Corundum from basaltic terrains: a mineral inclusion approach to the enigma |
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| Authors: | J Guo Suzanne Y O'Reilly William L Griffin |
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| Institution: | (1) Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia, AU;(2) CSIRO Division of Exploration and Mining, PO Box 136, North Ryde, NSW 2113, Australia, AU |
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| Abstract: | This paper investigates the origin of corundum megacrysts that occur in many basaltic terrains, and which are considered
to be eroded from basaltic rocks. Geochemical data for over 80 primary mineral inclusions within corundum megacrysts are used
to gain a new insight into the petrogenetic history of the corundum megacrysts. A wide spectrum of minerals is present as
inclusions in the corundum; the most common are Nb−Ta oxides (such as titaniferous columbite and uranpyrochlore), alkali feldspar,
low-Ca plagioclase (albite-oligoclase) and zircon. Rare inclusions include Fe,Cu-sulphide (low in Ni), cobalt-rich spinel,
Th,Ce-rich phosphate and uraninite. The similar chemistry of some inclusion minerals from corundum occurring in widely separated
areas suggests that the corundum megacrysts in basalts have a similar petrogenesis. Geochemical characteristics of the inclusions
indicate a bimodal grouping, which is best explained by a mixing-hybridisation process. This study indicates that the corundum
megacrysts are not cogenetic with their basaltic hosts but are crustal fragments accidentally incorporated into the erupting
magma. It is suggested that interactions between a silicic component and an intruding carbonatitic or similar Si-poor magma
is responsible for Al-oversaturation, resulting in locally distributed lenses of corundum-bearing rock of mixed paragenesis
(“hybrid rock hypothesis”). Feldspar exsolution textures provide strong evidence that this hybridisation occurred at mid-crustal
levels. Subsequent volcanic eruptions brought the corundum-bearing rocks (later disintegrated in the magma) up to the Earth's
surface. This petrogenetic model for corundum megacrysts is experimentally testable.
Received: 15 March 1995 / Accepted: 30 June 1995 |
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