Al-rich diopside in alluvial ruby and corundum-bearing xenoliths, Australian and SE Asian basalt fields

摘要

Alluvial rubies and sapphires are found in palaeodrainage deposits along the Cudgegong-Macquarie River system, central eastern New South Wales, Australia. A pink to red suite contains Cr (up to 0.6 wt.% Cr₂O₃) as the main chromophore, exceeding Fe (up to 0.5 wt.%Fe₂O₃). Corrosive etching suggests a prior xenocrystic Mesozoic-Cenozoic basaltic transport, while Cr₂O₃/Ga₂O₃ to Fe₂O₃/TiO₂ ratios indicate an original metamorphic source. Syngenetic mineral inclusions include Al-rich diopside, meionite and anatase. The Al-rich diopside (‘fassaite’) contains extremely high Al₂O₃ (20–21 wt.%). A blue-green suite contains Fe (up to 0.8 wt.% Fe₂O₃) as a dominant chromophore, while a rare nepheline-anorthoclase composite inclusion supports a magmatic phonolitic origin. The Cudgegong-Macquarie ruby formation is compared with a garnet granulite origin proposed for Thailand rubies and a xenolith of corundum-bearing garnet granulite from Ruby Hill, Bingara, Australia. Clinopyroxene-corundum thermometry suggests the Cudgegong-Macquarie rubies formed at T >1000–1300°C, a high equilibration T for proposed lithospheric granulites. These rubies form a distinctive suite compared to other rubies from Australian and SE Asian basalt fields, but have some similarities with eastern Thailand rubies.