PARAGENETIC TYPES OF CARBONATITE AS INDICATED BY THE DIVERSITY AND RELATIVE ABUNDANCES OF ASSOCIATED SILICATE ROCKS: EVIDENCE FROM A GLOBAL DATABASE

Abstract

Data on the diversity and relative abundance of igneous rock types associated with carbonatite have been compiled for 477 occurrences, which represent 90% of the 527 occurrences of carbonatite known to the authors. The carbonatites have been subdivided into magmatic carbonatites (84%) and carbohydrothermal carbonatites (16%) on the basis of their mineralogy and their intrusive or extrusive characteristics. The carbohydrothermal carbonatites are associated with a variety of silicate rock-types, but nepheline syenite and syenite are the characteristic ones. For the magmatic carbonatites, 24% of these localities have no associated igneous silicate rocks, whereas a diverse range of silicate rock-types are recognized to be associated with the other 76%. The silicate rocks associated with these carbonatites are listed, quantified and described. We distinguish seven main associations with silicate rocks. These are (in decreasing order of abundance): 1) nephelinite–ijolite, 2) phonolite – feldspathoidal syenite, 3) trachyte–syenite, 4) melilitite–melilitolite, 5) lamprophyre, 6) kimberlite, 7) basanite – alkali gabbro. A significant proportion of the nephelinite–ijolite, melilitite–melilitolite and carbonatite only associations also contain ultramafic rocks, which are interpreted as cumulates. Extrusive carbonatite is found at forty-six localities; these are characterized by the high proportion that contain mantle debris (xenoliths and xenocrysts) and the occurrence of melilite-bearing silicate rocks in more than one third of them. The presence of mantle debris in some carbonatite occurrences and the absence of associated silicate rocks from others are taken as evidence of derivation of these carbonatites directly from the mantle. Most other carbonatites are considered to have been generated by differentiation from magmas represented by the associated silicate rocks that are the result of partial melting in a metasomatized lithosphere. Carbonatites can be generated in a number of ways, but the close spatial and temporal association with a broad spectrum of the silicate melts implies a close relationship with them. However, certain associated silicate rocks are not strictly consanguineous with the carbonatite, but are interpreted as products of independent melting events at shallower levels in the lithosphere.