REINTERPRETING COMPOSITIONAL ZONING OF THE CENTRAL WHITNEY PLUTON, SIERRA NEVADA, CALIFORNIA

The Whitney pluton is the youngest, innermost member of the concentrically nested Mount Whitney Intrusive Suite. A transect across the center of the pluton reveals that it is symmetrically zoned, with ferromagnesian mineral abundances that decrease linearly inward for 2 km from each margin and are nearly constant across the body’s 10 km-wide interior. Mineral compositions define similar trends, with inward decreases in the An contents of plagioclase and increases in the Mg#s and Mn contents of hornblende and biotite.

Because elevation along the transect increases inward from the pluton’s margins to its center and the transition from its marginal to central zones consistently occurs at an elevation of ⁓3200 m, Hirt (2007) proposed that the pluton is vertically zoned with a uniform granite overlying a compositionally graded granodiorite. In conjunction with a thermal model which indicates that warming of the upper crust substantially lengthened the intervals during which magmas remained miscible as the suite grew, this zoning suggests that successive magma batches may have amalgamated to form a reservoir in the center of the Whitney pluton. About 10% fractionation of an assemblage dominated by plagioclase + hornblende + magnetite within this reservoir can account for the compositional differences between the central granite and the marginal granodiorite.

To test whether partial crystallization and melt loss from the marginal granodiorite could account for the pluton’s zoning, the procedure proposed by Barnes et al. (2019) was used to estimate the melt fractions lost from rocks along the transect. The estimates vary from 14-37%, have similar ranges in all samples, and are not systematically greater for the marginal granodiorite than for the central granite. This implies that these rocks are probably not related by crystal fractionation at or near the exposed level.

Zircon age spectra suggest that the magma which formed the northernmost part of the Whitney pluton interacted with and entrained crystals from the adjacent, more mafic Paradise Granodiorite prior to or during its emplacement. Similar interactions between the Whitney granite and surrounding Paradise granodiorite along the margins of the central part of the Whitney pluton can account for the body’s symmetrical compositional zoning.