MAJOR ADDITION OF MAGMA AT THE PYROXENITE MARKER IN THE WESTERN BUSHVELD COMPLEX, SOUTH-AFRICA

A major, but gradual, reversal in the cryptic variation pattern of the plagioclase and pyroxenes, of 13 mol% anorthite and 10 mol% Mg/(Mg + Fe) respectively, is documented in the Main Zone of the western Bushveld Complex. These changes are accompanied by a decrease in initial Sr-87/Sr-86 ratio from > 0.708 to < 0.707. The Pyroxenite Marker, a distinctive orthopyroxenite layer, occurs close to the top of this reversed differentiation sequence. This is attributed to addition of less differentiated magma. On the basis of a mass balance calculation of the initial Sr-87/Sr-86 ratios, it is estimated that the volume of magma added was comparable to that of the resident magma. Increases in the Fe2O3, TiO2, Al2O3, and Na2O contents of the pyroxenes above the level of magma addition indicate that the new magma had a lower silica activity and higher f(O2) than the resident magma. Quantification of the trace element and REE content of the two magmas is hampered by the very low proportion of trapped intercumulus component in these adcumulate rocks. However, semi-quantitative modelling indicates that the trace and REE signatures of the two magmas were similar, with moderate LREE enrichment and flat HREE profiles. The new magma had a slightly higher La/Sm ratio than the resident magma, consistent with its more alkaline nature. The new magma was probably added gradually, while 100-150 m of cumulates formed. It probably intruded at an intermediate level within an existing stratified magma chamber, where it cooled and crystallized, and composite packets of liquid plus crystals plunged to the base of the chamber. The cores of plagioclase grains formed during this mixing interval show a wider range of compositions than in other sections, and plagioclase primocrysts from both magmas may be preserved within single samples. Therefore, although intimate physical mixing of packets of unknown size of the two magmas occurred, re-equilibration of the major oxide composition of the plagioclase primocrysts was not achieved. However, the data and calculations based on diffusion rates indicate that partial Sr isotopic resetting of plagioclase may have occurred.