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New microbeam zircon dating insights on migmatites of the former Mooirivier Metamorphic Complex and the Neuhof Formation, Sinclair Group, Namibia
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- van Schijndel, Valby (författare)
- Gothenburg University,Göteborgs universitet,Institutionen för geovetenskaper,Department of Earth Sciences
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- Cornell, David H., 1948 (författare)
- Gothenburg University,Göteborgs universitet,Institutionen för geovetenskaper,Department of Earth Sciences
- Frei, Dirk (författare)
- (creator_code:org_t)
- 2010
- 2010
- Engelska.
- Ingår i: Abstract in ‘Out of Africa: a 140 years with Kevin Burke and Lew Ashwal’. ; 1:1, s. 203-204
- Konferensbidrag (övrigt vetenskapligt/konstnärligt)
Abstract
Ämnesord
Stäng
- New microbeam zircon dating insights on migmatites of the former Mooirivier metamorphic complex and the Neuhof formation, Sinclair group, Namibia Valby van Schijndel (1), David H. Cornell (1), Dirk Frei (2) 1) Department of Earth Sciences, The University of Gothenburg, SE40530, Sweden 2) GEUS, Denmark and Greenland Geological Survey, Copenhagen, Denmark In the 1.4-1.0 Ga Sinclair Group and its basement, located in Namibia, the relations and chronostatigraphic succession are not fully understood. This is certainly true for the former Mooirivier Metamorphic Complex (MMC) and the Neuhof Formation. Becker (2008) interpreted the MMC and the Neuhof Formation as part of the Kairab Formation (Hoal, 1990), a heterogeneous metamorphic basement overlain by metavolcanics that are intruded by subvolcanics. This basement is part of the Namaqua Metamorphic Complex and Hoal (1990) separated the metavolcanics from the apparently older basement. The comparison Becker (2008) made was based on the fact that the units lie close to each other and lithological similarities; the MMC seems to be dominated by gneissic intrusive rocks and the Neuhof Formation consists mainly of basic volcanic and sedimentary rocks with closely related intrusives. In this study we used the NordSIM ion probe and a high resolution laser ablation ICPMS to date zircons from two migmatised samples and a red aplite dyke that cuts a migmatite from the MMC on Hauchabfontein farm. This migmatite location is interpreted by Becker (2008) as the gneissic Hauchab Granodiorite. He states that this body intrudes an amphibolite which is indicated by the presence of amphibolite xenoliths in the granodiorite. The second migmatite is located close by to the west. Our field interpretation is that the ‗gneissic granodiorite‘ is the result of a migmatisation event caused by a gabbro intrusion mixing with melted granitoid material. The two migmatite samples are hybrid rocks, consisting of a mixture of mafic and felsic components. Zircons from these hybrid rocks showed large CLbright rims, indicating an overall metamorphic or migmatitic event. However most of the zircons show no age difference between rim and core. These dates are 1205 ±6 and 1220 ±8 Ma and reflect the gabbro intrusion and migmatisation event. Since the cores of the zircons were probably reset, a migmatisation event is favoured above a metamorphic one, since it requires very high temperatures. This also applies to the formation of hybrid mixtures between felsic and mafic components. The migmatisation age is slightly older than the red aplite dyke at 1201±16 Ma. The migmatisation corresponds in age to the Barby Formation which formed during northwestward subduction prior to the ~1200 Ma Namaqua collision. In the Hauchabfontein migmatite we found one concordant zircon core dated at 1367 Ma possibly related to tonalite intrusives at 1375 Ma like the Hammerstein (Becker, 2008) and the Aunis Tonalite (Hoal, 1990), that intrude the Neuhof and Kairab Formations respectively. The second migmatite sample contains four xenocrysts with a combined concordant age of 1748 ±21 Ma. This age indicates the presence of a basement older than the Kairab Formation, like the Elim Complex and Gaub Valley Formation to the north. South of Hachaubfontein on the border of Mooirivier Farm, we sampled a metasediment from the Neuhof Formation. This is a low grade, quartz-rich gneiss, thought to be of volcanic origin. Becker (2008) interpreted fine grained quartz-feldspatic gneisses that occur in the Neuhof Fomation as intrusives because of the young age (~1320 Ma) in comparison to the 1375 Ma Hammerstein Tonalite intrusive. However in our sample we found two age domains 69 at 1330±5 and 1291±9 Ma. This indicates that the sample was not an intrusive but more likely a metasedimentary rock containing detrial zircons derived from two magmatic units. References Becker, T., 2008. in R. McG. Miller (ed) The Geology of Namibia, Vol. 1 Archean to Mesoproterozoic, . Hoal, B.G., 1990. The geology and geochemistry of the Proterozoic Awasib Mountain Terrain, southern Namibia, Mem. Geol.Surv. Namibia, vol. 11, pp 163
Ämnesord
- NATURVETENSKAP -- Geovetenskap och miljövetenskap (hsv//swe)
- NATURAL SCIENCES -- Earth and Related Environmental Sciences (hsv//eng)
Nyckelord
- zircon dating mooirivier migmatite neuhof
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