FORTY-FIVE YEARS OF PLUTON STUDIES

This presentation addresses advice given to the author some 45 years ago. The focus is on three statements made by colleagues/advisors regarding the study of quartz-bearing plutonic rocks. Did these statements stand the test of time, as assessed by current study of plutonic rocks, with application of petrologic tools unavailable in distant past: e.g., rhyolite-MELTS, mineral chemography, and single-mineral thermobarometry?

“All plutons are cumulates.” Then, as now, bulk compositions of plutonic rocks were commonly considered to be equivalent to melt compositions, making petrogenetic interpretation and modelling straightforward. (The author subscribed to this idea at the time!) However, mineral-rock Fe-Mg Kd values for pyroxene and amphibole and Ca-Na Kd values for plagioclase indicate that many plutonic *rocks* are not in equilibrium with their constituent minerals. Instead, melts calculated from chemographic algorithms [1] are commonly in equilibrium with these minerals. Independent calculations using zircon saturation equations agree with this observation: many granitic plutons are cumulates.

“Mafic enclaves are not related to magma mixing.” Despite clear evidence of mixing/mingling from volcanic systems, mixing in plutonic environments was viewed skeptically. A variety of field evidence from plutons (e.g., pillowed enclaves, synplutonic dikes) along with ample evidence of disequilibrium among enclave compositions and constituent phenocrysts indicates that mixing/mingling is common and an important means of maintaining and/or defrosting mushy magmas.

“Granites are metamorphic rocks.” Clearly, elemental exchange and deuteric alteration affect many granitic rocks. Even so, single-mineral thermobarometry of augite, hornblende, and plagioclase yield magmatic temperatures, in agreement with rhyolite-MELTS results when calculated melt compositions are used as input. Moreover, immobile trace elements retain concentrations and zoning patterns that reflect petrogenetic processes.

Therefore, pay attention to conventional wisdom, but don’t feel obliged to believe it—test with rich data sets instead. Understand that it’s the rare pluton that was NOT an open system. Be grateful for a field of science that lets you conduct research outdoors.

[1] Zhang et al., AmMin (2017); Higgins et al., CMP (2022)