74 How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution (Posters)

This session integrates structural and petrologic views of plutonic systems, exploring the crustal response to pluton formation, and processes that control the physical and chemical characteristics of pluton-host rock systems from initial emplacement to solidification.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Contact Relationships and Inherited Zircon: Investigation of Pluton Emplacement in the Tuolumne Intrusive Suite, Sierra Nevada, California

Paper number: 74-4
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The emplacement of plutonic rocks in the upper crust has implications for the formation of continental crust and the generation and storage of arc magma beneath volcanic centers. The Tuolumne Intrusive Suite (TIS) in Yosemite National Park consists of five texturally distinct granodiorite units that are younger and progressively more felsic toward its center, and offers an ideal site to investigate two competing hypotheses for pluton emplacement: (1) kilometer-scale magma intrusions that mix in the upper crust, and (2) smaller sub-kilometer pulses that evolve in the lower–middle crust.

 

To investigate these hypotheses, we examined the Porphyritic Half Dome (pHD)-Cathedral Peak (CP) contacts near Volgesang, Evelyn, and Sunrise Lakes, where previous studies have mapped transitional units that suggest upper-crustal mixing. pHD displays abundant hornblende and inclusion-rich K-feldspar phenocrysts up to 3 cm in length. CP is distinguished by abruptly decreasing hornblende, and larger K-feldspar megacrysts with inclusions that are limited to K-feldspar cores if they are present at all. Across all sites, the pHD–CP contact is sharp and ranges from pencil-thin to several meters across, and it is commonly decorated by layered granite with oriented K-feldspar megacrysts and/or layers of clusters of unoriented K-feldspar megacrysts. Locally, layered granites and K-feldspar megacryst clusters intrude up to 30 meters into pHD.

 

To assess the possibility of mixing between pHD and CP units, we used plots of U/Ta vs. Nb/Ta in zircon to distinguish autocryst and inherited antecryst populations in CP samples. Across the CP, geochemical antecrysts yielded ages of >89 Ma, and overlap in age and geochemistry with zircon from older TIS units. Antecryst populations are highest (up to 19% of all analyses) in CP samples within 100 m of the pHD-CP boundary and are homogeneously lower (< 7%) in samples investigated across the interior of the CP. Marginal CP samples also display slightly older autocryst ages (88.7–88.0 Ma) whereas autocryst ages from the interior of the CP are younger (87.2-87.6 Ma). Increasing antecryst abundance and autocryst ages in CP near the pHD-CP is consistent with emplacement-level intra-unit mixing and chilled margins, or alternatively, may suggest that marginal CP represents a distinct TIS unit. Future work will focus on a suite of samples collected from a TIS transect near Evelyn Lake.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414364

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