Rubenach, Michael J. (2005) Relative timing of albitization and chlorine enrichment in biotite in Proterozoic schists, snake creek anticline, Mount Isa Inlier, northeastern Australia. Canadian Mineralogist, 43 (1). pp. 349-366.

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Abstract

In the Snake Creek area, near Cloncurry, Mount Isa Inlier, in northeastern Australia, multiple episodes of deformation, low-pressure metamorphism, intrusion of abundant mafic and granitic bodies, and extensive metasomatism occurred throughout the Mesoproterozoic Isan Orogeny. Albitites and associated metasomatic biotite-rich schists are abundant in the core of the Snake Creek Anticline, and are concentrated in five areas. Albitization peaked during D1 and was commonly localized along shear zones. Muscovite schists adjacent to albitites were altered to albite-bearing biotite-rich schists. Porphyroblast growth in metasomatic rocks commenced with cordierite and andalusite from late syn-D1, but episodic growth continued to post-D5, with the peak of metamorphism occurring late syn-D3 to early syn-D4. The timing of albitization was determined from the presence or absence of albite in various stages of andalusite growth, and similarly the Mg# [Mg/(Mg + Fe)] and Cl contents of biotite were examined over the span of porphyroblast-growth history. Differential mass-transport of Mg and Fe took place from the albitites to the adjacent schists predominantly during D1, and the Mg# for biotite in metasomatic rocks was largely fixed during D1. The Cl content of the matrix biotite varies considerably in the different areas of albitite development, from below detection to a maximum of 1.5 wt%. This could indicate that the Cl enrichment event was not related to albitization, but there is nevertheless an overall spatial relationship between them. In any particular sample, the Cl content in biotite is generally higher in the matrix than for inclusions in late andalusite, with inclusions in early andalusite being the poorest in Cl. Enrichment of Cl in biotite is a function of f(HCl)/f(H2O) rather than salinity, and so could be related to the replacement of muscovite by albite, as this involves release of H+. The ratio f(HCl)/f(H2O) would have been low during periods of high flux of fluid that produced the albitites, but increased during low flux, in the waning stages of albitization, resulting in maximum enrichment in Cl relatively late in the metasomatic history.

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