The origin of rhythmic magmatic layering in coarse-grained porphyritic S-type granite of the Peninsula pluton, Cape Granite Suite, South Africa

Ramphaka, Priscilla L. (2013-12)

Thesis (MSc)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: Rhythmic magmatic layering in granites is an intriguing feature that has been reported from plutons with contrasting chemical compositions from a wide range of tectonic settings. Layered granites are rare and occur in association with volumetrically dominant non-layered rocks having similar composition. Understanding the origin of such layering in granites, particularly from well exposed outcrops, may give crucial insights on the physical-chemical conditions and processes (such as fractional crystallization, size and composition of individual magma batches, efficiency of mixing between batches etc.) within magma chambers during their construction. In the Cape Granite Suite, rhythmic layering is exceptionally well preserved in outcrops of the S-type granodioritic to monzogranitic Peninsula pluton along the coastline of the small town of Llandudno. At Llandudno, the granodioritic facies of the Peninsula pluton is exposed as a coarse-grained, cordierite and K-feldspar phenocrystic granite (referred to as Llandudno granodiorite in this study). Layering within the Llandudno granodiorite occurs within several lens-shaped bodies, of which the largest, with a thickness of about 5 m. The studied rhythmic sequence comprises 50 layers with thicknesses ranging between 5 and 50 cm. Each layer is typically characterised by a sharp lower contact overlain by a biotite-rich portion containing approximately 50 vol. % biotite. This mafic lower portion, grades upwards into a strongly leucocratic upper portion enriched in quartz, plagioclase and K-feldspar. The layered rocks host a substantially lower abundance of K-feldspar megacrysts and cordierite than the surrounding Llandudno granodiorite. In particular, cordierite crystals are five times less abundant in the layered zone than the un-layered Llandudno granodiorite. The K-Feldspars megacrysts that do occur within the layering are generally concentrated in the mafic part of some of the layers and are commonly oriented parallel to the layering. The K-feldspar megacrysts are significantly smaller than those in Llandudno granodiorite, but were found to be similar in composition. The whole rock compositions of the layered rocks are peraluminous with A/CNK >1.4. They have lower Mg#s (51 to 58) than the Llandudno granodiorite (51 to 65). The Na2O/CaO ratio of mafic and leucocratic portions is higher than in the Peninsula pluton. The mafic portions show an enrichment of trace and rare earth elements relative to the leucocratic portions and Peninsula pluton. The difference in bulk rock composition as well as trace and rare earth elements composition shows that the layered rocks were not formed by magmas produced by differentiation of the Llandudno granodiorite. The differences in biotite composition in basal sections of adjacent layers suggest that each layer represents a separate magma pulse, with the mafic portions of the layers largely representing an accumulation of the crystals in the magma batch at the time of injection and that these crystals mostly consist of orthopyroxene and biotite. This is in agreement with the findings based on the whole rock chemistry of the layers compared with the Llandudno granodiorite. The inclusion of K-feldspar megacrysts from the host granite into the layering, as well as the exceptional state of preservation of the layering, suggests that the layering formed relatively late in the crystallization sequence of the granite. This, in combination with the evidence for subtle differences in the chemistry of the magmas that formed separate layers, suggests that the layers represent a frozen feeder zone in the Peninsula pluton recording small successive pulses of magma addition. Frequent occurrence of the less-preserved layered biotite schlieren in the pluton may represent the equivalents of the layering that have become partially digested and texturally equilibrated with the host magma.

AFRIKAANSE OPSOMMING: Ritmiese magmatiese gelaagdheid in graniet is ’n interessante kenmerk van plutone met teenstellende chemiese samestellings in ’n wye verskeidenheid tektoniese omgewings. Gelaagde graniet is seldsaam en kom saam met volumetries dominante, niegelaagde gesteentes met soortgelyke samestellings voor. ’n Begrip van die oorsprong van hierdie gelaagdheid in graniet, veral in goed sigbare dagsome, kan belangrike insig verleen in die fisies-chemiese omstandighede en prosesse (soos fraksionele kristallisasie, die grootte en samestelling van individuele volumes magma, die doeltreffendheid van volume-vermenging, ensovoorts) in magmakamers gedurende die vorming daarvan. In die Kaapse granietgroep het ritmiese gelaagdheid besonder goed behoue gebly in dagsome van die S-tipe granodioritiese tot monsogranitiese Skiereilandse plutoon langs die kuslyn van die voorstad Llandudno. Die granodioritiese fasies van die Skiereilandse plutoon by Llandudno manifesteer in die vorm van ’n grofkorrelrige, kordiëritiese en K-veldspatiese fenokris-graniet (wat in hierdie studie Llandudno-granodioriet genoem word). Gelaagdheid in die Llandudno-granodioriet kom voor in verskeie lensvormige massas, waarvan die grootste sowat 5 m dik is. Die bestudeerde ritmiese opeenvolging bestaan uit 50 lae met diktes van tussen 5 cm en 50 cm. Elke laag word gekenmerk deur ’n skerp onderste kontakvlak wat bedek is met ’n biotiet-ryke gedeelte wat uit sowat 50 vol % biotiet bestaan. Hierdie mafiese onderste gedeelte gradeer opwaarts in ’n sterk leukokratiese boonste gedeelte wat ryk is aan kwarts, plagioklaas en K-veldspaat. Die gelaagde gesteentes bevat beduidend minder K-veldspatiese megakriste en kordiëriet as die omliggende Llandudno-granodioriet. Die kordiëritiese kristalle in besonder kom vyf keer minder in die gelaagde sone as in die niegelaagde Llandudno-granodioriet voor. Die K-veldspatiese megakriste wat wél in die gelaagdheid voorkom, is oor die algemeen in die mafiese gedeelte van sommige lae gekonsentreer, en is meestal parallel met die gelaagdheid georiënteerd. Die K-veldspatiese megakriste is aansienlik kleiner as dié in die Llandudno-granodioriet, maar het ’n soortgelyke samestelling. Die heelrots-samestellings van die gelaagde gesteentes is peralumineus, met A/CNK >1,4. Dit toon ook laer Mg#s (51 tot 58) as die Llandudno-granodioriet (51 tot 65). Die Na2O/CaO-verhouding van die mafiese en leukokratiese gedeeltes is hoër as in die Skiereilandse plutoon. In die mafiese gedeeltes is daar ‘n verryking in spoor- en skaarsaarde-elemente relatief tot die leukokratiese gedeeltes sowel as die Skiereilandse plutoon. Die verskil in heelrots-samestelling sowel as spoor- en seldsame-aardelementsamestelling toon dat die gelaagde gesteentes nié gevorm is deur magmas wat uit differensiasie van die Llandudno-granodioriet ontstaan het nie. Uit die verskille in die biotiet-samestelling van basissnitte uit aanliggende lae word afgelei dat elke laag ’n afsonderlike magmapuls verteenwoordig, terwyl die mafiese gedeeltes van die lae hoofsaaklik ‘n versameling van kristalle verteenwoordig wat tydens inplasing van die magma volume gevorm het, meestal ortopirokseen en biotiet. Dít stem ooreen met die bevindinge rakende die heelrots-chemie van die lae in vergelyking met die Llandudno-granodioriet. Die insluiting van K-veldspatiese megakriste vanaf die moedergraniet by die gelaagdheid, dui daarop dat die gelaagdheid betreklik laat in die kristallisasie-orde van die graniet gevorm het. Dít, tesame met bewyse van subtiele verskille in die chemie van die magmas waaruit afsonderlike lae gevorm is, dui daarop dat die lae ’n bevrore toevoersone in die Skiereilandse plutoon uitmaak wat kort, opeenvolgende pulse van magmatoevoeging vasgevang het. Die gereelde voorkoms van swakker bewaarde, gelaagde biotiet-sliere in die plutoon kan moontlik dui op sones van die gelaagdheid wat gedeeltelik verteer is en tekstureel met die moedermagma ge-ekwilbreer het.

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