Secondary mineral assemblages in Eocene volcanic and volcaniclastic rocks of the Toa Baja drill hole proegressively change as a function of depth. Four metamorphic zones are delineated with increasing depth: Caclinoptilolite→heulandite→laumoniite→prehnite-pumpellyite zone. The deepest belongs to the prehnite-pumpellyite facies, whereas the three zones at shallower depths (>2350 m) belong to the zeolite facies. Ca-clinoptilolite and heulandite exhibit continuous solid solution with varying Si/Al (2.9--52) and Ca/(Ca+Na+K) ratio (0.45--0.89). Pumpellyites are characterized by extensive variation in the Fe/(Fe+Al+Mg) ratio (0.29--0.74) and extreme Fe-enrichment in the laumontite zone. Temperatures estimated from mineral assemblages and facies transitions suggest that the North Coast Basin, Puerto Rico, has been subject to a metamorphic field gradient of ~50 to 70¿C/km. ¿American Geophysical Union 1991