Alkaline rocks and gemstones, Australia: A review and synthesis
Titel:
Alkaline rocks and gemstones, Australia: A review and synthesis
Auteur:
Sutherland, F. L.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 43 (1996) nr. 3 pagina's 323-343
Jaar:
1996-06
Inhoud:
In Australia, valuable gemstones that occur in alkaline rocks include diamonds in lamproites and kimberlites and sapphires, zircons and rubies in alkali basalts. One gem zircon prospect is in carbonatite. This review combines geological and gemmological literature to discuss the tectonic settings and origins of Australian gem-bearing alkaline rocks. Marked contrasts exist between diamond and sapphire/zircon associations across the continent. More western cratonic areas exhibit episodic, sparse, deep alkaline activity from the diamond zone (2 Ga-20 Ma), while in eastern fold belt areas prolific Mesozoic/Cainozoic basalt volcanism carried up considerable sapphire and zircon (since 170 Ma). Some South Australian Mesozoic kimberlitic diamond events (180-170 Ma) represent ultra-deep material rising through the mantle transition zone. Eastern Australian diamonds are unusual and at present their origin is contentious; Palaeozoic subductions that form young, shallow-origin diamonds for later basaltic transport underpin a novel model. Several different models compete in explaining eastern sapphire/zircon formation. They range from eruptive plucking of metamorphosed subducted materials to crystallisation from felsic melts, or carbonatitic reactions. Pb-U isotope zircon ages favour formation during Phanerozoic basaltic activity and not during earlier Palaeozoic subduction or granitic intrusion events. A problem for crystallising zircon from highly fractionated basaltic melts is negligible europium depletion in Rare Earth Element patterns. A model proposed here favours sapphire/zircon crystallisation from relatively small volume, little evolved, felsic melts generated from metasomatised mantle as the lithosphere overruns subdued hot spot systems, initiated at Tasman-Coral Sea rift/spreading margins. A unique ruby, sapphire, sapphirine, spinel assemblage from the Barrington basalt shield in New South Wales, marks a separate ruby/pastel-coloured sapphire genesis.
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