The petrogenesis of group 2 ultrapotassic kimberlites from Finsch Mine, South Africa

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doi: 10.1016/0024-4937(92)90013-O
Authors:Fraser, K. J.; Hawkesworth, C. J.
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.327-345; 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, 23 anals.
Summary:Major, trace element and radiogenic isotope results are presented for a suite of hypabyssal kimberlites from a single pipe. These are group 2 kimberlites characterized by abundant phlogopite +- serpentine +- diopside; they are ultrabasic and ultrapotassic igneous rocks, they exhibit a wide range in major element chemistry with SiO2 = 27.6-41.9 wt.% and MgO = 10.4-33.4 wt.%. (87Sr/86Sr)i = 0.7089 to 0.7106, epsilon Nd is -6.2 to -9.7 and they have unradiogenic (207Pb/204Pb)i contents which ensure that they plot below the Pb- ore growth curve. They have high incompatible and compatible element contents, a positive array between Y and Nb which indicates that garnet was not involved in the within suite differentiation processes, and a negative trend between K/Nb and Nb contents which suggests that phlogopite was involved. In addition, some elements exhibit an unexpected order of relative incompatibility for different trace elements which suggests that the intra-kimberlite variations are not primarily due to variations in the degree of partial melting. The effects of fractional crystallization are difficult to establish because for the most part they have been masked by the entrainment of 50-60% mantle peridotite. The kimberlites are interpreted as mixtures of a melt component and entrained garnet peridotite, with no evidence for significant contamination with crustal material. The melt component was characterized by high incompatible element contents, which require both very small degrees of partial melting, and source regions with higher incompatible element contents than depleted or primitive mantle. The enriched Nd, Sr and Pb isotope ratios of the kimberlite are characteristic of the melt source region. The melt fractions were therefore derived from ancient, trace element enriched portions of the upper mantle, most probably situated within the sub-continental mantle lithosphere, and different from the low 87Sr/86Sr garnet peridotite xenoliths found at Finsch. Within the sub-continental mantle lithosphere old, incompatible element enriched source regions for the kimberlite melt fraction are inferred to have been overlain by depleted mantle material which became entrained in the kimberlite magma. [E.W.P.]
Subjects:Alkaline earth metals; Chemical ratios; Igneous rocks; Isotopes; Kimberlite; Lead; Major elements; Metals; Nd-144/Nd-143; Neodymium; Pb-206/Pb-204; Pb-207/Pb-204; Pb-208/Pb-204; Potassic composition; Radioactive isotopes; Rare earths; Sr-87/Sr-86; Stable isotopes; Strontium; Trace elements; Ultramafic composition; Ultrapotassic composition; Africa; Cape Province region; South Africa; Southern Africa; Cape Province South Africa; Finsch Mine
Abstract Numbers:94M/3662
Record ID:1993036169
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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