HIGH-TEMPERATURE FLUID-ROCK INTERACTION AND SCAPOLITIZATION IN AN EXTENSION-RELATED HYDROTHERMAL SYSTEM, MARY-KATHLEEN, AUSTRALIA

In the polymetamorphic Mary Kathleen Fold Belt, at the centre of the P roterozoic Mount Isa Inlier, Queensland, an early phase of extension ( 1760-1730 Ma) resulted in intrusion of voluminous granitic and dolerit ic magmas into the carbonate-evaporite-dominated Corella Formation at similar to 5-10 km depths. Widespread high-temperature metasomatism en sued, involving scapolitization in dolerite, formation of albite-scapo lite shear zones in granite, exo- and endoskarn formation, and a zone of K-Na-Ca alteration in the lowermost Corella Formation. Granites and dolerites were altered to an unusual Na-Ca-rich bulk composition, ref lecting high-temperature infiltration of highly saline, chemically rea ctive externally derived fluid. The alteration products and their dist ribution suggest not only reaction of magmatic/aqueous fluids with the country rocks but also extensive halite dissolution and recirculation of saline fluids back into the intrusive bodies. The bulk of fluid fl ow occurred at high temperatures (500-700 degrees C), and major elemen t and isotopic fronts were generally not smoothed out by the effects o f temperature gradients, with the exception of one part of the system which shows evidence for up-temperature fluid flow. Analysis of oxygen isotopic data and the position of isotopic and geochemical fronts rev eals time-integrated fluid fluxes of up to 2 x 10(4) m(3)/m(2) for the metasomatism. Although very high salinities (up to 50 mol% NaCl) were attained by evaporite dissolution, delta(18)O values of most alterati on products are in the range 7-12%, reflecting a predominance of oxyge n derived from an igneous fluid. The position and interrelationships o f metasomatic and isotopic fronts indicate an earlier stage of infiltr ation dominated by fluid released from crystallizing granite (with del ta(18)O 10-12 parts per thousand), and a later stage (delta(18)O 7-9 p arts per thousand) in which fluid had already interacted with halite a nd a mixed mafic-felsic igneous source or was repeatedly circulated be tween these rock types during alteration. The data reflect only a mini mal contribution from fluids produced by devolatilization of the abund ant carbonate-bearing rocks in the Corella Formation, and there are su bstantial areas of Corella Formation rocks that have escaped metasomat ism during this phase of intrusion-related hydrothermal activity and d uring the subsequent regional metamorphic overprint. Along with the re quirement that the fluids dissolved large amounts of halite from the s ame sequence, and the structural observations, we favour a model where fluid was preferentially channelled along specific permeable conduits , including former evaporite layers, before interaction with the now e xposed altered rocks. Fluid was probably driven by both convective cir culation and dilatancy-related deformation accompanying emplacement of magmas into a major crustal extensional decollement.