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

• U–Pb and Lu–Hf dating little-known Konkiep Terrane between Namaqua and Rehoboth Provinces.
• 160 M.y. variation in gabbro-granite hybrids related to Namaqua-Natal Wilson Cycle.
• Passive and active continental margin intrusions penetrated Palaeoproterozoic basement.

This work investigates a number of gabbro – granite hybrid rocks exposed in the Great Escarpment between 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 largely Palaeoproterozoic Rehoboth Province in southern Africa. Microbeam U–Pb and Lu–Hf analyses of zircon were used to investigate the geochronology of these rocks which have previously been assigned to the Mooirivier Metamorphic Complex or the Kairab Formation. The oldest hybrid rocks at Neuhof Valley are U–Pb dated at 1359 ± 6 Ma and cut by 1347 ± 8 Ma unfoliated granite dykes, but have the youngest Lu–Hf crustal 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 Namaqua Wilson Cycle.The hybrid rocks at Goede Hoop Foothills are U–Pb dated at 1230 ± 6 Ma with a number of xenocrysts up to 1532 Ma. Similar hybrid rocks at Hauchabfontein have 20–30 Ma younger U–Pb ages of 1211 ± 19 Ma and 1202 ± 7 Ma (two samples), they formed by gabbro intruding an 1195 ± 5 Ma granite (within error but older from field relations) and contain many xenocrysts with U–Pb ages 1755 ± 29 Ma (n = 7), 1367 and 1260 ± 17 Ma (n = 24). All the Goede Hoop Foothills and Hauchabfontein hybrid rocks have old Lu–Hf crustal 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 corresponds in age to the arc magmatism recognised in the Namaqua-Natal Province to the west and south of the Konkiep Terrane. The hybrid rocks probably formed during the closure of the Namaqua Ocean by eastward subduction under the Rehoboth Province, corresponding to the previously proposed Rehoboth Magmatic Arc 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 and the pre-collisional subduction phase of the Namaqua Wilson Cycle. The Arc-related magmatism ended abruptly 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 Konkiep Terrane did not experience crustal thickening during the Namaqua collision and terrane assembly. An undeformed 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 and younger Koras lavas in the Namaqua-Natal Province and these ∼1100 Ma rocks all extend the area of the plume-related Umkomdo Large Igneous Province documented on the Kaapvaal Craton. This part of the Konkiep Terrane originated in the Rehoboth Province and was then modified by magmatic intrusions coupled to the evolution of the Namaqua Natal Province.