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The anorthositic members of the Mealy Mountains Intrusive Suite (MMIS; Labrador, Canada) are host to 0� 5–5 m diameter pegmatitic, pod-like segregations, originally described as graphic granite pods. U–Pb zircon geochronology confirms that the pods are coeval with the 1650–1630 Ma emplacement age range for the MMIS, yielding ages of 1654 6 8 to 1628 6 3� 5 Ma. 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 Fepoor 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 SiO2), 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 anorthosite massifs and all have, at one time or another, been interpreted as the residual liquids of anorthosite crystallization. Our observation of in situ pods with similar compositions to all of the aforementioned lithologies, and displaying textures indicative of late-stage crystallization, supports the notion that all of these associated lithologies can be interpreted as comagmatic with, but variably contaminated and isolated residual

Positions suggested for the Grenville Front in eastern and central Labrador span a 100 km wide belt. The criteria used by previous workers are reviewed and the conclusion drawn that the various attempts to define the Grenville Front as a single line are not in agreement because different field or laboratory parameters have been employed.On the basis of recent mapping, a Grenville Front Zone is proposed. The northern boundary is considered to be the northern limit of widespread brittle deformation. The southern boundary is located along a series of geological contrasts, generally the northern limit of well-banded gneisses. In places, the Grenville Front Zone has been telescoped by thrusts or high-angle reverse faults.

A regional metamorphic gradient from upper greenschist to granulite facies is identified south of the Grenville front in the Double Mer – Lake Melville area of eastern Labrador. Mineral assemblages in politic–granitic gneiss, amphibole-bearing quartzo-feldspathic gneiss, and coronitic metagabbro allow three major metamorphic domains to be established. These are collectively divisible into 11 subdomains. Geothermobarometry applied to the higher grade domains suggests that each is characterized by specific P–T conditions, which achieved 1000–1100 MPa and 700–800 °C in the deepest level rocks.The problem of reconciling geochronological data (which record a major orogenic event at 1650 Ma) with the occurrence of high-grade mineral assemblages in 1426 Ma metagabbro (which suggests a pervasive Grenvillian event) is discussed in terms of three models. The preferred model envisages crustal stabilization at 1650–1600 Ma to give high-grade mineral assemblages seen in the host rocks and with which mineral assemblages in coronitic metagabbro equilibrated after their emplacement at 1426 Ma. During Grenvillian orogenesis (1080–920 Ma) the present structural configuration was achieved by thrust stacking. This imparted a sporadic metamorphic and structural overprint and Grenvillian ages in selected accessory minerals.

On the basis of geological, geophysical, and geochronological data, the Grenville Province has been divided into three first-order longitudinal belts, the Parautochthonous Belt (PB), Allochthonous Polycyclic Belt (APB), and Allochthonous Monocyclic Belt (AMB). These are set apart by three first-order tectonic boundaries, the Grenville Front (GF), Allochthon Boundary Thrust (ABT), and Monocyclic Belt Boundary Zone (MBBZ). The belts are subdivided into terranes based on internal lithological character. The GF separates the Archean to Proterozoic foreland northwest of the orogen from reworked equivalents to the southeast. Continuous at the scale of the orogen, its main characteristic is that of a crustal-scale contraction fault. The PB, although less clearly identified along the length of the orogen, in most places represents upgraded and tectonically reworked rocks of the adjacent foreland. The boundary between the PB and the APB to the southeast, the ABT, is most clearly delineated in the eastern half of the province. It is the locus of major crustal delamination along which high-grade, mostly middle Proterozoic, polycyclic terranes were tectonically transported northwest toward and onto the PB. The AMB comprises two separate areas underlain by the Wakeham Supergroup and what is currently known as the Grenville Supergroup, respectively; its basal contact, the MBBZ, is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks. This first-order zonation implies a tectonic polarity to the Grenville Province, superimposed on which are second-order features evident from contrasting tectonic styles and radiometric ages. These characteristics are consistent with a diachronous or oblique collisional model for the Grenville orogen.

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The Double Mer Formation is a sequence of red-bed deposits, confined to grabens within the Grenville Province in eastern Labrador, Canada. An outline of the regional distribution and structural setting of the Double Mer Formation is presented here, followed by a description of the red-beds and comments on their probable environment of deposition. Finally, the age of the sediments is discussed, using both stratigraphic and structural information.