RECOGNIZING THE IMPORTANCE OF CRYSTAL ACCUMULATION ON BULK-ROCK COMPOSITIONAL VARIABILITY IN SILICIC SYSTEMS

Fractional crystallization, magma mixing, and assimilation are commonly discerned through compositional variability of bulk-rock samples. However, crystal accumulation is rarely considered in conjunction with these processes. Accumulation has been difficult to recognize and quantify in silicic systems because accumulation may be cryptic at the outcrop scale and because accumulation trends follow a similar, yet opposite trajectory as crystal fractionation trends¹. Hornblende (Hbl) chemometry and Hbl-melt Fe/Mg tests provide a means to evaluate whether individual bulk-rock samples approximate an equilibrium melt composition or whether crystal accumulation is an important magmatic process. We compare bulk-rock compositions from several plutonic and volcanic systems to melt compositions calculated via Hbl chemometry. We also compare and contrast the range of melt compositions and crystallization temperatures observed between Hbl from plutonic and volcanic systems.

Hbl from plutonic and volcanic systems displays significant compositional overlap from Si (apfu) 6.4–7.2. However, volcanic Hbl extends to higher temperatures (1029°C) and lower Si whereas Hbl from plutons extends to lower temperatures (532°C) and higher Si. Only rarely does Hbl from volcanic systems yield temperatures < 750°C. Most calculated melts from plutonic Hbl yield rhyolitic compositions, indicating that the wide-range of observed bulk-rock compositions (SiO₂: 45–77 wt.%) may be due to varying amounts of crystal accumulation rather than chemical differentiation. The melts calculated from Hbl in intermediate volcanic rocks are instead more diverse, yielding SiO₂ contents that vary by up to 20 wt.% in a single sample. In some cases, these wider compositional ranges are bimodal, which is indicative of magma mixing. The general absence of intermediate melt compositions in plutons is in agreement with melt inclusion studies from volcanic suites, which suggest that intermediate melt compositions are rare². Our results indicate that crystal accumulation is an important process in granitic systems and should be considered in conjunction with other magmatic processes when discerning petrogenesis.

¹Deering, C. D., Bachmann, O. (2010). EPSL 297, 324-331.

²Reubi, O. & Blundy, J. (2009). Nature 461, 1269.