CRACKING DOWN: SOLIDIFICATION FRONT INSTABILITY AND THE SEGREGATION OF LEUCOGRANITE MELTS FROM GRANITIC PLUTONS

The Paradise and Whitney plutons of the eastern Sierra Nevada are regularly zoned from granodioritic to granitic compositions inward from their margins. Parallel changes in mineral compositions (e.g., lower An in plagioclase and higher Mg# in mafic silicates) suggest that this zoning resulted from large-scale crystal fractionation within these bodies. A zoning reversal occurs in the upper central part of each pluton, however, that is unsupported by corresponding changes in mineral chemistry. These abnormally mafic regions host numerous subhorizontal leucogranite dikes whose textures and compositions indicate they crystallized from extensively fractionated, volatile-rich melts.

The compositions of the central parts of the plutons predicted from zoning are reproduced by mixtures consisting of 15% of the leucogranites and 85% of the anomalously mafic rocks, suggesting that the reversals resulted from the segregation of interstitial melts in these areas. In the Paradise pluton, subhorizontal dikes of fine-grained granodiorite that are spatially associated with the leucogranites and more felsic than predicted by the overall zonation of the pluton are interpreted as melt-enriched regions.

Field relations and supporting analytical data suggest that both the dikes and compositional reversals are the products of solidification front instability (SFI) above the central, melt-rich parts of these intrusions. Delamination of a crystal-rich solidification front from the roof of the plutonic reservoir initially drew melt from the adjacent magma into a lens-shaped region above the front and produced the fine-grained granodiorite in the central part of the SFI. Continued sinking of the crystal-rich layer fractured the more extensively crystallized magma at the distal ends of the SFI and enabled volatile-rich interstitial melts to migrate outward to form the leucogranite dikes. Preservation of a SFI in a meter thick leucogranite dike provides insights into the mechanics of this segregation process, and the aspect ratio of the dike's SFI (width/height @ 10) is similar to the ratio between the widths of the compositionally reversed zones in the plutons and the thicknesses of the associated melt-enriched regions.