Compositional structures in two batholiths of circumpacific North America

Abstract

The compositional structure within an igneous body or cogenetic series depends on the number of end members that are required to account for a major part of its compositional variation.If a large part of the compositional variation can be accounted for by mixing of only a few materials (magmas or magmas and country rocks), the compositional structure is simple.Or, the compositional structure is simple if the variation can be explained by the separation of only a few mineral phases from a magma.Factor-variance diagrams constructed from data on major oxide constituents in samples from the Sierra Nevada and Alaska-Aleutian Range batholiths show that these bodies have fairly simple compositional structures, and, therefore, probably resulted from a few processes that were dominant over others.There is no need to call on highly complex mechanisms of fractional crystallization, assimilation, or magma mixing to account for the oxide data.In fact, the relatively simple compositional structures of the batholiths make these processes seem unlikely.Models containing three end members can account for most of the compositional variability in each batholith.One end member is a parent magma, or magma-source material, and the other two represent the extremes in a two-component range of mafic mineral assemblages that separated from the magma.The compositions of 228 samples from the Sierra Nevada batholith and of 158 samples from the Alaska-Aleutian Range batholith can be closely approximated as linear combinations of these end members.These approximations are somewhat improved for the Sierra Nevada if a fourth end member is used; the end member representing the magma, or magma-source material, is replaced by two end members that are apparently required to account for variation in the composition of the magmas that formed across the present site of the batholith.The principal variation among these magmas is in K2 O con tent, which was slightly higher in magmas formed at a distance from the continental margin.If it is assumed that the magmas were generated by partial melting of the crust or other material, the models may be used to estimate a range of compositions for the partial melts that formed each sample.The final composition of each sample was determined by the composition of this melt and by the composition of mafic mineral assemblages that were either retained in the melt after partial melting of the crust or precipitated from the melt during its ascension to the present site of each batholith.The melt compositions that are estimated on the assumption of minimum heat requirements correspond well with those that occur along cotectic lines on classical phase diagrams.Individual samples from the Alaska-Aleutian Range batholith required melting of 6 to 24 percent of the crust, with an average of about 9 percent melting.Most individual samples from the Sierra Nevada batholith require melting of 11 to 27 percent of the crust; the average degree of crustal melting required for individual groups of plutons (sequences) ranges from 17 to 21 percent.Although the models developed for the two batholiths are not unique, they appear to be in accord with major geologic observations that bear on the batholiths' origins.Other models could be developed that would account equally well for the compositional variabilities; however, these other models would also have to account for the fairly simple compositional structures present in each of the batholiths.