Abstract

The Ross Orogen of the Transantarctic Mountains developed in response to the early Paleozoic convergence between the paleo–Pacific plate and the Antarctic margin of Gondwana. The central Victoria Land sector of the orogen is characterized by the widespread occurrence of pink Irizar granite plutons and dikes and Vegetation lamprophyric dikes and sills, which were emplaced in a tensional regime during a restricted time interval of the latest Ross Orogeny, ca. 490 Ma, as documented by new geochronological zircon U-Pb and mineral 40Ar-39Ar data. The syenomonzogranitic Irizar granites-dikes and the Vegetation lamprophyres are all potassic and, despite the chemical gap between them, have overlapping 87Sr/86Sr(490 Ma) and ϵNd (490 Ma) values, within 0.7074–0.7092 and −4.4 to −7.5, respectively.

The genesis of Vegetation lamprophyres can be ascribed to the melting of previously enriched subcontinental lithospheric mantle further metasomatized by a subduction component during the Ross convergence. Melting was probably linked to asthenospheric upwelling during postcollisional slab rollback and convective thinning and/or delamination of overthickened lithosphere. On the other hand, the overlap of age, geochemical, and Sr-Nd isotope data between Vegetation and Irizar products, supported by geochemical modeling, suggests that the Irizar felsic magmas were derived by partial remelting of underplated material similar in composition to the Vegetation lamprophyres. This scenario provides new insights into the genesis of widespread postcollisional granites in orogens worldwide, implying significant net crustal growth by magma underplating in the very latest orogenic stages.

In the regional geodynamic framework, the NE strike shared by both mafic and felsic dikes along 300 km of the convergent margin points to NW-SE extension, which in turn suggests oblique convergence of paleo–Pacific and Antarctic plates during the latest orogenic stages. A comparison of latest igneous activity in the Ross Orogen with contemporary magmatism in southeastern Australia-Tasmania implies that different mechanisms triggered the magmatic activity: slab rollback in the Antarctic sector of the margin versus slab tear in the Australia-Tasmania sector.

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