ASSESSING THE ROLES UPPER VERSUS LOWER CRUSTAL PROCESSES IN CREATING DIVERSITY WITHIN FELSIC INTRUSIONS

Textural and compositional diversity in felsic intrusions reflect (1) changing conditions in their lower crustal sources and (2) upper crustal thermal histories that depend on the geometries, volumes and timing of the intrusive phases that built them. To better understand the roles these factors have played in the development of the Mount Whitney Intrusive Suite (MWIS), a composite intrusion in the eastern Sierra Nevada, measurements of enclave abundance, mafic mineral abundance and alkali-feldspar megacryst size have been used to map the intrusive phases that comprise the northern parts of the suite’s two younger members. Field relations suggest these nested tabular plutons grew as magmas spread laterally from dikes at an extensional step-over along the Sierra Crest shear zone and published U/Pb zircon dates indicate that both bodies young southward, toward their inferred “feeder” dikes.

The Paradise pluton apparently consists of three intrusive phases, each of which is defined by a southward decrease in the abundance of microdiorite enclaves. Mafic mineral abundance and megacryst size vary smoothly along the length of the pluton, however, independent of the boundaries inferred from enclave abundance. This lack of correlation suggests that these properties may, instead, reflect long-term changes in the composition or temperature of the suite’s lower crustal source—changes that also account for the more evolved Sr and Nd isotopic compositions of the suite’s younger members.

The younger Whitney pluton consists of two intrusive phases; the older is defined by steep southward decreases in the abundances of enclaves and mafic minerals and the sizes of megacrysts, whereas the younger is characterized by nearly constant enclave and mafic mineral abundances and a southward doubling of megacryst sizes.

Prior study of the central part of the MWIS suggested that warming of the upper crust during the body’s growth slowed the cooling of its successive intrusive phases and led to a transition from composite to internally-differentiated plutons. The presence of distinct intrusive phases in the distal parts of the two younger members reveals, however, that lower crustal processes had created significant textural and compositional diversity among the magmas that built the suite before upper crustal processes came into play.