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
87
Sr/
86
Sr
(i)
ratio of 0.70679 and a
143
Nd/
144
Nd
(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.