Geology, Geochemistry And Petrogenesis Of Middle Tertiary Volcanic Rocks Of The Queen Charlotte Islands, British Columbia (Canada)

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Journal Article: Geology, Geochemistry And Petrogenesis Of Middle Tertiary Volcanic Rocks Of The Queen Charlotte Islands, British Columbia (Canada)

Abstract
The volcanic rocks of the Middle Tertiary Masset Formation underlie an area greater than 5,000 km2 in the Queen Charlotte Islands (British Columbia) where their exposed sections are as much as 1.6 km thick. The volcanism, which is related to the development of an extensional basin, produced both subaerial and subaqueous units that are mainly bimodal with mafic types predominating. Two principal types of mafic rocks have been recognized - aphyric to slightly porphyritic tholeiitic basalts and subordinate strongly porphyritic calc-alkali basaltic andesites. The calc-alkali lavas occur mostly in the lower part of the succession although the two types are frequently intercalated. The tholeiites resemble T- and E-type MORB, with REE patterns ranging from slightly depleted to enriched in light REE. The calc-alkali basaltic andesites are enriched in light REE and Sr and do not show a distinct negative Nb anomaly. No difference exists in the Nd, Sr and Pb isotopic composition of the two rock types. The isotopic data indicate a heterogeneous mantle source compositionally similar to that of intraplate basalts from the adjacent NE Pacific seamounts and Cordillera. The differences between the tholeiitic and calc-alkali suites are attributed to polybaric fractional crystallization of different proportions of plagioclase to mafic minerals (depending on pressure) from a primitive tholeiitic magma. For the calc-alkali line of descent (fractionation at mid-crustal depth), plagioclase crystallization was partially suppressed, producing high-Al basalts. For the tholeiitic line of descent, crystallization of the primitive tholeiitic magma at lower pressures and fO2 conditions (probably within upper-crustal reservoirs) allowed fractionation of larger proportions of plagioclase; thus the magma evolved towards low-Al olivine tholeiites. The felsic rocks were derived primarily by extensive fractional crystallization of mafic magma.

Authors 
T. S. Hamilton and J. Dostal








Published Journal 
Journal of Volcanology and Geothermal Research, 1993





DOI 
Not Provided
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Citation

T. S. Hamilton,J. Dostal. 1993. Geology, Geochemistry And Petrogenesis Of Middle Tertiary Volcanic Rocks Of The Queen Charlotte Islands, British Columbia (Canada). Journal of Volcanology and Geothermal Research. (!) .