Primitive magmas representing mantle partial melts minimally affected by fractionation and assimilation are rare in the magmatic arc environment. Most examples are either associated with high rates of arc-parallel extension, or occur along faults and dykes perpendicular to the trend of the arc and related to arc compression. In two cases, the Vanuatu and Solomon Islands arcs, such arc compression is being caused by collision of seamounts. In the Antarctic Peninsula. primitive mafic dykes were emplaced perpendicular to the continental arc. Ar-Ar and K-Ar data suggest intrusion of the dykes at c. 126-106 Ma, possibly during mid-Cretaceous regional compression of the arc. The dykes form two compositional groups, One group has low La-N/Yb-N ratios (0.31-0.49), lower Nb/Yb and higher Th/Nb than N-MORB, age-corrected epsilonNd values of +7.3 to +7.9, and are interpreted as melts of subduction modified sub-arc asthenosphere. The other has high La-N/Yb-N ratios (3.86 8.92), higher Nb/Yb and Th/Nb than N-MORB, age-corrected epsilonNd values of -2.8 to +3.4, and are interpreted as melts of sub-arc lithosphere. The absence of dykes compositionally between these groups suggests that the primitive magmas avoided storage and mixing in magma chambers.
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