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Vertical variations in the effects of hydrothermal metamorphism in Chilean ophiolites: Their implications for ocean floor metamorphism

1979; Elsevier BV; Volume: 55; Issue: 1-2 Linguagem: Inglês

10.1016/0040-1951(79)90341-x

1879-3266

Charles R. Stern, Don Elthon,

High-pressure geophysics and materials

The metamorphic overprint on the pseudostratigraphy of ophiolite complexes in southern Chile exhibits a steep vertical metamorphic gradient passing from zeolite to actinolite facies in 2 km followed by a transition to fresh gabbros. Metamorphic boundaries are irregular and disequilibrium "retrograde" effects are common. These observations are best explained by hydrothermal metamorphism associated with circulation of seawater related to igneous and tectonic activity at a spreading center. Within the gabbros, both the intensity of metamorphic recrystallization and the extent of retrograde effects are low; epidote is sparse while secondary biotite and magnetite are common accessory minerals. In the overlying dikes both the extent of metamorphic recrystallization and the degree of retrograde effects are intense; epidote is abundant while magnetite and secondary biotite are absent. These petrologic and mineralogic differences are consis tent with a greatly decreased circulation of water within the gabbro unit so that reactions here have been buffered at the FMQ buffer of the igneous rock. At higher levels in the ophiolites metamorphic reactions occurred within an oxidizing environment resulting from the circulation of large volumes of water. The pseudostratigraphic variations in the proportions of major metamorphic minerals, even for rocks of the same facies, and the presence or absence of accessory phases such as biotite and orthoclase has resulted in distinct metasomatic effects at different pseudostratigraphic levels within Chilean ophiolites, particularly with respect to K2O and Rb. Where the gabbro unit makes an igneous contact with the overlying dikes, the observed petrochemical changes are abrupt and could produce a sharp seismic layer 2 to layer 3 transition.