Fluid-rock interaction in the Reynolds Range, central Australia: superimposed, episodic, and channelled fluid flow systems

Abstract

Abstract The Reynolds Range, central Australia, is a polymetamorphic Proterozoic terrain within the Arunta Inlier. The terrain comprises a diverse sequence of metasedimentary rocks (including pelites, psammites, quartzites, marls and marbles) intruded by with two generations of granites. The Reynolds Range preserves evidence of undergoing several metamorphic events, including: phases of contact metamorphism at 1.82 Ga and 1.78 Ga; regional metamorphism at 1.6 Ga that varied in grade from greenschist facies ( c. 400°C) to granuite facies (750–800°C) at 400–500 GPa; and metamorphism at up to amphibolite facies (550–600°C at 500–600 MPa) in the Alice Springs Orogeny at c. 334 Ma, the affects of which are recorded mainly within shear zones. Fluid flow occurred during both contact metamorphic events, cooling from the peak of regional metamorphism at 1.57–1.58 Ga, and additionally during Alice Springs shearing. By contrast, there is little fluid-rock interaction that can be attributed to the prograde stages of regional metamorphism, implying that fluids generated by metamorphic devolatilization at that time escaped relatively rapidly and did not interact with the rocks as a whole. Fluid flow changed the mineralogy and stable isotope ratios of rocks, and locally caused extensive metasomatism. During cooling from the peak of regional metamorphism, the fluids were derived from crystallization of partial melts and reflect internal fluid recycling. However, at least some fluid flow during contact metamorphism and shearing involved external fluids. In both contact metamorphic episodes, igneous and locally surface-derived fluids interacted with the country rocks adjacent to the granite plutons. During Alice Springs shearing, large volumes of surface-derived fluids infiltrated the middle crust. Much of the fluid flow was channelled on scales ranging from hecto- to millimetres as a result of variations in intrinsic permeability caused by deformation or reaction enhancement.