Geochemistry and petrogenesis of the Tibbit Hill metavolcanic suite of the Appalachian fold belt, Quebec-Vermont; a plume-related and fractionated assemblage

Abstract

The Late NeoProterozoic metavolcanic Tibbit Hill Formation (THF) covers a large crustal segment exposed along a belt about 250 km long in the Appalachian fold belt (Quebec-Vermont). It is predominantly basaltic in composition but contains a minor component of felsic and intermediate alkaline volcanic lithologies. Geochemically, the Tibbit Hill Formation forms a continuum in composition and exhibits a wide range of SiO2 (44-76 wt. percent), covering the entire spectrum from alkali basalt to trachyte and comendite. This mildly alkaline suite is relatively enriched in incompatible elements and exhibits a wide range of Zr (138-1493 ppm), Nb (15-139 ppm), and Y (18-185 ppm) concentrations, among other elements. The concentrations of the HFSE and the REE gradually increase toward the more evolved lithologies. The chondrite normalized REE patterns are fractionated (LREE-enriched over HREE), parallel to subparallel, and generally uniform but with negative Eu-anomalies developed in the more felsic varieties. These geochemical features underline the comagmatic nature of the entire suite and are consistent with a fractionated basalt to comendite suites. The incompat- ible element profiles suggest that most of these elements including Nb, Zr, Ti, Y, and the REE have not been affected by metamorphism, as they remained largely intact within the THF rocks. Chemical features of the mafic rocks are typical of within-plate basalts and suggest that their melts were derived from a fertile or plume-related mantle source. Chemical features of the felsic and intermediate rocks are typical of anorogenic A1 type suites, related to hotspots, mantle plumes, or continental rift zones. This is consistent with the regional geological context with the volcanism, associated with an Iapetan RRR triple junction, occurring shortly before the onset of seafloor spreading. Geochemical modelling shows that the THF basaltic magma was produced by a very small degree of batch partial melting (F 2.5 percent) of a garnet- bearing primitive mantle source (garnet lherzolite). A final basaltic melt segrega- tion depth is estimated at 80 to 100 km. Melting probably occurred within the thermal anomaly of a rising mantle plume beneath the Sutton Mountains triple junction (near the Quebec-Maine border). Fractionation of THF basaltic magma produced minor trachytic and comenditic magmas. The volcanic assemblage of the Afar rift (for example, Boina centre) appears to represent a modern analogue to the THF volcanic suite.