Ultramafic sills beneath the eastern Bushveld Complex; mobilized suspensions of early lower zone cumulates in a parental magma with boninitic affinities

Abstract

Two fundamental groups of marginal rocks occur along the basal contact of the eastern Bushveld Complex. The lower part of the complex, comprising the lower zone and the pyroxenite subzone of the critical zone, has a marginal group that is mostly made up of pyroxenites and norites, some of which are quench textured. The upper part of the complex, including the anorthositic portion of the critical zone and the main and upper zones, is bordered by two-pyroxene gabbros. All lithologies that make up the marginal facies of the complex have counterparts in the different groups of sills that intrude the underlying metasedimentary rocks of the Transvaal Sequence.Suites of peridotitc and quench-textured micropyroxenite sills are found adjacent to, and are members of, the pyroxenitic marginal group. The peridotite sills range from homogeneous harzburgites to differentiated dunite-harzburgite-orthopyroxenite bodies. Some have quenched and crescumulate members. Most peridotite sills are petrographically similar and consist of subhedral olivine grains (Fo (sub 83-90) ), poikilitically enclosed by orthopyroxene (En (sub 84-90) ) with accessory plagioclase (An (sub 59-76) ), chromite, clinopyroxene, and biotite. They have ubiquitously high contents of incompatible elements such as 10 to 45 ppm Zr, 5 to 30 ppm Rb, and 0.17 to 0.4 wt percent K 2 O. A family of quench-textured micropyroxenite sills and marginal rocks occurs both in close association with the ultramafic sills and deep below in the floor. These sills are characterized by orthopyroxene (En (sub 78-92) ), with rare olivine (Fo (sub 77-90) ) set in a groundmass of comb-textured plagioclase (An (sub 65-75) ), orthopyroxene, and devitrified glass. Crystallites of plagioclase and hollow, quenched olivine and orthopyroxene crystals occur in some samples. Clinopyroxene, magnetite, and chromite are common accessories; biotite is an essential constituent. The quench-textured micropyroxenites have high SiO 2 ( approximately 55 wt %) and MgO ( approximately 14 wt %) contents. Like the peridotite sills they are rich in incompatible elements and contain 70 to 230 ppm Zr, 20 to 60 ppm Rb, and 0.6 to 1.2 wt percent K 2 O. Members of the pyroxenitic marginal group and the peridotite sills together give an errorchron age of 2,015 + or - 211 m.y. and an initial 87 Sr/ 86 Sr isotope ratio of 0.7049. Olivine, or more commonly orthopyroxene, is the first mineral on the liquidi of melts of the quench-textured micropyroxenites at 1,250 degrees to 1,300 degrees C. Such melts have the requisite properties to have been parental to the lower zone, shown by experimental data and computer simulations of their one-atmosphere fractional crystallization sequences. Peridotite sills crystallized at f (sub O 2 ) conditions between nickel-nickel oxide (NNO) and quartz-fayalite-magnetite (QFM), but micropyroxenite sills crystallized under more reducing conditions, which caused the precipitation of abundant chromite to be suppressed and most Cr to enter orthopyroxene. The floor rocks carry metamorphic assemblages that equlibrated at 4 to 5.5 kb and 700 degrees to 800 degrees C.The compositions of olivine, orthopyroxene, chromite, and clinopyroxene in the peridotite sills are similar but are slightly more magnesian than minerals in harzburgites from the deepest exposed part of the lower zone. Compositional disequilibrium between rounded olivine grains and poikilitically enclosing orthopyroxene is common. Most peridotite sills are made up of a solitary cycle of dunite-harzburgite-bronzitite, but some are lithologically homogeneous. Rare contorted layering structures are indicative of flow banding. Such evidence suggests that many of the sills are solidified suspensions of olivine crystals in liquid. Simple mixing between an average micropyroxenite composition and olivine from the peridotite sills (with the trace element content calculated by olivine-liquid partition coefficients from a micropyroxenitic parent) reproduces the average peridotite sill composition for major, trace, and rare earth elements with approximately 60 percent olivine. The amount of plagioclase, clinopyroxene, and chromite in the sills was controlled by the dilution of parental liquid by olivine crystals. The mushes were derived from semisolid lower zone cumulates and their parental liquids, which were expelled from the Bushveld magma chamber at different periods (hence their range of compositions and stratigraphic levels) during tectonic disturbances, such as those that gave rise to the folding along the basal contact of the complex. A peridotite member of one such sill which has distinctive harrisitic textures originated by crescumulate growth, followed by homogenization of its bladed olivine crystals through subsolidus diffusion at high temperatures.