10-9 The spatial distribution of inherited zircon within the Tuolumne Intrusive Suite, Sierra Nevada, California

Session: How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution

Presenting Author:

Scott Johnston

Authors:

Johnston, Scott M.¹, Kylander-Clark, Andrew Robert Cooper², Lackey, Jade Star³, LeValley, Marissa⁴, Reali, Marie Clare⁵

(1) Cal Poly, SLO, San Luis Obispo, CA, USA, (2) UC Santa Barbara, Santa Barbara, CA, USA, (3) Pomona College, Claremont, CA, USA, (4) Cal Poly San Luis Obispo, Physics, San Luis Obispo, CA, USA, (5) California Polytechnic State University, San Luis Obispo, Santa Barbara, CA, USA,

Abstract:

The evolution and emplacement of granitic rocks in magmatic arcs have broad implications for the formation of continental crust and magma storage beneath volcanic centers. The Late Cretaceous Tuolumne Intrusive Suite (TIS) in the Sierra Nevada batholith exposes 5 texturally distinct granitic units that are inwardly more felsic and younger, and provide an opportunity to study plutonic magma geochemical evolution and emplacement. Competing models for TIS construction suggest it formed via numerous pulses of sub-km-scale intrusions not capable of mixing at the level of emplacement, or through several multi-km-scale pulses that periodically formed a dynamically mixing upper-crustal magma chamber. To test these models, we predict that if the TIS units mixed at the level of emplacement, inherited antecrystic mineral cargo should be more concentrated near TIS contacts. Alternatively, if mixing and magma evolution was limited to lower-crustal processes, we predict that there would be no spatial trend in the distribution of inherited material across individual TIS units.

 

We present field relationships and LA-ICPMS zircon geochronology and geochemistry in 19 samples collected from a transect across the TIS. Our results reveal that zircon < 89 Ma separated from the Cathedral Peak granodiorite (CP—within the interior of the TIS) displays high U/Ta and Nb/Ta, whereas analyses > 89 Ma are associated with distinctly lower U/Ta and Nb/Ta characteristic of zircon from older TIS units. When samples from across the CP are compared, three CP samples collected within 100 m of contacts with older TIS units are older (88–87 Ma) and contain at least two times as many antecrystic grains as five younger (87–86 Ma) CP samples collected from across the 10–15 km interior width of the CP. This spatial distribution of antecrystic zircon across the CP is consistent with mixing between TIS units at the emplacement level, although slightly older ages in marginal CP, and sharp contacts between marginal CP and older units suggest that upper-crustal mixing may have been spatially limited, or that marginal CP may represent its own unit. Planned zircon analyses from a new TIS sample transect as well as from several porphyritic intrusions displaying fine-grained groundmass with phenocrysts of amphibole and K-feldspar (locally megacrystic) will add to our understanding of TIS magma emplacement.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10389

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