Pacific-type transform and convergent margins: igneous rocks, geochemical contrasts and discriminant diagrams

Transform margins represent lithospheric plate boundaries with horizontal sliding of the oceanic plate, which in time and space replaced the subduction-related convergent margins. This happened due to the following: ridge-crest–trench intersection or ridge death along a continental margin (recent California and Baja California, Queen Charlotte–Northern Cordilleran, west of the Antarctic Peninsula, and probably Late Miocene–Pleistocene southernmost South America); change in the direction of oceanic plate movement (western Aleutian–Komandorsk and southernmost tip of the Andes); and island arc-continent collision (New Guinea Island). Post-subduction magmatism is related to a slab window that resulted from the spreading ridge collision (subduction) with a continental margin or slab tear formation after subduction cessation. Igneous magmatic series formed above the slab window or slab tear are similar in composition and show diversity of tholeiitic (sub-alkaline), alkaline, or even calc-alkaline and peraluminous rocks. The comprehensive geochemical dataset for igneous rocks (more than 2400 analyses) from the recent model geodynamic settings allowed us to build discriminant diagrams for the petrogenic oxides TiO₂ × 10–Fe₂O₃^Tot–MgO and trace elements Nb*5–Ba/La–Yb*10, which show distinctive rock features present on both convergent and Pacific-type transform margins. The author’s diagrams are capable of distinguishing volcanic and plutonic rocks formed above the subduction zones at an island arc and continental margin (related to convergent margins), from those formed in the strike-slip tectonic setting of transform margins along continents or island arcs.