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

Spicer, Esme Marelien (Stellenbosch : University of Stellenbosch, 2011-12)

Thesis (DSc)--University of Stellenbosch, 2011.

Thesis

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.

AFRIKAANSE OPSOMMING: Die 3 Mpa vloeistof-vrye gedeeltelike smelting van 4 metapeliete, gekollekteer van die hoogste graad rotse net onder die anatektiese sone van die Mt Stafford area, Arunta inlêer, sentraal Australië, is bestudeer. Die metapeliete van hierdie area word geinterpreteer dat hulle gedeeltelike smelting in die andalusiet stabiliteitsveld ondergaan het, moontlik as 'n resultaat van die verlaging van die metapeliet solidus as gevolg van die teenwoordigheid van boor. Twee van die monsters bestudeer was twee-mika metapeliete (MTS70 en MTS71) met beduidende hoeveelhede toermalyn en is dus boor-verryk. Die ander twee monsters was biotiet metapeliete, waarvan een spoorhoeveelhede toermalyn (MTS8) bevat het en die ander toermalyn vry was (MTS7). Ten spyte van verwagtinge dat muskoviet in die twee mika monsters sou afbreek via 'n subsolidus reaksie, was dit stabiel tot bo 750°C as gevolg van die vervanging van Ti, fengitiese en moontlik F komponente in die muskoviet struktuur. Tussen 750 en 800°C het muskoviet heeltemal gesmelt deur die vloeistof-vrye gekoppelde muskoviet+biotiet reaksie. In die monster met die meeste mika het hierdie reaksie ~ 60 % gesmelt by 80°C en lae hoeveelhede smelt is by hierdie lae temperature geproduseer. In kontras met die twee-mika metapeliete het die hoof puls van smeltproduksie in hierdie monsters plaasgevind tussen 850 en 950°C. In beide hierdie monsters het biotiet+smelt 150°C. Biotiet was steeds ongesmelt in MTS8 by 950°. Die hoë refraktoriese natuur van hierdie biotiet samestellings is hoogs waarskynlik 'n gevolg van die teenwoordigheid van 'n Ti-bufferende fase (ilmenite) en die afwesigheid van plagioklaas in die begin samestellings. Toermalyn is duidelik 'n reaktant in hierdie vloeistof-vrye gedeeltelike smelting studie, maar dra nie beduidend by tot die verlaging van die inkongruente smeltingsreaksies nie. Nie kwarts of andalusiet het heeltemal gesmelt oor die temperatuurreeks nie, wat aandui dat die andalusiet stabiel is by temperature hoër as die nat solidus. Die mineraalverspreidings verander nie veel met verhoging in temperatuur nie en mimiek dus die veld verwantskappe. Die vloeistof-vrye smeltings eksperimente het aangedui dat die hoofpuls van smelting tussen 850 en 950°C geskied het, wat aansienlik hoër is soos aangedui uit die veldgetuienis van 600 tot 675°C, dus is die moontlikheid van disekwilibrium gedurende die eksperimente 'n moontlikheid. Die moontlikheid dat vloeistof teenwoordig was tydens die smeltproses by Mt Stafford verskaf dus 'n oplossing vir die lae temperature wat tydens smelting bereik is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/18070
This item appears in the following collections: