Sapphirine is generally interpreted to be of metamorphic origin in highMg-Al rocks. Igneous sapphirine, i.e. sapphirine crystallized from melt, isvery rare. We examined sapphirine-bearing rocks in the FineroPhlogopite-Peridotite Massif, Western Italian Alps, to investigate a possibleigneous origin for sapphirine in a melt modified via melt-peridotiteinteraction. Sapphirine locally occurs in a melanocratic zone between aleucogabbroic vein and the host peridotite. The leucogabbroic vein cutsthe foliation and lithologic layering of the peridotite massif, which isdefined by alternating phlogopite-rich harzburgites and pyroxenites,indicating that its emplacement occurred after the main metasomaticevents in the massif. Melanocratic seams are observed on both sides ofthe leucogabbroic vein. These mainly consist of orthopyroxene andamphibole, and show a marked zoning in modal compositions: anorthopyroxene-rich zone overgrown on the host peridotite side (OPXzone), whereas an amphibole-rich zone occurs on the leucogabbroic veinside (AMPH zone). Sapphirine precipitated in the AMPH zone asindependent interstitial grains (up to 3 mm long), as independent grainswithin large amphibole grains or as overgrowth on spinel. The amphiboleswith sapphirine inclusions can also enclose spinel crystals that do not havesapphirine envelopes. Amphibole in the sapphirine-free melanocratic zoneis more abundant in incompatible elements, such as TiO2, and K2O, thanthat in the gabbroic veins and the OPX zone, excluding the development ofdiffusion-controlled subsolidus reaction. A pronounced enrichment inAl2O3 of the parent hydrous melts is indicated by the composition of theamphiboles and phlogopites of this study that show higher Al2O3 thanthose crystallised by basaltic melts. Mineral assemblages and chemistry inboth the melanocratic zone and the host peridotite can be explained bymelt-peridotite interactions, which resulted in replacement of peridotiteolivine by secondary orthopyroxene in the OPX zone, and by Al2O3, TiO2,FeO enrichments in the host peridotite beyond the recrystallization front.Interactions between peridotite and a hydrous, high Al2O3,orthopyroxene-oversaturated, mafic melt related to the formation of theleucogabbroic vein caused the formation of orthopyroxene at the expenseof peridotite olivine. This resulted in high MgO/FeO and high Al2O3/SiO2ratios in a modified melt, allowing for precipitation of igneous sapphirine.

Segregation of igneous Sapphirine in gabbroic veins cutting the Finero mantle sequence (Southern Alps): petrology, geochemistry and geodynamic context / Giovanardi, T.; Zanetti, A.; Mazzucchelli, Maurizio; Morishita, T.; Vannucci, R.. - In: EPITOME. - ISSN 1972-1552. - STAMPA. - 4:(2011), pp. 325-325. (Intervento presentato al convegno GeoItalia 2011 tenutosi a Torino nel 19-23/09/2011).

Segregation of igneous Sapphirine in gabbroic veins cutting the Finero mantle sequence (Southern Alps): petrology, geochemistry and geodynamic context.

GIOVANARDI T.;MAZZUCCHELLI, Maurizio;
2011

Abstract

Sapphirine is generally interpreted to be of metamorphic origin in highMg-Al rocks. Igneous sapphirine, i.e. sapphirine crystallized from melt, isvery rare. We examined sapphirine-bearing rocks in the FineroPhlogopite-Peridotite Massif, Western Italian Alps, to investigate a possibleigneous origin for sapphirine in a melt modified via melt-peridotiteinteraction. Sapphirine locally occurs in a melanocratic zone between aleucogabbroic vein and the host peridotite. The leucogabbroic vein cutsthe foliation and lithologic layering of the peridotite massif, which isdefined by alternating phlogopite-rich harzburgites and pyroxenites,indicating that its emplacement occurred after the main metasomaticevents in the massif. Melanocratic seams are observed on both sides ofthe leucogabbroic vein. These mainly consist of orthopyroxene andamphibole, and show a marked zoning in modal compositions: anorthopyroxene-rich zone overgrown on the host peridotite side (OPXzone), whereas an amphibole-rich zone occurs on the leucogabbroic veinside (AMPH zone). Sapphirine precipitated in the AMPH zone asindependent interstitial grains (up to 3 mm long), as independent grainswithin large amphibole grains or as overgrowth on spinel. The amphiboleswith sapphirine inclusions can also enclose spinel crystals that do not havesapphirine envelopes. Amphibole in the sapphirine-free melanocratic zoneis more abundant in incompatible elements, such as TiO2, and K2O, thanthat in the gabbroic veins and the OPX zone, excluding the development ofdiffusion-controlled subsolidus reaction. A pronounced enrichment inAl2O3 of the parent hydrous melts is indicated by the composition of theamphiboles and phlogopites of this study that show higher Al2O3 thanthose crystallised by basaltic melts. Mineral assemblages and chemistry inboth the melanocratic zone and the host peridotite can be explained bymelt-peridotite interactions, which resulted in replacement of peridotiteolivine by secondary orthopyroxene in the OPX zone, and by Al2O3, TiO2,FeO enrichments in the host peridotite beyond the recrystallization front.Interactions between peridotite and a hydrous, high Al2O3,orthopyroxene-oversaturated, mafic melt related to the formation of theleucogabbroic vein caused the formation of orthopyroxene at the expenseof peridotite olivine. This resulted in high MgO/FeO and high Al2O3/SiO2ratios in a modified melt, allowing for precipitation of igneous sapphirine.
2011
4
325
325
Giovanardi, T.; Zanetti, A.; Mazzucchelli, Maurizio; Morishita, T.; Vannucci, R.
Segregation of igneous Sapphirine in gabbroic veins cutting the Finero mantle sequence (Southern Alps): petrology, geochemistry and geodynamic context / Giovanardi, T.; Zanetti, A.; Mazzucchelli, Maurizio; Morishita, T.; Vannucci, R.. - In: EPITOME. - ISSN 1972-1552. - STAMPA. - 4:(2011), pp. 325-325. (Intervento presentato al convegno GeoItalia 2011 tenutosi a Torino nel 19-23/09/2011).
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