242-9 Local Crust Imparts its Geochemical Signature on Arc Magmas: A Regional Case Study of Shallow Mafic Intrusions Across the Sierra Nevada Batholith

Session: Crustal Petrology, Part I

Presenting Author:

Juliet Ryan-Davis

Authors:

Ryan-Davis, Juliet¹, Bucholz, Claire Elizabeth², Lackey, Jade Star³, Kylander-Clark, Andrew Robert Cooper⁴, Lewis, Madeline J.⁵, Kitajima, Kouki⁶, Valley, John W.⁷

(1) MIT, Cambridge, MA, USA; Boston College, Chestnut Hill, MA, USA, (2) California Institute of Technology, Pasadena, CA, USA, (3) Pomona College, Claremont, CA, USA, (4) UC Santa Barbara, Santa Barbara, CA, USA, (5) University of Wyoming, Laramie, WY, USA, (6) University of Wisconsin, Madison, Madison, WI, USA, (7) University of Wisconsin, Madison, Madison, WI, USA,

Abstract:

Contamination of subduction zone magmas by the upper plate crust is widely documented. However, quantifying how much material the crust contributes to arc magmas and the efficiency of the contamination process remains challenging, yet important for interpreting global (i.e., detrital zircon) datasets. The Sierra Nevada batholith is an ideal location to document the influence of the upper plate crust on arc magmas, as Mesozoic arc activity migrated through heterogeneous crust. Critically, Cretaceous intrusive activity migrated broadly from west to east across the batholith, resulting in interaction between magmas and well-documented arc-parallel belts of pre-existing crust and lithosphere with variable affinities—and thus unique isotopic signatures. This study focused on small (<35 km²) gabbro to diorite intrusions throughout the Sierra Nevada batholith, which represent the crystallized products of basalt to basaltic andesite magmas. Oxygen and hafnium isotope ratios in zircon (complemented by Sr and Nd isotopes of bulk-rock samples) were analyzed from Jurassic to Cretaceous mafic intrusions along a ~225 km arc-perpendicular transect across the batholith. The intrusions have distinctive isotopic signatures which are independent of their crystallization age and result from interaction of their parental magmas with local pre-existing crust. Because the pre-existing geology changes from juvenile, oceanic crust to ancient continental crust along this transect, the isotopic signatures vary greatly despite the amount of assimilation recorded by each mafic intrusion remaining similar (~20% on average). This transect of gabbroic and dioritic intrusions across the Sierra Nevada batholith provides a ~110-million-year record of contamination of continental arc magmas during early, mafic stages of their differentiation. Thus, the pre-existing geology of continental arc crust and lithosphere exerts a primary control on the isotopic composition of arc magmas. This implies that interpretation of global isotopic datasets must account for the type of pre-existing, assimilated crust to accurately quantify the amount of recycling.

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

doi: 10.1130/abs/2025AM-6717

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