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|Title:||Tectonic Significance of Nd Model Age Mapping in the Grenville Province of Western Quebec, Canada|
|Abstract:||<p>Detailed Nd model age mapping performed in the Grenville Province of western Quebec has identified biotite gneisses with crustal formation ages ranging from 1.4 to 2.8 Ga. These rocks represent different crustal units, which are now either in situ reworked terranes or allochthons.</p> <p>The presence of Archean crust in the Parautochthonous Belt has been known for some time. However, Nd model age data from this work, coupled with geochemical and aeromagnetic data indicate that the Archean Parautochthon extends beyond the proposed location of the Allochthon Boundary Thrust, the proposed southern limit of Archean crust by previous studies. As a result, Archean crust can be traced 120 km and 130 km south of the Grenville Front in the Kipawa-Mattawa and GrandRemous areas, respectively.</p> <p>Biotite gneisses with crustal formation ages of 1.8-1.9 Ga exposed to the south of Archean crust geochemically possess an orogenic affinity. They are thus interpreted to represent the remains of a reworked early Proterozoic arc, which was developed along the southern margin of the Laurentian continent and finally collided with the continent. Botween Archean crust and the arc-related material, an age boundary is defined. The age boundary is tentatively interpreted as a suture due to l=1ck of any evidence of shearing. The identification of plutons with model ages > 2.0 Ga to the north and < 2.0 Ga to the south of the age boundary also implies that the age boundary represents a division on a crustal scale.</p> <p>Metasediments overlying Archean crust in the study area yielded model ages of 2.0-2.4 Ga. The trace and isotope geochemistry indicates that the sediments probably represent mixing deposits in a proposed foreland basin developed during a collision between the proposed early Proterozoic arc and Laurentia. Since 1.74 Ga and 1.68 Ga detrital zircons have been found in the sediments, the foreland basin may have evolved to a late marginal basin formed on the basement of a ca. 1.8 Ga oregen, accompanying a north-dipping subduction that reversed from the previous south-dipping direction.</p> <p>In the central part of the study area, biotite gneisses and plutonic rocks from different allochthonous terranes yielded model ages spanning on a wide spectrum of 1.2-1.9 Ga. The rocks are lithologically and geochemically similar to plutonic rocks of the Coastal Batholith of Peru in the central Andes. Furthermore, in comparison with model age mapping results in the NW Central Gneiss Belt of Ontario, as a whole, the terranes show similar crustal age structure and thus represents crustal slices of a proposed Early-Mid Proterozoic continental margin. These crustal slices were probably thrust over from the south and stacked in the present site during Grenvillian thrusting and form an analogous structural complex to that of the NW Central Gneiss Belt.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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