OROGENIC WELTS, DELAMINATION, AND NON-STEADY-STATE EMPLACEMENT OF LARGE-VOLUME CONTINENTAL MARGIN BATHOLITHS: EXAMPLES FROM PERU, CALIFORNIA, AND NEW ZEALAND

Emplacement of subduction-related continental margin batholiths is a highly discontinuous, non-steady-state process. Large-volume high-flux melting events must occur over enormous regions of compositionally diverse source materials under varying P-T conditions – a complex nonlinear process. Tonalite-trondhjemite-granodiorite (TTG) magmatic flare-ups in circum-Pacific batholith belts pose particular problems in terms of source region characteristics and triggering mechanisms not adequately addressed by current models. Comparison of three contrasting but complementary TTG batholith belts suggest a common pattern of batholith formation: (1) the shallowly exposed (3-10 km) late Miocene Cordillera Blanca batholith (CBB) of the central Peruvian Andes, (2) intermediate crustal-level exposures (~10-20 km) of the Late Cretaceous eastern Peninsular Ranges batholith of California (PRB), and (3) deeply denuded (~30-40 km) exposures of Early Cretaceous Separation Point Suite rocks in western New Zealand (SPS). All three batholiths are dominated by compositions that reflect deep melting of fundamentally basaltic source regions; SiO2 variations in dominant lithologies, CBB=~72%, PRB=~63-68% and SPS=~52-58% may reflect density filtering during magma emplacement. We relate initiation of batholith production in each setting to major pre-batholithic horizontal shortening and crustal thickening events that provided deep crustal source rocks for melting. We speculate that subsequent delamination and detachment of mantle lithosphere and lower crust introduced a large input of heat into the crust from hot upwelling asthenosphere that drove large-scale TTG melting. Rapid post-emplacement tectonic/erosional batholith denudation is characteristic.