The Geology and Petrology of the Iron and Manitou Islands Alkaline Carbonatite at Nipissing Lake, Ontario

Abstract

The Iron and Manitou Islands complexes are 560 m.y. old. They were associated with a magma chamber lying at moderate depth below these complexes, On a regional scale the emplacement is controlled by the Nipissing graben system, an extension of the St. Lawrence rift system, along which an alkaline igneous province, 560 m.y. old, is well developed. A variety of rock types is developed in these complexes in spite of their small size. This is a result of extensive differentiation, interaction of late fractions with already crystallized early fractions, interaction of a "fluid phase" with country rocks, partial melting of country rocks, and finally various late hydrothermal alterations. The high degree of differentiation is probably a consequence of the high amount of volatile components in the magma which lowers its viscosity, extends its crystallization range, and consequently enhances fractional crystallization. In addition, liquid immiscibility played an important role in the differentiation. During differentiation the pyroxenes changed in composition from Ca-rich varieties through aegirine-augite to acmite, There is no evidence of an immiscibility gap between Ca-rich and Na-rich pyroxenes as has been proposed by Aoki (1964) but criticized by later investigators, Members of the phlogopite-biotite solid solution series exhibit a compositional range from annite-12 to annite-63. The characteristic assemblage melilite; Ca-rich pyroxene, and olivine (partially or completely replaced olivine phenocrysts) occurring in some lamprophyres indicates low silica activity, 10^(-1.6) to 10^(-1.2). However, the silica activity of the Lamprophyres is not restricted to the above range since the presence of sphene in some Lamprophyre dykes indicates silica activity above this range. The required condition of excess sodium silicate for the crystallization of acmite in synthetic melts seems to be required in nature too. Lamprophyres have a crystallization temperature of about 950°C. Fenitization of the country rocks takes place from relatively low temperature, probably 480°F, to about 740°C where partial melting of the rocks occur. The development of the graben system and the alkaline igneous activity are both believed to be related to devolatilization processes in the mantle.