Rates of production of the main magma types in the central Andes
Six new K-Ar dates for young volcanic rocks in the area between lat 21° and 22°S in the Andes of northern Chile have been determined. Two groups of volcanic rocks are present: extrusive lava flows and domes of andesitic composition and extensive sheets of ignimbrite. Both groups are less than 10 m.y. old. We estimate that 2 × 10 3 km 3 of andesitic rocks and 1.5 × 10 3 km 3 of ignimbrite lie within the study area. Andesitic rocks have been erupted at an estimated rate of between 1.7 × 10 −6 km 3 and 2.9 × 10 −6 km 3 /yr/1 km of length of the active belt, and ignimbrite at a rate of 1.3 × 10 −6 km 3 /yr/l km of length. The total volume of ignimbrite is less than is suggested by the huge area covered by ignimbrite sheets. In order to estimate the volume and rate of production of a comparable strip of the Coastal batholith of Peru, two extreme models were used for the shapes of batholiths in depth: one assumes a tabular shape with a thickness of 5 km, the other an inverted teardrop shape extending to a depth of 30 km. Rate of production estimates for these two models range from 2.9 × 10 −6 km 3 /yr/l km to 9.9 × 10 −6 km 3 /yr/l km of length. Whichever model is used, the rate of production is broadly comparable with that of the Sierra Nevada batholith in California. The rate of extrusive volcanism is about two orders of magnitude less than that at the Icelandic constructive plate margin and several times less than the rate of intrusion of batholithic material. Volumetric data do not explicitly support any particular one of the many hypotheses for the genesis of destructive plate margin magma types.