The browser you are using is not supported by this website. All versions of Internet Explorer are no longer supported, either by us or Microsoft (read more here: https://www.microsoft.com/en-us/microsoft-365/windows/end-of-ie-support).

Please use a modern browser to fully experience our website, such as the newest versions of Edge, Chrome, Firefox or Safari etc.

Partial melting of diverse crustal sources - Constraints from Sr-Nd-O isotope compositions of quartz diorite-granodiorite-leucogranite associations (Kaoko Belt, Namibia)

På svenska

Author

  • Stefan Jung
  • Peter Masberg
  • Daniela Mihm
  • Stephan Hoernes

Summary, in English

The Torrabaai-Koigabmond Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and titanite-bearing quartz diorites, metaluminous hornblende- and biotite-bearing granodiorites and peraluminous garnet- and muscovite-bearing leucogranites. Uranium-Pb zircon data obtained on the granodiorites and leucogranites indicate concordia upper intercept ages of 553 +/- 40 Ma although Pb-207/Pb-206 ages of ca. 650 Ma in zircon from the granodiorites suggest some inheritance of older material. Uranium-Pb monazite data obtained on the leucogranites give concordant ages of 550 Ma +/- 3 Ma. These ages are similar to the Rb-87/Sr-86 whole rock age of 584 +/- 35 Ma obtained on the granodiorites and leucogranites although the Rb-Sr age seems to be biased towards older ages due to limited assimilation of older material. In contrast to other plutonic complexes from the Kaoko Belt, the quartz diorites, granodiorites and granites show a restricted range in their initial Nd, Sr and O isotope compositions (quartz diorites: epsilon(Nd) ((init.)): -5.4 to -6.7; delta O-18: 8.3-9.4 parts per thousand; Sr-87/Sr-86: 0.7081-0.7098; granodiorites: epsilon(Nd) ((init.)): -6.1 to -7.7; delta O-18: 9.9-10.9 parts per thousand; Sr-87/Sr-86: 0.7071-0.7105; leucogranites: epsilon(Nd) ((init)): -4.9 to -8.7; delta O-18: 9.8-11.3 parts per thousand; Sr-87/Sr-86: 0.7060-0.7125). Enclaves are found in the granodiorites and leucogranites but not in the quartz diorites. They have a granodioritic composition with quartz, plagioclase, K-feldspar and hornblende and some have additional garnet. Relative to the country rock gneisses (the so called Nk Formation), enclaves are depleted in SiO2, Na2O, K2O, Sr, Ba and enriched in CaO, FeO(total), MgO, TiO2, Sc, V, Cr, Ni, Rb and Y. Rare garnet-bearing enclaves are additionally depleted in LREE and enriched in HREE relative to the granodiorites. These features are qualitatively consistent with the hypothesis that these enclaves may represent moderately depleted melting residues of Nk Formation gneisses. In comparison with experimentally derived melts and based on low Al2O3/ (FeO + MgO + TiO2) ratios and high Al2O3 + FeO + MgO + TiO2 values it is suggested that the quartz diorites are generated by dehydration melting of a mafic, amphibole- and plagioclase-bearing lower crustal source of Pan-African age. The granodiorites likely represent fractionation products of the quartz diorites. However, it is also possible that the granodiorites represent partial melting products of a mafic to intermediate lower crustal source but experienced likely slightly lower degrees of melting probably at water present conditions. The leucogranites display higher Al2O3/ (FeO + MgO + TiO2) ratios but lower Al2O3 + FeO + MgO + TiO2 values and are most likely generated by biotite dehydration melting of felsic crustal sources. Major and trace element and isotope variations indicate that fractional crystallization with only limited crustal contamination was the major rock-forming mechanism. it is suggested that most of the isotope variation reflects pre-existing heterogeneities of the sources. Consequently, interpretation of geochemicaland isotope data from the complex suggests that the Pan-African igneous activity in this part of the Damara-Kaoko Belt was not a major crust-forming episode and all rock types represent reprocessed crustal material. (C) 2008 Elsevier B.V. All rights reserved.

Publishing year

2009

Language

English

Pages

236-251

Publication/Series

Lithos

Volume

111

Issue

3-4

Document type

Conference paper

Publisher

Elsevier

Topic

  • Geology

Keywords

  • Partial melting
  • Quartz diorite-granite
  • Pan-African
  • Kaoko Belt
  • Sr-Nd-O isotopes

Conference name

Session on Formation and Evolution of Grantic Magmas held at the 17th VM Goldschmidt Conference

Conference date

2007-08-23 - 2007-08-24

Conference place

Cologne, Germany

Status

Published

ISBN/ISSN/Other

  • ISSN: 0024-4937
  • ISSN: 1872-6143