Constraining Slab Breakoff Induced Magmatism through Numerical Modelling

Abstract:

Post-collisional magmatism is markedly different in nature and composition than pre-collisional magmas. This is widely interpreted to mark a change in the thermal structure of the system due to the loss of the oceanic slab (slab breakoff), allowing a different source to melt. Early modelling studies suggest that when breakoff takes place at depths shallower than the overriding lithosphere, magmatism occurs through both the decompression of upwelling asthenopshere into the slab window and the thermal perturbation of the overriding lithosphere (Davies & von Blanckenburg, 1995; van de Zedde & Wortel, 2001). Interpretations of geochemical data which invoke slab breakoff as a means of generating magmatism mostly assume these shallow depths. However more recent modelling results suggest that slab breakoff is likely to occur deeper (e.g. Andrews & Billen, 2009; Duretz et al., 2011; van Hunen & Allen, 2011).

Here we test the extent to which slab breakoff is a viable mechanism for generating melting in post-collisional settings. Using 2-D numerical models we conduct a parametric study, producing models displaying a range of dynamics with breakoff depths ranging from 150 - 300 km. Key models are further analysed to assess the extent of melting. We consider the mantle wedge above the slab to be hydrated, and compute the melt fraction by using a simple parameterised solidus. Our models show that breakoff at shallow depths can generate a short-lived (< 3 Myr) pulse of mantle melting, through the hydration of hotter, undepleted asthenosphere flowing in from behind the detached slab. However, our results do not display the widespread, prolonged style of magmatism, observed in many post-collisional areas, suggesting that this magmatism may be generated via alternative mechanisms. This further implies that using magmatic observations to constrain slab breakoff is not straightforward.