Rb-Sr isotopic ages from Late Palaeozoic metamorphic rocks of central Chile.

Two metamorphic complexes of different lithology and structure occur in the coast ranges of central Chile. The Tanume metamorphic complex to the east belongs to a low P-T series of sillimanite-cordierite gneisses derived from a continental platform clastic sedimentary sequence and gives a whole-rock isochron age of 347 + or - 32 m.y. whereas the accreted Pichilemu complex to the west consists of stilpnomelane-bearing cherts, crossite-bearing metabasic rocks and siliceous metapelites, with an age of 311 + or - 10 m.y. Different provenance of the protoliths is indicated by the higher ⁸⁷Sr/ ⁸⁶Sr initial ratios of the Tanume rocks (0.7124 + or - 0.0007) relative to the Pichilemu rocks (0.7060 + or - 0.0005), which were more affected than the former by the main phase of deformation and metamorphism in the area.-R.A.H.