Large-scale evolutionary geochemical and isotopic patterns: How do igneous rocks at convergent margins evolve ?

Convergent continental margins are the main site of crustal growth. Changing geochemical and isotopic characteristics of igneous arc rocks can monitor their large-scale compositional evolution. We present examples from three Andean suites of arc rocks with systematically changing trace element, and Sr and Nd isotopic characteristics over time. (1) In southern Chile (40°-53°S), the evolution of the north Patagonian batholith (150 Ma to Present) shows two main stages. ⁸⁷Sr/^{86}Sr ratios first decrease from higher, crust-like to lower, island arc-like characteristics from 150-40 Ma, before they increase again to higher, crust-like ratios from 40 Ma to Present. Nd isotopes show a contemporaneous increase from low to higher ^{143}Nd/^{144}Nd ratios, before they drop again to lower ratios. (2) In central Chile (31°-34°S), the evolution of the coastal batholith (330-80 Ma) shows low and largely constant, island-arc like La/Yb ratios. ^{87}Sr/^{86}Sr ratios gradually decrease, from higher, crust-like to lower, island arc-like ratios, and ^{143}Nd/^{144}Nd ratios gradually increase. (3) In northern Chile (21°-26°S), a stepwise eastward-migrating main-arc axis from 200 Ma to present produced a collage of four, largely parallel, eastward-younging arc systems, each separated by a 5--10 m.y. magmatic gap of 50-100 km. Igneous rocks from the north Chilean arcs show evolutionary La/Yb ratios, and Sr and Nd isotope characteristics. La/Yb ratios show a repeated increase in La/Yb ratios and ^{87}Sr/^{86}Sr ratios, and decrease in ^{143}Nd/^{144}$Nd ratios within each arc system. Each individual arc system reflects crustal thickening over time, from an immature Jurassic island arc to the mature, orogenic continental arc of the central Andean Western Cordillera. Detailed studies show that the geochemical and isotopic characteristics at individual volcanic centers may be more complex, yet on a larger scale the geochemical and isotopic signatures follow distinct evolutionary patterns. The existence of such patterns may be controlled by geodynamic changes of the subducting slab (e.g. convergence rate, subduction angle, plateau subduction), but the changing geochemical and isotopic signatures imposed on the melt are due to interaction with changing thickness (and age and composition) of the overlying crust. As the recent thick, orogenic Andean crust is mainly a product of crustal shortening, the evolutionary patterns provide a sensitive tool for monitoring the different stages of the Andean Orogeny.