The Komati Formation of the Barberton Greenstone Belt is the type locality of the rock type known as Komatiites. Komatiites are ultramafic lavas that were generated and erupted mainly during the Archaean. They give insight into volcanism on the early Earth as well as the nature of the mantle and melting processes. During 2010-2011 the International Continental Drilling Programme (ICDP) Barberton Project drilled two cores (BARB 1 and BARB 2) into the Komati Formation to obtain continuous sections of the komatiite strata. These cores were drilled to gain knowledge about the structure, textures, compositions, processes and contact relationships of komatiite flows, which could not be obtained from surface outcrop because of lack of continuity and relatively poor exposure. The drill holes also intersected a volcanic tumulus unit, the first of its type recognized in komatiite lava flows, allowing insight into the processes that created the tumulus and the processes responsible for creating the differentiated komatiite flows.
The core was logged in detail, revealing a variety of rock-types and styles of volcanism. The rock-types encountered range from massive and differentiated ultramafic komatiites, through komatiitic basalts to mafic basalts. Some minor later intrusions of gabbros and dolerite are also present. The komatiites and komatiitic basalts are extrusive lavas and represent continuous eruptive sequences. The gabbros are typically intrusive, but can represent late stage crystallization. Contact relationships are evident in the core, where they have not been eroded by overlying flows, and are used to distinguish 85 individual flows in BARB 1 and 65 in BARB 2. Chill margins are typically between 5 and 50 cm thick and brecciated contacts are usually 5- 15 cm thick where present. Only rare examples of original mineralogy (olivine and pyroxene) are preserved because of pervasive alteration of the rocks. Alteration minerals are serpentine, chlorite, tremolite and magnetite. Early serpentine veining was followed by later stage magnesite veining. The opaque minerals – chromite with secondary magnetite overgrowth – are mostly located at olivine grain boundaries.
The tumulus unit in the BARB 1 core was created by upward doming of the upper skin of a lava tube. The unit is 90 m thick and consists of five textural sections: basal cumulates, harrisite, pyroxene spinifex, gabbro-pyroxenite and a hyaloclastite unit that caps the sequence. The cumulates contain macrocrystic olivine grains that reach 15 mm in length; they are elongated and rounded, aligned in certain horizons and are tightly packed, with a maximum of 20 % matrix. The harrisite is a form of skeletal olivine (with crystals up to 5 cm in length) that grew upwards from the underlying cumulate layer. Between the skeletal olivine crystals are small (< 2 mm) crescent-shaped pyroxenes contained within the melt residue. Pyroxene grains in the spinifex lava reach 20 cm in length and are surrounded by a fine-grained matrix. The gabbro and pyroxenite layers contained within the spinifex layer are interpreted to represent the last stage of crystallization within the structure since they are chemically related to the tumulus and no chill margins are present between the gabbro and surrounding pyroxene spinifex. Both the spinifex and gabbro contain unaltered pyroxene crystals and the gabbro also contains relatively fresh plagioclase. The hyaloclastite breccia consists of fragmented fine-grained chill margin blocks derived from the upper crust of the lava tube. The fragments are surrounded and supported by a glassy shard-like matrix. Inflation processes are evident in the tumulus and gave rise to multiple layers of large elongated olivine cumulates together with the upward and outward bulging of the upper crust to form a hyaloclastite breccia. Chemically the tumulus exhibits a Fo93 olivine control. Fractionation processes are clear in MgO vs. depth, binary diagrams and REE plots. Element concentrations are between 1 and 11 times chondrite with a very small LREE enrichment.
Differentiated komatiite flows are composed of three lithologies: basal cumulates, olivine spinifex and chill margin zones. The cumulus olivines have a crystal size of 0.5-1 mm, are euhedral and enclosed by a 30 to 60 % melt component. The spinifex olivine forms random or parallel sheets on a centimetre scale and is completely altered to serpentine. Between the olivine spinifex are chemically more evolved pyroxene spinifex blades, which are smaller (millimetre to centimetre scale) and altered to a combination of serpentine-chlorite-tremolite. The fine-grained chill margins of the flows are typically 1-10 cm wide and in some cases contain contact breccias or hyaloclastites.
Three packages of differentiated komatiite flows occurring at several stratigraphic intervals where sampled in detail. The lowest package BARB 1 (89-118 m) exhibits chemical trends that are interpreted to indicate a combined crystallization control by olivine and pyroxene. This is evident in the rock compositions and by petrographic studies that reveal the presence of two cumulus phases. The chemical compositions of the other two differentiated packages, BARB1 (378-420 m) and BARB 2 (252-274 m) are controlled by crystallization or accumulation of Fo93 and Fo94, respectively. These packages have olivine cumulates which are surrounded in some cases by pyroxene oikocrysts. In some samples the pyroxenes have unaltered cores. The REE plots are well constrained between 2 and 9 times chondrite values, and have slight LREE enrichment.
The tumulus structure and differentiated flow packages BARB 1 (378-420 m) and BARB 2 (252-274 m) have similar mineralogies and compositions but contain different textures. This is attributed to the size of the tumulus in comparison to the differentiated flows and implies similar magma processes and origins for all three units. The BARB 1 (89-118 m) interval appears to have undergone slightly different processes, as indicated by the presence of the two cumulus phases. This package is also altered to a greater extent than the others and no unaltered pyroxene is present.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/14936 |
| Date | 21 July 2014 |
| Creators | Coetzee, Grace |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf |
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