Strontium isotopic resolution of magma dynamics in a layered intrusion.

Abstract

Kalka is one of about 14 major layered mafic-ultramafic intrusions forming the Giles Complex in central Australia1. The stratigraphical section of the Kalka Intrusion passes from a basal pyroxenite zone (450 + m) through norite and olivine gabbro zones (3,500 m) to an uppermost anorthosite zone (800 + m)2. Further resolution into 21 cyclic units is derived from repeated mineral crystallization sequences and cyclical variation in plagioclase and olivine compositions. A conventional interpretation would have a basaltic magma progressively fractionating as crystallization proceeded from mafic base to leucocratic top, with periodic resetting to less evolved states by fresh incursions of primary magma or by convectional overturn within a sealed magma chamber. However, the reconnaissance isotopic data reported here indicate development of a more complex open system. High initial 87Sr/86Sr ratios in conjunction with normal 143Nd/144Nd ratios demonstrate massive contamination by country rock within the main part of the intrusion3. Furthermore, great variation in Sr initial ratios from 0.7049 to 0.7088 suggests substantial changes in magma composition beyond those induced by fractionation. Similar isotopic heterogeneity in the Newer Gabbros of Scotland4 and the Bushveld Complex5 has been attributed to variable contamination or the emplacement of magma batches of differing composition.