Studio di inclusioni policristalline in granati di granuliti dell'Athabasca Granulite Terrane (Canada)

Fluid inclusions in minerals represent an important tool to investigate the processes involved in the petrogenesis of rocks, including the fluid-rock-melt interactions during anataxis. In this work a detailed microstructural and microchemical study has been performed on polycrystalline inclusions trapped in peritectic garnets of felsic granulite gneisses from the Upper Deck domain, Athabasca Granulite Terrane (Canada). Inclusions have been characterized by SEM-EDS and FIB-SEM techniques, and by Raman spectroscopy. These inclusions are very small (≤ 10 μm), they are primary and do not show evidences of decrepitation. FIB-SEM analyses permitted to recognise the main crystalline phases and their distribution. Their polycrystalline assemblage is always made of ferroan magnesite, quartz and graphite, in association with minor amounts of corundum and Zn-spinel. Calcite and dolomite may be present, and pyrophyllite was found only in one inclusion. Quartz and corundum coexistence has been interpreted as the product of metastability. Raman spectroscopy revealed the presence of CO2, with traces of CH4 and N2. A good negative correlation between the CO2 density and the total amount of coexisting solid phases has been observed. The main minerals occur in a relatively constant amount in the investigated inclusions, suggesting the entrapment of a compositionally homogeneous fluid characterized by a high-carbon content. These tiny carbon-rich inclusions coexist in the same cluster with nanogranite inclusions. Because nanogranites are interpreted to be droplet of anatectic melt formed and trapped during incongruent melting of crustal rocks, the investigated polycrystalline inclusions prove the presence of a carbon-rich fluid during melting of these rocks, possibly in a situation of melt/melt or melt/fluid immiscibility. Additional melting experiments are needed to better constrain the nature of former fluid.