Geochemistry and Sr-Nd isotopes of amphibolite dykes of northern Victoria Land, Antarctica

Abstract

Mafic dykes cutting the gneisses and migmatites in the Deep Freeze Range high-grade metamorphic complex of northern Victoria Land, Antarctica, have undergone strong recrystallization and deformation during amphibolite-facies metamorphism. Metamorphic mobility mostly affected the large-ion lithophile elements (LILE). Rare Earth (RE) and the high field-strength elements (HFSE) were essentially immobile during metamorphism. Together with the major-element geochemistry, this suggests primary characteristics of evolved tholeiitic magmas and mafic cumulates. No precise ages of intrusion are available for the dykes, but geological evidence suggest emplacement during the time interval 800 to 900 Ma. The Rb-Sr isotopic system in some of the dykes were also variably affected by a later thermal event, probably coincident with the time of amphibolite metamorphism, ca. 500–550 Ma ago. This event can be correlated with the Ross Orogeny in the Transantarctic Mountains. Nd isotopes and trace element abundances indicate that the dykes were derived by different degrees of partial melting and fractionation of heterogeneous sub-continental lithospheric mantle. The Nd isotopic compositions range from depleted to enriched signatures (ϵ Nd computed back to 850 Ma = +4.5 to −11.61), and are coupled to different trace element normalized patterns characterized by a slight positive Nb anomaly in the former case to a strong negative Mb anomaly for the latter samples. On isotopic and chemical ground the depleted signature of the mantle source resembles that reported for E-type MORB. The nature of the enriched components cannot be uniquely stated; nevertheless, on the basis of isotopic and geochemical data, it could be represented by sediments recycled into the sub-continental mantle or by crustal contamination during underplating of mafic magmas, or a combination of the two processes.