An experimental investigation into the ‘fate’ of entrained peritectic minerals in I-type granite magmas intruded at below 2kbar.

Thesis (MSc)--Stellenbosch University, 2016.ENGLISH ABSTRACT: Entrainment of a peritectic mineral assemblage, formed through the incongruent melting of biotite and hornblende in a source of intermediate composition, has been proposed to account for the fact that I-type granites are commonly more mafic in composition than the melts from which they are derived. Magma consisting of variable proportions of melt and a distinct peritectic assemblage (Pl + Cpx + Opx + Ilm ± Grt) has been proposed to explain the substantial compositional range of I-type granitic rocks. Geochemical evidence to support the entrainment of a peritectic assemblage includes the strong, positive correlation between Ti vs maficity (atomic Fe + Mg) (with correlation coefficients typically higher than 0.9 for suites from individual plutons), as well as the very weak correlation between K and maficity in all granitic rocks. Despite this compelling geochemical evidence for the entrainment of a peritectic mineral assemblage, there is distinct lack of both mineral textural and mineral compositional evidence of these peritectic phases within I-type granites. In order to ascertain the ‘fate’ of the proposed entrained peritectic phases; the mechanisms and reactions by which these phases equilibrate with the surrounding magma, as well as the corresponding rates of these reactions needs to be established. This research used experimental techniques to determine the kinetic processes involved in the digestion of a high-pressure peritectic assemblage within a granodioritic magma under plutonic conditions within the upper crust (~2kbar and 763 to 720°C). A synthetic silicate gel (representative of the melt fraction) was mixed with a hypothetical, natural peritectic mineral assemblage (13.1Pl + 9.3Cpx + 1.1Opx + 0.8Ilm ± 5.7Grt) in a 7:3 ratio to form the experimental starting material. The peritectic mineral assemblage comprised of natural minerals of suitable compositions that were crushed and sieved to produce crystals with an average 200μm - 600μm size range. The 1.60 wt. % water within the starting composition was accommodated as kaolinite, which was added to the completed silicate gel in the correct stoichiometric proportions. The overall mineral-melt mixture was intended to represent the bulk composition of an I-type granodioritic magma produced by the fluid-absent melting of biotite and hornblende within an intermediate source composition at 10kbar and 850 - 900°C. The starting material (melt + mineral mixture) was loaded into gold capsules, which were then welded shut. The material was then reacted at pressures of 1.90kbar and 1.40kbar and at temperatures of 763 – 723°C within a cold-seal pressure vessel, with each experiment lasting 10 days. The findings of this research identified two principal reaction processes by which the high-pressure peritectic minerals react out within the experimental run products, namely; dissolution-precipitation and mineral-melt reactions. A coupled dissolution-precipitation mechanism is proposed to account for the rapid re-equilibration of ‘peritectic’ phases that are predicted (by phase equilibrium modelling) to be stable, but out of compositional equilibrium with the surrounding magma at the investigated PT conditions. ‘Peritectic’ plagioclase was seen to dissolve completely and reprecipitate a more albitic composition across all experimental pressure and temperature conditions, and within the experimental run-time (10 days). Consequently, a coupled dissolution-precipitation mechanism is inferred to be a sufficiently rapid and efficient process to account for the predominantly magmatic origin of feldspathic crystals within natural granitic rocks. At corresponding PT conditions, unstable mineral phases were seen to change both compositionally and texturally according to a separate reaction process, i.e. reaction with the surrounding melt portion. High pressure and temperature ferromagnesian minerals, such as garnet and orthopyroxene, which were not stable phases in the magma at the condition of the experiments were seen to react with the melt to produce reaction rims of biotite, which was a stable phase in the magma. The rapid growth rate of biotite resulting from this process suggests phaneritic textural features typically associated with granite bodies do not require long residency times to form within upper crustal magma chambers. The persistence of both garnet and orthopyroxene at the low pressure (1.40 – 1.90kbar) and temperature (763 - 720°C) conditions of experimentation indicates that at these conditions the mineral-melt reactions were too ‘sluggish’ to completely digest these phases. The rapid dissolution-precipitation rates of plagioclase within the experiments suggests that where garnet and orthopyroxene crystals are evident in natural granitic rocks these minerals bear no compositional or textural resemblance to the originally entrained ‘peritectic’ phases. The findings of this research corresponds well with the theory that I-type granites’ compositional trends form as a consequence of a peritectic mineral assemblage (Pl + Cpx + Opx + Ilm ± Grt) being entrained within a leucocratic melt. However, this research has far-reaching implications in terms of other formational theories, such as; fractional crystallization, magma mixing at depth and restite entrainment, that similarly involve the production of crystal-rich magmas at depth. Regardless of which process is responsible for generating the vast compositional heterogeneity of I-type granitic rocks, if the temperature conditions of the granitic magmas remain sufficiently high (>700°C) the magma will have the propensity to either partially or fully equilibrate any crystal component that is not in equilibrium with the surrounding magma.AFRIKAANSE OPSOMMING: Optel-en-meevoer van ‘n peritektiese mineraal versameling gevorm deur die ongelykvormige smelting van biotiet en hornblende in ‘n bron van intermediêre samestelling, was voorgestel om die I-tiepe graniet in ag te neem. Die I-tiepe graniet is oor die algemeen meer mafies in samestelling as wat smelt is. Magma bestaan uit verskillende hoeveelhede van smelt en ‘n spesifieke peritektiese versameling (Pl + Cpx + Opx + Ilm ± Grt) was voorgestel om die aansienlike komposisionele reeks van I-tiepe graniet smelt te verduidelik. Geochemiese bewyse om die optel-en-meevoer van die peritektiese versameling te ondersteun sluit in ‘n sterk, positiewe korrelasie tussen die Ti vs mafiese (atomies Fe + Mg) (met ‘n korrelasiekoëffisiënt wat gewoontlik hoër as 0.9 is vir rock groepe van individuele plutone) asook die baie swak korrelasie tussen K en mafiese in alle graniet klippe. Ten spyte van hierdie geochemiese bewyse van die optel-en-meevoer van die peritektiese mineraal versameling, is daar ‘n duidelike gebrek van bewyse vir beide mineraal tekstuur en mineraal komposisie van hierdie peritektiese fases in die I-tiepe graniet. Om die ‘lot’ van die voorgestelde optel-en-meevoer peritetiese fases te bepaal, moet die meganismes en reaksies van hierdie ewewigsfases, sowel as die magma in die omliggende en hul ooreenstemmende tempos, bepaal word. Hierdie navorsing het van eksperimentele tegnieke gebruik gemaak om die kinetiese prosesse te bepaal. Die prosesse is betrokke in die vertering van ‘n hoë druk perifiese mineraal versameling in ‘n granodioritiese magma onder plutoniese omstandighede in die boonste kors (~2kbar en 763 tot 720°C). ‘n Sintetiese silikahouende gel (verteenwoordiger van die smelt fraksie) was gemeng met ‘n hipotetiese, natuurlike peritektiese mineraal versameling (13.1Pl + 9.3Cpx + 1.1Opx + 0.8Ilm ± 5.7Grt) in ‘n 7:3 verhouding om die eksperimentele begin materiaal te vorm. Die peritektiese mineraal versameling bestaan uit natuurlike minerale van geskikte komposisies wat vergruis en gesif was sodat die kristalle ‘n gemiddeld grootte van 200μm - 600μm het. Die 1.6 wt. % water in die begin komposisie was geakkommodeer as kaoliniet, wat bygelas was om die silikahouende gel in die regte stoïgiometriese proporsies volledig te maak. Die hele mineraal mengsel was bedoel om die mederheid komposisies van ‘n I-tiepe granodiorite magma voor te stel en was geproduseer by die vloeistof-afwesige smelting van biotiet en hornblende in ‘n intermediêre bron komposisie by 10 kbar en 850 - 900°C. Die begin materiaal (smelt + mineraal mengsel) was in ‘n goue capsule gelaai, wat dan toe gesweis was. Die materiaal het gereageer by ‘n druk van 1.9 kbar en 1.4 kbar en by die temperatuur van 763 - 720°C wat in ‘n koue geseëlde drukvat geplaas was met elke eksperiment wat 10 dae lank geduur het. Die resultate van hierdie navorsing identifiseer twee beginsel reaksie prosesse, ontbinding-neerslag en minerale-smelt hidrasie reaksies, waar die hoë druk peritektiese minerale reageer in die eksperimentele resultante. ‘n Gekoppelde oplossings-presipitasie meganisme is voorgestel om die vinnige herewewigsproses van peritektiese fases in ag te neem wat voorspel is (deur fase ewewig modellering) om stabiel te wees, maar uit komposisionele ewewig saam met die magma in die omliggende gebied by die ondersoekte PT omstandighede. Peritektiese plagioklaas was gesien en het heeltemal opgelos en neerslag ‘n meer albeitiese komposisie oor al die eksperimentele druk en temperatuur omstandighede, asook in die eksperimentele looptyd (10 dae). Gevolglik is ‘n gekoppelde oplossings-presipitasie meganisme afgelei as ‘n effektiewe proses teen voldoende tempo om die oorwegende magmatiese oorsprong van feldspatiese kristalle in natuurlike graniet klip waar te neem. By ooreenkomstige PT omstandighede was daar onstabiele mineraal fases gesien wat volgens die afsonderlike reaksie fase in beide komposisionele en teksturele minerale verander het, bv. reaksies met die omgewind smelt proporsies. Hoë druk en temperatuur minerale met hoë Fe en Mg konsentrasies, soos granaat en ortopirokseen, was nie in stabiele fases in die magma by die omstandighede van die eksperiment nie en waargeneem dat dit reageer met die smelt om reaksie rande van ‘n stabiele biotiet fases te vorm. Dit was waargeneem dat ortopirokseen in laer druk eksperimente (1.40 kbar) reageer met die omliggende smelt proporsie om biotiet vervang ortopirokseen te vorm teen ‘n spoed van omtrent vier keer stadiger as die herewewig van peritektiese plagioklaas by die oplossings-presipitasie proses. Die vinnige groeitempo van biotiete, as gevolg van hieride proses, stel voor dat ‘phaneritic’ teksturele funksies wat tipies geassosieer word met graniet liggame, vereis nie lang verblyf tye om te vorm in die boonste kristal magma kamers nie. Die volharding van beide granaat en ortopirokseen by die lae druk (1.40 – 1.90kbar) en temperatuur (763 - 720°C) omstandighede van eksperimentasie toon dat by hieride omstandighede van eksperimentasie die mineraal-smelt hidrasie reaksies was te trae om volledig te verteer in hierdie fases. Die vinnige oplossings-presipitasie tempos van plagioklaas in die eksperimente dui duidelik aan dat granaat en ortopirokseen kristalle in natuurlike graniet klippe is. Hierdie minerale hou geen komposisionele of teksturele ooreenkoms met die oorspronklike opgetel en weggedra peritektiese fases nie. Die resultate van die navorsing stem goed ooreen met die teorie dat I-tiepe graniete komposisionele tendense vorm as ‘n gevolg van ‘n peritektiese mineraal versameling (Pl + Cpx + Opx + Ilm ± Grt) wat opgetel en weggedra is in ‘n leukokratiese smelt. Maar hierdie navorsing het verreikende gevolge in terme van ander magmatiese teorieë, bv. fraksionele kristalisering, magma vermenging in diepte en die optel-en-meevoer van die oorblywende minerale uit die bron, wat soortgelyk die produksie van kristalryk magmas in diepte insluit. Ongeag van watter van hierdie prosesse verantwoordelik is vir die generering van die oorgrote komposisionele heterogeniteit van I-tiepe graniet klippe, as die temperatuur omstandighede van die graniet magmas by voldoende hoë hoeveelhede bly (>700°C), dan sal die magma die geneigdheid hê om gedeeltlik of heeltemal ewewig te bereik met enige kristal komponent wat nie in komposisionele ewewig met die omliggende magma is nie