Contribution
Megacrysts in kimberlites from the Gibeon field, Namibia
Mitchell, Roger H.
Neues Jahrbuch für Mineralogie - Abhandlungen Band 157 Heft 3 (1987), p. 267 - 283
34 références bibliographiques
publié: Oct 15, 1987
DOI: 10.1127/njma/157/1987/267
ArtNo. ESP154015703003, Prix: 29.00 €
Abstract
A new suite of megacrysts from the Mukorob-1, Mukorob-2, Deutsche Erde-1 and Amalia kimberlites from the off-craton Gibeon field of the Southern African kimberlite province are described. Compositional data are presented for clinopyroxene, garnet, ilmenite and orthopyroxene discrete megacrysts, clinopyroxene-ilmenite lamellar intergrowths, clinopyroxene-phlogopite intergrowths and garnet exsolved from clinopyroxene. Significant interkimberlite differences are evident with respect to the megacrysts present and their compositional variation. Megacrysts are interpreted to be formed during the crystallization of kimberlitic magmas at high pressures. Fragmentation of several batches of such crystallization products followed by mixing during transport in the magma which formed their current host kimberlite is proposed as a means of generating the observed inter- and intra-kimberlite megacryst compositional variation. No significant differences were observed between the composition of megacrysts found in off- and on-craton kimberlites and this dichotomy is considered to be unwarranted. The absence of diamond in kimberlites emplaced in mobile melts is believed to be related to processes unconnected with kimberlite genesis.
Mots-clefs
Kimberlite • megacrysts • upper mantle • diamond; Gibeon • Southern Africa