P‐Tpaths from high temperature shear zones beneath ophiolites

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Abstract
Mctamorphic rocks of the St Anthony Complex of north‐western Newfoundland are best interpreted in terms of a high‐temperature shear zone formed between down‐going continental margin rocks and overriding oceanic lithosphere in a subduction zone. High‐grade rocks, immediately beneath the oceanic lithosphere peridotite, display retrograde meta‐morphism in high‐strain zones, whereas lower grade rocks, near the base of the metamorphic complex, display prograde metamorphism in high‐strain zones. Mylonite zones in meta‐basitcs at all levels in the complex contain the assemblage epidote‐hornblende‐albite‐sodic oligoclase. These observations suggest that the ‘inverted metamorphic gradient’within the St Anthony Complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone. The degree of re‐equilibration is strongly dependent on the degree of strain, and is best achieved in synmetamorphic mylonite zones. This interpretation of the St Anthony Complex can be extended to other sub‐ophiolite metamorphic sheets, which show very similar relationships. It is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones, theP‐Tpaths of which preserve records of burial and exhumation in subduction zones.