The Tatara–San Pedro volcanic complex, a Quaternary to Holocene composite volcano in central Chile, is underlain by late Miocene (6 Ma) plutons and Tertiary metavolcanic rocks. The plutons were intruded into greenschist facies metavolcanic rocks at a depth of about 4–5 km. Together, these rocks reveal important information about the development of continental arcs.

In some cases, apparent eruptive ages of metavolcanic rocks are younger than ages of intruding plutons, indicating that there was (1) pervasive reheating of metavolcanic rocks or (2) that metavolcanic rocks were locally derived and intruded by their own magma chambers (plutons). We have estimated uplift or denudation rates of nearly 1 mm/yr since late Miocene time. Normal faulting currently dominates the brittle behavior of the upper crust; therefore, uplift is probably related to magmatic addition to the crust rather than to compression and shortening. Although plutonic and metavolcanic rocks display expected arc geochemical affinities, the isotopic similarity of metavolcanic and plutonic rocks to Quaternary volcanic rocks is strong; this makes it difficult to use subvolcanic rocks as upper crustal contaminants to evaluate the igneous evolution of the Quaternary deposits. Most plutonic, metavolcanic, and Quaternary volcanic rocks exhibit the isotopic characteristics of juvenile crust. Some samples, however, exhibit sparse but unmistakable evidence (87Sr/86Sr and 40Ar/39Ar isotopic data) for the presence of older, probably Precambrian through Triassic radiogenic crust beneath the volcano.

Crustal structure in this arc can be described by a three-layer model in which a central section of Precambrian through Triassic rock is underplated by juvenile material, is intruded by juvenile material, or has juvenile material transported through it to be emplaced at or near the surface. Continued erosion of juvenile material at the surface may be balanced to some degree by continued volcanism and epizonal plutonism.

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