A non-basaltic experimental cotectic array for calc-alkaline batholiths

A non-basaltic cotectic array has been determined to constrain the origin of Andean- (Cordilleran) calc-alkaline batholiths that form at active continentalmargins in response to subduction. The cotectic array is traced after the anhydrous composition of experimental liquids formed at 800-1100 degrees C, 3 and 10 kbar, from a synthetic model parental magma of dioritic (andesitic) composition with initial water contents of 1 and 5 wt% H2O. A cotectic line is common for 3 and 10 kbar experiments forwater-poor systems containing 1wt% initialwater. For systems with 5 wt% initial water, liquids plot close to the common array at 3 kbar and slightly departs from the array at 10 kbar. The common array for 3 and 10 kbar, called here the main cotectic array (MCA) is almost coincident with the geochemical trends of Andean-type batholiths from California (Sierra Nevada and Peninsular Ranges) and South America (Patagonia). Comparisons are given in terms of MgO/CaO ratios, as these are the best proxy of phase relations in granite/granodiorite/tonalite systems. The results put into questionmodels based on basaltic magmas as the precursors of Andean-type batholiths. Also, water-fluxed melting of the lower crust at active continental margins is questioned. By contrast, the results of this preliminary study support derivation of batholiths by fractionation over a wide range of pressures, from lower to upper crust, of an intermediate, non-basaltic magma precursor. (C) 2020 Elsevier B.V. All rights reserved.