Browsing by Author "Muller, Jared"

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    Constraints on the size and temperature of magma batches that constructed the Peninsula pluton, based on the metamorphic record of metasedimentary xenoliths
    (Stellenbosch : Stellenbosch University, 2023-03) Muller, Jared; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: This study aimed to investigate the metamorphic histories of nine greenschist facies xenolith samples from the Malmesbury Group metasediments, which have an estimated age of deposition of approximately 575 to 550 Ma. These samples are hosted in the Peninsula Pluton granitoids (537.8 ± 1.6 Ma to 532.7 ± 1.9 Ma) of the Cape Granite Suite. The goal of the study was to put constraints on the size and temperature of the magma batches that formed the Peninsula Pluton granitoids. To achieve this, various techniques including petrography, field evidence, scanning electron microscopy, phase equilibria modelling using the freeware "Rcrust" and thermal modelling using the freeware "Energy2D" were employed to establish peak metamorphic mineral assemblages and constrain peak metamorphic conditions. The peak metamorphic assemblages for the xenoliths were determined to be quartz + biotite + plagioclase + cordierite ± K-feldspar ± ilmenite ± muscovite. The results of the bulk rock and mineral composition analyses revealed that the metasedimentary xenoliths of this study plot within a normal range of the Malmesbury group sediments and metasediments. Temperatures of formation for the magma batches that constructed the Peninsula Pluton have been noted to be between 800 and 900°C. Ti-in-biotite thermometry was used to constrain the temperature of peak metamorphism for the xenoliths of this study to be in the range of 665 to 725°C. Phase equilibria modelling revealed that the different xenoliths experienced peak temperatures between 530 and 730°C and pressures between 0 and 4.5 kbar. This has important implications for background magma temperature in the pluton and the size and temperature of the magma batches that entrained the xenoliths. Obviously if the magma arrived in the upper crust at ~ 700°C, batches could conceivably be any size. However, there is strong evidence for the Peninsula Pluton magmas having arisen by fluid absent melting involving biotite at >850°C. Two-dimensional conductive thermal modelling was used to explore the ranges of background magma temperature and magma batch sizes and temperatures that would be consistent with the xenoliths recording only amphibolite facies temperatures. The results indicate that the background magma temperature cannot be higher than the maximum temperatures recorded by the xenoliths. Additionally, if the magma batches that sampled the xenoliths and introduced them into the pluton were at 850°C, they could not have been thicker than 1m if intruded as 10m wide sills. Similarly, magma batches arriving at 750°C could not have been thicker than 2m if intruded as 10m wide sills. These size and temperature estimates contribute to the body of evidence supporting the incremental growth model of granitic plutons, as well as providing insight into the size and temperature of magma batches that were assimilated to form the Peninsula Pluton granitoids. The assimilation of these batches, as well as their frequency, can be better understood considering these data.