Disequilibrium melting of plagioclase during biotite fluid-absent anatexis of metapelites in the South Marginal Zone of the Limpopo belt, South Africa

Madlakana, Nonkuselo (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: This study investigated the behaviour of plagioclase during fluid-absent biotite melting in a metapelitic source from which peraluminous granite magma had segregated. The study focused on the Bandelierkop Quarry (South Marginal Zone of the Limpopo belt), where stromatic and discordant nebulous leucosomes contain peritectic garnet crystals produced by the following melting reactions; Bt + Sil + Qtz + Pl = Grt + Ilm + Melt; and, Bt + Qtz + Pl = Grt + Ilm + Opx + Crd + Melt. In rare cases, large (10 to 20mm) garnet crystals within both leucosome types are growth zoned. These garnet crystals host plagioclase inclusions, which are commonly euhedral and occur as single crystals, clusters of crystals and as polymineralic inclusions which typically consist of biotite, rutile and orthopyroxene. The inclusions display considerable chemical heterogeneity. Matrix plagioclase in both leucosome types ranges in composition from An27 to 35, whilst that in the residuum adjacent to the leucosomes ranges from An32 to 39. Euhedral plagioclase inclusions are considerably more calcic with compositions that range from An50 to 83. Composition of the inclusions shows no relationship with position within the garnet crystal, or with size of the plagioclase crystal. These crystals are also significantly zoned, yet their zoning is not systematic, with inclusions displaying both Na- and Ca-enriched rims. Garnet zonation around the inclusions was investigated in detail and demonstrates that the Ca-rich nature of the inclusions is not a consequence of Ca-uptake from garnet. In contrast, anhedral plagioclase within large amoeboid polymineralic inclusions displays compositions similar to the matrix plagioclase. Garnet also hosts very small (≤ 10μm) inclusions of quartz + plagioclase ± biotite. These micro-granite inclusions are interpreted as melt inclusions. The complexity of plagioclase behaviour recorded within the peritectic garnet crystals is interpreted to reflect disequilibrium during anatexis due to slow diffusion in plagioclase; only the outer portions of the plagioclase crystals participate in the melting reaction and they do so by combined dissolution and precipitation of new, Ca-rich crystals. Modelling demonstrates that the Ca-contents of these new crystals is inversely proportional to the amounts of plagioclase that participate in the reaction. Consequently, the euhedral plagioclase inclusions are interpreted to be peritectic, where these occur as polymineralic inclusions they are small and plagioclase is volumetrically dominant, so the melt volume in the inclusion was low. The more Ca-rich rims in euhedral plagioclase inclusions may reflect zonation produced by the melting reaction, whilst Na-rich rims may reflect crystals that are too calcic to equilibrate with melt within the time-scale available for melting , the volume of equilibrium is extremely locally controlled, which in turn induces Na-rich rims. The preservation of these garnet crystals, possibly due to rapid and efficient melt loss from the leucosome structures, provides a rare insight into the details of the anatectic process. In the vast majority of migmatitic granulites these are lost due to comprehensive extensive recrystallization and homogenization of mineral compositions.

AFRIKAANSE OPSOMMING: Hierdie studie ondersoek die gedrag van plagioklaas gedurende vloeistof-vrye biotiet smelting in ‘n metapelitiese bron vanwaar peraluminous graniet magma geskei het. Die studie fokus op die Bandelierkop Steengroef (Suid Marginale Zone van die Limpopo belt), waar stromatiese en afwykende newelagtige leukosome peritektiese granaatkristalle bevat wat produseer is deur die volgende smeltingsreaksies; Bt + Sil + Qtz + Pl = Grt + Ilm + Smelt; en, Bt + Qtz + Pl = Grt + Ilm + Opx + Crd + Smelt. In seldsame gevalle is groot (10 tot 20mm) granaatkristalle vanuit beide leukosoom tipes gesoneer tydens die groeiproses. Hierdie granaatkristalle bevat plagioklaas insluitings wat in die algemeen euëdries is en as enkel kristalle, kristal clusters en as polimineraliese insluitings wat tipies bestaan uit biotiet, rutiel en ortopirokseen. Die insluitings vertoon aansienlike chemiese heterogeniteit. Matriks plagioklaas in beide leukosoom tipes vertoon ‘n reeks komposisies van An 27 tot 35, waaartydens die in die residium aangrensend tot die leukosome verskil van An 32 tot 39. Euëdriese plagioklaaskristalle is aansienlik meer kalies met ‘n reeks komposisies van An 50 tot 83. Komposisies van die insluitings wys geen verhouding met posisie binne die granaat kristal of met die grootte van die plagioklaas kristal nie. Hierdie kristalle is ook merkwaardiglik gesoneer maar die sonering is nie sistematies nie, waar die insluitsels beide Na en Ca-ryke vellings vertoon. Granaat sonering rondom die insluitings was in detail ondersoek en dit demonstreer dat die Ca-ryke natuur van die insluitings nie ‘n oorsaak van Ca-opname vanaf granaat is nie. In kontras vertoon die oneievormige plagioklaas vanbinne groot amobidale polimineraliese insluitings komposisies soortgelyk aan die matriks plagioklaas. Granaat bevat ook baie fyn (≤ 10μm) isometriese insluitings van quartz + plagioklaas ± biotiet. Hierdie mikrogranietinsluitsels word as smeltingsinsluitsels geïnterpriteer. Die kompleksiteit van die gedrag van plagioklaas is aangeteken vanbinne die peritektiese granaatkristalle en is geïnterpriteer as die refleksie van onewewigtigheid gedurende anateksis as gevolg van stadige diffusie in plagioklaas. Slegs die buitenste gedeelte van die plagioklaaskristalle neem deel aan die smeltingsreaksie en hul doen dit deur gekombineerde ontbinding en presipitasie van nuwe Ca-ryke kristalle. Modellering demonstreer dat die Ca-inhoud van hierdie nuwe kirstalle ongekeerd eweredig is tot die hoeveelheid plagioklaas wat deel neem aan die reaksie. Gevolgelik is die euëdriese plagioklaas insluitings as peritekties geïnterpreteer. Waar hierdie voor kom as polimineraliese insluitings, is hulle klein en plagioklaas is volumetries dominant, so die smelt volume in die insluitsel was klein. Die meer Ca-ryke vellings in euëdriese plagioklaasinsluitings mag sonering as gevolg van die smeltingsreaksie reflekteer, waartydens Na-ryke vellings die kristalle reflekteer wat te kalies is om te ekwilibreer met die smelt. Die volume van ewewig is uiters plaaslik beheer wat dan Na-ryke vellings insluit. The bewaring van hierdie unieke granaatkristalle, moontlik as gevolg van spoedige en effektiewe verlies aan smelt van die leukosoomstrukture, bied ‘n rare insig in die proses van anatekse. In die oorweldige meerderheid van magmatiese granuliete is hierdie kirstalle verlore as gevolg van omvattende uitgebreide herkristallisasie en homogenisering van mineraalkomposisies.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98405
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