The low-pressure partial-melting behaviour of natural boron-bearing metapelites from the Mt Stafford area, Central Australia

[摘要] ENGLISH ABSTRACT: This study has examined the 3 kbar partial melting behaviour of 4 metapelites collected from the highest grade rocks occurring below the anatectic zone of the Mt Stafford area, Arunta Inlier, central Australia. In this area, metasediments are interpreted to have undergone partial melting within the andalusite stability field, possibly as a result of a lowering of the metapelite solidus by the presence of boron in the rocks. Two of the samples were two mica metapelites (MTS70 and MTS71) that both contained significant quantities of tourmaline and were thus boron enriched. The other two samples were biotite metapelites. One of these rocks contains only a trace of tourmaline (MTS8) and the other is tourmaline free (MTS7). Despite expectations that muscovite in the two mica samples would break down via a subsolidus reaction, muscovite was stable to above 750 C due to the incorporation of Ti, phengitic and possibly F components into its structure. Between 750 and 800 C, muscovite melted out completely via a coupled muscovite + biotite fluid-absent incongruent reaction. In the most mica-rich sample this reaction produced ~ 60 % melt at 800 C. In the biotite metapelites, biotite melting began at a temperature below 800 C and was accompanied by very modest melt production at this low temperature. In contrast to the two mica metapelites, the main pulse of melt production in these samples occurred at a temperature between 850 and 950 C. In both these samples biotite + melt coexistence persisted for a temperature range in excess of 150 C, and in MTS8, biotite was still in the run products at 950 C. The very refractory nature of these evolved biotite compositions is most likely a consequence of both the presence of a Ti buffering phase in the assemblage (ilmenite) and the essentially plagioclase-free nature of the starting compositions. Under the fluid-absent conditions of this study tourmaline is clearly a reactant in the partial melting process, but does not appear to shift the fluid-absent incongruent melting reactions markedly. Neither quartz, nor andalusite was completely consumed in the melting reactions, indicating the metastable persistence of andalusite to higher than the wet solidus temperatures. The assemblages do not change much with increasing temperature and mimic the field relationships. The fluid-absent melting experiments indicated that the main pulse of melting occurred between 850 and 950 °C, significantly higher than indicated by the field evidence of 600 to 675 °C, therefor disequilibrium in the experiments can not be ruled out. The presence of a fluid during partial melting at Mt Stafford provides therefor an explanation of the low temperatures at which melting occurred.

[发布机构] Stellenbosch University