=18.74–30.91; FeOtot=0.46–12.71), to a lesser extent for Mg (MgO=0.58–4.00), with a reciprocal variation of Al and Fetot which suggests that most Fe is Fe3+.Pumpellyite compositions can be related with the metamorphic conditions, particularly an increase of the Al/Fetot ratio with increasing pressure, but larger compositional variation for each facies in the Al-Fetot-Mg diagram than those previously described was recognized. Particularly the compositions of pumpellyite from the pumpellyite-actinolite and blueschist fades assemblages extend towards higher Fetot contents. Moreover, pumpellyites of the lawsonite-albite facies rocks result to be more aluminous than in blueschist facies; such relations appear to be inconsistent with that expected by the effects of pressure on the Al content.As the rocks are generally incompletely re-equilibrated, their bulk chemistry is not an important factor of pumpellyite composition; in fact the correlations of Al, Fetot and Mg are low. The local environment of crystallization, more evidently the composition of the precursor minerals, seems to be a major controlling factor. The observed topologic relations indicate that either the nature of the precursor mineral, or the presence of relict phases have a great influence on the pumpellyite composition. The observed variations within individual specimens, as well as the interspecimen ones, can be explained by small-scale inhomogeneities and in some cases by differences in the activity of O2.It can be therefore concluded that the composition of pumpellyite often reflects disequilibrium crystallization and cannot be generally used as an indicator of metamorphic conditions. |
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