LONGITUDINAL PETROLOGIC VARIATION OF THE NORTHERN PARADISE AND WHITNEY PLUTONS, CALIFORNIA: INSIGHTS INTO DEVELOPMENT OF COMPOSITE FELSIC INTRUSIONS

Petrologic variation in upper crustal magmatic systems results from the superposition of features produced at or near the level of emplacement onto those produced by secular changes in lower crustal sources. In the Mount Whitney Intrusive Suite (MWIS) of California's eastern Sierra Nevada, high-level processes have been inferred from spatial relationships between petrologic variations and the body's margins along a pair of transverse sections. Longitudinal variation of the suite's two younger members, the Paradise and Whitney plutons, is now being measured to better constrain the natures and areal extents of these processes.

Textural and compositional changes along a 40 km traverse from the northwestern margin of the Paradise pluton to the center of the Whitney pluton include: (1) a transition from equigranular to K-feldspar megacrystic textures in the first 2 km, followed by southeastward increases in the abundances and maximum lengths of megacrysts; (2) a decrease in the abundance of microdioritic enclaves from 0.5 to 0.2% in the Paradise pluton and a parallel decrease from 0.08 to 0.01% in the Whitney pluton; and (3) a decrease in color index from 15.5 to 10.5% in Paradise pluton and a parallel decrease from 6.5 to 5.5% in the Whitney pluton.

These observations support several preliminary conclusions about the intrusive and thermal history of the MWIS: (1) the similarity of rocks from the northernmost Paradise pluton and the older Sugarloaf-Lone Pine Creek pluton are consistent with the compositional continuity inferred between these units; (2) the northernmost part of the Paradise pluton may have cooled too quickly for the growth of K-feldspar megacrysts by textural coarsening, but slower cooling in the suite's interior promoted such growth; (3) enclave abundances indicate the amounts of mafic magmas that mingled with the intrusion decreased over time, perhaps due to entrapment beneath a growing low-density felsic reservoir; (4) southeastward decreases in the color indices of both plutons may reflect secular changes to more felsic compositions, more extensive fractionation in the warm interior of the growing intrusion, or both; and (5) compositional uniformity of the Whitney pluton suggests that the felsic “cap” encountered at elevations above 3200 m in the central part of the body may extend to its northern margin.