28 Nature of Magma Processes in Different Sized and Shaped Plutons

Friday, 19 May 2023
8:00 AM - 12:00 PM
Whitney Peak Ballroom 2

A COMPARISON OF MAGMA PROCESSES RECORDS IN THE TUOLUMNE INTRUSIVE COMPLEX, KUNA CREST LOBE, AND THE JACKASS LAKES PLUTON, SIERRA NEVADA BATHOLITH: SIZE, LONGEVITY, AND MAGMA FOCUSING MATTER

Paper number: 28-2
Program: Theme Sessions
Time: 8:05 AM – 8:25 AM

Authors

  • Valbone Memeti Presenting Author
    California State University Fullerton, Fullerton, CA

Abstract

Arc magma plumbing systems may develop thick magma columns as they mature over time. Magmas may stall at different crustal levels to form varying size magma storage sites that are active over myrs. The 1,100 km2, 95-85 Ma Tuolumne intrusive complex (TIC) represents such a site. It is composed of four nested, extensive granodiorite units of irregular shape that are largely separated by gradational contacts and wide hybrid zones. Local sharp contacts often coincide with structural truncations consistent with magmatic erosion. Mineral and whole rock geochemistry and geochronology indicate that magma mixing, recycling of older magma mush into younger, and melt loss were important. However, TIC initiation was distinct. Its record is preserved in a < 1 km wide sheeted complex that lines the 95-93 Ma, 70-80 km2 Kuna Crest lobe (KCL). It is composed of cm-m thin gabbroic to granodioritic sheets that did not interact. They crystallized within <500ka, indicating more punctuated, low volume, and compositional heterogeneous magma before they increasingly amalgamated to form bigger magma bodies, first in the KCL interior, and later in the TIC interior. How long-lived and prevalent were sheeting and magma-magma interactions before 95 Ma? Did large-scale magma mixing, homogenization, and erosion (MHE) occur outside the magma focusing zone?

The ~175 km2, 98-97 Ma Jackass Lakes pluton (JLP) south of the TIC is about one sixth of the TIC size and longevity and twice as large as the KCL but same longevity. It is trapezoidal in shape and has been interpreted to be sheeted. It is composed of the main porphyritic JLP granodiorite that was injected by younger, elongate to irregular shaped, more mafic granodiorites and quartz-diorites with sharp contacts. In the NW JLP, the younger granodiorites and diorites grade into one another indicating magma interconnectivity and hybridization between units, while they are reported to be more discreet sheets in the S and E. Although U-Pb zircon ages and geochemistry for the JLP will further test this hypothesis, the smaller, shorter lived JLP shows more KCL characteristics than interior TIC pattern. Observed heterogeneity in KCL and JLP likely represents original magma pulses during initial magma amalgamation while major MHE processes failed due to faster cooling and location outside the magma focusing zone.

Geological Society of America Abstracts with Programs. Vol. 55, No. 4

doi: 10.1130/abs/2023CD-387669

© Copyright 2023 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.