Field relations and strontium isotope systematics of the marginal rocks of the eastern Bushveld Complex

Abstract

The well-exposed basal contact of the eastern Bushveld Complex was mapped at scales of 1:10,000 and 1:20,000 along 130 km of strike and through a 9-km vertical section. Field relations, petrography, and geochemistry of the marginal rocks and associated sills have aided their subdivision into two main groups. A broadly pyroxenitic group rims the complex from its base to the top of the pyroxenitic subzone of the critical zone. A gabbroic group occurs above this level. Each group consists of a number of closely related rock types, most of which have sill analogs. Rocks of the first group (B1) include pyroxenites, norites, and peridotites, all of which may have quench textures. All contain orthopyroxene (>En 78 ), plagioclase (>An 65 ), and chromite, with subordinate clinopyroxene and biotite. Rare earth element patterns are mostly strongly fractionated (Ce N /Yb N approximately 10) and have 40X chondritic abundances at Ce. Their initial strontium isotope ratios at 2.05 b.y. fall in the range 0.7032 to 0.7057 and Rb/Sr ratios approximate 0.2. The gabbroic group consists of microcrystalline, two-pyroxene gabbros (B2N and B3N) and their coarser grained equivalents (B2 and B3) that border the upper critical and main zones, respectively. Mineral assemblages are dominated by plagioclase ( approximately An 55 ), clinopyroxene, and orthopyroxene ( approximately En 63 ), with abundant magnetite and traces of ilmenite and biotite. Their rare earth element patterns are less fractionated than B1 rocks. The B2N and B2 family have Ce N /Yb N approximately 6 and 30X chondritic abundances at Ce; the B3N and B3 rocks have Ce N /Yb N approximately 3, 6X chondritic abundances at Ce, and pronounced positive Eu anomalies. Strontium isotope initial ratios at 2.05 b.y. fall in the range 0.706 to 0.708 and the Rb/Sr ratios are approximately 0.02. Whole-rock and Sr isotope compositions of the peridotitc sills can be successfully simulated by a model involving olivine fractionation from a liquid with an initial Sr isotope ratio (R 0 ) similar to that of the Transition Zone (0.7056). A fraction of such liquid, enriched in cumulus olivine crystals, when mixed with up to 30 percent of a pyroxenitic liquid with low R 0 (0.7034), yields a product having the isotopic and compositional characteristics of the peridotite sills.A continuum exists in the initial Sr ratios through the pyroxenitic and gabbroic marginal rocks, suggesting that these represent mixtures of at least two sources with distinct initial Sr isotope ratio characteristics. It is argued that neither magma aging nor crustal contamination adequately accounts for the Sr isotope characteristics of the marginal rocks or their derived cumulates. The unusual chemistry of the pyroxenitic early liquids (SiO 2 approximately 55%, MgO approximately 13%, coupled with high concentrations of incompatible elements) should be considered primary.