Unraveling 800 Ma of pegmatite formation in the western Grenville Front tectonic Zone near Sudbury, Canada

Abstract

The origin of pegmatites has been at the core of petrological research for decades, but despite much effort their origin remains controversial – protracted fractionation of a causative felsic magma versus anatexis of an appropriate protolith. To address this controversy, we present the results of U-Pb analyses (zircon (n = 774), monazite (n = 230), xenotime (n = 295)) for 32 well characterized pegmatites from an extensive swarm of felsic dike rocks outcropping, along a 12 km transect ca. 12 km south of Sudbury (Ontario, Canada) in the westernmost part of the Grenville Front Tectonic Zone (GFTZ). Collectively these rocks exhibit a remarkably large morphological, textural and mineralogical variation related to the crystallization of three distinct pegmatite-forming events that span 800 Ma. The derived concordant U-Pb ages of the dated phases coincide with the late Paleoproterozoic (ca. 1740 Ma) Yavapai orogeny, the Mesoproterozoic (ca. 1450–1430 Ma) Chieflakian event, and the early Neoproterozoic (ca. 1000–970 Ma) Grenville orogeny. Field observations and related geochemical data suggest the two older events are genetically related to regional magmatism (i.e., fractionation of a progenitor), whereas the Neoproterozoic pegmatites were generated via partial melting of an appropriate protolith at depth. The data therefore support a duality of models for pegmatite genesis. We emphasize the important role of imaging (CL and BSE) as an integral part of our protocol, as this revealed widespread coupled dissolution-precipitation (CDP) replacement textures and metamictization in both zircon and xenotime, the latter due to high U contents (i.e., >1000 pm). Interestingly, the Neoproterozoic pegmatites show similar U(±Th), Nb(±Ta) and REE enrichments like their contemporaneous Sveconorwegian counterparts. Many zircons record U-Pb system resetting at ca. 220–200 Ma which is possibly related to the far-field effects of the onset of the Central Atlantic Magmatic Province (CAMP) related to the opening of the Atlantic Ocean. Finally, this study shows that anatexis is a viable process of rare-element pegmatite formation, even in areas rich in potential parental granites, which suggests that anatectic rare-element pegmatites might be globally under-recognized.