Small Scale Primary Cumulus Igneous Layering in the Kalka Layered Intrusion, Giles Complex, Central

Small scale primary cumulus layers occur in various parts of the Kalka Intrusion in central Australia, and generalized models have been developed from them to explain the formation of primary small scale layering features in stratiform gabbroic intrusions. Primary small scale layering appears to be produced by repeated bursts of discontinuous nucleation, followed by differential gravitational settling of pyroxene or olivine relative to plagioclase. Dependent on the distance of the nucleation zone above the pile of settled cumulus crystals, the layering may be mineral graded or isomodal as a result of either incomplete or complete separation of cumulus pyroxene or olivine and plagioclase on settling. Reverse or continuous mineral grading may result from settling overlap of plagioclase by pyroxene or olivine formed in later nucleation cycles. Conditions of continuous nucleation will generally result in massive unlayered sequences, irrespective of any differential settling which may occur between phases