An investigation of the petrogenesis of the Buddusò I-type granites and its mafic enclaves in Sardinia, Italy.

Soorajlal, Roxanne (2017-12)

Thesis (MSc)--Stellenbosch University, 2017.


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.

AFRIKAANSE OPSOMMING: Daar word steeds grootliks gedebateer rondom die petrogenese van I-tipe graniete met mafiese enklaves. Die graniet polemiek gee oor aan drie heersende prosesse/modelle rondom die tipiese chemise reeks wat voorgestel word deur algemene I-tipe graniete; 1) magma menging/vermenging, 2) differensieële inlaaiïng van material vanaf die oorsprong, en 3) fraksionele kristallisasie van/vanaf die oorsprong. Daar is drie predominerende teorieë wat die petrogenese van mafiese enklaves beskryf: 1) voorstellende kumulitiewe fragmente, 2) blobbe hibridiese magma wat ‘n mengsel van mafiese magma vermenging met ‘n felsiese hostie voorstel, en 3) fragmente van gerikristalliseerde metamorfiese klip wat van die oorsprong ge-erf is gedurende gedeeltelike smagmaing daarvan (die oorsprong). Mafiese enklaves word wyd oorweeg om magma menging/vermenging van ‘n mafiese magma en ‘n felsiese hostie magma voor te stel in I-tipe graniete. Die Buddusò Pluton is n perfekte voorbeeld van ‘n algemene I-tipe granitiese liggaam met mafiese enklaves. Hierdie studie beoog om 1) die oorsprong van die komposisionele variasie wat gesien word in die granitiese eenhede sowel as die mafiese enklaves te verduidelik, 2) om die mees konsekwente model vir die petrgenese van die Buddusò pluton op te stel. Hierdie studies al gebruik maak van ‘n mengsel geo-analitiese teknieke, veld verhoudinge, geheel klip geochemie, petrografie, mineral chemie, zircon geochronologie en Lu-Hf U-Pb isotopiese analiese van die zircons vanaf beide die mafiese enklaves en die graniete om hulp te bied om die doel van die studie te bereik. Die Buddusò pluton bestaan uit drie eenhede; ‘n binneste eenheid wat uit leukograniete bestaan, ‘n middelste eenheid met granitiese komposisies en ‘n buitenste eenheid wat uit granodiorite bestaan. Mafiese enklaves kom voor reg deur die pluton en verhoog in voorkoms vanaf die binneste tot die buitenste eenhede. Graniete van alle eenhede wys negatiewe korrelasie met alle hoof elemente behalwe K2O met betrekking tot SiO2. Hulle wys ‘n verhoging in Al2O3, CaO, FeO, MgO, TiO2 en P2O5 met dalende SiO2 vanaf die binneste eenheid deur tot die buitenste eenheid. Graniete wys ‘n stywer korrelasie met betrekking tot die hoof- en spoorelemente teenoor SiO2 in vergelyking met sy mafiese enklaves. Die mafiese enklaves wys ‘n soortgelyke minerale versameling as die graniete (sensu lato) met ‘n hoër verhouding van mafiese minerale, albei bevat kompleks gezoneerde plagioklaas kristalle. U-Pb isotope data dui ‘n kristallisasie ouderdom van 294±Mj vir beide graniete en mafiese enklaves en openbaar dat die ouderdom van die oorsprong heelwat na aan die ouderdom van die pluton it. ꜪHf(t) waardes vanaf Lu-Hf isotope analise suggesteer dat beide die granietes en mafiese enklaves het isotopiese handtekening wat vanaf die kors afgelei is en het kleinskaal isotopiese variasie binne-in die monsters. Die kleinskaal ƐHf(t) reeks word groter vanaf die binneste eenheid deur tot die middelste eenheid. Veld en petrografiese bewyse, kontak morfologieë, die teenwoordigheid van reaksie minerale, kristal verruilling, die teenwoordigheid van ‘acicular’ apatiet, lemvormige biotiet en komposisioneel gezoneerde plagioklaas almal suggesteer dat die mafiese enklave magma die interaksie met die granitiese magma gesien het, mees waarskynlik voor inplasing. Plagioklaas, biotiet en K-feldspar wys soortgelyke mineraal chemies in beide graniete en mafiese enklaves wat suggesteer dat die minerale reeds ekwilibrium bereik het. Hornblende wys verskillende chemiese komposisies in mafiese enklaves in vergyling met die graniete. ‘n vermengings model was ontwep wat elk van die enklaves komposisies gemeng het met die mees leukokratiese graniet (monster BG32) in 5wt. % inkremente. Die natuurlike monsters van die graniete en enklaves het nie op die vermengings lyne vir al die hoof elemente gelê nie. Die studie sluit af dat die mafiese enklaves verskillende grade van hibridisasie gesien het by die graniet magma. ‘n Fraksionele model was gedoen by 3Kbars met 3.4wt. % H2O vir drie verhitte paaie by 700, 800 en 900°C deur gebruik te maak van twee verskillende begin-komposisies om fraksionele kristallisasie uit te beeld. Magma was dan opeenvolgend in 5 wt. % inkremente onttrek totdat die magma nie in die sisteem bestaan het nie. Die tweede model was opgestel deur gebruik te maak van B27, ‘n granite van die middelste eenheid, as ‘n begin-komposisie het goed gepas. Hoewel die model ‘n waarskynlike inplasings-meganisme verduidelik sowel as ‘n proses wat die variasie in die granitiese eenheid veroorsaak, verduidelik dit nie die enclave komposisies nie. Die studie sluit af dat die mafiese enclave magma en die granitiese magma deur van kors afgelei is en komagneties gebaseer is on hul soortgelyke reeks in komposisie, soortgelyke magmatiese ouderdom en Hf isotopiese handtekening. Die magmas is voorgestel om deur parsieële smagmaing van ‘n andesitiese oorsprong geproduseer is. Die primêre meganisme wat die magmas vorm is die peritektiese mineral invoering en ko-invoering van bykomstige suite minerale wanneer smagmaing plaasvind. Die magma is ingespuit binne-in die magma kamer in twee pulse met ‘n klein tydgleuf tussen-in. Die mafiese enklave magma was eers ingespuit met ‘n hoër fraksie ingevoerde ferromagnesiese minerale en toe begin kristalliseer. Die granitiese magma was dan ingespuit met ‘n laer fraksie ingevoerde ferromagnesiese minerale wat ‘n komposisie na aan die van die intermediêre graniete tot gevolg het. Die mafiese enklave magma mush (kristalle + magma) het met die graniet magma ‘n interaksie ondergaan via chemiese verruilling, diffusie en meganiese oordrag gedurende daling voor inplasing plaasgevind het. Die mafiese enklave magma was gevolglik gehibridiseer en die meer viskeuse magma het oor gekristalliseerde velle van enklave magma gevloei en dit gevolglik in kleiner stukke gebreuk het. Tydens inplasing het die Buddusò Pluton ‘n vervormings gebeurtenis beleef en tot gevolg die magma kamer versteur. Die vervorming het ‘n lae temperatuur filter druk proses toegelaat om die magma van die graniet mush te druk (enklave gehibridiseerde blabbe van kristal en magma + minder mafiese graniet magma) en dit te mobiliseer na die lae-druk zones wat deur die differensieële stres van die vervorming geskep is. Die kristal ophoping was verteenwoordigend van die granodiorites se komposisies en dit het die magma wat verteenwoordigend van die leukogranietes se komposisies is afgedruk. Die hele pluton was nie geraak deur die vervorming nie en het daartoe tot gevolg dat sommige dele van die pluton die graniet mush se komposisie behou. Dit sou die drie granitiese eenhede tot gevolg hê; granodiorites met vollop mafiese enklaves, granietes met minder mafiese enklaves en leukogranietes met geen mafiese enklaves nie.

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