Magmatic immiscibility and fluid phase evolution in the Mount Genis granite (southeastern Sardinia, Italy)

The Mount Genis granite is one of the post-tectonic intrusives emplaced late in the magmatic history of the Hercynian batholith in Sardinia. Devitrified silicate melt inclusion are present in some (magmatic) rock-forming and miarolitic quartz. These magmatic remnants show initial melting at 680-720-degrees-C. High-temperature observations (700-800-degrees-C) revealed the presence in some of the inclusions of mixed hydrosaline melt (Li) and silicate melt (L2), with extremely variable L1/L2 ratios. Electron microprobe analyses indicate LI to be K-Na dominated chlorides. Inclusions of mixed silicate and hydrosaline melts are interpreted to have been formed by heterogeneous trapping of two immiscible fluid phases (silicate-hydrosaline) after second boiling, most likely during the final crystallization stage. Magma-derived brines (63 eq wt% NaCl) circulated at subsolidus conditions from almost-equal-to 600-degrees-C and were retained in the miarolitic cavities down to about 400-degrees-C. Fluid unmixing occurred locally in the miarolitic cavities from 550-degrees to 412-degrees-C. At temperatures of almost-equal-to 400-degrees to 100-degrees-C the microgranite was invaded by diluted waters (almost-equal-to 4-5 cq wt% NaCl). A possible model for fluid evolution begins with a hydrosaline melt exsolving from the magma at the late-magmatic stage. The absence of boiling within the volatile (hydrosaline) system shows that brines can occur by direct magmatic immiscibility. The comprehensive hydrothermal evolution suggests a nearly isobaric cooling path, with local boiling episodes in the miaroles, probably in coincidence with invasion of external waters.