Magma accumulation and segregation during regional-scale folding : the Holland’s dome granite injection complex, Damara belt, Namibia.

Kruger, Tolene Mia (2016-12)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Mid-crustal, upper amphibolite-facies rocks in the south Central Zone magmatic arc of the Damara belt in central Namibia contain kilometre-scale networks of intrusive, sheet-like leucogranites. These granite injection complexes are spatially and temporally closely associated with regional folds, particularly antiformal structures, and document the presence and geometries of magma permeability networks in suprasolidus (T ~700-750 °C, P ~5 kbar) crust, close to the anatectic zone. The Holland’s dome is a northeast-trending, upright, isoclinal, gently doubly plunging antiform that formed during northwest-southeast shortening in supracrustal rocks above the basement-cover contact in the south Central Zone. Highly fractionated leucogranite sheets have intruded the Holland’s dome as three main orthogonal sets. The predominant set of subvertical granite sheets is roughly axial planar to the fold dome and is intersected at right angles by a subvertical and subhorizontal set of leucogranites normal to the fold axial plane. Contacts between the three main sets are mostly continuous pointing to the broadly coeval emplacement of leucogranites, while the internal sheeting of granites documents the incremental growth of larger granite sheets through successively injected smaller magma batches. The granites constitute between 40-80 % of the outcrop area, but wall-rock fragments have undergone little or no rotation with little evidence for a whole-scale flow and disruption of the succession. All three leucogranite sets cross-cut subvertical wall rocks. This underlines the late-kinematic emplacement of the granite sets during fold lock-up and flattening of the fold. Structural relationships between leucogranite sheets and the Holland’s dome point to a stepwise evolution of the injection complex during folding from (1) the dilatancy-driven segregation and accumulation of granitic magmas in the core of the fold during fold amplification above the basement-cover detachment, followed by (2) fold tightening and compaction-driven segregation of a melt from the initially accumulated magma in the core of the fold. This stage corresponds to the formation of the injection complex at the present-day erosional level of the Holland’s dome. The successive assembly of the granite network through the injection of probably thousands of smaller melt batches ensures compatibility between regional strain rates during fold amplification and the rates of magma segregation and emplacement. The orientation of sheets closely reflects the regional stress field and folding of the cover sequence during regional shortening. Strain localization associated with the late-stage injection of a three-dimensional melt network also facilitates tightening of the Holland’s dome beyond the fold lock-up stage during ongoing regional shortening. Granite injection complexes similar to that of the Holland’s dome are common in the mid-crust of the Damara belt. The presence of the injection complexes highlights the significance of regional-scale folding for the formation of temporary magma accumulation sites and the redistribution of progressively more fractionated melts during deformation in suprasolidus crust.

AFRIKAANS OPSOMMING: Middel-aardkors, boonste-amfiboliet fasies gesteentes in die suid-Sentrale Sone magmatiese boog van die Damara gordel in sentraal Namibië bevat kilometer-skaalse netwerke van intrusiewe, leukograniet plate. Hierdie graniet injeksiekomplekse is ruimtelik en tydelik nou verwant aan regionale plooie, spesifiek antiforme strukture, en dokumenteer die teenwoordigheid en geometrie van magma deurlatendheid netwerke in bo-solidus (T ~700-750 °C, P ~5 kbar) aardkors, naby aan die anatektiese sone. Die Holland’s koepel is ‘n noordoos-strekkende, regop, isoklinale, liggies dubbel-duikende plooi wat tydens noordwes-suidoos verkorting van die bokorsgesteentes gevorm het bo die vloer-bedekkingsgesteente kontak in die suid-Sentral Sone. Hoogs gefraksioneerde leukograniet plate het die Holland’s koepel ingedring en vorm drie ortogonale stelle. Die dominante stel van subvertikale plate is asvlakplanêr met die koepel en word reghoekig gesny deur ‘n subvertikale en subhorisontale stel van leukograniete loodreg teenoor die plooi se asvlak. Kontakte tussen die drie hoof plaatstelle is meestal deurlopend en verwys na die gelyktydige inplasing van leukograniete, terwyl die interne plaatvorming van graniete die inkrementele groei van groter plate deur middel van agtereenvolgende injeksie van kleiner magma hoeveelhede dokumenteer. Die graniet maak tussen 40-80 % van die dagsoomarea op, maar wandgesteente fragmente het minimale of geen rotasie ondergaan nie, met amper geen bewyse van ‘n heelskaalse vloei en ontwrigting van die suksessie nie. Al drie leukograniet stelle kruissny subvertikale wandgesteentes. Dit beklemtoon die laat-kinematiese inplasing van die graniet stelle tydens plooisluiting en afplatting van die plooi. Strukturele verhoudings tussen die leukograniet plate en die Holland’s koepel dui op die stapgewyse evolusie van die injeksiekompleks tydens plooiing vanaf (1) die dilatante-gedrewe segregasie en akkumulasie van granietiese magmas in die kern van die plooi gedurende plooi versterking bo die vloer-bedekkingsgesteente afstropingsvlak, gevolg deur (2) plooi vernouing en kompaksiegedrewe segregasie van ‘n smeltsel van die aanvanklik ge-akkumuleerde magma in die kern van die plooi. Hierdie stadium korrespondeer met die vorming van die injeksiekompleks in die hedendaagse verweringsvlak van die Holland’s koepel. Die agtereenvolgende samestelling van die granietnetwerk deur middel van die injeksie van seker duisende kleiner smeltsel hoeveelhede verseker die verenigbaarheid tussen regionale vervormings tempo gedurende plooi versterking en die tempo van magma segregasie en inplasing. Die orientasie van plate reflekteer die streeksstresveld en die plooiing van die bedekkingsgesteentesuksessie gedurende regionale verkorting. Spannings lokalisering verwant aan die laat-stadium injeksie van ‘n drie-dimensionele smeltsel netwerk fasiliteer vernouing van die Holland’s koepel verby die plooisluitingsstadium gedurende aanhoudende regionale verkorting. Graniet injeksiekomplekse soortgelyk aan die van die Holland’s koepel is algemeen in die middel-aardkors van die Damara gordel. Die teenwoordigheid van die injeksiekomplekse beklemtoon die betekenis van die regionaalskaalse plooiing vir die vorming van tydelike magma akkumulasie gebiede en die herverdeling van progressiewe meer gefraksioneerde smeltsels gedurende vervorming van die bo-solidus aarkors.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/100046
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