Pegmatitic Pods in the Mealy Mountains Intrusive Suite, Canada: Clues to the Origin of the Olivine–Orthopyroxene Dichotomy in Proterozoic Anorthosites

• Published in: Journal of petrology 2015-05, Vol.56 (5), p.845-868
• Main Authors: Bybee, G M, Ashwal, L D, Gower, C F, Hamilton, M A
• Format: Article
• Language: English
• Subjects: Anorthosite; Crystallization; Liquids; Lithology; Massifs; Mountains; Surface layer; Texture
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egv019

The anorthositic members of the Mealy Mountains Intrusive Suite (MMIS; Labrador, Canada) are host to 0.5-5m diameter pegmatitic, pod-like segregations, originally described as graphic granite pods. U-Pb zircon geochronology confirms that the pods are coeval with the 1650-1630Ma emplacement age range for the MMIS, yielding ages of 1654 plus or minus 8 to 1628 plus or minus 3.5Ma. Petrographic and geochemical analysis of five pods from anorthositic rocks of the MMIS reveals that the pods have a diverse compositional range from monzodiorite to granite, varying from Fe-rich and Si-poor, to Fe-poor and Si-rich compositions. Fe-rich, Si-poor pods in the MMIS and other massifs (e.g. Laramie Anorthosite Complex) tend to be hosted by olivine-bearing anorthosites, whereas Si-rich, Fe-poor pods are hosted by pyroxene-bearing anorthosites. Each pod shows a range of graphic, myrmekitic and symplectitic textures, along with distinctive mineral assemblages (e.g. apatite and zircon) and highly enriched trace-element compositions. Evolved mineral assemblages, high concentrations of Fe, Ti and P (and in some cases SiO sub(2)), and 10-1000 chondrite enrichment in light rare earth elements, U, Th and Rb indicate that many of the pods are highly fractionated. The array of textural intergrowths provides clues about the final stages of crystallization in the pods and, by extension, the anorthosites. Macroscopic quartz-K-feldspar graphic intergrowths indicate high-viscosity, fluid-bearing and significantly undercooled magmatic conditions, whereas microscopic myrmekitic (plagioclase-quartz) and symplectitic (plagioclase-orthopyroxene) intergrowths on primary grain boundaries indicate replacement of phases in the presence of reactive fluids. In assessing the nature of these pegmatitic pods based on field, petrographic and geochemical evidence, we conclude that they represent the fluid-bearing, late-stage crystallization products of a residual liquid in the massif anorthosite system. The Fe and Si compositional variations observed in these late-stage pods can be linked to a fundamental olivine-pyroxene dichotomy observed in most Proterozoic anorthosite massifs, suggesting that pulses of magma experience variable contamination (in amount and/or composition) leading to varying differentiation paths. A range of lithologies (monzonites, monzonorites, ferrodiorites and jotunites) observed in similar pod-like structures, as well as dykes and plutons, has been observed in other Proterozoic