Browsing by Author "Soorajlal, Roxanne"

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    An investigation of the petrogenesis of the Buddusò I-type granites and its mafic enclaves in Sardinia, Italy.
    (Stellenbosch : Stellenbosch University, 2017-12) Soorajlal, Roxanne; Stevens, Gary; Farina, Federico; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: There is still great debate surrounding the petrogenesis of I-type granites with mafic enclaves. The granite controversy indulges three prevalent processes/models surrounding the typical range in chemistry portrayed by common I-type granites; 1) magma mixing/mingling, 2) differential entrainment of material from the source and 3) fractional crystallisation from the source. There are three predominant theories which describe the petrogenesis of mafic enclaves: 1) they represent cumulate fragments, 2) blobs of hybrid magma representing a mixture of a mafic magma mixing with a felsic host magma and 3) fragments of recrystallized metamorphic rocks inherited from the source during partial melting from a parent magma. Mafic enclaves are widely considered to represent evidence for magma mixing/ mingling of a mafic magma and felsic host magma in I-type granites. The Buddusò Pluton is a perfect example of a common I-type granitic body with mafic enclaves. This study aims to; 1) explain the origin of the compositional variation seen in the granitic units as well as the mafic enclaves, 2) constrain the most consistent model for the petrogenesis of the Buddusò pluton. This study will make use of a blend of geo-analytical techniques; field relations, whole rock geochemistry, petrography, mineral chemistry, zircon geochronology and Lu-Hf and U-Pb isotopic analysis of the zircons from both mafic enclaves and granites to aid in meeting the aims of the study. The Buddusò pluton is comprised of three units; an inner unit comprised of leucogranites, a middle unit comprised of granite compositions and an outer unit comprised of granodiorites. Mafic enclaves exist throughout the pluton, increasing in abundance from the inner to outer units. Granites from all units show negative correlations for all major elements except K2O with respect to SiO2. They show an increase in Al2O3, CaO, FeO, MgO, TiO2 and, P2O5 with decreasing SiO2 from the inner unit through to the outer unit. Granites show tighter correlations with respect to the major and trace elements vs. SiO2 with trends portrayed by the mafic enclaves. Mafic enclaves show a similar mineral assemblage to the granites (sensu lato) with a higher proportion of mafic minerals, both contain complexly zoned plagioclase crystals. U-Pb isotope data indicated a crystallisation age of 294±2Ma for both granites and mafic enclaves and revealed that the age of the source was fairly close in age to that of the pluton (292±5Ma (Del Moro et al., 1975)). ƐHf(t) values from Lu-Hf isotope analyses suggest that both the granites and mafic enclaves have crustally derived isotopic signatures and showed small scale isotopic variation within individual samples. The small scale ƐHf(t) range gets larger from the inner unit through to the middle unit. Field and petrographical evidence; contact morphologies, presence of reaction minerals, crystal exchange, presence of acicular apatite, bladed biotite, and compositionally zoned plagioclase all suggest that the mafic enclave magma did see interaction with the granitic magma most probably prior to emplacement. Plagioclase, biotite and K-feldspar show similar mineral chemistries in both granites and mafic enclaves suggesting that, for element ratios important to these minerals, the mafic magma largely equilibrated with the chemistry of the host granite magma. Hornblende shows differing chemical compositions in mafic enclaves in comparison to the granites. A mixing model was designed which mixed each of the enclave compositions with the most leucocratic granite (sample BG32) in 5wt. % increments in order to investigate if the hypothetical mixed magmas would overlap in composition with the compositional range portrayed by the granites. The study concluded that the mafic enclaves saw varying degrees of hybridisation by the granite magma and the range seen in granite compositions were not produced by mixing with the mafic to intermediate magmas which formed the enclaves. A fractionation model was run at 3Kbars using 3.4wt. % H2O for three heating paths at 700, 800 and 900°C using two different starting compositions as a means of modelling crystal fractionation. Melt was then extracted sequentially in 5 wt. % increments until melt no longer existed in the system. The compositions of the crystal enriched magma and the melt separated from it were compared with the compositions of the granites and enclaves. The second model set up using B27, a granite from the middle unit, as a starting composition achieved a good linear fit with respect to major element chemistry. However, the model explained a probable emplacement mechanism as well as a process causing the mineral variation in the granite unit, it did not explain the enclave compositions. The study concluded that the mafic enclave magma and granite magma are crustally derived and comagmatic based on their similar range in mineral compositions, similar magmatic age and Hf isotope signature. The magmas are proposed to be produced via partial melting of an andesitic source. The primary mechanism shaping the chemistry of the magmas is peritectic mineral entrainment and co-entrainment of accessory suite minerals when melting occurs. The magma is injected into the magma chamber in two pulses closely separated in time. The mafic enclave magma was injected first with a higher fraction of entrained ferromagnesian minerals and began to crystallise. This mafic enclave magma was considerably hotter and came from a deeper magma chamber. The granitic magma was then injected with a lower fraction of entrained ferromagnesian minerals resulting in a composition close to that of the intermediate granites. The mafic enclave magma mush (crystals + magma) interacted with the granite magma via chemical exchange, diffusion and mechanical transfer during ascent prior to emplacement. The mafic enclave magma was consequently hybridised and the more viscous granite magma flowed over crystallised sheets of enclave magma consequently breaking it up into smaller pieces. Upon emplacement, the Buddusò Pluton saw a deformation event consequently disturbing the magma chamber. This deformation allowed for a low temperature filter pressing process to squeeze melt of the granite mush (enclave hybridised blobs of crystal and melt + less mafic granite magma) and mobilise it into the low-pressure zones. The crystal accumulation was representative of the granodiorites’ compositions and the squeezed off melt was representative of the leucogranites’ compositions. The entire pluton was not affected by the deformation therefore some parts of the pluton did not undergo melt:crystal separation and consequently retained their original magma compositions. This would result in the three granitic units; granodiorites with abundant mafic enclaves; granites with fewer mafic enclaves and leucogranites with no mafic enclaves.