25 D8. The Origin and Spatiotemporal Evolution of Arc Magmas Recorded From Mineral to Plate-Boundary Scales II

Wednesday, 13 May 2020
1:30 PM - 5:30 PM
Plaza Room

ASHLAND PLUTON (KLAMATH MOUNTAIN PROVINCE) REVISITED: THE VIEW FROM MINERAL COMPOSITIONS AND ZONING

Paper number: 25-7
Program: Technical Sessions
Time: 3:10 PM – 3:30 PM

Authors

Abstract

The late Middle Jurassic Ashland pluton is a composite body that straddles the OR–CA border. The pluton and overlying Cretaceous and Tertiary sedimentary rocks are tilted gently to the east. Three magmatic units comprise the pluton. The tonalitic unit is structurally deepest, it crops out parallel to the western contact. The tonalite unit grades eastward to a heterogeneous unit of quartz monzodiorite and granodiorite (qmd unit), which hosts swarms of mafic enclaves. The qmd unit is intruded by the granitic unit: bodies of biotite ± hornblende granite that are locally host to enclave swarms. The tonalite unit has lower SiO2, K2O, and P2O5 contents than the qmd unit and the granitic unit contains high SiO2 and K2O but the lowest P2O5 values. Mafic enclaves are generally low in SiO2 but have high P2O5 and K2O values, similar to the qmd unit.

Trace element zoning in magmatic hornblende (Hbl) also distinguishes the three units. Hbl in the tonalite unit is richer in Ba and P and poorer in total REE, Y, Nb, Ta, and Mn than Hbl from the qmd unit. In addition, tonalite-unit Hbl is essentially identical to Hbl from coeval, broadly tonalitic plutons in the Klamath province, with most trace element values decreasing with decreasing Ti and temperature (T). In contrast, qmd-unit Hbl displays compositional arrays that are nearly orthogonal to those of tonalite-unit Hbl and are weakly correlated with Ti or T.

Trace element abundances and chemographic calculations indicate that (1) Hbl in all units crystallized from zircon-bearing rhyodacitic to rhyolitic melts, (2) in the qmd unit Ba and P were depleted prior to Hbl crystallization. Apatite in the qmd unit occurs mainly as clusters of fine inclusions in all primary phases, which suggests either thermal, or more probably compositional undercooling. The latter explanation is consistent with magma mixing during and after emplacement, and is consistent with the abundant swarms of enclaves (which are enriched in P2O5, REE, and Y) and the poor correlation of trace element variation with temperature in qmd-unit Hbl. Zircon saturation calculations [ref 1] indicate that rocks from the tonalite unit represent at least 40% crystal accumulation, qmd rocks represent 0–70% accumulation, and the granitic unit mainly reflects melt compositions.

[1] Barnes et al., Jour. Petrology, doi: 10.1093/petrology/egaa008.

Geological Society of America Abstracts with Programs. Vol. 52, No. 4

doi: 10.1130/abs/2020CD-347097

© Copyright 2020 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.