Crustal Influences in the Petrogenesis of the Naivasha Basalt--Comendite Complex: Combined Trace Element and Sr-Nd-Pb Isotope Constraints |
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| Authors: | DAVIES G R; MACDONALD R |
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| Institution: | 1Department of Earth Sciences, University of Leeds Leeds LS2 9JT
2Department of Environmental Science, University of Lancaster Lancaster LA1 4YQ |
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| Abstract: | The bimodal Naivasha complex (central Kenya) comprises 2 suitesof transitional basalts and 7 chemostratigraphic groups of comendites.The early basalt series (EBS) predates the Group 1 comenditeswith the later series (LBS) erupted between Groups 5 and 6.Basalts from both suites are notable for their relatively radiogenic207Pb/204Pb isotope ratios which are higher than in the majorityof ocean island basalt (OIB, Zindler & Hart, 1986), and87Sr/86Sr ratios more radiogenic than basalts from northernKenya. Both basalt suites exhibit systematic trace element andisotopic variations which appear related to greater assimilationof Proterozoic amphibolite facies crust by the chemically moreevolved rock types. Their mantle source regions show evidenceof residual plagioclase and have a ‘Dupal’-likeOIB trace element and Pb-Sr-Nd isotope signature (Hart, 1984).A contribution from the sub-continental lithosphere is proposedin basalt genesis. The seven comendite groups have distinct trace element and isotopesystematics. Hydration of comendite glass causes significantchanges in Sr and Pb isotope ratios. In terms of their Sr-Ndisotope relationships the unaltered comendites could be derivedfrom the basalts by an assimilation-fractional crystallization(AFC) process dominated by the fractional crystallization offeldspars. However, the Pb systematics clearly demonstrate thatthe basalts and comendites are not part of a cogenetic suite.Chemical variations within individual comendite groups are predominantlythe result of fractional crystallization of the observed phenocrystassemblages (i.e. alkali feldspar dominated) and minor crustalinteraction. The majority of the chemical and isotopic differencesbetween Groups 1–7 cannot be explained by fractional crystallizationand appear to represent crustal melts derived from close tothe interface between Pan African basement and the overlyingMiocene-Holocene volcanoclastic rocks, at approximately 6 kmdepth (KRISP working group, 1987). Halogens play a fundamental role in the petrogenesis of thecomendites (Cl+F<1?7 per cent) permitting small degree meltsof low viscosity to be extracted from the crust and causingthe breakdown of minor phases e.g, zircon. These factors explainthe extreme enrichment of certain incompatible trace elements(Zr<2500, Nb<700) in the comendites and coupled with theretention of zircon in the source of the halogen poor comendites(Group 1<0?6 per cent Cl+F) result in notable fractionationamong the HFSE (Zr/Nb 1?5–5?5). Halogens may be concentratedin the source region from the surrounding crust by the presentlyactive hydrothermal system. Each of the chemostratigraphic comenditegroups is chemically distinct, implying that partial meltingof the heterogeneous crust is on a limited scale and that noextensive magma chambers exist beneath Naivasha. |
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