INVESTIGATING COMPOSITIONAL CHARACTERISTICS OF MAFIC RECHARGE WITH ENCLAVES IN THE JACK MAIN INTRUSIVE COMPLEX, CENTRAL SIERRA NEVADA, CALIFORNIA

Plutons in the middle to upper crust have long lived magmatic histories (10s kyrs to myrs) that, in part, are sustained by the heat from magma recharge events. It is not well understood how much physical (crystal mixing) and chemical (melt mixing and/or diffusion) exchange takes place during magma mixing of recharge and host magmas, especially at the emplacement level. Evidence of magma mingling during mafic recharge events in the Jack Main Intrusive Complex (JMIC), central Sierra Nevada, CA, is preserved as mafic dikes and globules forming mafic microgranular enclaves (MME) entrained in intermediate to felsic host magmas.

The JMIC is a NW migrating, partially nested set of plutons composed of, from oldest to youngest, the Quartz Diorite of Mount Gibson (Kgi), the Granodiorite of Bearup Lake (Kbu), the Granodiorite of Lake Vernon (Klv), and the Boundary Lake Granite (Kbl). MME are non-uniformly distributed throughout JMIC units in order from most to least abundant in the Klv, Kbu, Kgi, and Kbl. It is unknown if the MME in the four units originated from the same magma source and to what extent these magmas affected host magma compositions through magma mixing and mingling.

Several MME (cm to few m size) populations are observed across the JMIC: most MME are fine-grained equigranular with grain sizes <1 mm; others are plagioclase porphyritic with phenocrysts between 1 and 4 mm. The overall, relatively uniform compositions across all units are intriguing. To test the hypothesis that MME in the JMIC are sourced from an identical source, including dikes from the Kgi and Kbu, petrography and XRF geochemistry of enclave samples and dikes are underway. To test the degree to which MME contributed to the compositional variability of JMIC magmas, we use petrography, cathodoluminescence and electron microprobe analyses to examine crystal zoning patterns of feldspars at the MME-host magma interface. Matching growth zones in the MME and host magmas suggest partial or full chemical equilibrium through melt mixing between the two magmas, while growth zones in crystals from MME populations found in the host magma (and vice versa) are indicative of crystal exchange. Thin sections at variable distances from the mingling interface will be examined to identify the spatial extent of crystal/melt mixing between MME and host magma.