Alkaline Rocks and Carbonatites of the World

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

Mount Megantic


Occurrence number: 
Longitude: -71.12, Latitude: 45.45

A mountain rising some 600-700 m above the surrounding plain is underlain by a central biotite granite around which, on the northern side, is an arcuate intrusion of gabbro/diorite around which again is a separate circular intrusion of syenite. The gabbro/diorite rocks comprise poikilitic kaersutite, plagioclase (An83-20), titanaugite, biotite and magnetite. The syenites are made of perthite, aegirine, augite and fayalite which may be rimmed by green amphibole. Up to 10% quartz is present with accessory apatite, sphene, allanite and zircon. Plagioclase- biotite lamprophyre dykes occur around the intrusion as well as feldspar-free analcime melteigites. Analyses for major and trace elements, including REE and Sr isotopes, and some mineral data will be found in Eby (1985a) and Pb isotopes in Eby (1985b).

Placer gold was once mined in the area; its source is not known but it may have been Mount Megantic.
Fission-track dating of apatite from gabbro gave 134±8 Ma and Rb-Sr 133 Ma (Eby, 1984b). A detailed palaeomagnetic study to try to elucidate ambiguities in the earlier radiometric ages has been made by Seguin and St-Hilaire (1985).
CURRIE, K.L. 1976a. The alkaline rocks of Canada. Bulletin, Geological Survey of Canada, 239: 1-228. EBY, G.N. 1984b. Geochronology of the Monteregian Hills alkaline igneous province, Quebec. Geology, 12: 468-70. EBY, G.N. 1985a. Monteregian Hills II. Petrography, major and trace element geochemistry, and strontium isotopic chemistry of the eastern intrusions: Mounts Shefford, Brome, and Megantic. Journal of Petrology, 26: 418-48. EBY, G.N. 1985b. Sr and Pb isotopes, U and Th chemistry of the alkaline Monteregian and White Mountain igneous provinces, eastern North America. Geochimica et Cosmochimica Acta, 49: 1143-53. SEGUIN, M.K.- and ST-HILAIRE, B. 1985. Magnetochronologie des formations rocheuses du Mont Megantic et des environs, Quebec meridionel. Canadian Journal of Earth Sciences, 22: 487-97.
Fig. 1_71 Mount Megantic (after Currie, 1976a, Fig. 16 - which is after Reid, 1961).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith