| Titre | Multistage deformation and metasomatism of c. 1.9 Ga eclogitic garnet revealed by laser ablation trace-element and electron backscattered diffraction maps |
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| Auteur | Cutts, J A ;
Graziani, R; Pehrsson, S; Regis, D; Petts, D; Knox, B; Smit, M |
| Source | North American workshop in laser ablation, abstracts; 2023, 1 feuille |
| Image |  |
| Année | 2023 |
| Séries alt. | Ressources naturelles Canada, Contribution externe 20230084 |
| Éditeur | Ressources naturelles Canada |
| Éditeur | Commission géologique des Territoires du Nord-Ouest |
| Éditeur | PCIGR |
| Réunion | North American workshop in laser ablation; Notre Dame; US; juin 5-9, 2023 |
| Document | livre |
| Lang. | anglais |
| Media | papier; numérique; en ligne |
| Formats | pdf |
| Province | Manitoba; Saskatchewan; Alberta; Territoires du Nord-Ouest; Nunavut |
| SNRC | 44K; 44L; 44M; 44N; 54I; 54J; 54K; 54L; 54M; 54N; 54O; 54P; 64I; 64J; 64K; 64L; 64M; 64N; 64O; 64P; 74K; 74L; 74M; 74N; 74O; 74P; 84I; 84J; 84O; 84P; 45C; 45D; 45E; 45F; 45L; 45K; 45M; 45N; 55; 65; 75; 85A;
85B; 85G; 85H; 85O; 85P; 85I; 85J; 46C; 46D; 46E; 46F; 46K; 46L; 46M; 46N; 56; 66; 76; 86A; 86B; 86G; 86H; 86I; 86J; 86O; 86P; 47B; 47C; 57A; 57B; 57C; 57D; 67A; 67B; 67C; 67D; 77A; 77B; 77C; 77D; 87A; 87D |
| Lat/Long OENS | -116.0000 -84.0000 70.0000 58.0000 |
| Sujets | déformation; métasomatose; ablation; géochimie; géochronologie |
| Illustrations | cartes de localisation |
| Programme | GEM-GéoNord : La géocartographie de l'énergie et des minéraux Architecture crustal et lithosphérique |
| Diffusé | 2023 06 05 |
| Résumé | (disponible en anglais seulement)
Bulk grain Lu-Hf dating of garnet is a well-established chronometry technique that provides key constraints on the timing of metamorphism. Of particular
importance in assessing the significance of garnet dates, is studying the preserved chemical zoning of its major (Mn, Ca, Fe, Mg) and trace-elements (e.g., Cr, Sc, V, Y, REE). These commonly form zonations that enable garnet growth, or portions
therein, to be linked to specific mineral reactions. Recent advances in laser ablation instrumentation allow for the routine acquisition of fully quantitative high resolution chemical maps, which provide unparalleled insight into the development of
chemical zoning in garnet, and other associated metamorphic minerals, and their influence on the interpretation of geochronology data. An underexplored aspect in interpreting bulk garnet ages is the role of deformation in overprinting chemical
zoning and disturbing chronometer systematics. To investigate the interplay between garnet chronometer systematics, chemical zoning, and deformation we couple bulk grain Lu-Hf garnet geochronology, with major- and trace-element mapping by LA-ICP-MS,
and electron back-scattered diffraction (EBSD) analyses on three highly-strained eclogite samples from the east Athabasca mylonite triangle in Saskatchewan, Canada. The area was affected by intense deformation at granulite to eclogite-facies
conditions, the timing of which has long been debated. The three samples yield broadly similar Lu-Hf garnet ages that are within 10 Myr of each other - precluding in situ analysis using novel MS/MS-based techniques given the typical uncertainty of
the method (>2 % 2RSD). The samples are retrogressed, with garnet showing varying degrees of fracturing, disagregation, and resorbtion, factors that could affect the interpretation of geochronology data and explain the observed variations. Chemical
maps preserve diffuse major-element zoning typical of rocks that experienced high temperatures. In contrast, trace elements Cr and the heavy rare early elements (HREE) show well-defined oscillatory zoning, which is a primary growth feature, and this
corresponds to areas showing a low degree of internal grain deformation as indicated by EBSD. The growth zoning is truncated by fracture networks coincident with a high density of internal grain deformation, and these correspond to areas with low Ca,
V, Ti, Mn, HREE, and locally Cr contents and elevated Zr; suggesting infiltration and/or scavenging of select elements during deformation of the rock. Some grain boundaries are irregular and cuspate, with Mn-Y-HREE-rich rims-features commonly
associated with resorption-while other grain boundaries are straight and oriented parallel to a finer-scaled intra-grain fracture network that show enrichments in large ion lithophile and and LREEs. The combination of observations from high
resolution trace-element maps and EBSD data allow us to establish a timeline for the growth and modification of garnet involving initial growth at eclogite facies conditions, at least two deformation and fracturing events, each associated with
metasomatic overprinting at suprasolidus conditions, and final resorption and re-enrichment of the HREE. While the infiltration and scavenging of trace-elements has minimal effect on the chronometer systematics, the final resorption and re-enrichment
of HREE can explain the variations in ages observed between the samples. |
| Sommaire | (Résumé en langage clair et simple, non publié)
Le grenat est un minéral qui est important pour dater l'âge de métamorphisme. Lorsque le grenat forme, il intègre des éléments qui forment des
schémas de zonage distinctifs, et ceux-ci sont nécessaires pour interpréter son âge. Les éléments présents en petites quantités sont généralement les plus utiles et sont analysés à l'aide d'un laser pour ablater la surface du minéral. Les avances
technologiques récentes dans le domaine des lasers permettent désormais la cartographie des éléments, ce qui offre de nouvelles perspectives sur la formation du grenat. Les cisaillements peuvent provoquer la déformation des minéraux, par former des
faiblesses qui peuvent permettre à la zonation chimique d'être disruptée et altérée par les fluides. Pour explorer comment la déformation affecte les déterminations d'âge du grenat, nous utilisons trois échantillons du nord de la Saskatchewan qui
montrent seulement de petites différences d'âge et nous avons cartographié le degré de déformation et la zonation chimique du grenat. Nos résultats montrent que bien que certains éléments soient extraits du grenat dans des zones déformées et
fracturées, la déformation et l'altération n'affectent pas les interprétations de l'âge. Cependant, de petites zones de grenat avec des enrichissements en éléments traces pourraient expliquer les variations d'âges entre les échantillons. |
| GEOSCAN ID | 331925 |
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