40AR/39AR AGES FOR INTRUSIVE ROCKS OF ST. CROIX

The island of St. Croix contains exposures of lithologies in a normal fault block that constitutes a southeastern portion of the Late Cretaceous arc of the Greater Antilles. Cretaceous sedimentary rocks of St. Croix are generally turbidites, and these are intruded by gabbroic to dioritic dikes, sills, and diatremes, that are in turn unconformably overlain by Miocene limestones and younger cover. The intrusive rocks have geochemical signatures typical of island arc volcanics (see Lidiak and Jolly, 1998). K/Ar dates for hornblende separates from sills that contain a cleavage formed by a regional deformation event define mean ages of ca. 66-71 Ma, with one older date of 75 Ma for an igneous clast in a boulder conglomerate stratum (Speed et al., 1979; Lidz, 1988), whereas younger plutons (that post-date the cleavage evident in the sills) give K/Ar ages of ca. 57-75 Ma (Smith et al., 1998). In order to test and refine the geochronologic constraints for the earlier intrusives, hornblende and plagioclase separates were prepared for six samples of coarsely grained diabase sills and analysis in the Auburn Noble Isotope Mass Analysis Laboratory (ANIMAL). ⁴⁰Ar/³⁹Ar fusion analyses of single hornblende phenocrysts with a CO₂ laser (with ca. 20 single crystal ages per sample) define dominant age modes of ca. 76-80 Ma, with some ages for single crystals as young as 58 Ma. The coarse size of the hornblende crystals (up to ca. 0.75 mm in diameter) permits incremental heating analysis of single hornblende crystals. The single crystal incremental heating analyses for hornblende typically result in young ages for initial heating steps that increase to plateau ages of ca. 78-80 Ma. We interpret these Campanian plateau ages of hornblende to represent the crystallization ages of the sills. The age spectra for the single crystal analyses are typical of ⁴⁰Ar* loss during an overprinting event, and are consistent with the overprinting deformation evident in field relationships and younger K/Ar ages for bulk samples of hornblende previously reported. The new 78-80 Ma ages of the present study are substantially older than the Paleocene ages of hornblende obtained for andesites of the Guaracanal Formation in Puerto Rico (Story et al., 2012; this GSA meeting), and appear to represent a Campanian stage of magmatism for the volcanic arc of the Greater Antilles.