Summary
The origin, whether primary (magmatic) or secondary (Ca-metasomatic) after elpidite, of the Ca-rich zirconosilicates armstrongite and gittinsite in the subsolvus granite of the Strange Lake peralkaline complex (Quebec-Labrador border, Canada) has been a well debated topic. These zirconosilicates, along with zircon and amphibole, account for most of the Zr in the associated Zr-, REE-, Y-, Nb-, and Be-rich ore deposit. In this study, cathodoluminescence (CL) examination and transmission electron microscopy (TEM) have been used to shed light on the relationships among and origin of these zirconosilicates. Optical and CL observations indicate that elpidite undergoes epitaxial replacement by armstrongite and, subsequently, by gittinsite. This gittinsite is generally associated with quartz and variable amounts of hematite. TEM results suggest the bulk replacement of elpidite by poorly crystalline armstrongite, followed by gittinsite and quartz, or zircon and quartz. The Ca-rich compositions of elpidite and its structural similarity with armstrongite imply the existence of a solid solution with armstrongite. Moreover, the enrichment of gittinsite in Mn and Fe (and in some cases Y), and armstrongite and zircon in Y, as determined by analytical electron microscopy (AEM), correlates with the presence of these elements in the Ca-metasomatic fluid. These structural and compositional relationships indicate that the primary, magmatic elpidite is replaced by armstrongite and, subsequently, by gittinsite and quartz during Ca metasomatism of the complex.
Résumé
L'origine, primaire (magmatique) ou secondaire après l'elpidite (metasomatose du Ca), des zirconosilicates riches en calcium, soit l'armstrongite et la gittinsite, dans le complexe granitique sub-solvus de Strange Lake (Québec-Labrador, Canada) a fait l'objet de controverses. Ces zirconosilicates, ainsi que le zircon et l'amphibole, contiennent la majeur partie du zirconium présent dans ce complexe qui comprend un gisement important de Zr, Terres-Rares, Y, Nb et Be. Dans cette étude, un examen des relations texturales entres les zirconosilicates par cathodoluminescence (CL) et par microscope électronique en transmission (MET) a été effectué afin de clarifier leur origine. Les observations en optique naturelle et en CL montrent que l'armstrongite et, subséquemment, la gittinsite, se sont formés par remplacement épitaxique de l'elpidite. La gittinsite est généralement accompagnée par du quartz et par des quantités variables d'hématite. Les résultats de la MET suggèrent un remplacement de l'elpidite par de l'armstrongite pauvrement cristallisé, suivi, soit par de 1a gittinsite et du quartz, soit par du zircon et du quartz. La présence d'elpidite riche en Ca et sa structure cristalline similaire a celle de l'armstrongite indique q'il existe une solution solide depuis l'elpidite vers l'armstrongite. De plus, un enrichissement en Mn et en Fe de la gittinsite (parfois aussi en Y), et en Y de l'armstrongite et du zircon, déterminé par analyse au microscope électronique (AEM), est corrélé avec la présence de ces éléments dans un fluide responsable de la métasomatose du Ca. Les relations de structure et de composition entre l'elpidite primaire, magmatique, et l'armstrongite, la gittinsite et le quartz, indiquent que l'elpidite a été remplacée par ces derniers durant un épisode de métasomatose du Ca.
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Roelofsen, J.N., Veblen, D.R. Relationships among zirconosilicates: examination by cathodoluminescence and transmission electron microscopy .. Mineralogy and Petrology 67, 71–84 (1999). https://doi.org/10.1007/BF01165117
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DOI: https://doi.org/10.1007/BF01165117