CRYSTAL ACCUMULATION AND COMPOSITIONAL TRENDS IN A TILTED CALC-ALKALINE PLUTON

The Wooley Creek batholith, a Late Jurassic, arc-related, calc-alkaline plutonic complex, exposes ~12 km of structural relief owing to syn(?)- and post-emplacement tilting and erosion. The structurally lower zone (~159.1 Ma) was assembled by piecemeal emplacement of many pyroxenitic to tonalitic magma batches, whereas the structurally higher upper zone (~158.2 Ma) grades from lower quartz diorite to upper granite.

Lower-zone pyroxenes are too Fe-rich to be in equilibrium with a melt with the composition of the host rock. Mass balance and rhyolite-MELTS simulations show these rocks to be pyroxene + plag ± olivine ± apatite ± oxide cumulates. Interstitial melt varied from ~7.5–83%. Cumulate plagioclase/pyroxene ratios vary widely in pyroxenites to diorites, but are relatively uniform in quartz diorites to tonalites. This near-constant ratio results in compositional trends that mimic a liquid line of descent. Expulsion of melt from lower zone cumulates was probably the result of a filter-pressing-like processes caused by episodic emplacement of magma batches, with consequent upward migration of fractionated magmas.

Upper-zone bulk-rock trends reflect differentiation of andesitic magma. Mass balance and rhyolite-MELTS calculations indicate that structurally lower parts of the upper zone are cumulates of hornblende + plagioclase ± biotite + accessory minerals, with 37–80% trapped melt. The cumulate plag/(plag + mafic minerals) ratio of ~0.48 mimics a liquid line of descent. Accumulation was probably due to hindered settling. Structurally higher upper zone rocks formed from the magmas extracted from the subjacent cumulates, with melt fractions from 54–92%.

We conclude that compositional variation in many calc-alkaline plutons is representative of crystal accumulation instead of, or in addition to fractional crystallization. If so, the assumption that bulk compositions of arc plutonic rocks geochemically resemble frozen liquids is problematic. In addition, geochemical comparisons of plutons with potentially comagmatic volcanic rocks must account for accumulation in the plutons.