Modification of an oceanic plateau, Aruba, Dutch Caribbean: Implications for the generation of continental crust

Abstract

The generation of the continental crust may be connected to mantle plume activity. However, the nature of this link, and the processes involved, are not well constrained. An obstacle to understanding relationships between plume-related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships, particularly in ancient greenstone belts. Studies of younger analogous regions may help to clarify these relationships. On the island of Aruba in the southern Caribbean, a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed. The mafic lavas show geochemical and isotopic affinities with other basaltic, picritic and komatiitic rocks that crop out elsewhere in the Caribbean—these are well documented as belonging to an 88–91 Ma plume-related oceanic plateau, which is allochthonous with respect to the Americas, and is thought to have been formed in the Pacific region. The ∼85 to ∼82 Ma tonalitic rocks share some geochemical characteristics (high Sr and Ba, low Nb and Y) with Archaean tonalite–trondhjemite–granodiorite (TTG) suites. Field relationships suggest that deformation of the plateau sequence, possibly related to collision with a subduction zone, was synchronous with intrusion of the Aruba batholith. New incremental heating 40Ar/ 39Ar dates, combined with existing palaeontological evidence, show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence. Sr–Nd isotopic data for both rock suites are uniform ( 87Sr/ 86Sr i≈0.7035 , εNd i ≈+7), whereas Pb isotopes are more variable (Plateau sequence: 206Pb/ 204Pb =18.6–19.1 , 207Pb/ 204Pb =15.54–15.60 , 208Pb/ 204Pb =38.3–38.75 ; Aruba batholith: 206Pb/ 204Pb =18.4–18.9 , 207Pb/ 204Pb =15.51–15.56 , 208Pb/ 204Pb =38.0–38.5 ). This suggests that there has been a minor sedimentary input into the source region of the batholith. However, the limited time interval between basaltic and tonalitic magmatism makes a normal subduction-related origin for the tonalites improbable. Instead, models involving derivation of tonalite from partial melting of the plateau sequence, or alternatively, genesis in an unusual subduction zone environment, are investigated.