Agangi, and two co-authors, reported in 2010 that rhyolites of the Mesoproterozoic Gawler Range Volcanics (GRV) of South Australia are characterized by high concentrations of some trace elements (REE, Y, HFSE, Rb and F, in particular). Whole rock geochemical data suggest that these elements were incompatible during magma crystallization. Accessory minerals (fluorite, zircon, REE-F-carbonate, Ti oxide, apatite, and titanite) can account for most of the trace element content of the rocks.
These minerals occur in vesicles, micromiaroles, lithophysal vugs and in interstices between major mineral phases (quartz and feldspar as both phenocrysts and groundmass). Such textural evidence indicates that accessory minerals crystallized late in the history of the magma and that they were deposited from a volatile-rich (fluid) phase. These features are explained by the following sequence of events:
1) F dissolved in the magma lowered the crystallization temperature of accessory minerals, causing trace elements (REE, Y, and HFSE) to behave as incompatible elements.
2) Protracted crystallization of major mineral phases (quartz, feldspar, and oxides) formed a volatile- and trace element-enriched residual liquid.
3) A volatile element (H2O, F, and CO2)-rich phase (late-stage magmatic fluid) evolved from the magma. High concentration of fluorine and other complexing agents in this phase allowed trace elements to be transported in solution.
4) Accessory minerals crystallized from such a phase in vesicles, micromiaroles and interstices between the major mineral phases.
The authors concluded that accessory mineral assemblages contained in micromiaroles, vesicles and lithophysal vugs or occupying interstitial positions in the GRV include significant amounts of REE (lanthanides, U, and Th), Y, HFSE (Ti, Zr, and Nb) and transition metals (Cu, Zn, Mo, and W). These elements are mainly concentrated in REE-fluoro-carbonate, zircon, niobian Ti oxide, fluorite, titanite and, to a lesser extent, apatite and allanite. Textures and mineral associations indicate that these accessory minerals formed at a late magmatic stage, rather than in super-solidus conditions and that they crystallized from a volatile (F, H2O, CO2, ±P, and ±S)-rich low-viscosity (fluid) phase. It is inferred that fluorine played the dual role of promoting incompatible behavior and concentration of lithophile trace elements in the residual magmatic liquid and, together with other complexing agents (CO2, OH , and P), allowing the transport of the same “immobile” elements in a fluid in the late magmatic stage.
Experimental data (London et al., 1988; Webster, 1990; Keppler and Wyllie, 1991) and melt inclusion studies in pegmatites (Thomas et al., 2000; 2009) have shown that silicate melts can contain very high amounts of water. Such a high solubility of silicate and volatile component allows the existence of fluid phases with intermediate
properties between an aqueous fluid and a silicate melt in terms of both composition and physical characteristics. The presence of REE and HFSE-bearing minerals (e.g. zircon, xenotime, fluorite, fluorocarbonate) described by Thomas et al. (2009) in pegmatites, implies that this kind of fluids are able to transport significant amounts of
lithophile elements. Such a volatile- and silicate-rich fluid phase could account for the textural, mineralogical and compositional data described here.
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