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| Titel |
Nature, geochemistry and petrogenesis of the syn-tectonic Amspoort suite (Pan-African Boundary Igneous Complex, Kaoko Belt, NW Namibia) |
| VerfasserIn |
Vojtěch Janoušek, Jiří Konopásek, Stanislav Ulrich |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039720
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| Zusammenfassung |
| Crucial information on the Neoproterozoic–Cambrian amalgamation of Western Gondwana is
provided by studies of the large Pan-African collisional belt in central–northern Namibia.
This so-called Damara Orogen (Miller, 1983) can be subdivided into two branches, the
SW–NE trending Damara Belt and a roughly perpendicular, NNW–SSE trending Kaoko Belt
further north. The Kaoko Belt consists of two principal crustal units. The easterly part has a
Congo Craton affinity (a basement built mostly by -¥ 1.5 Ga granitic gneisses with
Neoproterozoic metasedimentary cover), whereas the westerly Coastal Terrane consists of
Neoproterozoic (c.850–650 Ma) metapsammites and minor metabasic bodies; no exposures
of the basement were found. The at least 180 km long, NNW–SSE trending suture between
both units was intruded by numerous syn-tectonic magmatic bodies with ages spanning the
interval 580–550 Ma (Seth et al., 1998; Kröner et al., 2004) designated as the Boundary
Igneous Complex by Konopásek et al. (2008). The most typical representatives of this
syn-collision igneous association are c.550 Ma old K-feldspar-phyric, Bt ± Cam
granites–granodiorites of the Amspoort suite, with minor Cpx gabbro and rare two-pyroxene
dolerite bodies.
The petrological character, whole-rock geochemistry and Sr–Nd isotopic signatures of the
scarce Opx–Cpx–Bt dolerites indicate an origin from a CHUR-like mantle-derived melts
(87Sr/86Sr550 ~ 0.7045, ÉNd550 ~ 0) modified by extensive (?Ol–) Cpx fractionation. The
rest of the suite is interpreted as a product of a high-temperature anatexis of a heterogeneous
lower crust, built mainly by immature metapsammites – rich in arc-derived detritus – with
minor metabasite and intermediate metaigneous bodies. The most likely source appears to be
the anatectic Coastal Terrane gneisses. Yet, partial melting of the so far little constrained
Congo Craton cover, if formed by immature and youthful detritus unrelated to the basement,
cannot be discounted. In any case, the rather primitive Sr–Nd isotopic compositions
of the Amspoort suite (apart from dolerites; ÉNd550 = -3.4 to -5.3, 87Sr/86Sr550=
0.7063–0.711), rule out any major role for the Congo Craton-basement derived
material.
On a much broader scale, the contribution of Congo-derived material in the Pan-African
granitoids in the Kaoko Belt seems to increase southwards from nearly nil in the
studied suite, through minimal in Hoanib Valley (Seth et al., 2002) to maximum in
southern extremity of the Belt (Jung et al., 2009 and, in particular, van de Flierdt et al.,
2003).
This research was financially supported by GAÄR Project 205/07/1409 (to JK).
Jung, S., et al., 2009. Lithos, 111: 220–235.
Konopásek, J., et al., 2008. Journal of the Geological Society, 165, 153–165.
Kröner, S., et al., 2004. South African Journal of Geology, 107, 455–476.
Miller, R.M., 1983. In: Miller, R.M. (Ed.), Evolution of the Damara Orogen of South
West Africa/Namibia. Geological Society of South Africa Special Publications 11,
431–515.
Seth, B., et al., 1998. Precambrian Research, 92, 341–363.
Seth, B., et al., 2002. South African Journal of Geology, 105, 179-192.
van de Flierdt, T. , et al., 2003. Lithos, 67, 205–226. |
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