Geochemistry of the Trans-Labrador granitoid belt, Canada. A quantitative comparative study of a Proterozoic batholith and possible Phanerozoic counterparts

Abstract

Abstract The Trans-Labrador granitoid belt (TLGB) of eastern Canada is a 600 km-long magmatic province, that forms part of a wider belt of Lower to Middle Proterozoic crust along the southern margin of the pre-1800 Ma cratonic nucleus of North America. The size, geometry and tectonic position of the TLGB invite comparison with continental margin batholiths such as those of circum-Pacific areas. Quantitative comparative studies using a large, statistically representative, database suggest that there are significant contrasts between the compositional spectra and differentiation trends of the TLGB and typical continental arc batholiths. Specifically, the TLGB is dominated volumetrically by alkali-calcic granites (s.s.), showing locally agpaitic tendencies and relatively enhanced levels of F, Zr and REE. Such trace element characteristics invite comparison with “A-type”, “anorogenic” or “within-plate” granites, and some components of the TLGB fall into this category. In terms of bulk compositional spectra, the TLGB is similar empirically to post-tectonic and anorogenic intrusions of the Newfoundland Appalachians. It is suggested that a continental arc setting is inappropriate for the TLGB, and that a more likely environment is a post-orogenic setting dominated by crustal thickening, uplift and transcurrent faults. It is also possible, however, that the TLGB was generated in an intraplate environment by melting and vertical differentiation of continental crust in response to thermal input from the mantle. It is suggested that the use of large, representative databases, coupled with quantitative comparative studies of compositional spectra and differentiation trends, may be a useful approach in the discrimination of batholiths from different tectonic environments. At the present time, however, studies of this type are hampered by the lack of representative data from “type” areas.