COMPOSITION AND COLOR OF BIOTITE FROM GRANITES - 2 USEFUL PROPERTIES IN THE CHARACTERIZATION OF PLUTONIC SUITES FROM THE HEPBURN INTERNAL ZONE OF WOPMAY OROGEN, NORTHWEST-TERRITORIES

A combined compositional and optical spectrophotometric study of 24 bi otite specimens from the granitic rocks of the Hepburn and Bishop intr usive suites of the early Proterozoic Wopmay orogen, Northwest Territo ries, shows that the chemical composition and the color of this minera l strongly reflect the tectonic origin of its host. In the continental -collision-related Hepburn suite, biotite is enriched in total Al and Fe and is Fe3+-poor, consistent with anatexis or assimilation of reduc ed metasedimentary material. In the Bishop suite, formed within a cont inental arc, biotite is Al-poor, Mg- and Fe3+-rich, indicating relativ ely more oxidizing - conditions and a less important metasedimentary c ontribution. The biotite quadrilateral (annite - siderophyllite - phlo gopite eastonite diagram) effectively portrays the compositional trend s of micas from continental-collision- and arc-related granites. The b right red color of biotite from peraluminous collisional granitic plut ons reflects a high total Fe content with low Fe3+/(Fe2++Fe3+). and pr obably also the presence of Ti3+. Green or brown biotite from arc-rela ted granites is Mg- and Fe3+-rich.