Major-element chemical compositions of the Utuado, Morovis, San Lorenzo, Rio Blanco, Rio Vivi, Torrecilla, and Guyon intrusions in Puerto Rico and the nearby Virgin Islands batholith indicate that there was a decrease in K2O in the intrusive magmas over most of the last 40 m.y. of eastern Greater Antilles island-arc evolution (80 to 40 m.y. B.P.) with a possible slight increase after 40 m.y. B.P. The average rate of change of K2O content was about −0.06% K2O/m.y. This compositional evolution is similar to that observed in the intrusive rocks of the south-western United States. By analogy to interpretations made for that area, the observed compositional changes could have been caused by gradual flattening of the angle of subduction of the Caribbean plate beneath the south side of the eastern Greater Antilles until 40 m.y. B.P. and subsequent initiation of subduction along a new trench to the north.

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