Abstract

The Cordillera Blanca batholith, Peru is a metaluminous 'I'-type whose dominant high silica rocks (>70%) are Na-rich with many of the characteristics attributed to subducted oceanic slab melts. However, the position of the batholith and the age of the oceanic crust at the trench during the Miocene preclude slab melting, and the batholith rocks result instead from partial melting of newly underplated Miocene crust. In this dynamic model, basaltic material is melted to produce high Na, low HREE, high-Al trondhjemitic type melts with residues of garnet, clinopyroxene and amphibole. Such Na-rich magmas are characteristic of thick Andean crust (+ 60 km) and differ significantly from typical calc-alkaline, tonalite-granodiorite magmas formed in normal crust (ca. 30 km). 'Archean' signatures in Mesozoic (and younger) plutonic rocks from Antarctica, Chile, the Cascades and New Zealand, are also thought to reflect partial melting of basaltic lower crust and calls into question the traditional model of TTG formation in the Archean by slab melting. We propose a revised model of Archean TTG formation where rapid partial melting in magmatically thickened 'greenstone' crust produces partial melts of TTG composition similar to that proposed for the Mesozoic-recent TTG batholiths of Antarctica and the Andes. Transition from TTD to high-K tonalites and granodiorites does not require pre-existing evolved continental crust and can be achieved simply by melting mafic-intermediate K-rich rocks at source. Rapid melting of basaltic underplate by successive emplacement of numerous, thin (< 1 km thick) mafic sills overcomes the thermal 'problem' of melt generation at source. Emplacement of TTG magmas as tabular sheets (as opposed to diapiric blobs) at higher structural levels within greenstone basins ameliorates the space 'problem'.