Geochronology of Mesoproterozoic hybrid intrusions in the Konkiep Terrane, Namibia, from passive to active continental margin in the Namaqua-Natal Wilson Cycle

• tThis work investigates a number of gabbro – granite hybrid rocks exposed in the Great Escarpmentbetween the coastal Namib desert and the inland plateau of Namibia. They form part of the tectonostratigraphic Konkiep Terrane which lies between the Mesoproterozoic Namaqua-Natal Province and largelyPalaeoproterozoic Rehoboth Province in southern Africa. Microbeam U–Pb and Lu–Hf analyses of zirconwere used to investigate the geochronology of these rocks which have previously been assigned to theMooirivier Metamorphic Complex or the Kairab Formation. The oldest hybrid rocks at Neuhof Valley areU–Pb dated at 1359 ± 6 Ma and cut by 1347 ± 8 Ma unfoliated granite dykes, but have the youngest Lu–Hfcrustal residence ages between 1690 and 1430 Ma. Together with nearby volcanic rocks of the 1333 ± 6 Ma Neuhof and 1327 ± 10 Ma Welverdiend Formations which we dated, they are thought to reflect rifting of the Rehoboth Province followed by the ocean development (by seafloor spreading) phase of the NamaquaWilson Cycle.The hybrid rocks at Goede Hoop Foothills are U–Pb dated at 1230 ± 6 Ma with a number of xenocrysts upto 1532 Ma. Similar hybrid rocks at Hauchabfontein have 20–30 Ma younger U–Pb ages of 1211 ± 19 Maand 1202 ± 7 Ma (two samples), they formed by gabbro intruding an 1195 ± 5 Ma granite (within errorbut older from field relations) and contain many xenocrysts with U–Pb ages 1755 ± 29 Ma (n = 7), 1367and 1260 ± 17 Ma (n = 24). All the Goede Hoop Foothills and Hauchabfontein hybrid rocks have old Lu–Hfcrustal residence ages from 1935 to 2000 Ma and one sample up to 2320 Ma, reflecting involvement of Palaeoproterozoic Rehoboth Province crust. This 1230 to 1200 Ma group of hybrid rocks correspondsin age to the arc magmatism recognised in the Namaqua-Natal Province to the west and south of theKonkiep Terrane. The hybrid rocks probably formed during the closure of the Namaqua Ocean by eastwardsubduction under the Rehoboth Province, corresponding to the previously proposed Rehoboth MagmaticArc and forming the Konkiep Terrane by massive intrusions into the margin of the Rehoboth Province.The hybrid rocks we investigated in the Konkiep Terrane are thus related to the rifting phase andthe pre-collisional subduction phase of the Namaqua Wilson Cycle. The Arc-related magmatism endedabruptly at 1200 Ma as the Namaqua-Natal terranes were assembled by collisions and subduction ceased.We did not find evidence for the 1200 to 1150 Ma post-collisional metamorphic cycle and magmatism which prevailed in the adjacent Grünau Terrane of the Namaqua Province, suggesting that the KonkiepTerrane did not experience crustal thickening during the Namaqua collision and terrane assembly. Anundeformed felsic lava at Sukses which we dated at 1105 ± 10 Ma corresponds in age to the Langberg Formation in the Rehoboth Terrane. It is coeval with the Keimoes Suite of post-tectonic intrusions andyounger Koras lavas in the Namaqua-Natal Province and these ∼1100 Ma rocks all extend the area of theplume-related Umkomdo Large Igneous Province documented on the Kaapvaal Craton. This part of theKonkiep Terrane originated in the Rehoboth Province and was then modified by magmatic intrusionscoupled to the evolution of the Namaqua Natal Province.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

Nyckelord

Konkiep TerraneNamaqua-Natal Province

Rehoboth Province

Zircon dating

U–Pb

Lu–Hf

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