Abstract

The Coastal Batholith of Peru shows how diorite, tonalite, granodiorite, and granite magmas rose through their last few kilometers by a process of magmatic stoping by their fluidized upper surface. The magmas successively displaced both older, more basic plutons and their own volcanic debris downward and came to rest within 3 km of the Earth's surface after loosing volatiles by eruptions through fissures and calderas. There was a continuous association between volcanic eruptions and plutonic intrusions from at least 100 to 30 m.y. ago in a narrow belt parallel with the continental margin.

The batholith is 50 km wide, more than 1,100 km long, and probably 15 km or less thick. It has steep walls and an extensively preserved flat roof exposed in mountainous desert with relief of 4,500 m. It consists of plutons and sheets which were intruded in five distinct episodes into the same tabular belt by repeated cauldron subsidence. Each subsidence was preceded by the formation of small shear zones which were fractured and fluidized, and accompanied by the rise of corrosive, turbulent gas-liquid-solid mixtures. Individual plutons form relatively thin tabular bodies with flat roofs and floors and steep walls which pass downward into ring dikes and upward into ring dikes and calderas.

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