Apatite, allanite, titanite and monazite characteristics in S-, I- A-type Cape Granites

[摘要] ENGLISH ABSTRACT:This study focussed on the comparison of accessory mineral chemistry and paragenesis inthe S-, I- and A-type granites of the Cape Granite Suite. The objective of the study was touse differences in accessory mineral chemistry and petrography to give insight in theevolution, recycling and formation of continental crust as affected by the Cape GraniteSuite. Because of the high partition coefficients of the REE and trace elements intoaccessory minerals these minerals play an important role to explain granite evolution.The accessory mineral features are used as discriminators between barren andmineralized S-, I- and A-type granites in the suite.The petrography of the suite reflects the allanite-monazite dichtonomy with allanite andtitanite 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 allthe 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 relicsof sedimentary source rocks. Concentric growth- and sectoral zoning, as observed withCL and SEM, are common features in minerals that crystallized in barren plutons. Theoverprinting of magmatic textures reflects secondary processes, such as those thatoccurred in mineralized plutons, by patchy zoning and irregular alteration rims(coronas) in the mineralized plutons' accessory minerals. CL and SEM observationsrevealed that REE are redistributed into these coronas.Mineral chemistry of the accessory minerals reflects mostly the whole rock chemistry andphysical conditions of the magmas. (Al~ Fe) substitution in titanite is controlled by P-Tconditions, together with Ca, Mn and Mg substitution which is controlled by whole-rockchemistry, are good discriminators in S- and I-type granites. LREE and Sr content inallanite discriminate between the plutons and reflect the whole-rock chemistry. Apatite,because it occurs in all the plutons, is the most useful accessory mineral fordiscriminating between the plutons. From previous studies it is known that ASI controlsthe two main substitutions in apatite: Ca+P~Si+REE and Na+REE~2Ca, Fe and Mncontent in apatite (0,1 pfu Mn and 0,05 pfu Fe contents are the cut-off between S-and Itypegranites) are controlled by oxidation state of the magma and Sr, REE and Mg reflectwhole-rock chemistry. The content of these elements in apatite can be used as discriminators between the plutons as their ASI, oxidation states and whole-rockchemistry differ. REE patterns of monazite and allanite are LREE enriched withoutexception, while apatite and titanite REE patterns are mostly birdwing profiles withoccurrences of LREE or HREE enrichment. These patterns are influenced bycrystallization of coexisting REE-bearing phases, fractionation history of the pluton andby crystallization sequence of the accessory minerals.Phase relationships were investigated experimentally for monazite and allanite undermagmatic 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 H20where added to peraluminous melts (ASI> 1 ). Monazite broke down to Cl-apatite andcorona textures were observed. Allanite was tested in peraluminous (ASI> 1) andmetaluminous (ASI=1) melts with different P20 5 (0.08 - 0.25 wt%) concentrations.Allanite became unstable at high phosphorus and peraluminous melt conditions andbroke down to LREE-P± Al, Ca, K phases. Corona (kelyphitic) textures were observed. Itis also clear that phosphorus played an important role, with Al, in the melt structure ascan 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 Tof the granitic melts' liquidus. Because of an interaction of phosphorus with Si networksand formation of complexes it also depolymerize aluminosilicate melts. The solubility ofmonazite 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) chlorideswere very reactive and dissolved the monazite completely, while solutions of CaCb wereless reactive and only partly dissolved the monazite. These experiments demonstrate theconcentrations required in hydrothermal solutions to destabilize monazite and explaintextures found in natural rocks.Accessory minerals are useful discriminators between S-, I- and A-type granites and alsotheir mineralized counterparts. Discrimination does depend on what accessory mineralsare 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.

[发布机构] Stellenbosch University