Crustal segmentation and the isotopic significance of the Abancay Deflection: Northern Central Andes (9-20 degrees S)

The Abancay Deflection (13 degrees S) is a continental trench- normal structure that marks the northern limit of the central volcanic zone in Peru, the northern limit of exposed Precambrian basement, and the continental extension of the oceanic Nazca Ridge, In order to assess the potential influence of this structure on magma compositions emplaced across it, strontium and neodymium isotope data on the Mio-Pliocene Cordillera Blanca batholith north of the Abancay Deflection (9- 11 degrees S) are compared with volcanic rocks of similar age and composition from the Central Andes of southern Peru and northern Chile (16-20 degrees S), The Cordillera Blanca magmas show no evidence of contamination by mature continental basement, in spite of having been intruded through continental crust in excess of 50 km thick. In contrast, Central Andean volcanic rocks of similar age, intruded through crust of similar thickness have elevated initial strontium isotope ratios (Sr,) and negative epsilon(Nd) values consistent with contamination by crustal or lithospheric material. The authors consider these contrasting variations in isotopic composition relate to differences in the composition of the continental crust along strike in this sector of the Andean chain, with old Arequipa-type basement dominating in the south, while farther north, the lower to mid crust is made up mostly of young, mantle-derived basaltic material. The boundary between them (the Abancay Deflection) thus, represents a deep and possibly long-lived feature separating crustal segments of different composition north and south of 13 degrees S.