Bimodal magmatism in northeast Palmer Land, Antarctic Peninsula: Geochemical evidence for a Jurassic ensialic back-arc basin
Major- and trace-element (XRF and INAA) and Nd-isotope analyses are presented on a Jurassic bimodal association of basic greenstones and silicic metavolcanic rocks from the Black Coast, northeast Palmer Land, Antarctic Peninsula. The greenstones are divided into three sub-groups, indistinct in the field, but which have geochemical characteristics of island arc tholeiites (group I), E-type MORB (group II), and continental arc basalts (group III). The tholeiites of group I and II have a similar range of ϵNd values (+3.7 to −1.2) and were produced from a heterogeneous, large ion lithophile element (LILE)-enriched, mantle source. The rocks of group III show a much stronger enrichment in LILE than groups I and II, and were derived from a mantle source with slightly lower ϵNd values (−2.3 to −5.0). The silicic volcanic rocks have low ϵNd values (−7.1 to −8.7) indicating a dominant crustal source, although trace element concentrations show a within-plate affinity. The origin and tectonic setting of this bimodal suite is discussed in relation to Mesozoic subduction along the proto-Pacific margin of the Antarctic Peninsula and southern South America and intra-continental extension associated with the break-up of Gondwana. It is concluded that the mafic greenstones and silicic metavolcanic rocks formed in an ensialic back-arc basin setting where, during a period of continental lithospheric attenuation, the rise of a mantle diapir may have caused widespread bimodal magmatism. The geographical extent of the Palmer Land basin is unknown at present, but it may be part of a much larger Weddell Sea or proto-Weddell Sea back-arc basin system.