Metamorphic plumbing system in Proterozoic calc-silicates, Queensland, Australia

Abstract

A calc-silicate–rich terrain in northwest Queensland exhibits spectacular examples of heterogeneous fluid/rock interaction attending regional amphibolite facies metamorphism and deformation. Large areas of pristine calc-silicate rocks show evidence for internal buffering of fluid composition and low fluid/rock ratios (F/R), as inferred from δ 18 O and δ 13 C signatures typical of metamorphosed marine carbonates, the progress of calc-silicate reactions, and scapolite compositional variations between layers. In contrast, high-variance assemblages are developed in pretectonic intrusive rocks, as well as in metasomatic zones in calc-silicates around their margins. Scapolitized metadolerites, albitized calc-silicates, and large calcite pods and veins with 18 O- and 13 C-depleted values are typical of these regions and indicate localized throughput of predominantly externally derived fluid, at high F/R, accompanying the metamorphic peak. Major competency contrasts between meta-intrusive and calc-silicate rock types influenced stress and strain patterns around the igneous bodies, which in turn controlled fracture permeability and the focusing of the metamorphic fluid. The relatively brittle behavior of the meta-intrusive rocks favored the development of small-scale fracture permeability and consequent pervasive alteration of these bodies. Calcite vein systems and intensely albitized calc-silicate breccias are localized in shear zones and dilatant areas around the meta-intrusions, either where shear stresses were high or where all stresses were low, resulting in shear and tensile failure, respectively. Thus, permeability enhancement and the development of major fluid pathways during regional metamorphism of the terrain are systematically related to variations in the stress field accompanying deformation. Similar factors may strongly influence the channeling of fluids during metamorphism of other terrains.