The Fontaine Pluton: An early Ross Orogeny calc‐alkaline gabbro from southern Victoria Land, Antarctica

Abstract

The Fontaine Pluton is a previously undescribed mafic intrusion outcropping at Fontaine Bluff on the south side of the Carlyon Glacier in southern Victoria Land, Antarctica. It is the southern‐most member of a laterally extensive mafic suite emplaced at mid‐crustal depths during the initial stages of the Ross Orogeny. The pluton comprises recrystal‐lised hornblende‐biotite gabbro, which in places shows well‐defined centimetre to metre scale primary igneous layering. Recrystallised ultramafic enclaves composed of amphibole‐chlorite‐talc are inferred to be remnants of a chemically and mineralogically distinct cumulate fraction. The intrusion has a 87Sr/86Sr(i) ratio of 0.70679 and a 143Nd/144Nd(i) ratio of 0.51187 (ϵNd(i) = ‐1.2). This, coupled with other geochemical data, implies that the Fontaine Pluton was formed by c. 15% partial melting of a depleted mantle source that was subsequently contaminated by continental crust. Preliminary U‐Pb geochronology on zircon suggests an emplacement age for the pluton of 546 ±10 Ma. These new data indicate that Ross Orogeny magmatism in this area of southern Victoria Land was initiated in the late Neoproterozoic along a subducting plate margin.