U-series disequilibria of trachyandesites from minor volcanic centers in the Central Andes

Young trachyandesite lavas from minor volcanic centers in the Central Andes record the magma differentiation processes at the base of the lower continental crust. Here we report U-series disequilibrium data for the historical lavas from the Andagua Valley in Southern Peru to define the time-scale and processes of magmatism from melting in the mantle wedge to differentiation in the crust. The Andagua lavas show ( ²³⁰Th)/( ²³⁸U), ( ²³¹Pa)/( ²³⁵U), and ( ²²⁶Ra)/( ²³⁰Th) above unity except for one more evolved lava with ²³⁰Th depletion likely owing to fractional crystallization of accessory minerals. The ²²⁶Ra excess indicates that the time elapsed since magma emplacement and differentiation in the deep crust is within 8000 years. Based on the correlations of U-series disequilibria with SiO ₂ content and ratios of incompatible elements, we argue that the Andagua lavas were produced by mixing of fresh mantle-derived magma with felsic melt of earlier emplaced basalts in the deep crust. Because of the lack of sediment in the Chile-Peru trench, there is no direct link of recycled slabs with ²³⁰Th and ²³¹Pa excesses in the Andagua lavas. Instead, ²³⁰Th and ²³¹Pa excesses are better explained by in-growth melting in the upper mantle followed by magma differentiation in the crust. Such processes also produced the ²²⁶Ra excess and the positive correlations among ( ²²⁶Ra)/( ²³⁰Th), Sr/Th, and Ba/Th in the Andagua lavas. The time-scale of mantle wedge melting should be close to the half-life of ²³¹Pa (ca. 33 ka), while it takes less than a few thousand years for magma differentiation to form intermediate volcanic rocks at a convergent margin.