Fluid infiltration and volume change during mid‐crustal mylonitization of Proterozoic granite, King Island, Tasmania

The development of 10–30 m wide mylonite zones at mid‐crustal depths in late Proterozoic granitoids on King Island, Tasmania, was associated with pervasive infiltration of low δ 18 O‐fluids (+5 to +7) on the scale of the shear zones. Syndeformational fluid–rock interaction produced substantial differences in mineral composition and bulk rock chemistry among several adjacent shear zones which are hosted by the same granite. In a shear zone at Cape Wickham with a normal slip component, changes in whole‐rock chemistry between granite and mylonites indicate a gain of Ca, and losses of K and Na during deformation, which was nearly isovolumetric. Notable losses of K, Rb and Si occurred during partial retrograde alteration of mylonites near the western margin of this shear zone. The alteration suggests a component of up‐temperature fluid flow. In contrast, 3 km to the south east, in a strike‐slip shear zone at Disappointment Bay, complete albitization of plagioclase was associated with Na‐gain and Ca‐loss. Deformation also involved losses of Mg and Fe. Up to 60% volume gain occurred during the formation of closely spaced mesoscopic to microscopic quartz veins during mylonitization. The substantial silica‐gain in this, as well as in two mylonite zones further south east, is interpreted to have been associated with upward flow of aqueous fluids along these shear zones. On the basis of a gradient reaction model, minimum time‐integrated fluid‐fluxes of 10 6  m 3 /m 2 are estimated for the Disappointment Bay (West) Shear Zone.