Structural and microthermometric studies of fluid inclusions in the Gallura intrusive complex (N Sardinia)

Abstract

A study of brittle deformation within a segment of the Hercynian intrusive complex of Gallura (comprising porphyritic biotite monzogranitic and granodioritic intrusions) reveals a quite regular network of fluid inclusion planes (interpreted as healed paleocracks) and open microcracks. Fluid inclusion planes and microcracks in 9 oriented samples show a consistent orientation across different intrusions; two principal, almost orthogonal sets have been recognized: N20-60 o E-trending and N100-120 o E-trending directions; both mainly consist of subvertical planes. A subhorizontal set is nearly always present as healed microcracks; it occurs as open cracks only in one sample. Microthermometric investigations on fluid inclusions show that Na dominated aqueous fluids (salinity from 0.9 to 12.3 wt.% NaCl eq.; ThL=111-257 o C) circulated throughout the intrusive complex, along almost all the microfracture sets during the late-stage hydrothermal history. Locally, subordinate Li- or Ca-, K- and Na-bearing brines have been detected along N100 o E- and N160 o E-trending paleocracks. Li-bearing brines have salinities ranging from 11 to 17 wt.% LiCl eq. and ThL from 74 to 171 o C; salinity in Ca-dominated brines varies from 4.9 to 21.8 wt.% NaCl eq. and ThL from 130 to 217 o C. The principal set directions (N20-60 o E and N100-120 o E) are often shared by fluid inclusion planes and open microcracks: it is suggested that they belonged to the same brittle deformation episode, related to late Hercynian batholith cooling and uplift