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

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 - 12:00 PM
HBGCC, 216AB

Growth and Internal Evolution of the Box Springs Plutonic Complex, Peninsular Ranges Batholith, Riverside, California

Paper number: 10-8
Program: Technical Sessions
Time: 9:55 AM – 10:10 AM

Authors

Abstract

The Box Springs Plutonic Complex (BSPC) in southern California exhibits reverse zoning with a core of biotite tonalite surrounded by foliated tonalites and granodiorites. It is characterized by inward-dipping, sub-concentric intrusive units and concentric fabrics discordant with the regional NW trend, forming a funnel-shaped geometry typical of a lopolith. To reconstruct the BSPC’s emplacement history, we integrated detailed mapping, petrography, structural analysis, geochemistry, and zircon U-Pb geochronology.

The intrusion sequence of early felsic magmas followed by deeper, less evolved magmas, together with textural evidence, Eu anomalies, trace elements, and Sr isotopes, suggests that most fractionation occurred prior to emplacement from a deeper, vertically stratified magma chamber.

We propose a four-stage emplacement model. Stage 1: Kbgg granodiorite (102.35–100.96 Ma; Sri = 0.7054–0.7050) was emplaced first as a relatively high-SiO₂, K-rich unit, forming a marginal dynamothermal aureole with intense foliation. Stage 2: Kbfg tonalite (99.88–98.47 Ma; Sri = 0.7050–0.7054) intruded next, displacing Kbgg and enhancing foliation. Stage 3: less differentiated Kbt tonalite (98.77 Ma; Sri = 0.7049) intruded as an undeformed core. Stage 4: Kbft tonalite (98.78–97.41 Ma; Sri = 0.7054), geochemically similar to Kbt, intruded the northern and southern margins.

Magmatic fabrics, concentric contacts, schlieren, and contact metamorphism support magma wedging and lateral shortening, while two subvertical, inward-dipping ductile shear zones reflect floor subsidence as the main mass transfer processes. Although previously interpreted as ballooning, BSPC's features support its emplacement as an incrementally assembled lopolith derived from a deeper vertically zoned magma source.

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

doi: 10.1130/abs/2025AM-414929

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