TRACING THE TRANSITION FROM HORNBLENDE- TO TITANITE-DOMINATED FRACTIONATION OF RARE-EARTH ELEMENTS DURING CRYSTALLIZATION OF A GRANITIC PLUTON

It is often difficult to trace the liquid line of descent in granitic plutons because it is unclear which, if any, of the rocks they contain approximate melt compositions. Two of the three lithologies preserved in the Whitney pluton of the eastern Sierra Nevada, however, may reasonably be inferred to represent near-liquid compositions. Tracing rare-earth element (REE) differences among these rocks indicates how fractionating phases changed during the pluton's solidification.

The marginal granodioritic phase of the pluton gives way to an inner granitic phase across a subhorizontal boundary at 3300m elevation. The granitic phase is interpreted to be a melt-rich magma that formed by sidewall crystallization in the underlying reservoir and accumulated beneath its domical roof. Its major-element composition can be derived from that of the granodiorite by about 10% crystallization of an assemblage dominated by Pl + Hb + Mt, which leads to an increase in (La/Sm)_N from 5.5 to 7.0 and to a decrease in (Sm/Yb)_N from 4.4 to 3.5.

The central part of the granitic phase, in turn, is cut by subhorizontal leucogranite dikes that are thought to sample highly-fractionated interstitial liquids segregated from the semi-solid granite as crystal-rich solidification fronts tore away from the chamber roof. Miarolitic cavities indicate that the leucogranite liquids were vapor saturated, and their compositions plot along the water-saturated 4-phase curve in the Ab-Or-Qz system at 200MPa. The leucogranites have (La/Sm)_N = 10 to 42, and (Sm/Yb)_N = 1.4 to 2.6. Fractionation of the small amount of Hb found in granite cannot produce these ratios. However they can be explained by removal of a small amount of titanite (Ttn) which fractionates middle REEs even more strongly than Hb.

It is suggested that REE evolution in the Whitney pluton occurred in two stages: (1) initial sidewall crystallization dominated by Hb fractionation raised La/Sm and lowered Sm/Yb in the body's granitic "cap"; and (2) within the granite, the decline in the amount of Hb crystallizing relative to Ttn produced a strong enrichment of La/Sm and depletion of Sm/Yb in the interstitial melt. Subsequent segregation of this melt to form the leucogranite dikes led to notable decreases in La/Sm in the parts of the granite from which the interstitial liquids were withdrawn.