On the onset and evolution of the Ross-orogeny magmatism in North Victoria Land — Antarctica F. Giacomini a, ⁎ , M. Tiepolo b , L. Dallai c , C. Ghezzo a a Dipartimento di Scienze della Terra, Università di Siena, via Laterina 8, 53100, Siena, Italy b CNR–Istituto di Geoscienze e Georisorse (IGG) Unità di Pavia, via Ferrata 1, 27100, Pavia, Italy c CNR–Istituto di Geoscienze e Georisorse (IGG) Unità di Pisa, Via Moruzzi 1, 56124 Pisa, Italy Received 22 December 2005; received in revised form 10 January 2007; accepted 3 February 2007 Editor: S.L. Goldstein Abstract An extensive geochemical (major-, trace-element and oxygen isotope) and geochronological (U/Pb geochronology on zircon) characterisation of Ol-bearing ultramafic–mafic cumulates and gabbro-diorites recently discovered in the southwestern region of the Wilson Terrane (North Victoria Land — Antarctica) was undertaken in order to constrain source characteristics, magma evolution and emplacement history in this sector of the Cambro–Ordovician Ross Orogeny. Textural and chemical data indicate disequilibria among mineral phases of these Ol-bearing cumulate rocks. Corroded Cr-rich clinopyroxene with high Mg# (0.82) and Ca-rich plagioclase likely represent xenocrysts from an old magmatic system, and are not in equilibrium with the host cumulate assemblage (Ol + Opx + Amp + Bt + Pl). The calculated liquid in equilibrium with the xenocrysts (“melt 1”) exhibits exceptionally high La/Yb ratios and Th–U concentrations, suggesting a strong sediment influx in the mantle source, possibly from the subducted slab. Because of the extensive fractional crystallisation (Fo 70 ; Mg# = 76) and crustal contamination (e.g., δ 18 O Opx = 7.13–7.47‰) the trace element composition of the liquid in equilibrium with the cumulate assemblage (“melt 2”) does not reveal the nature of the mantle source not the differentiation processes. The gabbroic parental liquid for the main cumulate assemblage was not produced by assimilation and fractional crystallisation (AFC) of the equilibrium “melt 1” calculated from the xenocrystic paragenesis. Rather, it likely represents a new pulse of magma originated from a different mantle source, which then evolved through AFC into gabbro-diorites. Zircon U–Pb dating of one gabbro-diorite yields a crystallisation age of 489 ± 4 Ma. Zircon in the Ol-bearing cumulates yielded two ages populations at 521 ± 2 Ma and 502 ± 3 Ma, which we interpret to represent the actual age difference between the two magmatic systems. This age difference suggests that poorly differentiated melts with adakite-type signature intruded the crust prior to the generation of the large volumes of gabbroic and dioritic magmas. These early pulses represent up to now the oldest proof of subduction related mantle melts in North Victoria Land and predate the diffuse igneous activity dominated by intermediate- to felsic products. © 2007 Elsevier B.V. All rights reserved. Keywords: Zircon geochronology; Ultramafic-cumulates; Antarctica; Adakite-type mantle melts; Laser ablation; Equilibrium Chemical Geology 240 (2007) 103 – 128 www.elsevier.com/locate/chemgeo ⁎ Corresponding author. Tel.: +39 0577 233 802; fax: +39 0577 233 938. E-mail address: giacomini@unisi.it (F. Giacomini). 0009-2541/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2007.02.005