A study of macro-rhythmic layering and cumulate processes in the jimberlana intrusion, Western Australia. part I

Abstract

The upper layered series of the Jimberlana Intrusion rests unconformably on the lower layered series and has resulted from a major new pulse of magma which entered the magma chamber during the final stages of crystallization of the lower series. All parameters which vary systematically with fractionation are sharply reversed at the contact between the two layered series.The lower portion of the ultramafic zone of the upper layered series is composed of a repeated macro-rhythmic succession of olivine cumulates, bronzite-olivine cumulates, and bronzite cumulates. A detailed investigation was made of this sequence so that the contacts between the units could be compared with the contact between the upper and lower layered series. Cu, Ni, Cr, P, and U were measured on a whole rock basis; the Mg/Mg÷Fe ratios of the pyroxenes and olivines, the Cr content of pyroxenes and the Ni content of olivines were determined by the electron microprobe. Each of the macro-rhythmic units is associated with a reversal in mineral variation and in the Ni-Cr trends and has a zone of high sulphide values at its base. These features are also found at the contact between the upper and lower layered series and, if the upper series is due to a new influx of magma, it follows that the macro-units are also due to multiple injection. This conclusion is strongly supported by the size of the reversals which are too large to be explained by the alternative hypotheses to multiple injection suggested by Jackson (1961) and Wager (1959).The anomalous sulphide-rich zones at the contacts between the upper and lower layered series and between the macro-rhythmic units are thought to have formed from a narrow supercooled zone which developed for a brief period at the bottom of the magma chamber following each new magma pulse. The olivines from these sulphide-rich zones are depleted in Ni, suggesting that Ni has been scavenged by the sulphides. This is only possible if the olivine grains crystallized with the sulphides in the restricted supercooled zone at the bottom of the magma chamber. Show more