ARE MOST GRANITOID ROCKS CUMULATES? DEDUCTIONS AND CONUNDRUMS FROM AMPHIBOLE COMPOSITIONS AND ZIRCON SATURATION

When applied to coarse-grained rocks, zircon saturation equations commonly yield temperatures at or below the wet solidus, despite inclusions of zircon in high-T magmatic phases, and the common presence of inherited, xenocrystic, or antecrystic zircons. Three arc-related magmatic systems in California (English Peak pluton, Wooley Creek batholith, and Tuolumne Intrusive Complex; all with Zr contents in the 40–190 ppm range) yield zircon saturation T <725°C and as low as 617°C (Boehnke et al., 2013 calibration). In the same rocks, magmatic amphibole displays core-to-rim decreases in Zr, Hf, and Zr/Hf, which indicate zircon fractionated during amphibole crystallization, at temperatures greater than 800°C. These higher T estimates agree with maximum Ti-in-zircon T estimates of ca. 835°C and therefore indicate that Zr contents of the melts were higher, and possibly much higher, than Zr contents of the rocks. Zr concentrations in the melts were estimated by (1) applying saturation equations to bulk compositions, (2) applying saturation equations to calculated melt compositions, and (3) using T-dependent amphibole/melt partition coefficients (d) for Zr (d values from Nandedkar et al., 2016). Results indicate that bulk-rock Zr concentrations are consistently low relative to calculated melt concentrations. The fact that bulk rock values commonly give Zr-saturation temperatures below solidus temperatures indicates that Zr-rich melt was lost during crystallization, conservatively as much as 40%.

We tested this approach on samples from the lower-crustal Malaspina Pluton (Fiordland, New Zealand) in which Ti-in-zircon temperatures reach ca. 950°C. In amphibole, abundances of Zr and Hf, and Zr/Hf ratios all decrease with decreasing Ti, consistent with initial zircon fractionation at ca. 925°C. Calculated Zr (melt) contents at ca. 925°C are ~ 230 ppm, which suggests loss of 0–50% melt. These results indicate that bulk compositions of many granitic rocks reflect crystal accumulation. These bulk compositions are therefore inappropriate for use in thermodynamic calculations and in direct comparison of potentially consanguineous volcanic and plutonic suites.

Boehnke et al. (2013) Chem. Geol. 351, 324–334. Nandedkar et al. (2016) Contr.Min.Pet. 171:71.