Abstract

Divergent double subduction, involving removal of an oceanic basin from both sides and collision, has distinct fingerprints of magmatism and significant implications for the tectono-magmatic evolution of orogenic belts. The complete evolution of a divergent double subduction system involves four stages: (1) initial interaction between the two divergent subduction zones on both sides of a single oceanic plate; (2) closure of the oceanic basin when the two overriding plates meet, followed by (3) detachment of the oceanic lithosphere from the overlying crust and sedimentary sections; (4) sinking and disappearance of the oceanic slab into the mantle. Detachment of the oceanic slab leads to intense decompressional melting of mantle and triggers melting of accretionary-wedge strata and lower crust, resulting in large-scale magmatic and volcanic activity. The main consequences of a divergent double subduction system are that it produces opposed thrust systems, extensive long-lived granitoid magmatism with mantle isotopic signature, and volcanism that evolves toward bimodalism. This model can be applied to the western half of the Paleozoic Lachlan fold belt (southeastern Australia) to explain the wide-scale Silurian to early Carboniferous granitoid magmatism, its spatial and temporal relationships, and late bimodal magmatism. Closure of the oceanic basin is thought to have occurred during the Early to Middle Devonian. Detachment of the oceanic slab led to felsic to intermediate-composition magmatism. Bimodal volcanism involving local basaltic flows is a reflection of subsequent sinking of the slab.

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