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

Reverse Zonation in the Bucks Lake Pluton: Field and Petrographic Evidence for Magma Mingling and Mineral Accumulation at Map to Mineral Scale

Paper number: 74-2
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

  • Matthew Connor Presenting Author
    Department of Geosciences, Texas Tech University, Lubbock, Texas
  • Katie Ardill
    Department of Geosciences, Texas Tech University, Lubbock, TX
  • Ethan Langton
    Department of Geosciences, Texas Tech University, Lubbock, TX

Abstract

Reversely zoned plutons potentially record localized variations in magma process not captured by more commonly unzoned or normally zoned plutons. Understanding these systems provides insights into the efficiency of fractional crystallization, crystal segregation, and magma mingling to produce the final compositional diversity of plutons. The Cretaceous Bucks Lake pluton in the northern Sierra Nevada exhibits reverse compositional zonation from two-pyroxene diorite in the Central Zone to hornblende-biotite diorite and quartz diorite in the Outer Zone (Hietanen, 1973).

This study presents an updated 1:10,000 scale geologic map and petrographic and initial trace element analysis of 23 samples containing hornblende to evaluate the extent of emplacement level magma interactions. Six lithologic subunits are identified within the Central and Outer zones based on mineral modes, grain sizes, enclave morphology, and foliation patterns. The more felsic Outer Zone contains diorite to quartz-diorite with two main subunits: a hornblende-dominated margin exhibiting strong foliation (3-4M) with ellipsoidal enclaves and leucocratic segregations, and an inner biotite-dominated subunit with weaker foliation (1-3M) lacking abundant hornblende and enclaves. A 20-100 meter wide gradational hornblende-biotite-diorite Transition Zone separates the Central and Outer zones and contains pyroxene-hornblende clots and mafic enclaves. The mafic Central Zone contains dominantly pyroxene and hornblende with minimal biotite and weak foliation (0-1M). The Spanish Peak and Peninsular mafic subunits break this broadly concentric zonation pattern. Located within the Outer Zone, these subunits contain pyroxene and distinctive skeletal hornblende habits with weak to no foliation.

The gradation in mineral assemblages, textures, and enclave characteristics from margin to center indicate magma mingling within the Transition Zone, with mineral accumulation interpreted to create local variations between subunits.  Enclaves and non-concentric mafic bodies are permissive of separate magma injections that may relate to the Central Zone or record deeper level source magmas. Enclave populations show trace element signatures in hornblende (e.g., Ti, Sr, REE patterns) genetically linking Outer Zone enclaves to host rocks while Central Zone enclaves display distinct fractionation histories to Outer Zone enclaves. At this stage, emplacement level magma differentiation is visible at field and thin section scales, particularly in the Transition Zone and enclaves. Future geochemical analyses will expand the dataset across newly designated subunits to investigate the role of these processes in producing the overall zonation of the Bucks Lake pluton.

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

doi: 10.1130/abs/2025AM-413223

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