FURTHER FORAYS INTO “FTP ROCKS” ASSOCIATED WITH MASSIF ANORTHOSITES OF THE CRUML BELT, GRENVILLE PROVINCE, QUEBEC: MULTIPLE ORIGINS FOR JOTUNITES, OAGNS, NELSONITES, AND MASSIVE OXIDE DEPOSITS

A wide variety of rocks rich in iron, titanium, and phosphorus --- so-called FTP-rocks --- have been recognized in association with the Mattawa and Lac Piché anorthosite massifs, Grenville Province, Quebec. At Mattawa, OAGNs (oxide-apatite gabbronorite) occur along the northern, southern, and western margins of the massif, and each occurrence has a distinct texture, mineralogy, and chemical composition (e.g., different values of An-Or, mesoperthite vs. separate plag and Kf, opx/cpx proportions). Xenolithic blocks of anorthosite contained therein together with diking relationships demonstrate the relatively younger age of the OAGNs. Also found within outcrops of the northern OAGN are gradational layers of nelsonite (in this case, ilmenite+magnetite-apatite rock containing minor silicates). Intrusive sheets of magnetite-ilmenite rock as well as separate bodies of massive hemoilmenite rock occur nearby. Jotunites (pyroxene-bearing monzodiorite) form narrow dikes within the massif, are interlayered with OAGNs in the south, and form discordant, intrusive sheets at the western boundary of the anorthosite. At Lac Piché, OAGNs (with ilmenite+magnetite) form intrusive sheets within anorthosite but are most common as mappable horizons (segregation layers and lenses) in mangerite (=pyroxene-bearing granodiorite) found a few km from the anorthosite. Jotunites (with ilmenite+magnetite) form a more-or-less continuous rim around the anorthosite massif. At one locality, massive hemoilmenite rock and nelsonite (in this case hemoilmenite+apatite rock, lacking silicate) occur with jotunite. These finding reveal the diversity and complexity of FTP rocks, and it seems unimaginable that they could all form on the liquid line-of-descent of a common parental magma that also produced anorthosite. For example, compositional variations within the northern Mattawa OAGNs are controlled by mixing-unmixing of ~An₄₀ plagioclase, whereas the anorthosite massif consists mainly of ~An₃₀ plagioclase. Important differences in FeTi-oxide assemblage (hemoilmenite vs. ilmenite + magnetite), feldspar compositions among different OAGNs and jotunite, the proportions of silicate material in nelsonite, suggest that these rocks are polygenetic, adding further complications to the already thorny “anorthosite problem.”