Petrogenesis of the Marginal Sill Phase and Lower Zone in the Eastern Limb of the Bushveld Complex: Incremental development of igneous layering and syn-magmatic emplacement of peridotite intrusions and sills

Abstract

Abstract The petrogenesis of the mafic-ultramafic layered rocks of the Bushveld Complex, i.e., the Rustenburg Layered Suite (RLS) remains uncertain despite more than a century of intensive research. An investigation of a 100 km strike section in the Eastern Limb has led us to propose that igneous layering in the lowermost part of the RLS developed from a succession of intrusive events. Zonal boundaries and geochemical discontinuities are explained by radical switches in the composition of parental magmas. Magmas were sourced from deep staging chambers, and we find no evidence for the existence of a large magma reservoir in the shallow crust. At the base of the RLS is the Marginal Sill Phase (MSP), a stacked sequence of noritic-gabbronoritic and pyroxenitic sills that form a continuum with the syn-Bushveld sills in the floor rocks. The MSP does not constitute the chilled carapace of a magma chamber and the postulated so-called “B1” (tholeiitic) parental magmas have little direct bearing on the overlying components of the RLS. The Lower Zone (LZ) reflects a switch to ultramafic parental magmas. Magmas were fed into isolated subchambers, each of which has a unique igneous stratigraphy. The primary component of the LZ is an adcumulate comprised of >98.5 modal % orthopyroxene (En87-83) and from which Cr-spinel is conspicuously absent. Chemical equilibrium was maintained as the orthopyroxene-saturated magmas were intruded, not as sills, but as thin magma laminae. The increments of magma laminae matched the rate of batch crystallisation, and there is no evidence of a fractionated residue having been ejected. In a later phase of magmatism, peridotite magmas were injected into the floor rocks, where they formed complexly layered peridotite intrusions with their anastomosing groups of sills. Formation of peridotite bodies such as the Aapiesdoorndraai Intrusion with distinct layers of dunite, harzburgite and olivine orthopyroxenite (Fo92-83) (+Cr-spinel) result from near-equilibrium crystallisation of the olivine- and Cr-spinel-saturated magmas. Some peridotite sills stepped up through the thermal aureole and intruded the earlier-formed MSP and LZ, crystallising as syn-magmatic sills with broadly similar compositions. Development of the trough-like subchambers with thick sequences of the MSP and LZ triggered extensive deformation and thermal metamorphism of the floor rocks. Sinking of subchambers was compensated by intervening domes and upwarps, the crests of which reveal tectonically thinned sequences of the MSP and from which the LZ is absent. This early phase of syn-Bushveld tectonism has resulted in the Lower Critical Zone (LCZ) discordantly overlying either the MSP (on the crests of domes and upwarps) or the LZ (in subchambers). Field relationships thus demonstrate that the LCZ developed entirely independently of the MSP and LZ, from a new series of intrusive events and a new lineage of parental magma.