Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Poohbah Lake


Occurrence number: 
Longitude: -91.5, Latitude: 48.25

The Poohbah Lake complex is an elliptical body intruded conformably into Archaean biotite schists and gneisses; there is a subsidiary intrusion 500 m to the west. The principal rock types, in order of intrusion, are malignite, porphyritic syenite and amphibole syenite, with some biotite pyroxenite, which is assumed to be a cumulate from the same parental magma as the malignite. The malignites, which are also considered to be cumulates, comprise diopsidic aegirine-augite (50-80%), poikilitic orthoclase (10-40%), poikilitic nepheline (5-10%), apatite (1- 10%), with replacement of pyroxene by andradite/melanite and green biotite. Nepheline is sometimes altered to natrolite-thomsonite, and vermiform intergrowths of orthoclase and nepheline occur. The porphyritic syenites contain phenocrysts of perthitic Ba-rich feldspar in a groundmass of biotite, pyroxene and amphibole, while the amphibole syenite consists of an equigranular perthite syenite with a blue-green amphibole. The biotite pyroxenite is free of feldspar and nepheline and apparently grades into malignite. Mitchell and Platt (1979a) give a detailed description of the malignites including rock and mineral analyses and a full discussion of the term 'malignite'.

The K-Ar isochron age is 2706±46 Ma (Mitchell, 1976).

MITCHELL, R.H. 1976. Potassium-argon geochronology of the Poohbah Lake alkaline complex, northwestern Ontario. Canadian Journal of Earth Sciences, 13: 1456-9.
MITCHELL, R.H. and PLATT, R.G. 1979a. Nepheline-bearing rocks from the Poobah Lake complex, Ontario: malignites and malignites. Contributions to Mineralogy and Petrology, 69: 255-64.

Fig. 1_27 Poohbah Lake (after Mitchell and Platt, 1979a, Fig.1).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith