10-3 Crustal Controls on Systematic Variability of Mafic Intrusions Across the Sierra Nevada Batholith

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

Presenting Author:

Juliet Ryan-Davis

Authors:

Ryan-Davis, Juliet¹, Bucholz, Claire Elizabeth², Lackey, Jade Star³, Lewis, Madeline J.⁴, Kylander-Clark, Andrew Robert Cooper⁵, Wilner, Oliver⁶

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

Abstract:

Upper-crustal mafic intrusions of the Sierra Nevada batholith record spatiotemporal variability in their styles of emplacement, petrology, and geochemistry across a tectonically assembled framework of heterogeneous upper plate lithosphere and crust. This framework is best described as four distinct batholith-parallel belts with variable lithospheric affinity—the pre-existing host-rock for Mesozoic Sierra Nevada magmatism. Although typically small (≤35 km² exposure area), gabbro and diorite intrusions are commonly found throughout the upper crust (~2–3 kbar) of the Sierra Nevada batholith. They provide a record of initial stages of differentiation (compositionally), as they are the crystallized products of mafic arc magmas. We summarize field relationships, geochronology, and whole-rock geochemistry of 46 mafic intrusions spanning ~225 km (west to east) across all four crustal belts in the central the Sierra Nevada batholith. The mafic magmas parental to all the upper-crustal mafic intrusions are not primary, mantle-equilibrated compositions; rather they are low-MgO basaltic compositions produced through prior differentiation of mantle-derived magmas (in the lower crust). Ages of the mafic intrusions across the transect range from Jurassic to Cretaceous (~194–88 Ma). These ages indicate that Jurassic mafic magmatism occurred in both the western and eastern belts of the batholith, and that Cretaceous mafic magmatism migrated west to east across the belts, with final stages of activity in the high Sierra (just west of the range crest), contemporaneous with voluminous felsic magmatism of the batholith. In the western and central belts, gabbro and mafic cumulates are more common, and geochemistry indicates thinner crust. In the eastern and high Sierra belts, mafic intrusions have geochemistry that reflects a source beneath thick crust. In particular, the youngest mafic intrusions of the high Sierra comingle with their felsic host intrusions and are compositionally evolved (diorite, with rare gabbro). These systematic variations in geochemistry and style of emplacement of mafic intrusions across the Sierra reflect the pre-existing upper-plate crustal thickness, composition, and strength. These parameters that vary systematically by belt thus have a first-order control on the involvement of mafic magmas in production of the silicic batholith.

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

doi: 10.1130/abs/2025AM-5467

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