CORUNDUM FROM BASALTIC TERRAINS - A MINERAL INCLUSION APPROACH TO THEENIGMA

This paper investigates the origin of corundum megacrysts that occur i n many basaltic terrains, and which are considered to be eroded from b asaltic rocks. Geochemical data for over 80 primary mineral inclusions within corundum megacrysts are used to gain a new insight into the pe trogenetic history of the corundum megacrysts. A wide spectrum of mine rals 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 (albiteoligoclase) and zircon. Rare inclu sions include Fe,Cu-sulphide (low in Ni), cobalt-rich spinel, Th,Ce-ri ch phosphate and uraninite. The similar chemistry of some inclusion mi nerals from corundum occurring in widely separated areas suggests that the corundum megacrysts in basalts have a similar petrogenesis. Geoch emical 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 ba saltic hosts but are crustal fragments accidentally incorporated into the erupting magma. It is suggested that interactions between a silici c component and an intruding carbonatitic or similar Si-poor magma is responsible for Al-oversaturation, resulting in locally distributed le nses of corundum-bearing rock of mixed paragenesis (''hybrid rock hypo thesis''). Feldspar exsolution textures provide strong evidence that t his hybridisation occurred at mid-crustal levels. Subsequent volcanic eruptions brought the corundum-bearing rocks (later disintegrated in t he magma) up to the Earth's surface. This petrogenetic model for corun dum megacrysts is experimentally testable.