Petrogenesis of a two-mica ignimbrite suite: the Macusani Volcanics, SE Peru

ABSTRACTThe Miocene-Pliocene Macusani ash-flow tuffs and glasses from SE Peru are a rare example of two-mica felsic peraluminous volcanic rocks. They outcrop in three separate tectonic basins of the Cordillera Oriental in the Central Andes. In the Macusani field, the rocks are characterised by andalusite and muscovite phenocrysts. Compositions are both very felsic and peraluminous, similar to two-mica granites. Field relations, age differences and isotopic heterogeneities suggest that several distinct magma batches were involved. Two separate magmatic stages are recognised: (1) partial melting and evolution at or near the source region, and (2) shallow-level crystallisation and eruption. Magma genesis involved partial melting of metapelitic materials, with internally controlled. High heat flux, rapid heating, elevated temperatures and F-rich compositions were essential for the production of these mobile, H2O-undersaturated magmas. Chemical variations between the erupted products can be attributed to different degrees either of partial melting in somewhat variable source materials and (or) of fractional crystallisation at shallow levels. We discuss some important differences between the magmatic evolution of the Macusani Volcanics and of Hercynian and Himalayan two-mica granites.