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

Hinchey, Stephen (2010) 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. Masters thesis, Memorial University of Newfoundland.

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Abstract

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. -- The overall structural pattern within the Barth Concentric Plutonic Suite is roughly concentric, assuming a north-south long axis before fault offset, with all three-dimensionally assessed structures dipping inward except for one dipping outward, an internal intrusive contact within the charnockitic rock-type predominancy south of Nain Bay. -- Olivine forsterite contents generally do not vary significantly within thin-sections of Ol-gabbroic rock. South of Nain Bay, where thin-sectioning is relatively dense and regular, a locally concentric pattern in the distribution of forsterite content in olivine may be interpreted as recording contamination near the charnockitic contact combined with northward proceeding boundary layer fractional crystallisation of olivine, or, as recording two east-west emplacements, the older one having undergone boundary layer fractional crystallisation of olivine proceeding northwards, the younger having intervened between the older and the comagmatic charnockitic neighbor to the south. -- 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. -- Thermal pulses caused by nearby later intrusion have the potential for allowing internal redistribution of Pb and Pb loss in zircon and baddeleyite grains crystallized sufficiently late-stage and therefore having plausibly experienced strain by deformation of their host crystal framework to accommodate crystallisation-contraction. With this possibility in mind, previous U-Pb isotopic results obtained using TIMS for baddeleyite and zircon and present results obtained using LA-ICPMS for zircon have been interpreted as indicating that the emplacement history of the Barth Concentric Plutonic Suite spans at least ~46 My, from ~1337 to ~1292 Ma, representing the middle and late intervals of known Nain Plutonic Supersuite vitality. Four time-clusters of ages are presently recognised: 1) the oldest time cluster, spanning 1337 ± 5 (1-sigma) to 1328 ± 6.4 Ma (1-sigma), representing some rocks of the Fe-rich, Ol-free anorthogabbroic, and Ol-gabbroic rock-type predominancies; 2) the second oldest cluster, spanning 1321 ± 1 (2-sigma) to 1317.2 ± ~2 Ma (2-sigma), representing some rocks of the Fe-rich, charnockitic, and Ol-gabbroic predominancies; 3) the second youngest cluster, spanning 1302.2 ± 4.3 (1-sigma) to 1299 ± 6 Ma (1-sigma), representing some rocks of the Fe-rich and charnockitic predominancies; and 4) the youngest cluster, consisting of one age determination at 1291.8 ± 3.9 Ma (1-sigma), representing a rock of the charnockitic predominancy. The Barth Concentric Plutonic Suite therefore serves as a warning to geologists to abandon simplistic assumptions of synchronity for structurally and compositionally unified bodies of plutonic rock. -- The plagioclase-dominated gabbroic Hosenbein pluton, located 2.5 km southwest of the Barth Concentric Plutonic Suite, is locally Ol-gabbroic as well as intraplutonically brecciated along its central western margin. Both Ol-gabbroic and Ol-free gabbroic rocks occur as breccia blocks and breccia matrix phases. LA-ICPMS U-Pb isotopic analyses of zircon from an Ol-free leucogabbro sample from the central Hosenbein pluton, where for hundreds of metres rock type and texture appear more or less homogeneous to the unaided eye, yield an age of 1338.7 ± 2.8 Ma (1-sigma). Assuming that the sample is of the same age within error as the Ol-free breccia matrices to the west, the breccia block Ol-gabbroic rocks of the western central margin of the Hosenbein pluton are at present the oldest known Ol-gabbroic rocks of the Nain Plutonic Supersuite, being at least as old as ~1339 Ma. -- 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.

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