Three metapelitic xenolith suites in the Neogene Volcanic Province (NVP) of SE Spain (from SW to NE: El Hoyazo, Mazarron and Mar Menor) originated by partial melting at different crustal depths, decreasing from 20-25 km in the SW to 9-12 km in the NE. Peak temperatures reached c. 900 degrees C. The xenolith source level is equated with the base of a felsic upper crust of high melting potential ('fertility'). At El Hoyazo, this matches a thin, intracrustal low-velocity zone recently inferred from seismic studies. Isostatic calculations indicate that this zone increases in thickness from SW to NE. A model of increasing upper crustal thinning from SW to NE in the NVP, accompanied by mafic underplating, is consistent with the 9 Ma petrological data, with current heat flow, seismic data and gravimetry. It is concluded that significant crustal extension occurred in the NVP in the late Miocene, i.e. after the main phase of widespread extension, exhumation of high-pressure rocks and formation of the Alboran Sea.