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All the Mesozoic and Cenozoic volcanic rocks of the Central Andes (from southern Ecuador to central Chile), except Recent ones, have been affected by episodes of regional metamorphism, without change in textwe and structure. The metamorphism, which ranges from low zeolite to greenschist facies. can be classified as burial metamorphism because there is an overall increase in metamorphic grade with stratigraphic depth in the individual volcanic sequences separated by regional unconformities. Some sequences display metamorphic patterns transitional to ocean-floor and to geothermal field types, refleding variations along and across the Andes in tectonic setting and thermal gradients.Volcanism was closely followed by metamorphism during each cycle characterizing the geological history of the Central Andes. The episodic nature of the metamorphism has led to breaks in metamorphic grade at regional unconformities and repetition of facies series, where strata of higher grade may even overlie those of lower grade. The existence of permeability-controlled distribution patterns of secondary minerals within individual flows shows that gradients of chemical activity, rate of reaction and Pwd were acting, in addition to temperature and P, overall gradients, during the regional metamorphism. The alteration is accompanied by chemical changes and disturbances of the K-Ar and Rb-Sr isotope systems. Similarities between Mesozoic facies series in the western and eastern flanks of the Andes are consistent with a mechanism of ensialic spreading-subsidence.

Summary A low-grade, non-deformational, regional metamorphism ranging from burial to ocean-floor type characterizes the Andean range. Longitudinal variations exist, coinciding with changes in the chemistry of the volcanic suites involved, both trends being controlled by the geotectonic setting. The study of Cretaceous volcanic rocks in Colombia, Peru and Chile indicates that: (i) ocean-floor metamorphism (high- T gradients) correlates with tholeiitic volcanism in marginal basins where the continental crust was eliminated or extremely attenuated; (ii) burial metamorphism (low- to moderate- T gradients) correlates with calc-alkaline to shoshonitic volcanism in ensialic, aborted, marginal basins with moderate thinning of the continental crust. The metamorphism in the Andes, which is characterized by a counter-clockwise P-T-t path, started during the initial stages of basin formation and preceded the orogenic activity, contrary to the development in compressional geotectonic settings.