A clinoenstatite-bearing cumulate Olivine Pyroxenite from Howqua,Victoria |
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| Authors: | Anthony J Crawford |
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| Institution: | (1) Department of Geology, University of Melbourne, 3052 Parkville, Victoria, Australia |
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| Abstract: | The Howqua Olivine Pyroxenite of eastern Victoria, Australia, intrudes a metamorphosed sequence of Cambrian high-Mg lavas. It crystallized an unusual mineral assemblage: Cr-rich magnesiochromite, olivine (Fo94), and protoenstatite (now inverted to polysynthetically-twinned clinoenstatite). Residual liquid crystallized strongly-zoned interstitial pyroxenes followed by pargasite. Pargasite, often showing quench habit, crystallized in interstitial glass which is now altered to serpentine.The extremely refractory nature of the cumulus phases indicates a very high temperature of crystallization for liquidus olivine and chromite from a high-MgO, low-Al2O3 parent liquid similar in some respects to Archaean peridotitic komatiites. The suggested origin by hydrous melting of depleted mantle peridotite, plus other compositional and mineralogical similarities (especially the olivine-liquid reaction producing protoenstatite) indicate that the parent magma of the Howqua Olivine Pyroxenite had many features in common with the high-SiO2, high-MgO clinoenstatite-bearing boninitic lavas of the Western Pacific. It is interpreted as a more extreme melt with affinities to boninite and it demonstrates that ultramafic magmas existed in the Cambrian. |
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