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Kimberlites in a Karoo graben of northern Mozambique: Tectonic setting, mineralogy and Rb-Sr geochronology

BGS, Murchison House, West Mains Road, Edinburgh, Scotland EH9 3LA, e-mail: rmk{at}bgs.ac.uk

B. Bingen

Geological Survey of Norway (NGU), N-7491 Trondheim, Norway, e-mail: Bernard.Bingen{at}NGU.NO

E. Barton

Lithosphere Dynamics Group (Group Exploration), De Beers Group Services (Pty) Limited, PO Box 82232 Southdale 2135, South Africa, Presently at MSA Geoservices (Pty), PO Box 81356 Parkhurst 2120, South Africa, e-mail: erika{at}msageoservices.co.za

E.X.F. Daudi, S. Manuel and A. Moniz

DNG, PO Box 217, Maputo, Mozambique, e-mail: edaudi{at}teledata.mz

Kimberlites in northern Mozambique are aligned with the northeastern extremity of a trans-continental northeast to southwest trending kimberlite corridor that stretches from southern Namibia across Botswana into central Zimbabwe. The northern Mozambique kimberlites occur as dykes up to 3 m thick and as small pipes in the southern part of a northeast to southwest trending Karoo graben across the Mozambique Belt. U-Pb zircon dates from the Mozambique Belt basement of northern Mozambique indicate that the Karoo graben hosting the kimberlites covers a major north-northeast to south-southwest trending tectonic boundary in the basement. To the northwest of the graben, Palaeoproterozoic (>1.95 Ga) crust is recorded while Mesoproterozoic (<1.07 Ga) crust is widespread to the southeast. The kimberlites are reported along four-northwest to southeast trending zones, and one east-northeast to west-southwest trending zone. They are macrocrystic hypabyssal Group Ia kimberlites based on their mineralogy and whole rock geochemistry. The kimberlites show a typical crystallization history with early olivine, perovskite, ilmenite and phlogopite growth succeeded by rutile and spinel crystallization. Serpentine and calcite are the youngest primary minerals although there is also secondary growth of these two minerals. Pyrope occurs as rounded xenocrysts. SEM analyses of various macrocrysts and groundmass minerals, as well as Sr isotope data confirms the Group I classification of these kimberlites. Insufficient mineral-chemistry data is presently available to decide whether the kimberlites are likely to be diamondiferous.

A phlogopite Rb-Sr mineral isochron provides a precise Lower Cretaceous age of 140.5 ± 1.7 Ma (2) and is interpreted to reflect the time of emplacement of one small kimberlite pipe. The two principal trends of the dykes and the age of 140.5 Ma strongly suggest that kimberlite emplacement was controlled by stresses associated with Lower Cretaceous break-up of Gondwanaland, involving dextral offset of Madagascar with respect to mainland Africa. These stresses reactivated bounding faults to the Karoo graben hosting the kimberlites. Shearing along the bounding faults produced Riedel fractures within the graben that were infilled by kimberlite dykes. This tectonic control on the emplacement of the kimberlites is thought to explain why the majority of the intrusions form dykes rather than pipes.