Geology and geochronology of the Barth Concentric Plutonic Suite and the central western margin of the Hosenbein pluton, Nain Plutonic Supersuite, Labrador: a fundamentally descriptive study detailing previous and new data, including new observations on the petrographic context of baddeleyite and zircon in olivine-gabbroic rocks of the Barth Concentric Plutonic Suite.

摘要

The Barth Concentric Plutonic Suite of the Mesoproterozoic Nain Plutonic Supersuite, northern Labrador, is an oblong, sinistrally offset body underlying 35 km2 of land occupying western and central Barth Island and the shores of Nain Bay to the northwest and southeast. The Barth Concentric Plutonic Suite is bounded by intrusive contacts and is subdivided into concentrically disposed units bounded by intrusive contacts. With some minor exceptions, each unit is characterised by one of four categories of rock type: 1) mineralogically Fe-rich diorite and gabbroic; 2) Ol-gabbroic (includes troctolite); 3) charnockitic; or 4) Ol-free anorthosite and gabbroic. The sum of all rock volumes, documented and undocumented, belonging to each category of rock type are here called rock-type clans, and the sum of all units each rock-type clan characterises are here called rock-type predominancies. The intrusive contacts between the units of the Barth Concentric Plutonic Suite variously exhibit sharp and straight intrusion, chilling, magma mingling, magma mixing, hybridisation, and interleaving. Most such contacts occur between rock-type predominancies, and therefore generally between disparate rock types, the exception being that some physically opposing rock types are similar in composition due to mixing. Intrusive contacts have also been recognised within several units. Extreme differentiation is not evident in any of the major units.;Zircon and baddeleyite in the Ol-gabbroic rock-type clan have not been observed to contact each other, despite grains of both being abundant and generally co-occurring in the same thin-sections. Approximately 90% of occurrences contact ilmenite with which they commonly show intimate textural relationships, specifically ilmenite-baddeleyite composite grains and zircon rims against ilmenite. Ilmenite is therefore suspected of constituting a thermally divisive, non-intermediate phase between baddeleyite and zircon. Melt compositional differences such that thermally divided phases can co-occur in the same rock may be introduced by progressive occlusion of pore spaces and the differentiation that results as sparsely nucleated liquidus or equilibrating minerals become enclosed in some pore spaces but not in others, thereby producing pore spaces of differing equilibrating bulk composition.;Textures that may be interpreted as recording a thermal pulse from a nearby intrusion during the partially crystalline magmatic state include optically continuous coarser-than-matrix, highly irregular in shape and outline, pyroxene oikocrysts (OCCIPO) in many rocks of the Fe-rich rock-type clan and redissolution textures in some rocks of the Ol-gabbroic rock-type clan. Successive, synmagmatic intrusion is therefore in evidence. Other textures in the Ol-gabbroic rock-type clan may, with less certainty, be interpreted as resulting from textural equilibration in the solid state due to relatively high thermal input at sufficiently high temperatures, namely, transgressive biotite slivers, plagioclase septum texture involving relatively Ti-rich hornblende and ilmenite, and olivine-oxidation symplectite of relatively uniform coarseness and lacking the appearance of fingerprints.;General and theoretical conclusions of the present work include: that the term "pluton" may be used in a non-genetic sense nonetheless consistent with present interpretive usage of "pluton"; that grainsizes may be more reliably and comprehensively described and communicated by providing indications of the proportions of metric size classes; that non-standard, special terms (e.g. anorthositic, troctolitic, ferrodiorite) are not necessary to describe in detail the rocks of the Nain Plutonic Supersuite; that the new concept "plutonic perimetron" could be useful in the future to subdivide plutonic rocks; that the new terms "suboikocrystic", "plastomorphic", and "simple zonation" should be useful in the future for describing plutonic rocks; and, that a descriptive foundational approach is scientifically valuable because it establishes the detailed context within which multiple possible interpretations become apparent. (Abstract shortened by UMI.).