203 Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder

We encourage papers that challenge and give insights into the interplay between timescales, methodologies, and processes to build a cohesive understanding of the lifespan, evolution, storage, and eruption of magma, from plutonic to volcanic systems.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 216AB

Long-lived magma hybridization in the composite, Late Jurassic pluton, Ashland pluton, Klamath Mtns., CA and OR

Paper number: 203-12
Program: Technical Sessions
Time: 4:30 PM – 4:45 PM

Authors

Abstract

Emplacement of the tilted, compositionally zoned Ashland pluton spanned 9 m.y., with tonalite, diorite, and gabbro in the deepest levels, central quartz monzodiorite (QMD), and shallowest biotite granite. The QMD unit varies from monzonite to granodiorite, displays crude upward increase in K2O, and is characterized by abundant mafic magmatic enclaves (MME). U-Pb (zircon) ages indicate QMD magmatism spanned 3 m.y.

Plagioclase in the QMD and biotite granite is weakly zoned, varying from An40–An20 over all samples. Large plagioclase crystals in the MME are identical, with the exception of a few Ca-richer cores of An68–An43. RhyoliteMELTS models indicate that melts of QMD and MME composition should crystallize liquidus plagioclase more calcic than ca. An50. The QMD and its MME are also characterized by high P2O5 contents (0.3-0.5 wt% in QMD and 0.6-1.15 wt% in MME), approximately 2x the P2O5 contents of coeval Klamath plutons. In the QMD, these high P2O5 values are expressed as abundant small, short apatite inclusions in hornblende, biotite ± cpx, and scant interstitial grains. In the MME, the high P2O5 is due to abundant acicular apatite inclusions, mainly in groundmass plagioclase and quartz.

Mass balance mixing models of QMD and biotite granite compositions thought to represent melts (based on zircon solubility: Barnes et al., 2019 [1]) are consistent with simple mixing of average P2O5-rich MME with K2O-rich rhyolite, in proportions of 48 ± 11% mafic end member. Most probably, the felsic end member contributed sodic plagioclase to hybrid QMD and the MME, with the mafic end member contributing augite to the QMD. In this scenario, apatite-rich hornblende and biotite in the QMD crystallized from a P2O5-rich hybrid melt, explaining the abundance of stubby apatite inclusions.

The P2O5-rich MME also display higher abundances of HFSE than typical mafic rocks in the province. We suggest that these unusual features result from deep-seated fractionation and recharge of primitive arc magmas in the lower crust (cf. Streck & Grunder, 1999 [2]). Thus, development of the QMD suite requires mixing in deep- and middle-crustal levels, in a magma plumbing system active for circa 3 m.y.

[1] J. Petrology v. 60, p. 2227; [2] Contributions v. 136, p. 193

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

doi: 10.1130/abs/2025AM-413137

© Copyright 2025 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.