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Abstract:The geology and geochemistry of the Canton Saint-Onge wollastonite deposit indicate that it is a skarn formed by silica metasomatism of dolomite-rich rocks. Oxygen isotopic compositions of the skarn rocks and the nearby plutons show that the fluids responsible for the metasomatism were not meteoric, but were probably associated with the Du Bras granitic pluton. However, the granite is not in contact with the wollastonite rocks at the present level of exposure, hence the fluids must have been released at greater depths. Wollastonite-rich parts of the skarn rocks are isotopically lighter than diopside-rich rocks, suggesting that wollastonite formed in regions of higher fluid flux.Résumé : La géologie et la géochimie du dépôt de wollastonite du canton Saint-Onge indiquent qu'il s'agit d'un skarn formé par le métasomatisme de la silice de roches riches en dolomie. Les compositions isotopiques de l'oxygène des roches skarnifiées et des plutons avoisinants montrent que les fluides responsables du métasomatisme n'étaient pas d'origine météorique mais qu'ils étaient plutôt associés au pluton granitique de Du Bras. Toutefois, le granite n'est pas en contact avec les roches à wollastonite au présent niveau d'affleurement; les fluides auraient donc été relâchés à de plus grandes profondeurs. Les parties riches en wollastonite des roches du skarn sont isotopiquement plus légères que les roches à diopside, indiquant que la wollastonite s'est formée dans des régions à très grand écoulement.[Traduit par la Rédaction] 1140Higgins et al.

Volcanic basement recovered at Hole 765D is characterized by nonpervasive, oxidative alteration, typical of seafloor weathering. Chilled margins and the mesostasis of the lavas are variably altered to assemblages of celadonite, Fe-oxyhydroxides, zeolites, and calcite with trace saponite. Plagioclase is partially altered to Ca-Na zeolites and/or albite. Well-developed alteration halos parallel fracture surfaces and extend several centimeters into the surrounding rock. These clay-rich halos are enriched in K2O and Fe2θ3 relative to the adjacent clay-poor rock. The halos and adjacent rock are characterized by δ 18 θ values 2% 0 -3‰ higher than those of fresh MORB. The "freshness" of the samples and the scarcity of saponite suggest that the duration of seawater circulation was short-lived. Albitization of plagioclase indicates that the volcanic rocks were altered initially at low temperatures and were subsequently reheated off-axis in a closed environment. Reheating did not result in significant modification of the bulk composition of the crust.

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