SPATIO-TEMPORAL MAGMA MIXING IN THE PYRAMID PEAK PLUTON, SIERRA NEVADA, USA: AN INTEGRATED FIELD AND GEOCHRONOLOGY-GEOCHEMISTRY STUDY

Magma mixing is an important process that can trigger volcanic eruptions and contribute to the compositional range of magmas found in arc settings. The Pyramid Peak Pluton in the northern Sierra Nevada Batholith preserves mafic and felsic magma mixing and mingling textures in three areas within the pluton: the Maud Lake (ML), the Lake Sylvia (LS), and the Island Lake (IL) zones. Previous studies solely differentiated between granite and gabbro-diorite map units although wide compositional heterogeneity has been noted. This study combines new detailed mapping, petrography, whole rock chemistry, and U-Pb geochronology to better understand the spatiotemporal processes of magma mixing in the Pyramid Peak Pluton. New mapping of the whole pluton (1:20,000 scale) and detailed maps (1:5,000 scale) of the ML and LS zones reveal the presence of multiple hybrid and granite units in each zone. Field observations show that while mafic and felsic magmas were coeval, granitic magmas were continuously resupplied into the reservoir as indicated by both non-hybrid and hybrid granitic intrusions displaying both sharp and diffuse contacts with mafic magmas as well as autointrusions within formerly emplaced granite. Petrographic observations show widespread disequilibrium textures in hybrid units such as ocellar quartz, mafic clots, rapakivi feldspars, acicular apatite, resorption textures, and multiple mono-mineralic populuations, which are indicative of extensive mingling between magmas. Whole rock chemistry of representative granites and hybrid units in the ML, LS, and the IL zones indicate compositionally similar felsic and mafic magmas between zones. Measured granites range from 70.5 – 74.4 SiO₂ wt. %, hybrids range from 51.0 – 70.5 SiO₂ wt. %, and mafics range from 50.6 – 56.3 SiO₂ wt. %. Almost all analyzed samples have a negative Eu anomaly (0.46 – 0.98), apart from the lack of an Eu anomaly in one monzo-gabbro from unit JdiA (ML zone), and a positive Eu anomaly (1.12) in hybrid quartz diorite JhA (ML zone). Granites Jpg3 (LS zone) and JpgC (ML zone) show distinctly high La/Yb (34.5-36.08) compared to granites Jpg2 (LS zone) and JpgB (ML zone; 11.24-16.6). Upcoming LA-ICP-MS zircon U-Pb analyses will constrain the age of the pluton and will provide insights into whether the mixing zones at Maud Lake and Lake Sylvia were active during the same time or have distinct periods of magma interaction.