GEOCHEMISTRY OF THE HEREFORD MEADOW AMPHIBOLITE: POSSIBLE TECTONIC SETTING AND RELATIONSHIP TO LATE JURASSIC SUBDUCATION

The informally named Hereford Meadow amphibolite (MacDonald, 2006) is a coarse- to fine-grained amphibolite, with lesser felsic orthogneiss, located within the Manastash inlier, central Cascades, Washington. It is intruded by the ca. 157 Ma Quartz Mountain stock and is steeply faulted against the other inlier units. Relic screens, chill margins and dike tips can be seen on surfaces oriented perpendicular to foliation and lineation. An ⁴⁰Ar/³⁹Ar total gas age of 142.8 ± 0.74 Ma (1σ) has been obtained from hornblende of a medium-grained well foliated amphibolite; however, this sample was most likely reset by the intrusion of the tonalitic Quartz Mt. stock.

18 amphibolites and 7 orthogneisses where analyzed for major and trace elements. Amphibolites are basaltic to basaltic andesites, with one andesite. Ta/Yb ratios, chondrite-, N-MORB-, and fertile MORB mantle-normalized diagrams suggest that the amphibolites originate from a depleted mantle source. A few samples have a more enriched mantle source. Their trace element chemistry suggests that they formed from the partial melting of a mantle source that was enriched by subducation. Orthogneiss chemistry suggests they were originally tonalities and trondhjemites, with 1 granodiorite. They are calcic and magnesian; and, their Ta/Yb, chondrite-, N-MORB-, and primitive mantle-normalized diagrams suggest that they originate from a moderately enriched mantle source. Their chemistry suggests they formed in a primitive arc setting.

It is suggested that the Hereford Meadow amphibolite is a highly dismember section of oceanic crust; while the orthogneiss formed in an arc setting. The geochemistry of the amphibolite and orthogneiss suggest that they did not originate from the same source. Possible tectonic settings for these rocks include: a supra-subduction zone and an intrusive arc; a subducting ridge and an intrusive arc; or, a forearc setting. The Quartz Mt. stock is interpreted to have formed in a forearc as a result of a ridge subduction. Based on the intrusive nature of this stock, the Hereford Meadow amphibolite was either formed in, or emplaced into a forearc before the Late Jurassic. Obducted oceanic crust has been identified within other ridge subduction settings (e.g., Taitao ophiolite, Chile, and Knight Island and Resurrection Peninsula ophiolites, Alaska).