Most Granitoid Rocks are Cumulates: Deductions from Hornblende Compositions and Zircon Saturation

Abstract

Abstract Cumulate processes in granitic magma systems are thought by some to be negligible and by others to be common and widespread. Because most granitic rocks lack obvious evidence of accumulation, such as modal layering, other means of identifying cumulate rocks and estimating proportions of melt lost must be developed. The approach presented here utilizes major and trace element compositions of hornblende to estimate melt compositions necessary for zircon saturation. It then compares these estimates with bulk-rock compositions to estimate proportions of extracted melt. Data from three arc-related magmatic systems were used (English Peak pluton, Wooley Creek batholith, and Tuolumne Intrusive Complex). In all three systems, magmatic hornblende displays core-to-rim decreases in Zr, Hf, and Zr/Hf. This zoning indicates that zircon must have fractionated during crystallization of hornblende, at temperatures greater than 800 °C. This T estimate is in agreement with Ti-in-zircon thermometry, which yields a maximum T estimate of 855 °C. On the basis of this evidence, concentrations of Zr in melts from which hornblende and zircon crystallized were calculated by (1) applying saturation equations to bulk-rock compositions, (2) applying saturation equations to calculated melt compositions, and (3) using hornblende/melt partition coefficients for Zr. The results indicate that melt was lost during crystallization of the granitic magmas, conservatively at least as much as 40 %. These results are in agreement with published estimates of melt loss from other plutonic systems and suggest that bulk-rock compositions of many granitic rocks reflect crystal accumulation and are therefore inappropriate for use in thermodynamic calculations and in direct comparison of potentially consanguineous volcanic and plutonic suites.