Browsing by Author "Zarrebini, Sara Kathryn"
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- ItemAn experimental investigation into the ‘fate’ of entrained peritectic minerals in I-type granite magmas intruded at below 2kbar.(Stellenbosch : Stellenbosch University, 2016-04) Zarrebini, Sara Kathryn; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.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.