The western Hayfork terrane: Remnants of the Middle Jurassic arc in the Klamath Mountain province, California and Oregon

Abstract

Arc magmatism was widespread in the Cordillera of North America during Middle Jurassic time. The predominant representative of this arc magmatism in the Klamath Mountain province is the western Hayfork terrane (WHT). This terrane is primarily metasedimentary, consisting mainly of crystal-lithic arenite, argillitic sediments and lahar deposits, rare lavas, and sparse quartz-rich arenite. Because lavas are rare, petrologic study using bulk-rock compositions is restricted to analysis of cobbles in lahar deposits. Moreover, the WHT underwent greenschist-facies regional metamorphism with consequent modification of bulk-rock compositions. However, many of the sandstones preserve igneous clinopyroxene and calcic amphibole, which were phenocrysts in the original volcanic rocks. Major- and trace-element compositions of the magmatic pyroxene and amphibole permit reconstruction of the range of rock types eroded from the arc, specifically scant basalt, volumetrically dominant basaltic andesite and andesite, and smaller but significant amounts of dacite and rhyodacite. Eruptive temperatures reached ~1180 °C and may have been as low as ~800 °C on the basis of pyroxene and amphibole thermometry, with most eruptive temperatures >1000 °C. On the basis of augite compositions, WHT magmatism is divided into two suites. One features high-Mg augite with high abundances of Cr and Sr, high Sr/Y and Nd/Yb values, and low Y and heavy rare-earth elements (REE). These compositions are typical of high-Mg andesite and dacite suites in which garnet is a residual mineral, most probably in a metasomatized upper mantle setting. The other suite contains augite with lower Sr, Sr/Y, and Nd/Yb; these features are typical of normal calc-alkaline magmas. Augite from a coeval pluton emplaced inboard of the western Hayfork outcrop belt is similar to augite from the low-Sr group of WHT samples. In contrast, augite from the Ironside Mountain pluton, previously consid­ered the plutonic equivalent of WHT sediments, is Fe-rich, with low Cr and Sr and relatively high Zr and REE. Previous suggestions that the Ironside Mountain pluton is correlative with the WHT are not supported by these data. The magmatic diversity of the WHT is typical of the modern Aleutian and Cascade arcs, among others, and could reflect subduction of relatively young oceanic lithosphere or fragmentation of the subduction slab. Although we favor the former setting, present data cannot rule out the latter. The presence of scant quartz-rich sedimentary rocks within the predominantly volcanogenic WHT is consistent with deposition as a sedimentary apron associated with a west-facing magmatic arc with late-stage input from cratonal sources. The results of this study indicate that detailed petrographic study of arc-derived sedimentary rocks, including major- and trace-element analysis of preserved magmatic phases, yields information about magmatic affinities, processes, and temperatures.

Tectonic Evolution of the Condrey Mountain Schist: An Intact Record of Late Jurassic to Early Cretaceous Franciscan Subduction and Underplating

“…Magmatic zircons from the Scraggy Mountain epidote blueschist lens (Figure 2a) date to 169 ± 2 (magmatic zircon TIMS U‐Pb). This age is comparable to the Ironside Mountain batholith in the Klamaths (Allen & Barnes, 2006; Barnes & Barnes, 2020).…”

“…The Rattlesnake Creek/Western Hayfork terranes sutured to the inboard Klamath terranes ca. 170 Ma (Barnes & Barnes, 2020; Saleeby, 1990) and underwent transtensional backarc spreading that resulted in formation of the Josephine Ophiolite (161–165 Ma) (Hacker et al., 1995; Saleeby & Harper, 1993; Yule et al., 2006). The Galice Formation flysch (part of the Western Klamath terrane; Figure 1b) was deposited in the Josephine basin following spreading (151–158 Ma) (Frost et al., 2006; LaMaskin et al., 2021; Macdonald et al., 2006; Surpless et al., 2023).…”

Tracking Deep Sediment Underplating in a Fossil Subduction Margin: Implications for Interface Rheology and Mass and Volatile Recycling

“…The Condrey Mountain Schist is primarily in thrust contact with an ophiolitic melange (Rattlesnake Creek terrane), but is locally juxtaposed against a west‐facing arc with associated plutons (Western Hayfork terrane) and a fossil accretionary wedge (Eastern Hayfork terrane) (Figures 2a and 2c) (C. G. Barnes et al., 2006, 2020; Frost et al., 2006; Gray, 1986; Hacker et al., 1995; Helper, 1986; Snoke & Barnes, 2006; Wright, 1982; Wright & Wyld, 1994). These terranes sutured to the inboard Klamath terranes ≥170 Ma (Saleeby, 1990; Snoke & Barnes, 2006), but plutons that cross‐cut the terrane bounding thrusts suggest ongoing arc activity over an east‐dipping slab (Figure 2c) (Wright & Wyld, 1994).…”

A crucial geologic test of Late Jurassic exotic collision versus endemic re-accretion in the Klamath Mountains Province, western United States, with implications for the assembly of western North America

“…170 Ma calc-alkaline plutons (Fig. 4A; Wright, 1982;Gray, 1986;Wright and Fahan, 1988;Barnes and Barnes, 2020). The Western Hayfork terrane lies structurally beneath the Eastern Hayfork terrane along the Wilson Point thrust and is thrust over the Rattlesnake Creek terrane along the Salt Creek thrust (Figs.…”

“…170-169 Ma multigrain thermal ionization mass spectrometry (TIMS) zircon U-Pb ages on the Ironside Mountain batholith, which intrudes the Wilson Point thrust (Figs. 3 and 4A;Wright and Fahan, 1988;Barnes and Barnes, 2020).…”