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TitleThe Caldwell Group lavas of southern Quebec: MORB-like tholeiites associated with the opening of Iapetus Ocean
AuthorBedard, J; Stevenson, R
SourceCanadian Journal of Earth Sciences 36, 6, 1999 p. 999-1019, Open Access logo Open Access
Alt SeriesGeological Survey of Canada, Contribution Series 1998150
PublisherCanadian Science Publishing
Mediapaper; on-line; digital
File formatpdf
NTS21E; 21L; 31H; 31I
AreaDrummondville; Chaudière River; St.-Anne; St-Fabien; St-Anselme
Lat/Long WENS -74.0000 -70.0000 47.0000 45.0000
Subjectsigneous and metamorphic petrology; geochemistry; tectonics; igneous rocks; volcanic rocks; lavas; tholeiites; mid-ocean ridges; basalts; Archean; petrography; isotope geochemistry; fractional crystallization; Maquereau Group; Shickshock Group; Ruisseau Gagnon Basalts; St.-Anne River Nappe Basalts; St-Fabien Basalts; St-Anselme Basalts; Chaudière River Nappe Basalts; Drummondville Olistostrome Lavas; Tibbit Hill Lavas; Caldwell Group; Iapetus Ocean; Humber Zone; Rosaire Group; Proterozoic; Precambrian
Illustrationssketch maps; tables; ternary diagrams; graphs
AbstractThe Caldwell Group belongs to the Internal Nappe Domain of the Humber Zone and consists of basaltic lavas, quartzo-feldspathic sandstones, and mudslates. The lavas are clinopyroxene ± plagioclase ± olivine-phyric tholeiites, and are typically altered to epidote-, chlorite-, carbonate-, and (or) hematite-rich secondary assemblages. In most cases, the high field strength elements do not appear to have been perturbed by the alteration, and preserve magmatic signatures. Most Caldwell basalts exhibit coupled major and trace element variations compatible with low- to medium-pressure ([Formula: see text] 10 kbar, where 1 kbar = 100 MPa) fractional crystallization. Paleotectonic discriminants imply an ocean-floor or normal mid-ocean ridge basalt (N-MORB) affinity. Most basalts have flat N-MORB-normalized profiles, except for the highly incompatible elements (Ba, Th, Nb), which show slight relative enrichment. Melting models suggest that most of these lavas formed by about 20% melting from a mantle slightly less depleted than fertile MORB mantle (FMM). Subpopulations of Caldwell lavas (types 1b and 1a) are characterized by slightly higher incompatible element abundances, with similarly shaped N-MORB-normalized profiles, and can be modeled by slightly smaller degrees of melting (6-15%) of a similar source mantle. The Caldwell basalts erupted in the final stages of Iapetus rifting, when the predominant mantle source involved in melting was the depleted asthenosphere. Isotopic data preclude significant crustal contamination, yet the basalts are associated with sandstones, implying that a mature continental crust was present nearby. Nd isotopic data on the sandstones suggest erosion of an ancient Archean-Proterozoic composite terrane.

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