Abstract
Secondary mineral assemblages in Eocene volcanic and volcaniclastic rocks of the Toa Baja drill hole progressively change as a function of depth. Four metamorphic zones are delineated with increasing depth: Caclinoptilolite→heulandite→laumontite→prehnite–purnpellyite 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 (2.9–5.2) and Ca/(Ca+Na+K) ratios (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 fades transitions suggest that the North Coast Basin, Puerto Rico, has been subject to a metamorphic field gradient of ∼50 to 70°C/km.
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