Geology of the Kranzberg syncline and emplacement controls of the Usakos pegmatite field, Damara belt, Namibia

[摘要] ENGLISH ABSTRACT: The Central Zone (CZ) of the Damara belt in central Namibia is underlain byvoluminous Pan-African granites and is host to numerous pegmatite occurrences,some of which have economic importance and have been mined extensively. Thisstudy discusses the occurrence, geometry, relative timing and emplacementmechanisms for the Usakos pegmatite field, located between the towns of Karibiband Usakos and within the core of the regional-scale Kranzberg syncline.Lithological mapping of the Kuiseb Formation in the core of the Kranzberg synclineidentified four litho-units that form an up to 800 m thick succession ofmetaturbidites describing an overall coarsening upward trend. This coarseningupwards trend suggests sedimentation of the formation's upper parts may haveoccurred during crustal convergence and basin closure between the Kalahari andCongo Cratons, rather than during continued spreading as previously thought.The Kranzberg syncline is a regional-scale NW verging, NE-SW trending, stronglynon-cylindrical structure that consists of a moderately SE dipping, normal NW limband a steep- to overturned SE limb. First- and lower-order folds show relativelyconsistent E - SE plunges at moderate angles and stretching lineations andboudinage of competent layers point to a fold-parallel stretch during folding.Folding is associated with a moderate- to steep SE dipping transecting foliation thatshows a consistent anticlockwise rotation with respect to the axial plane of the fold.The transecting cleavage and a component of non-coaxial shear along theoverturned limb suggest that folding was accomopanied by a dextral component ofshear thought to be related to the SW-directed extrusion of the adjacent Usakosdome during regional NW-SE directed shortening. It is further suggested that theKranzberg syncline evolved within the overall regional pattern of regional dome andsyncline structures in the sCZ, and not as a forced fold in response to the formationof neighbouring dome structures.Based on cross-cutting relationships and deformation, four main generations ofbedding-concordant sills and bedding-discordant pegmatite dykes were identified.Along the normal limb, shallowly-dipping sills dominate, highlighting thesignificance of bedding anisotropies for sheet propagation. Along the overturnedlimb, interconnected dyke and sill geometries co-exist. Here, pegmatiteemplacement appears to have been influenced by (1) the regional strain, (2)differing wall rock rheologies; (3) the orientation of pre-existing anisotropies; and(4) driving melt pressures.Dykes within the Usakos pegmatite field formed within dilational sites, at highangles to the regional stretch, whereas sills formed at high angles to the regionalshortening strain and in contractional sites. Where driving pressures for melt ascentwere high enough, an interconnectivity of dykes and sills and subsequent melttransfer from contractional into dilational sites is developed. Where melt pressuresdropped below a critical value pegmatites were arrested, thus preserving the ascentpathways of the melts. These complex intersecting melt pathways are developedthroughout the Kranzberg syncline. This suggests the existence of fairly stable meltnetworks in the continental crust. This geometrical complexity also accounts for thestockwork-like structures observed in pegmatite fields.

[发布机构] Stellenbosch University