The flow of surface-derived fluids through Alice Springs age middle-crustal ductile shear zones, Reynolds Range, central Australia

The south-east Reynolds Range, central Australia, is cut by steep north-wes t-trending Alice Springs age (c. 334 Ma) shear zones that are up to hundred s of metres wide and several kilometres long with reverse senses of movemen t. Amphibolite facies (550-600 degrees C, 500-600 MPa) shear zones cut meta pelites, while greenschist facies shear zones (420-535 degrees C, 400-650 M Pa) cut metagranites. The sheared rocks commonly underwent metasomatism imp lying that the shear zones were the pathways of significant fluid flow. Alt ered granites within greenschist facies shear zones have gained Si and K bu t lost Ca and Na relative to their unsheared counterparts, suggesting that the fluid flowed down-temperature land hence probably upward) through the s hear zones. Time-integrated fluid fluxes calculated from silica addition ar e up to 2.1 x 10(10) mol m(-2) (c. 4.2 x 10(5) m(3) m(-2)). Similar time-in tegrated fluid fluxes are also estimated from changes in K and Na. The shea red granitic rocks locally have delta(18)O values as low as 0 parts per tho usand which is much lower than the delta(18)O values of the adjacent unshea red granites (7 to 9 parts per thousand), implying that the fluid which flo wed through these shear zones was derived from the surface. For the estimat ed time-integrated fluid fluxes, the fluids would be able to retain their i sotopic signature for many tens to hundreds of kilometres. The flow of surf ace-derived fluids into the ductile middle crust, with subsequent expulsion upwards through the shear zones, may have been driven by seismic activity accompanying the Alice Springs deformation.