A multidisciplinary investigation of the formation, growth and evolution of Neoarchean crust, Snare River terrane: southwestern Slave Province

Bennett, Venessa Rose Charlene (2006) A multidisciplinary investigation of the formation, growth and evolution of Neoarchean crust, Snare River terrane: southwestern Slave Province. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The hypothesis that Neoarchean crust (~ 3.0 - 2.5 Ga) is the expression of a global thermotectonic event that led to cratonization of crustal nuclei and the transition to a 'modern' style of plate tectonic processes is tested through a multidisciplinary study of the Snare River terrane, a ca. 2.67-2.56 Ga crustal fragment in the southwestern Slave Province. Integration of datasets defining the temporal, geochemical, architectural and thermal evolution reveals crust formation was initiated in an oceanic tectonic regime (TR1) and terminated with collision with an older Mesoarchean crustal fragment (TR2). -- U-Pb zircon crystallization ages for thirteen intrusive units provide tight constraints on the timing of crust formation and orogenic evolution. Seven metaluminous plutons were emplaced over ca. 90 M.y. from ca. 2674-2589 Ma, whereas six peraluminous plutons were emplaced over ca. 15 M.y. from ca. 2598-2585 Ma. Collectively they define four stages of crustal development: (i) 2674-2635 Ma — proto-arc formation and early turbidite sedimentation; (ii) 2635-2608 Ma —juxtaposition of turbidites and proto-arc; (iii) 2608-2597 Ma — collision of proto-arc with Central Slave Basement Complex, crustal thickening, metamorphism and plutonism; and (iv) 2597-2586 Ma — orogenic collapse and mid-crustal exhumation. -- Deformation histories of low- and high-grade domains illustrate younging of structural age with paleodepth and a progression from steep structures in the upper crust to gently-dipping structures in the mid crust due to diachronous polyphase deformation. A migmatitic transition zone is characterized by structural elements from both crustal levels. The proposed tectonic model involves: (i) protracted thin-skinned shortening (DI); (ii) crustal thickening by nappe tectonics and orogenic plutonism (D2); and (iii) post-orogenic collapse and mid-crustal exhumation (D3). -- Geochemical and Nd isotopic data for magmatic units support initiation of crust formation by plume-related mafic magmatism, evolution to a protoarc by subcretion, and establishment of subduction magmatism. Protoarc-continent collision heralded emplacement of sanukitoid, metaluminous and peraluminous suites, whereas only sanukitoid and peraluminous magmatism accompanied orogenic collapse. Mantle-melting events demarcate the four stages of crustal evolution, implying linkage between crustal and mantle tectonics, and a time-space analysis demonstrates that the oldest magmas occur in the upper crust and younger syn-orogenic magmas intruded at mid-crustal levels. -- Ages of inherited zircon cores are used to isotopically profile Neoarchean crustal evolution. The datasets illustrate that metaluminous and peraluminous magmas were derived from discrete lower- and mid-crustal reservoirs at different times. Secular changes in inherited zircon in metaluminous magmas fingerprint lower crustal growth from both 'local' magmatic additions and intercalated 'exotic' material, and the absence of Mesoarchean inherited cores in metaluminous magmas indicates crust formation was distal from the Central Slave Basement Complex. With respect to peraluminous magmas, the similarity between their inheritance profiles and detrital zircon spectra confirm metaturbidites as the source for peraluminous magmatism. -- Zircon saturation thermometry is used to constrain the thermal contributions of magmas to the crustal heat budget, after refinement of the raw data using whole-rock geochemistry, BSE imaging and petrographic analysis. Results, confirmed by independent metamorphic data, reveal moderate-temperature magmatism during crustal growth, both low-temperature H₂O-rich and high-temperature H₂O-poor magmatism during the orogenic phase, and low-temperature magmatism in the post-orogenic stage. Low-temperature plutons emplaced into the upper crust did not significantly affect the existing thermal regime, whereas high-temperature plutons in the mid crust provided the principal heat source for metamorphism. -- Recognition of mid- and upper-crustal levels separated by a transition zone has permitted reconstruction of a 'crustal stratigraphy' that exhibits: (i) depth-dependent magmatic growth, (ii) compositional zonation; (iii) secular magmatic evolution; (iv) metamorphic zonation, and (v) diachronous polyphase deformation. Collectively, these results demonstrate intimate feedback between crustal levels and the interplay among magmatism, sedimentation, metamorphism and deformation in crustal evolution. Neoarchean crustal growth in the Snare Rive terrane was characterized by a prolonged period of lateral accretion in an oceanic realm (TR1), followed by rapid crustal thickening during accretion of the oceanic terranes to the Central Slave Basement Complex (TR2). Thus a primary control on the Neoarchean crustal evolution was the presence of a Mesoarchean nucleus around which cratonization occurred.

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