The petrogenesis of high grade contact metamorphic mineral assemblages in calc-silicate xenoliths, Eastern bushveld complex, South Africa

Abstract

One of the most spectacular but least studied thermal metamorphic aureoles is undoubtedly that surrounding the intrusive rocks of the Bushveld Complex. Apart from displaying a vast range of metamorphic assemblages in pelitic rocks of the Pretoria Group, xenoliths of calcsilicates within the intrusive rocks of the complex host some unique assemblages unknown from other localities. The degree of thermal metamorphism belongs to the highest ever recorded. A petrogenetic grid is proposed, which covers the highest degrees of metamorphism occurring in the system Ca0-Mg0-Sio2-co2. This petrogenetic grid involves 65 stable and metastable mineral reactions and serves to predict the fate of siliceous dolomites under extreme contact metamorphic conditions. With increasing temperature akermanite, monticellite, merwinite and periclase formed from mainly irreversible decarbonation reactions in which calcite was an important reactant. From diagnostic mineral parageneses overburden load pressures of 1.1 - 2.2 kbar for the emplacement of the critical zone magma and 0.6- 1.6 kbar for the marginal zone magma can be inferred. Forsterite exsolutions in monticellite with an unusual optical positive sign and dehydroxylated Ba-phlogopite indicate magma temperatures higher than 1200°c, while the lack of melting of the present high-temperature mineral assemblages gives an upper temperature limit of 1400°c. The minerals and mineral assemblages found in the marginal and critical zone xenoliths were hardly affected by hydration during retrograde metamorphism. The upper zone xenoliths consist mainly of vesuvianite-, grandite- and melilite-bearing mineral assemblages, which became stable during relatively late stages of retrograde metamorphism. In the upper zone xenoliths most of the minerals present belong to the system CaO-MgO-Si02-A1 2o 3-H2o-co2. There are strong indications that they formed from minerals similar to those found in the high temperature mineral assemblages of the marginal zone xenoliths. The average Al 2o3 content in the melilites originally present in the upper zone xenoliths was higher than in the marginal zone xenoliths. This, together with an influx of a H20-rich fluid phase during retrograde metamorphism led to the formation of the vesuvianite- and grandite-bearing mineral assemblages. Microprobe analyses of different coloured Mg-Fe-vesuvianites, garnets of different composition, prehnite and xanthophyllite show unusual variations in their chemical composition.