Evolution of the Phlogopite-Chrome Diopside-Spinel Pools in the Ultramafic Nodules of Pipe 200, Northern Lesotho

Abstract

Peridotites are ultramafic igneous plutonic rocks consisting of predominantly ferromagnesian minerals. Their mineralogy reflects their stability field at different pressures and temperatures.

olivine-2 pyroxenes-garnet: high pressure assemblage

olivine-2 pyroxenes-spinel: lower pressure assemblage

olivine-pyroxenes-plag: lowest pressure assemblage

Peridotites are generally of mantle origin.

The sampling of the upper mantle by kimberlite permits the study of mantle rocks; their depth of origin, mineral assemblages, and the processes responsible for their formation. Partial melting of peridotite produces responsible for their formation. Partial melting of peridotite produces a basaltic melt and leaves behind a depleted peridotite. Depending upon subsequent events, these peridotites may be metasomatized and altered prior to incorporation into the kimberlite (primary metasomatism) and during transportation in the kimberlite (secondary metasomatism). If the depleted peridotites are metasomatised, however, it is difficult to recognize and make distinction between the new properties acquired during primary and secondary metasomatism. Two of the criteria that have been used to separate the primary and secondary events are, (a) phlogopite chemistry, (b) textural evidence such as grain deformation, kelyphytization and polygonization. The garnet lherzolites of the kimberlites from Pipe 200 Lesotho have been extensively metasomatised. Although both primary and secondary metasomatising events are recognised in the "subset nodules" of Pipe 200, there are some rocks that lack evidence for primary metasomatism in the other PTH nodules.

The subsequent nodules differ from the rest of the garnet lherzolites by the presence of "pools". The term "pool" refers to a mineral assemblage consisting of phlogopite + chrome diopside + spinel. Pool shapes are roughly spherical, with a mosaic texture, and the presence of phlogopites suggests metasomatic origin. The pools are thought to be the result of a combination of processes:

Solid-state metamorphic reaction Mg2SiO4 + CaMg2Al2Si3O12 d CaMgSi2O6 + 2MgSiO3 + MgAl2O2 metasomatising fluid promoted the development of phlogopite-rich pools with the chrome diopsides and spinels.

The presence of the pools is a strong indication of metamorphism followed by metasomatism. Phlogopites in the pools are strained, suggesting crystallization prior to incorporation into the kimberlite. The lack of pools in some nodules suggests that metasomating events were local phenomena to some degree, and did not affect all the nodules to the same extent.

Keywords: Pages: 83 Supervisor: D. Barrie Clarke