Compositions and distributions of mafic dykes in the Antarctic Peninsula continental arc show that tapping of several mantle sources was tectonically controlled. In the Cretaceous to Tertiary, between 135 Ma and 55 Ma, calc-alkaline dykes intruded the arc. In the late Cretaceous, however, between 95 Ma and 65 Ma, there was a pulse of compositionally diverse magmatism. This change resulted from collision of an ocean spreading centre with the trench. As a consequence, non-partitioned dextral transtensional shear in the overriding plate became partitioned into strike-slip and extensional domains. Calc-alkaline magmatism was, therefore, replaced by strike-slip-related shoshonitic magmatism towards the rear-arc and extensionrelated tholeiitic magmatism towards the fore-arc. OIB-like dykes were emplaced because of the break in otherwise continuous subduction. During the early Tertiary subduction continued but ceased after a late Tertiary ridge-trench collision.