ANALYZING MAFIC ROCKS AND THEIR INTERACTIONS WITH GRANODIORITIC HOST MAGMA IN THE JACKASS LAKES PLUTON OF THE CENTRAL SIERRA NEVADA BATHOLITH

The intrusion of mafic magmas has often been interpreted to trigger volcanic eruptions. Thus, understanding interactions between mafic and felsic magmas in plutons is important.

The 175 km2, 98-97 Ma Jackass Lakes pluton (JLP) in the central Sierra Nevada batholith, a slightly older pluton just south of the 1,100 km2, 95-85 Ma Tuolumne intrusive complex, is compositionally more heterogeneous at map to outcrop scale. It is largely composed of compositionally varying and mappable equigranular to porphyritic granodiorites. In addition, cm- to several km-long quartz diorite bodies are found across the JLP. McNulty et al. (1996) suggested that these mafic magmas represent dikes or sheets, while Pignotta et al. (2010) proposed that they occur in irregular shapes and mixed/mingled with host magmas.

This study analyzed seventeen samples from map- to cm-scale mafic enclaves from the NW- central JLP with petrography, CL, XRF whole rock major element, and LA-ICPMS trace and rare earth element analyses to examine 1) if these mafic magmas are compositionally homogeneous and likely derived from the same source, and 2) to what degree they interacted with host granodiorites. Most mafic rock outcrops observed in the study area occur in cm- to m-scale irregular bodies made of quartz diorite.

Preliminary petrography has revealed that of the five most prominent minerals typical in intermediate granitoids, hornblende and plagioclase feldspar vary most in abundance, ranging from 12-40% and 5-27% in area. Most samples are porphyritic, with plagioclase, quartz, and alkali feldspar making up most phenocrysts (the latter two likely deriving from the granodioritic host). Plagioclase varies most in grain size (0.20-20 mm) and forms sizeable clusters. Sericitized plagioclase cores indicate higher An-content derived from earlier more mafic magmas.

Zoned plagioclase grains from the diorite and host granodiorites will be imaged with CL and compared to one another. Although mixing between these two magma types is apparent, XRF and LA-ICPMS whole rock element concentrations will help determine the degree of such mixing (and crystal recycling).

While the same mineralogy and similar mineral abundances allude to a potential shared magma source, this hypothesis will also be further tested with XRF and LA-ICPMS whole rock element data.