Phlogopite in the generation of olivine-melilitites from Namaqualand, South Africa and implications for element fractionation processes in the upper mantle

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doi: 10.1016/0024-4937(92)90014-P
Authors:Rogers, N. W.; Hawkesworth, C. J.; Palacz, Z. A.
Author Affiliations:Primary:
Open Univ., Dep. Earth Sci., Milton Keynes, United Kingdom
Volume Title:Potassic and ultrapotassic magmas and their origin; Sixth meeting of the European Union of Geosciences (EUG VI)
Volume Authors:Foley, S., editor; Peccerillo, A.
Source:Lithos (Oslo), 28(3-6), p.347-365; Sixth meeting of the European Union of Geosciences; Potassic and ultrapotassic magmas and their origin, Strasbourg, France, March 1991, edited by S. Foley and A. Peccerillo. Publisher: Elsevier, Amsterdam, International. ISSN: 0024-4937
Publication Date:1992
Note:In English; illus. incl. 3 tables, 13 anals.
Summary:Major and trace element and Sr, Nd and Pb isotope analyses are presented for 13 olivine-melilitites. Major element variations are consistent with derivation from carbonated garnet peridotite at depths of at least 100 km and trace element abundances indicate melt fractions of <= 4%. Ubiquitous negative K anomalies and low, buffered K2O concentrations are interpreted to reflect the effect of residual phlogopite during melting. It is suggested that phologopite stability and low melt K saturation concentrations are enhanced by high CO2/(CO2 + H2O) conditions. Residual phlogopite can also account for low measured Rb/Sr, Ba/Sr and Th/U ratios. REE abundances are controlled by residual garnet and hence Sm/Nd ratios are low. U/Pb ratios vary from 0.05 to 5 and are a function of Pb concentration which is in turn controlled by a residual Pb-rich phase. Nd and Sr isotopes are comparable with OIB from St. Helena, although two samples extend to higher 87Sr/86Sr ratios. Trace element characteristics of the melilitites are distinct from those of within-plate potassic magmas despite both being derived from phlogopite-bearing, enriched mantle source regions. This can be attributed to the depth at which source enrichment occurred and the subsequent control exerted by phlogopite and carbonate during melting. The incompatible element ratios are similar to those inferred for HIMU OIB source regions and it is suggested that such basalts may be derived from sources that have been subject to enrichment by a melt generated in the presence of residual phlogopite. [E.W.P.]
Subjects:Alkali basalts; Alkaline earth metals; Basalts; Chemical fractionation; Chemical ratios; Igneous rocks; Isotopes; Lead; Major elements; Mantle; Melilitite; Metals; Mica group; Nd-144/Nd-143; Neodymium; Nesosilicates; Olivine; Olivine group; Orthosilicates; Pb-206/Pb-204; Pb-207/Pb-204; Pb-208/Pb-204; Phlogopite; Radioactive isotopes; Rare earths; Sheet silicates; Silicates; Sr-87/Sr-86; Stable isotopes; Strontium; Trace elements; Ultramafic composition; Upper mantle; Volcanic rocks; Africa; Cape Province region; Namaqualand; South Africa; Southern Africa; Cape Province South Africa
Abstract Numbers:94M/3663
Record ID:1993036170
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique
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