ANATOMY OF A FOSSIL MAGMA CHAMBER: ZONING AND CLOSED-SYSTEM FRACTIONATION IN THE WHITNEY PLUTON, SIERRA NEVADA, CALIFORNIA

The 83Ma Whitney pluton is the youngest member of the Mount Whitney Intrusive Suite. Unlike the two older members that surround it and may have prolonged its crystallization time by slowing its cooling, the Whitney pluton is symmetrically zoned from granodioritic margins to a granitic center. Samples throughout the body have similar initial Sr ratios (0.7073-0.7075) and uniformly low (0.01 volume %) abundances of enclaves that preserve mingled mafic magmas. The pluton’s lack of obvious internal contacts, isotopic homogeneity, and low abundance of admixed mafic magmas suggest that: (1) it solidified from a single relatively well-mixed batch of granodioritic magma; and (2) its internal compositional zoning is primarily the result of closed-system crystal fractionation.

Modal variations suggest that hornblende (Hb), biotite (Bt), magnetite (Mt), and plagioclase (Pl) are the major fractionating phases in the pluton, and the abundances of elements enriched in these minerals decrease steadily inward. The most precise data (for Fe and Sc) indicate, however, that the abundances of these elements decrease most steeply in the outermost 2km of the pluton and are nearly constant across its 10km-wide interior. Conversely, the abundances of Sr and Rb change most in the innermost 4km of the body where an antithetic increase in Rb and decrease in Sr raise the baseline Rb/Sr ratio by 50%. The degree of LREE enrichment remains nearly constant across the pluton ([La/Yb]_N=25), but MREE are depleted relative to both LREE and HREE inward ([Sm/La]_N drops from 0.176 to 0.147). The compositions of several minerals also vary regularly across the pluton, with the An contents of Pl decreasing inward and the Mg#s and Mn contents of Hb and Bt increasing.

Modeling indicates that fractionation of an assemblage initially dominated by Hb+Bt+Mt and later containing a larger fraction of Pl reproduces the observed trace-element trends. These models only fit major-element data well, however, if Hb and Bt become more magnesian as crystallization progresses. This suggests that fractional crystallization rather than segregation of an “inherited” crystalline assemblage with a fixed composition provides the best explanation for the zoning of the Whitney pluton.