The generation of sodic granite magmas, western Palmer Land, Antarctic Peninsula

Subduction-related Mesozoic to Cainozoic granites s.l. in western Palmer Land, Antarctic Peninsula, have similar chemical compositions to Archean tonalite-trondhjemite-granodiorite (TTG) suites, Phanerozoic slab-melts (adakites), and to experimental partial melts of basaltic material in equilibrium with amphibole ± pyroxene ± garnet. They are predominantly sodic, metaluminous and most have Al2O3 > 15 wt% and Y < 18 ppm. All are light rare earth element (LREE)-enriched (2 < La/Ybn <30) and most have small Eu anomalies. They have a wide range of initial ɛNd(t) (−6.8 to +4.5) and ɛSr(t) (+293.4 to −3.7), but most Pb isotope compositions deviate by 36 km. Between Triassic and Tertiary times the initial ɛNd(t) of the magmatism increased and ɛSr(t) decreased, suggesting that new continental crust was produced during this period. Underplating by mafic magma was an important crustal growth mechanism in the arc: the generation of abnormally thick crust, and its subse quent fusion, is considered to be a consequence of ca. ≥ 180 Ma of subduction and associated magmatism in the region. An implication of the model is that dense garnet-amphibolite and eclogite residues from partial melting of the lower crust will accumulate. In theory, the setting was appropriate for such residues to detach from the base of the crust and to sink into the convecting mantle. Such a process would leave the rest of the crust enriched in large ion lithophile elements/LREE, but depleted in HREE+Y.

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