THE WESTERN HAYFORK TERRANE: MIDDLE JURASSIC ARC WITH ADAKITIC AFFINITIES IN THE KLAMATH MOUNTAIN PROVINCE, OR AND CA

The western Hayfork terrane (wHt) is a thick sequence of arc-related volcanogenic sandstone and siltstone (arc apron) with sparse mafic to intermediate lavas and air-fall deposits that crop out from the southern Klamath Mountain province to the latitude of Grants Pass, OR. It was deposited on ophiolitic mélange of the Rattlesnake Creek terrane and depositional ages determined by ⁴⁰Ar/³⁹Ar dating of detrital (magmatic) hornblende range from 179 to 169 Ma[1]. The end of wHt magmatism is coincident with a regional high-grade (~9–10 kbar, 965–980°C)[2] metamorphic event.

The bulk of the wHt consists of fine to coarse metasandstone with clasts of volcanic rock fragments, plagioclase, augite, and hornblende. Graded bedding is common and is locally truncated by channels filled with volcanic cobble-rich lahar deposits. The lack of volcanic rocks in the wHt makes characterization of Middle Jurassic arc magmatism difficult. However, magmatic compositions of the arc may be characterized by major and trace element analysis of augite and hornblende phenocrysts and detrital grains. Augite is relative calcic (Wo > 0.4) and magnesian (mg# 0.79–0.92). Most augites have strongly concave-downward REE patterns with highest chondrite-normalized abundance at Nd; most lack a Eu anomaly. Normalized heavy REE abundances are variable from as much as 20x to as little as 1.5x chondrites. Augite with low HREE also has low Y and high Sr contents (to 110 ppm) and calculation of magmatic trace element patterns of these samples shows an affinity to mafic adakite, with Sr/Y ratios suggestive of both slab melting and deep-crustal melting[3]. Among the numerous proposed origins for adakites, the wHt examples are best explained as slab melts modified by interaction with the mantle wedge. If this was the case, then the wHt arc was evidently related to subduction of a young, hot slab. It is possible that wHt magmatism ended as a spreading ridge reached the trench. Subduction of the trailing edge of the slab could then have resulted in a slab window which provided heat for the subsequent high-grade metamorphism.

[1]Hacker et al., Tectonics 14, 677. [2]Medaris et al., GSA Abst. Prog. 41, 590. [3]Drummond & Defant, JGR 95, 21503.