INFERRING THE TIMING OF MEGACRYST GROWTH AND INTERNAL DIFFERENTIATION IN THE WHITNEY PLUTON, EASTERN SIERRA NEVADA, CALIFORNIA

The Whitney pluton is one of four large granitic intrusions that form the young central members of the nested Sierra Crest intrusive suites. Alkali-feldspar (af) megacrysts up to 10 cm-long comprise about 10 volume % of this rock, which grades from granodiorite to granite inward from the pluton’s margins and upward towards its roof across the center of the body. Prior studies of af megacrysts in this pluton and the similar Cathedral Peak pluton suggest megacrysts grew during episodic recharge of persistent magma reservoirs (Moore and Sisson, Geosphere, 2008) over hundreds of thousands of years (Chambers et al., Geology, 2020). Megacryst “sizes” (areas) are nearly constant across the central part of the Whitney pluton, however, implying that megacryst growth either preceded or occurred independently of the crystal fractionation that produced the pluton’s compositional zonation. To understand when and how megacrysts grew in the Whitney pluton it is useful to examine the sizes and shapes of the original igneous af crystals preserved in dikes associated with it.

At least three large granite porphyry dikes cut the pluton and the mineral assemblages and compositions of these dikes suggest they sample magmas from deeper in the pluton that rose into fractures in its solid carapace and the surrounding wall rocks 0.8-1.1 Ma after crystallization at the exposed level. The sizes and shapes of af crystals preserved in the dikes are correlated: large crystals are faceted whereas small ones are anhedral and round. This is a signature of textural coarsening (Holness, Con. Min. Pet., 2018), a process that can occur when heating leads to preferential dissolution of small crystals and subsequent cooling leads to precipitation of the dissolved material onto the larger surviving grains (Johnson and Glazner, Con. Min. Pet, 2010).

Coarsening of af due to repeated dissolution-precipitation is consistent with Moore and Sisson’s proposal that episodic recharge heated and stirred up crystals in the magma reservoir of the central Whitney pluton. The constancy of megacryst sizes across this body suggests, however, that megacrysts grew relatively early and that only after the dynamic process of recharge had begun to wane did differentiation become dominant.