FLUID MIGRATION AND VEIN FORMATION DURING DEFORMATION AND GREENSCHISTFACIES METAMORPHISM AT ORMISTON GORGE, CENTRAL AUSTRALIA

During the Alice Springs Orogeny, deformation at Ormiston Gorge, centr al Australia, occurred under lower- to middle-greenschist facies condi tions. Dolomites of the Bitter Springs Formation and quartzites, metag reywackes, and metapelites of the Heavitree Quartzite contain abundant early-, syn-, and post-tectonic veins. However, though vein densities locally approach 15%, the distribution of veins and the oxygen isotop e geochemistry of wallrocks and veins suggest that fluid movement was on a local scale. The Heavitree Quartzite contains quartz veins that, even along the main thrust plane, have similar deltaO-18 values (13.5- 16.0 parts per thousand) to those of their wallrocks (13.6-16.9 parts per thousand), with DELTAO-18(vein-wallrock) values of -0.6 to 0.4 par ts per thousand. In contrast, the Bitter Springs Formation contains pr edominantly dolomite veins that have deltaO-18 values of 23.4 to 27.7 parts per thousand. These differences are observed even at the boundar y between the Heavitree and Bitter Springs rocks, implying that signif icant fluid exchange between these rocks has not occurred, or that flu id flow was channelled through areas outside those sampled for this st udy. By contrast with the Heavitree Quartzite, deltaO-18 values of wal lrocks in individual samples of the Bitter Springs Formation are signi ficantly higher (23.3-29.1 parts per thousand) than those of the veins , with DELTAO-18(vein-wallrock) values up to -4 parts per thousand. (a verage of -2.1 parts per thousand). These systematic differences in de ltaO-18 values most likely result from oxygen isotope fractionation ca used by fluid immiscibility or disequilibrium dissolution. Smaller dif ferences in deltaC-13 values between some dolomite veins and wallrocks [DELTAC-13(vein-wallrock) up to -1.9 parts per thousand, average of - 0.5 parts per thousand] are also explained by these processes. This st udy indicates that large volumes of veins may be produced by repeated fracturing and fluid migration within particular rock units, without i nvolving large volumes of externally derived fluids.