VARIATION IN THE COMPOSITION OF APATITE IN THE MUNNI MUNNI COMPLEX AND ASSOCIATED INTRUSIONS OF THE WEST PILBARA BLOCK, WESTERN-AUSTRALIA

Apatite is a ubiquitous intercumulus mineral in the Archean Munni Munni, Radio Hill, and Mount Sholl layered igneous complexes of the west Pilbara Block in Western Australia. Apatite compositions from the three intrusions are relatively rich in the fluorapatite endmember and become more so in cumulates derived from compositionally evolved magmas. The most extensive stratigraphic coverage is from the Munni Munni complex, where the mole fraction of fluorapatite increases from approximately 60-70% in cumulates of the ultramafic zone to over 90% near the top of the gabbroic zone. The chlorapatite component decreases from a high of 20-30 mol% in the ultramafic zone to less than 10% in the uppermost part of the gabbroic zone. The REE content of apatite parallels stratigraphic changes in whole rock incompatible element (Zr, Rb) concentrations and is consistent with equilibration with the bulk assemblage. The stratigraphic decrease in the Cl/F and OH/F ratio of apatite can be modeled by assuming that fluid saturation of intercumulus liquids is a function of pressure and occurs prior to apatite saturation. Imposed on the stratigraphic trend are the effects of the combined processes of continuing fluid loss and fractional crystallization of apatite (and locally biotite) during solidification of the intercumulus liquid. These processes produced relative compositional variations in endmember components on the order of 50 mol% or more in individual thin sections. Because apatite crystallizes late in the solidification of intercumulus liquids, it is estimated that the Cl/F ratio of the original trapped parent liquid prior to degassing could have been more than 2 orders of magnitude higher than is now recorded by intercumulus apatite compositions. A comparison of literature apatite compositions from other intrusions shows that, with the exception of the unusually Cl-rich assemblages of the Stillwater and Bushveld complexes. processes affecting halogen geochemistry of layered intrusions do not appear to have changed significantly over the past 3 Ga.