An experimental investigation of the “fate” of entrained peritectic minerals during adiabatic ascent of S-type granite magmas

Mthethwa, Mbali Pertunia (2018-03)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: S-type granites generally display a wide range of trace and major element compositional variation, from tonalitic to leucogranitic compositions. A substantial volume of past research has investigated the mechanisms by which granitic magmas attain the wide compositional range that they display. The proposed theories fall into two broad categories: (i) processes that occur during magma ascent and crystallization, such as fractional crystallization and magma mixing, and (ii) processes that involve entrainment of crystals, or fragments of the source rock, to form part of the magma in the source during segregation. S-type granite magma compositions have been proposed to represent mixtures, in variable proportions, between melt and the peritectic assemblage produced by fluid-absent incongruent melting of metasediments. Despite strong geochemical arguments in support of this process, peritectic minerals are not commonly present in granites. This study used an experimental approach to investigate the fate of an entrained peritectic mineral assemblage, produced by the incongruent fluid-absent melting of biotite, in a monzogranitic S-type magma emplaced in the upper crust. The experiments were conducted at upper crustal, magmatic pressure-temperature conditions, in water pressurized, externally heated, Inconel cold-seal vessels using gold capsules. The starting materials consisted of 70 wt% of a synthetic silicate oxide and mineral mixture, which reacted to form the melt component of the magma at the experimental conditions, and 30 wt% of an assemblage of natural plagioclase, ilmenite and garnet, which was intended to represent the peritectic assemblage, with which the melt coexisted in the source. The results of phase equilibrium modelling and prior experimental studies indicate that the “melt” composition and peritectic assemblage used are consistent with the products of incongruent partial melting of a metapelite at approximately 10 kbar and 850 °C. The resultant starting material had an S-type monzogranitic composition. Experiments were carried out over a temperature range of 743 to 804 °C at a pressure of 1 kbar, with a run duration of 20 days. The results demonstrate that under the investigated P-T conditions, the magma undergoes two reaction mechanisms to bring the out-of-equilibrium, entrained high-P-T peritectic crystals into equilibrium with the magma. These are, (i) a coupled dissolution-precipitation mechanism, which resulted in efficient equilibration of plagioclase and ilmenite. (ii) mineral-melt reactions that led to the replacement of the high-P-T phases by minerals that are stable at the conditions of the experiment. This process partially removed garnet from the run products, with biotite and cordierite produced at the expense of garnet. However, the garnet-melt reactions are significantly less efficient than dissolution-precipitation in bringing the garnet crystals into equilibrium with the magma. This is evident by the preservation of garnet cores armored by biotite reaction rims. The experiments show that, even including the slower reactions involving garnet, the rates of equilibration of the hypothetical peritectic crystals with the granitic magma at shallow crustal levels is very rapid. These processes appear to be highly efficient, and account for the predominantly magmatic nature of the above-mentioned minerals in crystalline granitic rocks. In the experimental run products, the magma was shown to have a sufficiently high melt volume, at the P-T conditions close to the solidus, to facilitate processes and reactions that bring out-of-equilibrium crystals into equilibrium with the magma. Furthermore, the results suggest that, where garnet crystals are present in crystalline granitic rocks they most likely represent a metastable phase at the conditions of intrusion. They also do not compositionally or texturally resemble the high-P-T peritectic phases, due to recrystallization in the deeper portions of the magma plumbing system. This research has explained why peritectic minerals are absent or very rare in crystalline granitic rocks. Additionally, the findings provide an understanding of why granitic rocks do not commonly contain restitic minerals, early formed magmatic phases that are not stable under the conditions of intrusion, nor peritectic minerals.

AFRIKAANSE OPSOMMING: S-tipe graniet rotse algemeen vertoon 'n wye groot en spoorelement komposisionele variasie van tonalitic om leucogranitic komposisies. Daar is 'n baie navorsing tot op hede te dokumenteer hoe granitiese magmas bereik die wye komposisionele variasie dat hulle vertoon. Die voorgestelde teorieë val in twee breë kategorieë: (i) prosesse wat plaasvind tydens magma trap en kristallisasie en (ii) prosesse wat meevoeren van kristal fragmente betrek om die magma in die bron. Maar laasgenoemde is die hoofstroom prosesse. Hierdie studie ondersoek deur 'n eksperimentele benadering die lot van entrained peritectic minerale versameling, wat deur die indruis vloeistof-afwesig smelt van biotiet, in 'n monzogranitic S-tipe magma ingeplaas in die boonste kors. Meevoeren van wisselende verhoudings van 'n peritectic minerale versameling in die smelt in die bron streek, voor magma trap, is vermoedelik 'n belangrike rol in graniet petrogenese speel. Daar word voorgestel dat die samestellingsvariasies vertoon deur gemiddelde tot S-tipe graniet rotse mafiese, is 'n gevolg van meevoeren van peritectic granaat, plagioklaas, ilmeniet, kwarts, cordierite en sillimanite om die smelt. Groot-element komposisionele tendense soos: (i) 'n sterk positiewe korrelasie tussen Ti en maficity (mol Fe + Mg.), 'N tendens wat algemeen in al granietrotse. (Ii) Toename in Ca en A / CNK met 'n toenemende maficity. (Iii) Die afname in Si en Ki met dalende maficity. Hierdie tendense ondersteun die lewensvatbaarheid van die peritectic samevoeging model. Maar mineralogiese en teksturele bewyse van hierdie peritectic fases is skaars in kristallyne granitiese gesteentes. Om vas te stel wat word van die entrained peritectic kristalle op vlak kors vlakke, is 'n sintetiese S-tipe monzogranitic magma samestelling gebruik as 'n vertrekpunt materiaal vir alle eksperimente. Die begin materiaal is saamgestel uit 70 wt.% sintetiese silikaatgel - (bedoel om die smelt fraksie verteenwoordig) en 30 wt.% hipotetiese hoë-P-T peritectic minerale versameling bestaande uit plagioklaas, granaat, en ilmeniet. Die begin materiaal is gesintetiseer sodat op die bereiking van eksperimentele P-T toestande, sal herhaal 'n silikaat smelt gegenereer by 850 °C en 10 kbar deur vloeistof-afwesig smelt van biotiet, saam sit met 'n peritectic versameling. Eksperimente is oor 'n temperatuur van 743-804 °C en 'n konstante druk van 1 kbar gedra, met elke eksperiment hardloop vir 20 dae. Die resultate toon dat onder die ondersoek P-T toestande, die magma ondergaan drie reaksiemeganismes te buite-ewewig hipotetiese, entrained hoë-P-T peritectic kristalle in ewewig te bring. Dit is, (i) 'n gepaardgaande ontbinding-neerslag meganisme, wat-re evenwichtige komposisies van die hipoteties entrained plagioklaas en ilmeniet oor al ondersoek P-T voorwaardes en oorblyfsel granaat in die loop onder 804 °C. (ii) minerale-smelt reaksies waarin die granaat kristalle reageer met die smelt om biotiet, cordierite en hercynite-geproduseer produseer, net in die aanloop produk teen 804 °C. Maar die granaat-smelt reaksies is stadiger in vergelyking met die ontbinding-neerslag meganisme. Dit is duidelik deur die behoud van granaat kerne gepantserde deur biotiet reaksie vellings. Die eksperimente toon dat, afgesien van die oënskynlike stadiger reaksies met granaat, die pryse van ewewig van grof-gegranuleerde minerale met graniet magma op vlak kors vlakke is baie vinnig. Hierdie prosesse verskyn hoogs doeltreffend te wees, en is verantwoordelik vir die oorwegend magmatiese aard van die bogenoemde minerale in 'n hoë-vlak granitiese gesteentes. In die eksperimentele lopie produkte, is die magma getoon dat 'n voldoende hoë smelt volume het, op die P-T toestande naby aan die solidus, om prosesse of reaksies wat buite-ewewig kristalle bring in ewewig te fasiliteer. Verder het die resultate dui daarop dat, waar granaat kristalle teenwoordig is in kristallyne granitiese gesteentes hulle waarskynlik metastabiele fases in die magma wat ingedring en gekristalliseer is, en hulle doen ook nie compositioneel of texturally lyk die hoë-P-T entrained peritectic fases. Hierdie navorsing het duidelik waarom peritectic minerale afwesig of baie skaars in kristallyne granitiese gesteentes. Daarbenewens het die bevindinge toon hoekom ons nie moet verwag om óf restitic komponente, vroeg gevorm magmatiese fases of die peritectic minerale versameling in 'n hoë-vlak granitiese gesteentes sien.

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