Petrogenesis of Cretaceous mafic intrusive rocks, Fosdick Mountains, West Antarctica: Melting of the sub-continental arc mantle along the Gondwana margin

Abstract

A diorite pluton and widely distributed mafic dykes occur in the Fosdick migmatite–granite complex, which is interpreted to represent middle-to-lower crustal rocks of the paleo-Pacific active continental margin of Gondwana. The mafic dykes exhibit a variety of relationships with host rocks in the field ranging from undeformed dykes with sharp contacts with host gneisses to dismembered dykes with commingled textures and numerous back-veins of leucosome intruded from host migmatitic gneisses suggestive of significant interaction with crustal rocks. New U–Pb ages for magmatic zircon in these rocks yield Cretaceous crystallization ages ranging from ca 113Ma to ca 98Ma for the mafic dykes and ca 100Ma for the diorite pluton. These mafic intrusive rocks, which contain abundant hydrous minerals, are medium- to high-K-series calc-alkaline rocks with basic–intermediate compositions (47–59wt.% SiO2 for the mafic dykes and 52–56wt.% SiO2 for the diorite pluton). They have trace element patterns characterized by LILE enrichments and negative Nb anomalies indicating an origin from a hydrous mantle source metasomatized by slab-derived components. The samples without evidence of interaction with crustal rocks (11 of 14 samples), which are likely to better reflect the mantle source composition, have positive εSr(100Ma) values (+8.1 to +14.5) and negative to slightly positive εNd(100Ma) values (−1.6 to +2.5) consistent with derivation from an enriched mantle source. These eleven samples may be divided into two groups either characterized by higher LILE/HFSE ratios, less radiogenic εSr(100Ma) values and more radiogenic εNd(100Ma) values, or characterized by relatively lower LILE/HFSE ratios, more radiogenic εSr(100Ma) values and less radiogenic εNd(100Ma) values suggesting differences in the mantle source. The results of this study are consistent with the melting of a variably metasomatized sub-arc mantle source during a transition from a wrench to a transtensional tectonic setting, but are inconsistent with a mantle plume origin.