Reconnaissance geochemistry of the Lassiter Coast Intrusive Suite, southern Antarctic Peninsula

Abstract

Calc-alkaline plutonic rocks of the Lower Cretaceous Lassiter Coast Intrusive Suite are the predominant rocks exposed in the Lassiter, Orville, and southern Black Coasts and eastern Ellsworth Land of West Antarctica. Compositions range from granite to gabbro. K-Ar and Rb-Sr isotopic ages are in the range 130–95 m.y., Early Cretaceous. Magmas were formed in the roots of an Andean-type magmatic arc developed on continental crust during Pacific plate subduction and were emplaced at high crustal levels as one-mica (biotite) granitoids. Our data do not demonstrate transverse asymmetric geochemical variation across the southern Antarctic Peninsula batholith complex in either the southern Black Coast or Lassiter Coast. We have, however, few samples from the eastern half of the batholith in this area. In eastern Ellsworth Land, however, where our samples span the entire width of the batholith, K 2 O index [1,000 K 2 O/(SiO 2 -45)] increases to the south away from the site of the former subduction zone. The batholithic rocks of the southern Antarctic Peninsula are similar in form, regional extent, composite nature, over-all composition, and geochemistry to the plutonic rocks of the northern Antarctic Peninsula, to southeastern Australian I-type granitoids, and to other circum-Pacific batholith terrenes. Geochemical features of the southern Antarctic Peninsula batholith complex that are shared with other circum-Pacific subduction-related magmatic arcs include moderate to low iron enrichment, late K 2 O enrichment, transition from diopside-normative to corundum-normative rocks in the range 63% to 69% SiO 2 , high levels of large-ion lithophile elements (Ba, K, LREE, Rb, Sr, Th), high Ba/Sr, high large ion lithophile element/high field strength element ratios, and low K/Rb ratios. Chondrite-normalized, rare-earth–element trends in the southern Antarctic Peninsula are similar to those of many western North and South American batholith complexes. Light rare-earth elements are only slightly enriched over heavy rare-earth elements in the most mafic samples, increase in abundance (to as much as 160 times chondrite values) and are more strongly enriched in intermediate and felsic rocks, but decline slightly in abundance in the most felsic compositions. Europium anomalies are negligible in the most mafic rocks but become moderately negative in intermediate and felsic samples. Variation in mineral assemblages is controlled by fractionation of clinopyroxene and plagioclase in the most mafic rocks, by hornblende and plagioclase in intermediate and felsic rocks, and by plagioclase and rare-earth-element–rich accessory minerals in the most felsic rocks.