Abstract This chapter provides an overview of the current state of research on orogenic andesites. While their importance as proxies to the evolution of the continental crust has long been recognized, andesite genesis has remained highly controversial with a broader consensus yet to be reached. The controversy is fuelled by the question of whether orogenic andesites are primary melts of slab and mantle materials, or instead derivative products of basaltic mantle melts that differentiate in the overlying crust. These hypotheses are addressed in three sections of the book devoted to slab–mantle processes, the complexities of melt differentiation at crustal levels, and models pertaining to arc crustal growth. We believe that cross-fertilization and discussion among seemingly opposite and irreconcilable hypotheses will smooth the pathway towards a holistic communal model of andesite petrogenesis.

Abstract A fundamental question in the formation of orogenic andesites is whether their high melt SiO 2 reflects the recycling of silicic melts from the subducted slab or the processing of basaltic mantle melts in the overlying crust. The latter model is widely favoured, because most arc magmas lack the ‘garnet’ signature of partial slab melts. Here we present new trace element data from Holocene high-Mg# >64–72 calc-alkaline basalts to andesites (50–62 wt% SiO 2 ) from the central Mexican Volcanic Belt that crystallize high-Ni olivines with the high 3 He/ 4 He=7–8 of the upper mantle. These magmas have been proposed to be partial melts from ‘reaction pyroxenites’, which formed by hybridization of mantle peridotite ( c. 82–85%) and heavy rare earth element-depleted silicic slab melt (>15–18%). Forward and inverse models suggest that the absence of a garnet signature in these melts reflects the efficient buffering of the heavy rare earth elements (Ho to Lu) in the subarc mantle. In contrast, all elements more incompatible than Ho – excepting TiO 2 – are more or less strongly controlled by the silicic slab flux that also directly contributes to the silicic arc magma formation. Our study emphasizes the strong link between slab recycling and the genesis of orogenic andesites. Supplementary material: Methods, additional data and modelling parameters are available at http://www.geolsoc.org.uk/SUP18686

Abstract The western Mexican subduction zone is characterized by steep subduction of the Rivera plate, and by the existence of a continental rift at the rear arc under which the slab rests at >300 km deep. Mafic magmatism at the volcanic front is potassic lamprophyric, interpreted to be influenced by deep and hot slab melts or supercritical fluids. In contrast, mafic rocks at the rear arc are intraplate-like basalts that derive from low extents of melting of a dryer mantle source. Although a transition from a volcanic arc front to an extensional rear arc is apparent, calc-alkaline andesitic stratovolcanoes with trace element characteristics that suggest a key role of residual amphibole have been constructed at the rear arc during the past ∼200 ka. Crystal fractionation of basalts and partial melting of crustal amphibolites are not viable mechanisms for andesites, whereas melting of slab amphibolites beneath the rear arc is also problematic because the oceanic plate rests too deep. We thus suggest that andesites are partial melts of rising diapirs made by mixtures of hydrous mantle, sediments, and possibly eroded crustal blocks, which detach buoyantly from the downgoing slab as discrete plumes that ‘pollute’ the upwelling regime of a continental rift.