Strontium isotopic evidence against magma addition in the Upper Zone of the Bushveld Complex

Details Strontium isotopic evidence against magma addition in the Upper Zone of the Bushveld Complex Strontium isotopic evidence against magma addition in the Upper Zone of the Bushveld Complex

Jun 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987e%26psl..84...51k&link_type=abstract

Earth and Planetary Science Letters, Volume 84, Issue 1, p. 51-58.

Mathematics

Logic

12

Scientific paper

Sr-isotopic data from the Main and Upper Zones of the Bushveld Complex show that the evolution of the Upper Zone started with a large influx of magma close to the level of the ``Pyroxenite Marker'', a distinctive orthopyroxenite layer in otherwise relatively uniform gabbronorites. Whole rock samples, which span the complete stratigraphic succession (ca. 2100 m) above this layer, fall on a single Rb-Sr isochron (2066 +/- 58 Ma) and hence have a common initial ratio of 0.7073 +/- 1. This ratio is significantly lower than those of the Main Zone (ca. 0.7085), below the level of the Pyroxenite Marker.
The entire Upper Zone crystallized from a mixed magma which was thoroughly blended before crystallization. This magma had an isotopic ratio intermediate between that of the Main Zone and the added magma which had an initial ratio of ca. 0.7067. Further significant magma additions during crystallization are precluded unless they were of the same isotopic composition as the blended magma, which is considered improbable. Hence the layering and mineralogical diversity of the Upper Zone was produced by internal processes and not produced by magma influxes during crystallization.
The lithological, compositional and isotopic changes at the Pyroxenite Marker and the petrological coherence of all rocks above this horizon support the placing of the Upper Zone boundary at this point in the stratigraphy.

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