Thesis (MSc)--Stellenbosch University, 2001. / ENGLISH ABSTRACT: This study focussed on the comparison of accessory mineral chemistry and paragenesis in
the S-, I- and A-type granites of the Cape Granite Suite. The objective of the study was to
use differences in accessory mineral chemistry and petrography to give insight in the
evolution, recycling and formation of continental crust as affected by the Cape Granite
Suite. Because of the high partition coefficients of the REE and trace elements into
accessory minerals these minerals play an important role to explain granite evolution.
The accessory mineral features are used as discriminators between barren and
mineralized S-, I- and A-type granites in the suite.
The petrography of the suite reflects the allanite-monazite dichtonomy with allanite and
titanite occurring in the I -type granites while monazite occurs in S-type granites.
Monazite becomes unstable in high Ca melts such as I-type granites. Apatite occurs in all
the plutons which reflects its stability over a wide range of geological conditions.
Rounded crystal habits of apatite and monazite in S-type granites indicate they are relics
of sedimentary source rocks. Concentric growth- and sectoral zoning, as observed with
CL and SEM, are common features in minerals that crystallized in barren plutons. The
overprinting of magmatic textures reflects secondary processes, such as those that
occurred in mineralized plutons, by "patchy" zoning and irregular alteration rims
(coronas) in the mineralized plutons' accessory minerals. CL and SEM observations
revealed that REE are redistributed into these coronas.
Mineral chemistry of the accessory minerals reflects mostly the whole rock chemistry and
physical conditions of the magmas. (Al~ Fe) substitution in titanite is controlled by P-T
conditions, together with Ca, Mn and Mg substitution which is controlled by whole-rock
chemistry, are good discriminators in S- and I-type granites. LREE and Sr content in
allanite discriminate between the plutons and reflect the whole-rock chemistry. Apatite,
because it occurs in all the plutons, is the most useful accessory mineral for
discriminating between the plutons. From previous studies it is known that ASI controls
the two main substitutions in apatite: Ca+P~Si+REE and Na+REE~2Ca, Fe and Mn
content in apatite (0,1 pfu Mn and 0,05 pfu Fe contents are the cut-off between S-and Itype
granites) are controlled by oxidation state of the magma and Sr, REE and Mg reflect
whole-rock chemistry. The content of these elements in apatite can be used as discriminators between the plutons as their ASI, oxidation states and whole-rock
chemistry differ. REE patterns of monazite and allanite are LREE enriched without
exception, while apatite and titanite REE patterns are mostly birdwing profiles with
occurrences of LREE or HREE enrichment. These patterns are influenced by
crystallization of coexisting REE-bearing phases, fractionation history of the pluton and
by crystallization sequence of the accessory minerals.
Phase relationships were investigated experimentally for monazite and allanite under
magmatic conditions (870 °C, 1,8 kbar) in peraluminous to metaluminous granitic melts.
Monazite became unstable when aqueous CaCh solutions of 0.7-7 g CaCh/10cc H20
where added to peraluminous melts (ASI> 1 ). Monazite broke down to Cl-apatite and
corona textures were observed. Allanite was tested in peraluminous (ASI> 1) and
metaluminous (ASI=1) melts with different P20 5 (0.08 - 0.25 wt%) concentrations.
Allanite became unstable at high phosphorus and peraluminous melt conditions and
broke down to LREE-P± Al, Ca, K phases. Corona (kelyphitic) textures were observed. It
is also clear that phosphorus played an important role, with Al, in the melt structure as
can be seen from the absence or presence of crystals in the glasses of the different melts.
This is possible because adding of phosphorus to the melt results in a depression of the T
of the granitic melts' liquidus. Because of an interaction of phosphorus with Si networks
and formation of complexes it also depolymerize aluminosilicate melts. The solubility of
monazite was also tested in aqueous solutions under atmospheric conditions and low T
(100-350 °C) to investigate low TIP alteration. Solutions ofCaCb +NaCl (1:1) chlorides
were very reactive and dissolved the monazite completely, while solutions of CaCb were
less reactive and only partly dissolved the monazite. These experiments demonstrate the
concentrations required in hydrothermal solutions to destabilize monazite and explain
textures found in natural rocks.
Accessory minerals are useful discriminators between S-, I- and A-type granites and also
their mineralized counterparts. Discrimination does depend on what accessory minerals
are present and therefore apatite is the best mineral because it occurs in all the plutons.
Petrography of these minerals is an indicator of primary or secondary processes. / AFRIKAANSE OPSOMMING: Die fokus van hierdie studie was om die mineraalchemie en paragenese van bykomstige
minerale in S-, 1- en A-tipe graniete van die Kaapse Graniet Suite te vergelyk. Die
doelwit van hierdie studie was om die verskille in chemie en petrografie van bykomstige
minerale te gebruik as insig in die evolusie, herwinning en ontstaan van kontinentale kors
soos geaffekteer deur die Kaapse Graniet Suite. Omdat SAE en spoorelemente hoe
partisiekoeffisiente het vir bykomstige minerale speel hierdie minerale 'n belangrike rol
om graniet evolusie te verklaar. Genoemde kenmerke van bykomstige minerale is ook
gebruik om te onderskei tussen ongemineraliseerde en gemineraliseerde S-, 1- en A-tipe
graniete in die suite.
Die petrografie van die Kaapse Graniet Suite weerspieel die tweeledigheid van allanietmonasiet
deurdat allaniet en titaniet in 1-tipe graniete en monasiet in S-tipe graniete
voorkom. Monasiet word dus onstabiel in hoe Ca, 1-tipe, graniete. Apatiet kom in al drie
tipes voor wat die mineraal se stabiliteit in verskeie geologiese omgewings weerspieel.
Geronde kristalvorme , of reliekteksture, van apatiet en monasiet in S-tipe graniete
weerspieel die sedimentere oorsprong van hierdie graniete. Konsentriese groei - en
sektorale sonering kom algemeen voor in bykomstige minerale in ongemineraliseerde
plutone. Sekondere veranderings rande (koronas) en onreelmatige sonering in
gemineraliseerde plutone se bykomstige minerale is 'n aanduiding dat primere teksture
gedeeltelik vemietig is deur sekondere prosesse. Katodeluminisensie en skandeerelektron
mikroskopie ondersoeke het bewys dat SAE gehermobiliseer word na die
koronas.
Heelrotschemie en fisiese toestande van die magma word weerspieel in die
mineraalchemie van bykomstige minerale. (Al~Fe) substitusie in titaniet word beheer
deur P-T toestande en is, saam met Ca, Mn en Mg inhoud wat heelrotschemie weerspieel,
goeie diskriminators in S- en 1-tipe graniete. LSAE en Sr inhoud in allaniet onderskei
goed tussen plutone omdat hierdie elemente die heelrotschemie weerspieel. Omdat
apatiet in al die plutone voorkom is dit die bruikbaarste mineraal om as diskriminant te
gebruik. V anuit vorige werk is dit bekend dat die aluminium versadigings indeks die twee
hoofsubstitusies: Ca+P~Si+SAE en Na+SAE~2Ca beheer, Fe en Mn inhoud in apatiet
(0,1 pfu Mn en 0,05 pfu Fe is die afsnypunt tussen S- en 1-tipe graniete) weerspieel die oksidasietoestand van die magma en Sr, SAE en Mg weerspieel heelrotschemie. Saam
kan hierdie elemente dus gebruik word as diskriminatore tussen die verskillende plutone.
SAE patrone van allaniet en monasiet is sonder uitsondering verryk in die LSAE, terwyl
apatiet en titaniet meestal "birdwing" profiele wys, maar kan ook verryk wees in LSAE
of SSAE. Hierdie patrone word beinvloed deur kristallisasie van ander SAE-draende
minerale, fraksionering van minerale uit die magma en die kristallisasie volgorde van die
mineral e.
Faseverwantskappe is eksperimenteel getoets tussen monasiet en allaniet in magmatiese
toestande (780 °C en 1,8 kbar). Monasiet word onstabiel in 'n peralumineuse smelt
(Aluminium versadigingsindeks >1) as waterig oplossings met konsentrasies van 0.7-7 g
CaCh/1 0 cc H20 bygevoeg word. Cl-apatiet vorm as veranderingsproduk om die rande
(koronas ). Allaniet is getoets in peralumineuse (Aluminium versadigingsindeks > 1) en
metalumineuse smelte (Aluminium versadigingsindeks =1) met wisselende konsentrasies
P20s (0.08 - 0.25 wt%). Allaniet het onstabiel geraak in peralumineuse smelte en hoe
fosfor konsentrasies en het afgebreek na fases van LSAE+P± Ca, Al, K. Korona
(kelifitiese) teksture is waargeneem. Hierdie eksperimente bewys dat fosfor, saam met
Al, 'n belangrike rol speel in smeltstruktuur. Dit kan gesien word in die teenwoordig- of
afwesigheid van kristalle in die glas. Dit is moontlik deurdat die byvoeging van fosfor 'n
verlaging in die graniet likwidus temperatuur veroorsaak. Fosfor depolimeriseer ook
aluminiumsilikaat smelte deur interaksie en kompleksvorming tussen fosfor en silika
netwerke. Die oplosbaarheid van monasiet is ook getoets in waterige oplossings onder
atmosferiese toestande en lae T (100-350 °C) om lae PIT veranderinge te ondersoek. 'n
Versadigde oplossing van CaCh en NaCl (1:1) chloried het monasiet heeltemal opgelos
terwyl 'n versadigde oplossing van CaCh monasiet net gedeeltelik opgelos het. Hierdie
eksperimente dui op die konsentrasies nodig vir hidrotermale vloeistowwe waar
bykomstige minerale onstabiel raak en verklaar teksture in natuurlike rotse.
Bykomstige minerale kan dus gebruik word as diskriminators tussen ongemineraliseerde
en gemineraliseerde plutone en ook tussen S-, I- en A-tipe graniete. Hulle kan egter net
gebruik word as hulle teenwoordig is en daarom is apatiet die beste omdat dit in al die
plutone aanwesig is. Petrografie is 'n aanduiding van magmatiese of sekondere prosesse.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52120 |
| Date | 12 1900 |
| Creators | Spicer, Esme M. (Esme Marelien) |
| Contributors | Scheepers, R., Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 93 p. : ill. |
| Rights | Stellenbosch University |
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