Across‐arc versus along‐arc S r‐Nd‐Pb isotope variations in the E cuadorian volcanic arc

Previous studies of the Ecuadorian arc (1°N–2°S) have revealed across‐arc geochemical trends that are consistent with a decrease in mantle melting and slab dehydration away from the trench. The aim of this work is to evaluate how these processes vary along the arc in response to small‐scale changes in the age of the subducted plate, subduction angle, and continental crustal basement. We use an extensive database of 1437 samples containing 71 new analyses, of major and trace elements as well as Sr‐Nd‐Pb isotopes from Ecuadorian and South Colombian volcanic centers. Large geochemical variations are found to occur along the Ecuadorian arc, in particular along the front arc, which encompasses 99% and 71% of the total variations in 206 Pb/ 204 Pb and 87 Sr/ 86 Sr ratios of Quaternary Ecuadorian volcanics, respectively. The front arc volcanoes also show two major latitudinal trends: (1) the southward increase of 207 Pb/ 204 Pb and decrease of 143 Nd/ 144 Nd reflect more extensive crustal contamination of magma in the southern part (up to 14%); and (2) the increase of 206 Pb/ 204 Pb and decrease of Ba/Th away from ∼0.5°S result from the changing nature of metasomatism in the subarc mantle wedge with the aqueous fluid/siliceous slab melt ratio decreasing away from 0.5°S. Subduction of a younger and warmer oceanic crust in the Northern part of the arc might promote slab melting. Conversely, the subduction of a colder oceanic crust south of the Grijalva Fracture Zone and higher crustal assimilation lead to the reduction of slab contribution in southern part of the arc. Subduction of a younger oceanic crust in the northern arc promotes slab melting Southward increase of crustal contamination along the front arc (from 3% to 14%) The slab structure (Grijalva Fracture Zone, age) and shape (flexure along the Grijalva Fracture Zone) influences magmatism