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Geochemistry and geochronology of the igneous rocks of the Belt-Purcell Supergroup of the northern Belt Basin, Alberta and southeastern British Columbia, Canada
Abstract
Intrusive and extrusive igneous rocks occur in the Proterozoic Purcell Supergroup of Alberta and British Columbia. Mafic intrusions are Proterozoic and can be subdivided into a porphyritic-type, an amygdaloidal-type, a type "A" diabase and a type "B" diabase. The Proterozoic mafic extrusions are Purcell and Sheppard lavas. Felsic intrusions can be subdivided into two distinct Mesozoic groups. A 1400 Ma $\sp{40}$Ar/$\sp{39}$Ar plateau age for a biotite separate from a sill intruding the upper Appekunny Formation is interpreted as the minimum age of emplacement. Field relationships place the time of intrusion between the emplacement of the porphyritic intrusions and diabasic intrusions. Type "A" diabases which comprise an extensive suite of mafic rocks that occur as one or more sills in the Siyeh Formation. Two K/Ar hornblende ages of 796 and 797 Ma for type "A" intrusion specimens are interpreted as a minimum age. $\sp{40}$Ar/$\sp{39}$Ar minimum whole-rock ages for type "B" range from 558-727 Ma. The two Mesozoic felsic suites are monzodiorites and monzonites. A K/Ar data of 138 Ma for a hornblende separate is interpreted as the minimum emplacement date. Whole-rock K/Ar apparent ages range from 79 to 89 Ma. In general, Proterozoic rocks plot as alkaline basalts in classification diagrams. REE patterns of all intrusions are parallel to subparallel, similar to alkaline basalts, LREE-enriched, have slight negative anomalies. The type "B" diabasic sill within the Roosville Formation has the lowest C/C$\sb{\rm m}$ value. Two chemically-distinct Mesozoic intrusive suites, separated in time of emplacement by approximately 60 million years. The Purcell and the Sheppard lavas constitute the Purcell Supergroup extrusive rocks. Both lavas have chemistry similar to those of the Proterozoic intrusions. Chemical analyses of the Proterozoic rocks indicate that they possibly had analkali olivine magma source and were produced by fractional crystallization. The tectonic setting was that of an incipient rift. The data suggest episodic intervals of crustal extension. The two younger intrusive suites were probably produced by a partial melting of the existing rocks during a period of mid-Jurassic to mid-Cretaceous regional faulting.
Subject Area
Geochemistry
Recommended Citation
Ghazi, Ali Mohamad, "Geochemistry and geochronology of the igneous rocks of the Belt-Purcell Supergroup of the northern Belt Basin, Alberta and southeastern British Columbia, Canada" (1992). ETD collection for University of Nebraska-Lincoln. AAI9233400.
https://digitalcommons.unl.edu/dissertations/AAI9233400