Geochemical constraints on the origin of mafic and ultrapotassic dykes from the southern Manicouagan area, Grenville province

Valverde Cardenas, Carolina (2009) Geochemical constraints on the origin of mafic and ultrapotassic dykes from the southern Manicouagan area, Grenville province. Masters thesis, Memorial University of Newfoundland.

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The Grenville Province has a long record of magmatic activity, including arc magmatism related to the pre-Grenvillian evolution of eastern Laurentia and magmatism during the final collision that shaped the orogen. This study focuses on recently identified mafic dykes exposed in the Gagnon Terrane and the Canyon Domain as well as late tectonic, ultrapotassic dykes exposed in the Canyon Domain, in the central part of the Province. -- In the Gagnon Terrane, mafic dykes inferred to be ~1.74 Ga old, intrude Archean orthogneisses east of a major shear zone. These dykes are foliated and recrystallized under upper amphibolite to granulite facies conditions. These rocks have a geochemical signature of subalkaline tholeiitic basalts to andesites and, some, show primitive Mg#, Cr and Ni contents. Epsilon Nd values (at 1.75 Ga) range from +1.1 to +2.1, to highly negative (-17.9). The geochemical data and presumed age suggest that the dykes represent mantle-derived melts variably contaminated by crust. The ~1.74 Ga magmatic event in Gagnon Terrane is coeval with late orogenic magmatism in the Makkovik Province and provides a hint for potentially widespread 1.7 Ga magmatism into the Laurentian margin. -- In the Canyon Domain, mafic dykes intrude a ~1.4 Ga volcanoclastic sequence, inferred to represent remnants of an island arc, in the vicinity of a ~1.2 Ga bimodal sequence inferred to have formed in a back arc setting. These dykes are variably deformed and recrystallized under medium-P granulite facies conditions. These dykes have a geochemical signature of subalkaline tholeiitic basalts to andesites, similar REE patterns and εNd¹³⁰⁰ values from +4.6 to +6.2. However, two groups have been recognized based on the presence (Group 2) or absence (Group 1) of pronounced negative Nb anomalies. The dykes of Group 2 have an arc geochemical signature and may have been emplaced during late stages of the development of the ~1.4 Ga arc. In contrast, the dykes of Group 1 have a within-plate geochemical signature and may be correlative with the ~1.2 Ga (Elzevirian) bimodal meta-volcanic sequence exposed east of the Canyon Domain. It is therefore suggested that the ~1.2 Ga volcanic sequence developed in proximity to the Canyon Domain and that their juxtaposition is not the result of Grenvillian age tectonic transport. -- A swarm of ~0.98 Ga late tectonic, ultrapotassic dykes occur in the central and southern Canyon Domain. These dykes crosscut the foliation, but have an internal fabric in the north. Farther south they are massive and display rims of granitic pegmatite. The ultrapotassic dykes consist of biotite-quartz-apatite ± K-feldspar ± plagioclase ± clinopyroxene ± orthopyroxene ± amphibole with highly variable proportions. Those lacking internal fabric preserve an igneous texture with large biotite and apatite in interstitial pools dominated by K-feldspar and quartz. -- Some ultrapotassic dykes are characterized by primitive Mg#, Cr and Ni contents. They have extremely steep rare earth patterns and high concentrations of Ba (2000-9000 ppm) and Rb (65-260 ppm). However, concentrations of Nb and Ta are depleted relative to the Ba, Rb and La values on primitive mantle normalized plots. Th contents are variable, showing both enrichment and depletion relative to La. The dykes are characterized by low εNd (-2.6 to -14.6) and εSr, (6.4 to 20.86) isotopic signatures (at T=1 Ga). Based on the geochemical data, the most likely source region would be a metasomatized subcontinental lithospheric mantle. Ultrapotassic dykes emplaced within the age range of 990-970 Ma are also known in the ~1010 Ma Labrieville anorthosite to the south, and the Pinware and Lac Melville Terranes farther east. It is therefore suggested that a late orogenic (Grenvillian) magmatic event, involving a metasomatized subcontinental lithospheric mantle source, took place in the hinterland of the central eastern Grenville Province.

Item Type: Thesis (Masters)
Item ID: 9152
Additional Information: Includes bibliographical references (leaves R.1-R.13)
Department(s): Science, Faculty of > Earth Sciences
Date: 2009
Date Type: Submission
Geographic Location: Canada--Quebec (Province)--Grenville
Library of Congress Subject Heading: Analytical geochemistry--Quebec (Province)--Grenville; Dikes (Geology)--Quebec (Province)--Grenville; Ultrabasic rocks--Quebec (Province)--Grenville

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