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

The generation of the continental crust may be connected to mantle plume ac tivity. However, the nature of this link, and the processes involved, are n ot well constrained. An obstacle to understanding relationships between plu me-related mafic material and associated silicic rocks is that later tecton ic 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 Car ibbean, a sequence of partly deformed mafic volcanic rocks intruded by a pr edominantly tonalitic batholith is exposed. The mafic lavas show geochemica l and isotopic affinities with other basaltic, picritic and komatiitic rock s that crop out elsewhere in the Caribbean-these are well documented as bel onging 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 Pa cific region. The similar to 85 to similar to 82 Ma tonalitic rocks share s ome geochemical characteristics (high Sr and Ba, low Nb and Y) with Archaea n tonalite-trondhjemite-granodiorite (TTG) suites. Field relationships sugg est that deformation of the plateau sequence, possibly related to collision with a subduction zone, was synchronous with intrusion of the Aruba bathol ith. New incremental heating Ar-40/Ar-39 dates, combined with existing pala eontological 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 (Sr-87/Sr-86(i) approximate to 0.7035, epsilon Nd- i approximate to +7), whereas Pb isotopes are more variable (Plateau sequen ce: Pb-206/Pb-204 = 18.6-19.1, Pb-207/Pb-204 = 15.54-15.60, Pb-208/(204)b = 38.3-38.75; Aruba batholith: Pb-206/Pb-204 = 18.4-18.9, Pb-207/ Pb-204 = 1 5.51-15.56, Pb-208/Pb-204 = 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, mod els involving derivation of tonalite from partial melting of the plateau se quence, or alternatively, genesis in an unusual subduction zone environment , are investigated. (C) 1999 Elsevier Science B.V. All rights reserved.