The mineralogy of the Gosse pile ultramafic intrusion,central Australia. II. Pyroxenes

The orthopyroxenes of Gosse Pile, a layered ultramafic sequence within the Giles Complex of central Australia, show unusual exsolution features such as the development of ‘hour‐glass’ shapes by clinopyroxene lamellae, and also the exsolution of spinel and rutile. Both orthopyroxene and clinopyroxene have high R2O3 contents and their distribution coefficients for Mg and Fe are high (average KD = 0.88). Some, if not all, of these features are considered to indicate that the Gosse Pile rocks crystallized under high pressure. This interpretation also explains the predominance of orthopyroxene over olivine as an early cumulus phase. Estimated conditions during crystallization are 10–14 kilobars and between 1,200‐l,400°C.