Comparison of fluid inclusion studies in cordierite-andalusite-rich leucogranite dykes and tourmaline-bearing aplite-pegmatite dykes (Elba, Italy)

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Abstract

The late magmatic-hydrothermal stage of the Monte Capanne pluton (Elba, Italy) is associated with abundant fluid sources, of which the remnants are present in numerous fluid inclusion assemblages in associated leucogranite dykes, and aplite-pegmatite dykes and veins. They are mainly located in the pluton and its thermometamorphic aureole close to the pluton's contact. The studied dykes were sampled in serpentinitic host rock. The leucogranite dyke contains abundant andalusite and cordierite (peraluminous). The pegmatite vein is mainly composed of coarse-grained quartz, plagioclase and foitic tourmaline. A variety of fluid inclusion assemblages were observed in andalusite-plagioclase-quartz (leucogranite) and tourmaline-quartz (aplite-pegmatite). The contents of fluid inclusions were detected with microthermometry and Raman spectroscopy. Complex gas mixtures of CO2, CH4, H2, N2, and locally H2S and alkanes were detected in the vapour phase of all types of inclusions. Anhydrous inclusions in andalusite contain a low density CO2-rich gas-mixture and abundant accidentally trapped mica's. Some inclusions contain diaspore and quartz, which are reaction products of H2O (completely consumed) and host mineral. Type 1 inclusions in quartz contain a mixture of H2O and H2, and usually an accidentally trapped mica, which can only be detected by Raman. Type 2 inclusions have a CO2-CH4-rich vapour phase with up to 16 mole% H2 and N2. Some minor amounts of dissolved sassolite (H3BO, < 2.5 mass%) and NaCl are detected in the liquid phase. Tourmaline from the aplite-pegmatite contains exceptional large fluid inclusions. The liquid phase contains higher concentrations of sassolite (4 to 6 mass%), which nucleate to crystals by Raman laser irradiation at room temperatures. Arsenolite and arsenic was observed in some inclusions. The bulk fluid properties are similar to Type 2 from quartz in the leucogranite. Also the fluid inclusions in quartz from the aplite-pegmatite contain traces of sassolite (H3BO3 < 2.5 mass%), and the vapour phase is CO2-rich, with minor amounts of CH4. Other gases were not detected in these inclusions. Both types of rock (leucogranite and aplite-pegmatite) reveal similar fluids in inclusions, which may imply a similar origin. H2O is the most important component of all fluid inclusion assemblage. The absence of melt inclusions in both rock types emphasizes their hydrothermal origin. Details about accessory components such as NaCl, H3BO3 and gas-compositions in different fluid inclusion assemblages illustrate the development of rock-forming conditions. Whereas the CO2-rich fluids in andalusite suggest a magmatic source, a highly reduced fluid (H2 and CH4) that is found in early assemblages in quartz and plagioclase illustrates the strong influence of fluid properties from the surrounding host rock (serpentinites). The fluid sources of both rock types are poor in NaCl. The solvus boundary of a Boron-H2O rich granitic melt, the presence of andalusite, and the specific molar volume of homogeneous fluid inclusion assemblage restrict formation conditions of both leucogranite and aplite-pegmatite to a narrow T-P window at about 650 ˚C and 300 MPa.

Details

Original languageEnglish
Title of host publicationMitt. Österr. Miner. Ges.
Publication statusPublished - 2015
EventMinPet 2015 - Leoben, Austria
Duration: 10 Sep 201512 Sep 2015

Conference

ConferenceMinPet 2015
CountryAustria
CityLeoben
Period10/09/1512/09/15