Browsing by Author "Kruger, Tolene Mia"

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    Magma accumulation and segregation during regional-scale folding : the Holland’s dome granite injection complex, Damara belt, Namibia.
    (Stellenbosch : Stellenbosch University, 2016-12) Kruger, Tolene Mia; Kisters, Alexander F. M.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    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.