San Jacinto Intrusive Complex: 3. Constraints on crustal magma chamber processes from strontium isotope heterogeneity

Abstract

Strontium isotope data from three large plutons interpreted as derived from dynamic magma chambers (Hill, this issue; Hill et al., this issue) show the existence of pronounced isotopic heterogeneity within each unit. Ranges of calculated initial ^(87)Sr/^(86)Sr are unit I, 0.7060–0.7076; unit II, 0.7060–0.7074; unit III, 0.7058–0.7073. A limited sample of mafic inclusions and dyke rocks, interpreted as samples of liquid added to the various chambers during inflation, have initial ^(87)Sr/^(86)Sr of 0.7068–0.7084. These variations are regular at the kilometer scale within each pluton and show no identified correlation with any other observed geochemical or petrological parameter. Slightly older small intrusions that span the compositional range olivine gabbro to granite show a systematic increase in initial ^(87)Sr/^(86)Sr from 0.7057 in the southwest to 0.7077 in the northeast, expressing the pattern previously reported for the northern 600 km of the Peninsular Ranges batholith. The Sr isotope data indicate that melt production, aggregation, transport, and crystallization processes were far from capable of completely homogenizing initial variations in initial ^(87)Sr/^(86)Sr within the liquids from which these rocks crystallized. Whole chamber convection is apparently ruled out by these data; more complex patterns, including double-diffusively induced horizontal stratification, are permitted.

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