A lithological, petrographic and geochemical investigation of the M4 borehole core, Morokweng Impact Structure, South Africa

Wela, Slindile Sthembile
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This study investigates the mineralogical, petrographic and geochemical characteristics of target rocks and impact-formed breccias (impactites) intersected by the 368 m long M4 drillcore located 18 km NNW from the estimated centre of the 145 ± 2 Ma, Morokweng impact structure (MIS), South Africa. M4 is the only core from the central parts of the Morokweng impact structure not to intersect fractionated granophyric impact melt directly beneath 35-100 m of Cenozoic Kalahari Group sediments. Instead it intersects highly fractured, cataclased and shocked, crystalline target rocks that are cut by mm- to m-scale melt-matrix breccia and suevite dykes. The target rocks comprise granitic, granodioritic, trondhjemitic and dioritic Archaean gneisses, metadolerite and dolerite. The gneisses and metadolerite show signs of quartz veining and metasomatism linked to localised mylonitic to brittle fault deformation that predated the impact. The suevite and meltmatrix breccia dykes make up ~10% of the core. All rocks show signs of low-T hydrothermal effects that occurred after the impact. The target rocks contain a complex network of shear fractures that contain cataclasite and which grade into monomict lithic breccia. The cataclasite contains shocked mineral fragments, which indicates that the shear fracturing postdated the initial shock stage of the impact. The melt-matrix breccia and suevite dykes show signs that they intruded along the fractures, although there is also evidence that shear fracturing continued after quenching of the melt. This suggests that the intrusion of the dykes overlapped the brittle deformation of the target rocks. Shock features in the M4 core lithologies include planar fractures, feather features, decorated planar deformation features (PDF), mosaic extinction and toasting in quartz; oblique lamellae, reduced birefringence and patchy (mosaic) extinction in plagioclase, and chevron-style spindleshaped lamellae in microcline, as well as kink bands in biotite and planar fractures in titanite and zircon. Universal Stage measurements of PDF sets in quartz from 8 target rocks and 6 impactite dykes revealed four dominant sets: 0°(0001), 22.95°{ 3 1 10 }, 17.62°{ 4 1 10 }, 32.42°{ 2 1 10 }; with no significant change in shock intensity with depth nor significant differences in PDF orientations or intensity between melt-matrix breccias, suevites and target rocks. Based on these observations the average peak shock pressures are estimated at 10 - 25 GPa. Apart from one suevite dyke that contains exotic clasts and an unusual bulk composition, all suevite and melt-matrix breccia dykes show major, trace and REE compositions and lithic and mineral clasts that indicate that they were formed from the target rocks found in the M4 core. The individual impactite dykes show good compositional correlation with their wallrocks, which supports limited transport of the melt and suevite. This is also supported by evidence of small-scale variation of the melt composition in the melt-matrix breccias, which indicates that not enough time was available for complete mixing to happen. The similarity in matrix composition and in lithic and mineral clast types in the melt-matrix breccias to their wallrocks, is consistent with a friction melt origin. These dykes are thus interpreted as pseudotachylite. Macroscopic and microscopic evidence suggests that the melts intruded cataclasite-filled fractures and that interfingering and infolding between the melts and incohesive cataclasite allowed the melt to assimilate cataclasite. The melt clasts in the suevite show the same composition and clast features as the melt-matrix breccias. Based on this evidence it is proposed that the melt clasts in the suevite in the M4 core are fragments of quenched pseudotachylite that became separated and mechanically mixed into the cataclasite matrix when movement continued along the cataclasite-bearing fractures after the melt quenched. This was possible because the cataclasite was still incohesive and because strong vertical and horizontal displacements of the entire M4 sequence happened during the crater modification stage of the impact, possibly for 1-2 minutes after the impact. The melt-matrix breccias are compositionally distinct from the Morokweng granophyric impact-melt rock intersected in the other central borehole cores. Melt particles are pervasively hydrothermally altered to a secondary mineral assemblage of zeolites and smectites, attributed to impact-induced hydrothermal fluid circulation in the MIS. The upper parts of the core are marked by abundant haematite but in the deeper levels of the core, chlorite-epidote-andradite garnet is found, which may indicate a vertically-zoned hydrothermal system after the impact. The hydrothermal effects also explain the abundance of decorated PDF in shocked quartz grains and the lack of glass in the PDF in quartz. The 10-25 GPa shock levels in the target rocks support them lying close to the transient crater floor and initially close (<10 km) to the point of impact. The high structural position of the rocks relative to the impact-melt sheet suggests that the M4 sequence represents part of the peak ring of the Morokweng impact structure. The rocks of the peak ring would have experienced strong vertical and centrifugal displacement during the crater excavation and modification stages, which can explain the intense shear fracturing and cataclasis, brecciation and friction melting as well as the strong block movements that could disrupt and disperse the pseudotachylite melt dykes to produce suevite. A peak ring radius of 18 km would suggest that the original Morokweng crater rim diameter would have been >70 km, but between 1 and 2 km of post-impact erosion before the deposition of the Kalahari Group means that this could be a minimum estimate.
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science. September, 2017.
Wela, Slindile Sthembile (2017) A lithological, petrographic and geochemical investigation of the M4 borehole core, Morokweng Impact Structure, South Africa, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/24994>