METAMORPHIC PATTERNS OF ACCRETIONARY OROGENS

Accretionary orogenic systems (AOS) form by plate convergence without collision; they represent sites of continental growth and include Archaean greenstone belts, and Proterozoic, Neoproterozoic-Palaeozoic and Mesozoic-Cenozoic orogens. AOS dominate during supercontinent break-up, in contrast to collisional orogenic systems, which form by ocean closure during assembly of supercontinents and are superimposed on AOS.

Extensional-contractional AOS form by subduction in dominantly extensional arcs, where accretionary wedges contain protoliths derived from the subducting plate and eroded from the facing plate. In Phanerozoic extensional-contractional AOS (e.g., Lachlan, Australia), LP-HT metamorphism is dominant, with hairpin or (ambiguous) CW or CCW or looping P-T-t paths and peak metamorphic mineral growth late in relation to tectonic fabrics. UHT and HP granulites are absent, and although blueschists may occur early in the orogenic cycle UHPM is not recorded; extensive granite magmatism accompanies metamorphism. Short-lived phases of orogenesis are caused by features on the ocean plate that interrupt the continuity of subduction and flatten the slab, and may cause outboard-stepping of the magmatic arc/subduction zone.

Terrane-dominated AOS involve accretion of arcs, supra-subduction zone ophiolites and oceanic plateaus. In Phanerozoic terrane-dominated AOS (e.g., Ryoke-Sambagawa, Japan), ‘paired' metamorphic belts are supposed to characterize the metamorphic pattern. However, this is a false construct resulting from failure to recognize orogen-parallel terrane migration and limitations of particular chronological datasets. Whereas a HP-LT (blueschist-eclogite) metamorphic belt occurs outboard, it is always separated from an inboard LP-HT (andalusite-sillimanite type) metamorphic belt by a terrane boundary. In some systems an additional feature of the orogenic process is ridge subduction, which is reflected in the spatial-temporal pattern of LP-HT metamorphism and the type of magmatism (adakitic). Granulites may occur at the highest grade in the LP-HT belt, where granite magmatism is common, but UHPM is rare in the outboard HP-LT belt. Flattening of the slab by ridge or plateau subduction is important, but terrane accretion may choke the subduction zone, leading to termination of orogenesis and outboard-stepping of the magmatic arc/subduction zone.