Timing and nature of fluid flow and alteration during Mesoproterozoic shear zone formation, Olary Domain, South Australia

The development of shear zones at mid-crustal levels in the Proterozoic Willyama Supergroup was synchronous with widespread fluid flow resulting in albitization and calcsilicate alteration. Monazite dating of shear zone fabrics reveal that they formed at 1582 +/- 22 Ma, at the end of the Olarian D3 deformational event and immediately prior to the emplacement of regional S-type granites. Two stages of fluid flow are identified in the area: first an albitizing event which involved the addition of Na and loss of Si, K and Fe; and a second phase of calcsilicate alteration with additions of Ca, Fe, Mg and Si and removal of Na. Fluid fluxes calculated for albitization and calcsilicate alteration were 5.56 x 10(9) to 1.02 x 10(10) mol m(-2) and 2.57 x 10(8)-5.20 x 10(9) mol m(-2) respectively. These fluxes are consistent with estimates for fluid flow through mid-crustal shear zones in other terranes. The fluids associated with shearing and alteration are calculated to have delta(18)O and delta D values ranging between +8 and +11%, and -33 and -42%, respectively, and epsilon Nd values between -2.24 and -8.11. Our results indicate that fluids were derived from metamorphic dehydration of the Willyama Supergroup metasediments. Fluid generation occurred during prograde metamorphism of deeper crustal rocks at or near peak pressure conditions. Shear zones acted as conduits for major crustal fluid flow to shallow levels where peak metamorphic conditions had been attained earlier leading to the apparent 'retrograde' fluid-flow event. Thus, the peak metamorphism conditions at upper and lower crustal levels were achieved at differing times, prior to regional granite formation, during the same orogenic cycle leading to the formation of retrograde mineral assemblages during shearing.