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A high spatial resolution Ar–Ar dating study of compositionally zoned micas using UV laser extraction has been used to investigate the effect of composition and compositional boundaries upon argon diffusion in mica. The crystals are characterised by muscovite cores and Li–F–Rb–Cs-rich lepidolite rims produced by the interaction of a residual melt-vapour phase with the original muscovite during the late stages of pegmatite crystallisation. Single crystals exhibit dramatic intra-grain age variations, with a maximum range of 2,880–2,117 Ma. Backscattered electron images combined with UV-laser analyses reveal that apparent ages younger than 2,600 Ma are directly associated with the compositionally distinct rims, suggesting that Ar diffusion was faster in the lepidolite than in muscovite. Although it is not possible to be fully quantitative using the present data, it seems that argon diffusion rates in lepidolite are similar to those in biotite. Major rearrangements of the crystal structure needed to accommodate high concentrations of lithium, fluorine and large interlayer cations (Rb+, Cs+) in muscovite account for lower argon retentivity of the crystal rims. In most cases the age profiles show no disturbance at the compositional boundary, despite the transition from a di-octahedral to tri-octahedral mica, indicating that in general the boundaries do not act as fast diffusion pathways. However, in some cases there is a clear drop in ages at the compositional boundary, which acts as an effective sub-grain boundary. The difference between the two types may be related to the internal structure of the compositional boundaries or their degree of contact with the grain boundary network of the rock.

Liddicoatite, a rare species of tourmaline, has been found in the McCombe pegmatite, of complex type and spodumene subtype, an example of the LCT (lithium, cesium, tantalum) family. It is one of a group of beryl- and complex-type pegmatites exposed along the Uchi–English River subprovincial boundary-zone, Ontario, in the Superior Province of the Canadian Shield. The mineral assemblage containing zoned "fluor-elbaite"i?½liddicoatite crystals was the last product of primary, near-solidus crystallization of the pegmatite and is unique not only to this subprovincial boundary, but also throughout the vast number of rare-element pegmatites distributed across Ontario. The most calcic composition of liddicoatite analyzed here corresponds to (Ca0.5Na0.40.1) (Li1.5Al1.2Mn0.2Fe0.1) Al6(Si6O18) (BO3)3 (OH3.4F0.6). Other calcium-enriched species found in the McCombe pegmatite include microlite (Ca1.2Na0.60.2)(Ta1.6Nb0.3Sn0.1)O6(F0.7OH0.3), Ca-enriched almandine (Alm44.6Sps26.4Grs26.2), Ca-enriched spessartine (Sps69.3Alm19.8Grs10.9), and fluorapatite. Evidence from progressively evolved units of the pegmatite and mineral zonation shows that contamination of the pegmatite-forming melt occurred, and that fluids from the pegmatite migrated out into the mafic metavolcanic host-rocks, creating a Li-, Cs-, and B-enriched metasomatic aureole. The evolution of the pegmatite-forming melt resulted in increased activity of fluorine, and the essentially in situ fractionation of the contaminated melt led to the crystallization of the rare-element-enriched minerals. Patterns of zonation seen in back-scattered-electron images show that primary crystallization of columbite–tantalite, garnet and tourmaline species involved multiple episodes of growth and dissolution in a dynamic system. Liddicoatite was able to form, not because of any one of these processes, but because they all acted together to concentrate both Li and Ca, a sequence of events that is apparently very rare in nature.

U-Pb isotopic analyses using TIMS and Laser Ablation-Multi Collector-ICP-MS were carried out on columbite-tantalite minerals from three suites of rare-element (Li, Cs, Ta) pegmatites in the Superior Province of Canada. Conventional TIMS analyses of these columbite-tantalite crystals produce scattered data and reverse discordance even after HF leaching of the grains prior to dissolution, possibly reflecting the incomplete removal of the open-system metamict segments during sample preparation. LA-MC-ICP-MS analyses of unleached, primary columbite free from inclusions and alteration give consistent and precise (

Tourmaline in petalite-subtype granitic pegmatites from the Pakeagama Lake and Separation Lake areas of northwestern Ontario provides evidence for the compositional evolution of pegmatite-forming melts through magmatic crystallization and pegmatite melt – host-rock interactions. At the former locality, tourmaline from the pegmatite units is essentially magmatic, whereas at the latter locality, tourmaline in many of the Fe-suite pegmatites displays characteristics (such as elevated Mg, Ti and Ca contents) that indicate significant contamination through the incorporation and digestion of components of amphibolite (mafic metavolcanic units) and banded iron-formation (BIF) host-rocks. The presence of tourmaline in mafic host-rocks indicates that cation migration also involved the introduction of B, Li and Al from pegmatite-generated fluids to form metasomatic haloes. The Li + Mn versus Mg + Ti + Ca diagram is introduced to discriminate tourmaline with a dominantly magmatic signature from tourmaline with a signature influenced by pegmatite – host-rock interaction. The behavior of Ca in tourmaline is also shown to be potentially useful in recognizing late-stage Ca-enrichment in pegmatite-forming melts as a result of Ca–F complexing, but can give similar trends to tourmaline that has suffered Ca-metasomatism as a result of interaction with granitic host-rocks. The tourmaline data suggest that at Separation Rapids, pegmatites mainly crystallized in an open system (i.e., pegmatite – host-rock interaction was common), whereas at Pakeagama Lake, the pegmatite units were predominantly unaffected by interaction with host rocks (i.e., they crystallized in a closed system). This has implications for concentrating rare-elements such as Rb and Cs, which otherwise would be dispersed throughout pegmatite and host rocks (as at Separation Lake). The potential for a buried pollucite deposit associated with the SE pegmatite at Pakeagama Lake has increased as a result of these observations, particularly as Cs is elevated in associated K-feldspar and mica, and is also present in a late pollucite-bearing vein.

The Separation Lake area is host to the most important rare-element pegmatites in Ontario, Canada. They include the Big Whopper and Big Mack petalite pegmatite systems which potentially represent the world's second largest lithium deposit of this type. The pegmatites occur in two distinct clusters adjacent to the Separation Rapids pluton which is thought to be the source of the rare-elements. Beryl-type and complex-, petalite-subtype pegmatites are the most common and a few pegmatites have characteristics similar to the lepidolite-subtype. This study reveals that columbite-tantalite in the pegmatites has an extremely wide range of composition from primitive ferrocolumbite to evolved, almost end-member manganotantalite. Evidence is provided that melt evolution resulted in increased fluorine activity (as seen in microlite compositions) and that in situ fractionation of magma within individual pegmatites often led to the crystallization of rare-element-enriched, Li mica-fluorapatite-cleavelandite pods. Zonation patterns seen in backscattered electron images show primary compositions of columbite-tantalite were modified by secondary processes related to extreme fractionation and involving the late stage development of albitic units in individual pegmatites. This alteration led to recrystallization of columbite-tantalite and produced compositions with lower Ta contents, but with little change in Mn content.

U-Pb isotopic data for zircon, monazite, and titanite are used to assess the evolution of the late Archean English River Subprovince (ERS) and adjacent domains of the Winnipeg River Subprovince in the northern Superior Province, Ontario. The ERS consists of turbiditic metasedimentary rocks, deposited during the final stages of magmatic and tectonic accretion within the greenstone-granite Uchi Subprovince to the north at around 2720 to 2710 Ma. The sedimentary rocks were intruded by a suite of calc-alkalic plutons at 2698 Ma. Major regional deformation, amphibolite to granulite facies metamorphism, anatexis, and emplacement of an extensive peraluminous granitic suite culminated at 2691 Ma. Late episodes of metamorphism, metasomatism, and emplacement of pegmatites occurred locally at circa 2680 and 2669 Ma. In contrast, the Winnipeg River Subprovince to the south consists of widespread tonalitic suites formed mainly between 3200 and 2800 Ma and granodioritic to granitic intrusions formed between 2710 and 2690 Ma. Metamorphism occurred at around 2710 Ma in southern domains, and a distinct granulite facies event at around 2680 Ma affected central and northern domains of the Winnipeg River Subprovince. The rapid sequence of events recorded in the ERS supports the notion that metamorphism was caused by thermal perturbations related to injection of granitic magmas. These melts were generated through partial melting of the lower crust by a thermal anomaly presumably caused by mantle upwelling related to collapse of the inactivated subducting plate. The short-lived nature of the metamorphic events and their cyclicity follow the behaviour predicted by recent models linking the evolution of strain rates to episodes of segregation and upward magma transfer in the course of prolonged orogenic events.