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The ultramafic rocks of the Appalachian region in the Eastern Townships of Quebec consist mainly of serpentinized harzburgite and dunite. They are intruded by dikes of dioritic rocks, quartz monzonites (adamellites), pegmatite, and albitite. Near contacts with the dikes the serpentinite with 7-Å serpentines has been transformed to a zone with 14-Å chlorites with higher Al/Si ratio; serpentinite may also be converted into diopside, talc, anthophyllite, and so forth, indicating dehydration and loss of water into the intruding dike. The granitic and dioritic rocks are modified especially at the borders to assemblages consisting of grossularite, diopside, vesuvianite, prehnite, zoisite, and calcite, forming a rodingite.

In general, the water-rich and silica-deficient environment of serpentinized ultramafic rock is responsible for affecting the normal sequence of crystallization of the dikes. This leads to the conversion of pyroxene to hornblende and finally to chlorite and (or) biotite; at the same time the crystallizing plagioclase becomes albitic. The lime and alumina that would normally form clinopyroxene or hornblende and plagioclase remain in the residual hydrous melt to form abundant lime-rich hydrothermal minerals. The lime in these rodingites is, therefore, not derived by serpentinization of the ultramafic wall rocks.

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