SIGNIFICANCE OF THE COMPOSITIONAL HETEROGENEITY IN THE KUNA CREST UNIT OF THE EASTERN MARGIN OF THE TUOLUMNE INTRUSIVE COMPLEX, SIERRA NEVADA, CA

Understanding the emplacement and evolution of large and long-lived plutons in the middle crust is essential in comprehending igneous crustal growth, how arcs operate and are physically and compositionally connected to the source and volcano. One challenge is that the plutonic rock record seldom preserves the initial stages of pluton growth, which is often overprinted by subsequent magmatism. The initial stages of growth, however, may give us clues on the size and compositions of the first intruding magma batches and if or when they coalesce to form interconnected magma mush bodies.

Early stages of pluton growth are preserved in the 95-93 Kuna Crest margin located in the southeast lobe of the 95-85 Ma, 1,100 km², normal zoned Tuolumne Intrusive complex (TIC) in the central Sierra Nevada batholith. The Kuna Crest is composed of cm- to m-scale sheets at the margins to ≤ km-scale, irregular bodies toward the interior of the lobe. Compositions range from fine- to medium-grained granodiorite, medium- to coarse-grained tonalite, diorite, and gabbro with knife sharp to gradational contacts. Color index ranges from 10% to 32%.

To determine the significance of intra-unit mineralogical and geochemical variation observed in the field, we conducted detailed petrographic and whole rock and mineral-scale geochemical analysis. Petrographic analysis reveals varying mineral abundances and textural variation across subunits, indicating different histories, some containing sparse cpx-hbl-bio-oxide glomerocrysts. Whole rock element chemistry across units and REE element patterns of hornblende cannot be explained by fractional crystallization and suggest only intra-unit differentiation of compositionally diverse and isolated magmas. Sr and Nd isotopes from the Kuna Crest lobe, however, show all magmas originated from a homogenous and rather primitive source. Isotope data from the sheeted margin will reveal how much homogeneity is preserved there. This may help resolve if magma amalgamation and homogenization occurs at emplacement levels.

Our preliminary conclusion is that each Kuna Crest subunit – from single sheets to larger interconnected areas- was emplaced as increments to grow a magma mush that underwent varying degrees of fractionation at the emplacement level without amalgamating to even bigger magma mush bodies.