The Kalka layered intrusion,central Australia

The Kalka Intrusion, central Australia has a 5000 m-thick layered sequence comprising Pyroxenite, Norite and Anorthosite Zones; an Olivine Gabbro Zone is enclosed by, and has a facies relationship with, the Norite Zone. Contamination is evidenced by high initial ⁸⁷Sr/⁸⁶Sr ratios ( 0.708) in the lower four-fifths of the intrusion, and resulted in pyroxenite rather than peridotite as a basal crystal accumulate. After an early phase of erratic buildup in contamination due to assimilation of ragged granulite wall-rock, armouring of the walls and mixing produced an homogeneous Norite Zone (threshold) magma body crystallising opx-cpx-plag. Within the succeeding 3500 m of section plagioclase An values have a general decline up sequence (An_74-60-feldspar threshold) with superimposed short term digressions to more calcic compositions. Initial ⁸⁷Sr/⁸⁶Sr ratios also fall very gradually (0.7081-0.7078 — isotopic threshold) with transient fluctuations to distinctly lower values. Maxima in plagioclase An contents and ⁸⁷Sr/ ⁸⁶Sr minima may be correlated with the spasmodic appearance of olivine and pronounced lithological variation. Such features are explained by the ponding of fresh bodies of uncontaminated magma on the floor of the chamber; these formed a hybrid with threshold magma that temporarily dominated crystallisation thereby perturbing plagioclase and isotopic compositions; eventually, mixing of the hybrid into the overlying threshold magma returned crystallisation to its initial state. The facies-controlled Olivine Gabbro Zone is the physical expression of ponded basal hybrid magmas. The onset of the Anorthosite Zone is marked by a pronounced decline in ⁸⁷Sr/⁸⁶Sr ratios to around 0.705 registering a major influx of new magma. In this instance the new magma dominated the system and a change to plagioclase as the dominant liquidus phase indicates a drastic change in input magma composition. The evolution of Kalka was determined by an interplay of crystal fractionation, fresh magma pulses, and magma mixing, with the latter effects producing both increases and decreases in plagioclase An contents; the complete process can only be revealed by combined mineral composition and isotopic data.