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

A study of brittle deformation within a segment of the Hercynian intrusive complex of Gallura reveals a fairly regular network of fluid inclusion planes and open microcracks. Fluid inclusion planes and microcracks in nine oriented samples show a consistent orientation across different intrusions; two principal, almost orthogonal sets have been recognized: both consist mainly of subvertical planes. A subhorizontal set is nearly always present as healed microcracks. Microthermometric investigations on fluid inclusions show that Na dominated aqueous fluids (salinity 0.9- 12.3 eq.wt.% NaCl; T (sub h) L = 111-257 degrees C) (T (sub h) in liquid phase) circulated throughout the 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 palaeocracks. Li-bearing brines have salinities of 11-17 eq.wt.% LiCl and T (sub h) L of 74-171 degrees C; salinity in Ca-dominated brines varies from 4.9 to 21.8 eq.wt.% NaCl and T (sub h) L from 130 to 217 degrees C. The principal set directions 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 coolingand uplift.