Alkaline Rocks and Carbonatites of the World

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Shefford Mountain

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Occurrence number: 
031-00-138
Country: 
Canada
Region: 
Quebec
Location: 
Longitude: -72.62, Latitude: 45.37

Emplaced in Cambro-Ordovician sedimentary rocks, the Shefford Complex has a hornfelsed aureole which is poorly exposed. Around a dioritic and a gabbroic core are concentric intrusions of more salic rocks which show inward dipping foliation and some banding, possibly about two centres. The core of the complex is formed of diorites and gabbros which have gradational relationships and include poikilitic gabbros and diorites, monzodiorite and quartz monzonite. Diorites are more abundant than gabbros and both consist of andesine or labradorite, kaersutite and minor augite and olivine, and with increase in alkali feldspar and biotite they grade into monzonites. Small areas of monzodiorite and nepheline diorite outcrop within pulaskite and are cut by pulaskite dykes. They consist of andesine zoned to oligoclase, kaersutite and ferrohastingsite, and accessory nepheline, cancrinite, sodalite, sphene, up to 10% in some rocks, apatite and opaques. Pulaskites and micropulaskites grade into nepheline-rich varieties of foyaite and laurdalite, but the average rock type contains 90% perthite and less than 5% nepheline, with accessory ferrohastingsite, augite/aegirine-augite, apatite, sphene, zircon and opaques. Quartz-bearing syenites (nordmarkite) develop a larvikitic facies at the contacts with diorite and gabbro, which consists of alkali feldspar, less than 2% quartz and aegirine-augite, ferrohastingsite, altered fayalite and biotite. The central part of the west substructure contains a heterogeneous assemblage of breccias and porphyries including fragments of latite and trachyte in a matrix of plagioclase porphyry, chemically identical to the gabbros and diorites. A varied suite of dyke rocks includes types similar to the principal rock types of the complex. Rock analyses, including REE and Sr isotopic data, as well as pyroxene and olivine data are available in Eby (1985a) and Pb isotopes in Eby (1985b). Frisch (1970, 1972) has given a detailed account of amphiboles, pyroxenes and iron knebelite from Shefford.

Age: 
Eby (1984b) reports ages of 119+-8 and 131+-8 Ma by fission- track dating of apatite from nepheline diorite and diorite, and 129 Ma by Rb-Sr.
References: 
DRESSER, J.A. 1902. Report on the geology and petrography of Shefford Mountain Quebec. Annual Report, Geological Survey of Canada, 13,L: 1-35. 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. FRISCH, T. 1970. Chemical variations among the amphiboles of Shefford Mountain, a Monteregian intrusion in southern Quebec. Canadian Mineralogist, 10: 553-70. FRISCH, T. 1972. Iron knebelite from the nordmarkite of Shefford Mountain, Quebec. Canadian Mineralogist, 11: 552-3. WOUSSEN, G. and VALIQUETTE, G. 1972. The geology of the Brome and Shefford igneous complexes. 24th International Geological Congress, Canada. Excursion Guide, B-13: 1-15
Fig. 69 Shefford Mountain (after Woussen and Valiquette, 1972, Fig. 3).
Location: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith