Structural study and geochronology in the Hook Batholith, Central Zambia

The Pan-African Hook batholith is emplaced N of the Mwembeshi dislocation, a regional scale structure at the contact between Zambezi Belt and Lufilian Arc in Central Zambia. Exposed over 12000 km2 the batholith is composed mainly of fine-grained and coarse-grained porphyritic granites and leucogranites affected by solid-state deformation along high-strain zones. Two main zones of deformation were investigated - the Itezhi-Tezhi Zone (ITZ) in the SW part of the batholith and the Nalusanga Zone (NZ) to the NE. The 2.5 km wide, N-S trending, subvertical ITZ is a medium-grade, pure shear dominated structure, reflecting probably regional scale E-W shortening. In the central part of the zone, augen-gneiss textures developed. Mineral lineations plunging ~40° S are recorded occasionally. The deformed feldspar porphyroclasts show symmetrical tails and rarely sinistral stair-stepping. In the SE part of the Hook batholith the continuation of the ITZ trends E-W. This orientation can be explained by rotation of the original ITZ trend by N-S shortening that also has been recorded in the siliciclastic metasediments S of the contact. S dipping, up to 15 cm wide thrust zones observed in the ITZ area were probably formed during this tectonic event. The 3 km wide NZ is a subvertical to steeply SSW dipping structure, parallel to the NE contact of the batholith, with well-developed foliation and mineral stretching lineations. Field and microstructural analyses defined the NZ as a medium-grade, non-coaxial, sinistral strike-slip shear zone. The transition from weak foliated granite to S-C mylonites and ultramylonites was observed. The sinistral shearing is consistent with E-W shortening in agreement with the tectonic framework of the ITZ. The low grade metasediments to the E of the granite are folded in N to NNW trending structures also implying E-W shortening. Temperature conditions during the deformation in ITZ and NZ inferred from microstructural analyses are about 500°-550°C. The metamorphism in the country rocks E of the batholith is in the lower greenschist facies indicating that deformation along the ITZ and NZ occurred during the cooling of the granite. U-Pb zircon LA-SF-ICP-MS analyses reveal that the coarse-grained and fine-grained granites in the NE part of the batholith have the same age of 549×2 Ma. The age of an undeformed aplite that truncates the NZ's foliation brackets the strike-slip shearing between 549×2 Ma and 541×3 Ma. In the SE margin of the batholith deformed coarse-grained granite is dated at 544×2 Ma and an undeformed granitic vein gave an age of 543×3 Ma, thus relating the fabric formation to the same time interval. To the SW the deformed granite in the ITZ is dated at 533×3 Ma indicating that the E-W shortening was still active at this time. This study reports two deformational stages recorded in the Hook batholith and its country rocks. E-W shortening folded the sediments form the E margin of the granite and formed the solid-state fabric in the batholith. The following N-S shortening cold be related to the final docking of the Zambezi sequence to the Lufilian Arc along the Mwembeshi dislocation.