The Saint-Urbain and Big Island rutile-bearing ilmenite Fe-Ti oxide deposits are located
in the composite 450 km² Saint-Urbain anorthosite (1055-1046 Ma, U-Pb zircon) and in
the Lac Allard intrusion (1057-1062 Ma, U-Pb zircon) of the 11,000 km² Havre-Saint
Pierre anorthosite suite, respectively, in the Grenville Province of Eastern Canada. Slow
cooling rates of 3-4°C/m.y. are estimated for both anorthosites, based on combined U-Pb
zircon/rutile/apatite and ⁴⁰Ar/³⁹ Ar biotite/plagioclase geochronology, and resulted from
emplacement during the active Ottawan Orogeny. Slow cooling facilitated (1) diffusion
of Zr from ilmenite and rutile, producing thin (10-100 microns) zircon rims on these
minerals, and (2) formation of sapphirine via sub-so lidus reactions of the type: spinel +
orthopyroxene + rutile ± corundum → sapphirine + ilmenite. New chemical and
analytical methods were developed to determine the trace element concentrations and Hf
isotopic compositions of Ti-based oxides. Rutile is a magmatic phase in the deposits
with minimum crystallization temperatures of 781°C to 1016°C, calculated by Zr-in
rutile thermometry. Ilmenite present in rutile-free samples has higher Xhem (hematite
proportion in ilmenite), higher high field strength element concentrations (Xhem = 30-17;
Nb = 16.1-30.5 ppm; Ta 1.28-1.70 ppm), and crystallized at higher temperatures than
ilmenite with more fractionated compositions (Xhem = 21-11; Nb = 1.36-3.11 ppm; Ta =
<0.18 ppm) from rutile-bearing rocks. The oxide deposits formed by density segregation
and accumulation at the bottom of magma reservoirs, in conditions closed to oxygen,
from magmas enriched in Fe and Ti. The initial ¹⁷⁶Hf/¹⁷⁷ Hf of rutile and ilmenite (Saint
Urbain [SU] = 0.28219-0.28227, Big Island [BI] = 0.28218-0.28222), and the initial Pb
isotopic ratios (e.g.²⁰⁶Pb/²⁰⁴ Pb: SU = 17.134-17.164, BI = 17.012-17.036) and ⁸⁷Sr/⁸⁶ Sr
(SU = 0.70399-0.70532, BI = 0.70412-0.70427) of plagioclase from the deposits overlap
with the initial isotopic ratios of ilmenite and plagioclase from each host anorthosite,
which indicates that they have common parent magmas and sources. The parent magmas
were derived from a relatively depleted mantle reservoir that appears to be the primary
source of all Grenvillian anorthosite massifs and existed for --600 m.y. along the margin
of Laurentia during the Proterozoic.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/2842 |
| Date | 11 1900 |
| Creators | Morisset, Caroline-Emmanuelle |
| Publisher | University of British Columbia |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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