The volcanic rocks of the Afro-Arabian volcanic province chart the nature and evolution of the mantle plume source responsible for a major continental flood basalt event and the manner in which it interacted with the lithosphere over the course of continental separation. The flood basalts of the Ethiopian Plateau constitute the larger part of this province and are associated with melting in the head of the Afar Plume. High-Ti picrites from the eastern part of the NW Plateau are among the earliest uncontaminated outpourings of flood volcanism; their compositions are therefore considered to be close to that of the primary melt. These are exposed in a stratigraphically coherent sequence with variably porphyritic high-Ti olivine-rich basalts and ankaramites, which together provide a continuous record of volcanism reflecting a transition from high- to low-flux magmatism. Phase-relations differ fromthose in other tholeiitic flood basalts in that clinopyroxene and magnetite crystallisebefore plagioclase. This is typical of ferropicrites and reflects a high-Fe-Ti-, and low AI-source. The olivine phenocrysts exhibit a range of morphologies and compositions which reflect fractionation and magma-mixing at varying depths prior to eruption, and the introduction of water into the magmas at shallow levels is evident from pervasive iddingsitization. The lavas have similar major element chemistry to MORB but show higher concentrations of incompatible elements inherited from a HIMU-type lithospheric component. and enrichments in LREE relative to HREE which reflect the presence of residual garnet in the source. The picrites and ankaramites have lower AI<sub>2</sub>O<sub>3</sub> and higher K<sub>2</sub>O, P<sub>2</sub>O<sub>5</sub>, TiO<sub>2</sub> and Fe<sub>2</sub>O<sub>3</sub><sup>(tot)</sup> contents than any reported lavas with equivalent MgO content from other flood basalt provinces. These least evolved lavas were derived from oxidised (QFM+1), deep-seated magmas generated by different degrees of partial melting of the primary melt, whereas the olivine basalt magmas evolved by crystal fractionation at a shallow level from a melt similar in composition to the ankaramites. The MgO content of the primary melt calculated from the most Mg-rich olivines (Fo<sub>90</sub>) is between 17 and 18 %, and it is likely that this ponded at the base of the crust where it evolved at pressures near to 1.2 GPa. Primary melt fractions of 3 - 6 % were generated in the plume at pressures of 4 - 5 GPa and temperatures in excess of 1600 °C, implying that melting occurred beneath thick (120- 150 km) lithosphere prior to extension. Sub-chondritic initial <sup>187</sup>Os/<sup>188</sup>Os ratios (0.125 - 0.126) indicate derivation from a depleted mantle source, and high <sup>3</sup>He/<sup>4</sup>He (18.6 R<sub>a</sub>) and solar-like Ne isotopic ratios imply an origin in undegassed mantle.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:504253 |
| Date | January 2008 |
| Creators | Davies, Marc Kerry |
| Publisher | Open University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://oro.open.ac.uk/54645/ |
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