Formation conditions of leucogranite dykes and aplite pegmatite dykes in the eastern Mt. Capanne plutonic complex (Elba, Italy): fluid inclusion studies in quartz, tourmaline, andalusite and plagioclase

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Abstract

Leucogranite and aplite-pegmatite dykes are associated with theMt. Capanne pluton (Elba) and partly occur in the thermally metamorphosed host rock (serpentinites).
Crystallization conditions of these dykes in the late magmatic-hydrothermal stage are estimated from fluid inclusion studies and mineralogical characterisation, obtained from detailed microthermometry, Raman spectroscopy, and electron microprobe analyses. Fluid inclusion assemblages are analysed in andalusite, quartz, and plagioclase from the leucogranite dykes, and in tourmaline and quartz from the aplitepegmatite dykes. The fluid inclusion assemblages record multiple pulses of low salinity H2O-rich magmatic and reduced metamorphic fluid stages. Magmatic fluids are characterized by the presence of minor amounts of CO2 and H3BO3, whereas the metamorphic fluids contain CH4 and H2. The highly reduced conditions are also inferred from the presence of native arsenic in some fluid inclusions. Several fluid inclusion assemblages reveal fluid compositions that must have resulted from mixing of both fluid sources. In leucogranite dykes, magmatic andalusite contains a low-density magmatic CO2-rich gas mixture with minor amounts of CH4 and H2. Accidentally trapped crystals (mica) and step-daughters (quartz and diaspore) are
detected in some inclusions in andalusite. The first generation of inclusions in quartz that crystallized after andalusite contains a highly reduced H2O-H2 mixture and micas. The second type of inclusions in quartz from the leucogranite is similar to the
primary inclusion assemblage in tourmaline from the aplitepegmatite, and contains up to 4.2 mass% H3BO3, present as a sassolite daughter crystal or dissolved ions, in addition to a CO2-CH4 gas mixture, with traces of H2, N2, H2S, and C2H6. H2O is the main component of all these fluids (x=0.91 to 0.96) with maximally 7 mass% NaCl. Some accidentally trapped arsenolite and native arsenic are also detected. These fluids
were trapped in the leucogranite at about 670–720 °C and 270–310 MPa, which is determined by combining isochore reconstruction, limits of the stability field of magmatic andalusite, and the water-saturated granite solidus. Trapping conditions
of the aplite-pegmatites are at similar temperatures and slightly lower pressures, at 230–270 MPa. The third type of fluid inclusion assemblages in quartz in the leucogranite represents a continuously trapping event down to 200 °C and 50 MPa, whereas the aplite pegmatite records a distinct trapping event of inclusions in quartz at about 250–300 °C and 50–100 MPa

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Original languageEnglish
JournalMineralogy and Petrology
DOIs
Publication statusE-pub ahead of print - 25 Nov 2015