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The interaction between rocks and water during low-grade metamorphism leads to the growth of a predominantly hydrated secondary mineral assemblage and results in the elevation of whole-rock oxygen isotope values. Rb-Sr whole-rock isotope systems are also disturbed during low-grade metamorphism and give reset metamorphic ages. The process of isotope disturbance during low-grade metamorphism is examined by comparing the behavior of the oxygen isotope systems in rocks of similar composition that have undergone differing levels of metamorphic recrystallization.

Two intrusions from the Ordovician volcanic province of North Wales represent metamorphically undisturbed systems. They give low oxygen isotope whole-rock values of δ18OSMOW (standard mean ocean water) = 7.3‰ ± 0.3‰, their Rb-Sr whole-rock systems are undisturbed and they show minimal metamorphic recrystallization. By comparison, other Ordovician igneous rocks have elevated oxygen isotope values enriched between 1‰ and 7‰ over undisturbed rocks and give metamorphic Rb-Sr whole-rock regression ages. Interaction with water is shown to be the main influence on the resetting of Rb-Sr systems during low-grade metamorphism. The metamorphic pressure-temperature conditions are not a controlling factor.

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