Melting Relations of MORB–Sediment Mélanges in Underplated Mantle Wedge Plumes; Implications for the Origin of Cordilleran-type Batholiths

• Published in: Journal of petrology 2010-06, Vol.51 (6), p.1267-1295
• Main Authors: Castro, Antonio, Gerya, Taras, García-Casco, Antonio, Fernández, Carlos, Díaz-Alvarado, Juan, Moreno-Ventas, Ignacio, Löw, Irena
• Format: Article
• Language: English
• Subjects: cordilleran batholiths; granodiorite; Marine; melting experiments; mélange; silicic plumes; subduction; tonalite
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egq019

This paper gives the results of a set of laboratory experiments designed to analyse the petrological implications of mantle wedge plumes—large buoyant structures predicted by thermomechanical numerical modelling of subduction zones. A particular design of layered capsule was used to simulate the complex multilayer formed by intense flow within the mantle wedge as predicted by numerical models. A basaltic [mid-ocean ridge basalt (MORB)-derived amphibolite] component was sandwiched between two adjacent layers of a sedimentary (Bt-rich metagreywacke) component. Conditions were fixed at temperatures of 1000–1200°C at pressures of 1·5–2·0 GPa. Our results suggest that significant volumes of hybrid, Cordilleran-type granodioritic magmas can be generated by sub-lithospheric partial melting of a mechanically mixed source. Partial melting of the end-members is not buffered, forming granitic (melting of metasediment) and trondhjemitic (melting of MORB) melts in high-variance assemblages Melt + Grt + Pl and Melt + Grt + Cpx, respectively. However, the composition of melts formed from partial melting of metasediment–MORB mélanges is buffered for sediment-to-MORB ratios ranging from 3:1 to 1:3, producing liquids of granodiorite to tonalite composition along a cotectic with the lower-variance phase assemblage Melt + Grt + Cpx + Pl. Our model explains the geochemical and isotopic characteristics of Cordilleran batholiths. In particular, it accounts for the observed decoupling between major element and isotopic compositions. Large variations in isotopic ratios can be inherited from a compositionally heterogeneous source; however, major element compositions are more strongly dependent on the temperature of melting rather than on the composition of the source.