Graduate Thesis Or Dissertation
 

Vail_Scott_Garret_1977_Plate_I.tif

Public Deposited

Downloadable Content

Download image
https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vd66w273j

Descriptions

Attribute NameValues
Creator
Abstract
  • The Oregon Mountain area contains an ophiolitic assemblage of rocks which can he divided into two major zones on the basis of lithology: (1) spilite and diabase, with minor chert, and (2) plagiogranite, hornblende gabbro, cumulate gabbro and cumulate ultramafic rocks. The two zones have a total estimated thickness of about 4.6 km. The lower contact of the cumulate zone is gradational into the Josephine Peridotite, an extensive body of alpine-type harzburgite. The uppermost spilites are depositionally overlain by the Galice Formation (Jurassic, Kimmeridgian). Rocks in the diabase-spilite zone have undergone metamorphism similar to sea-floor burial metamorphism. Metamorphic grade progresses from zeolite facies through chlorite, epidote and actinolite zones of the greenschist facies to amphibolite facies. Prehnite-pumpellyite facies metamorphism has been superimposed on the lowest temperature assemblages, probably owing to later burial by the Galice Formation. The plutonic zone consists of a sequence of ultramafic and mafic cumulate rocks which culminates in a residue of granophyric plagiogranite. Hornbiende gabbro, in part cumulate and in part intrusive, occurs near the top of the sequence. The Josephine Peridotite consists of olivine-rich harzburgite and minor amounts of dunite, chromitite and pyroxenite, all with tectonite textures. It is essentially indistinguishable from other occurrences of alpine-type harzburgite. The Galice Formation is predominantly slaty shale. Lithic graywacke and pebbly conglomerate are volumetrically minor components. The graywacke contains abundant quartz, volcanic fragments with minor feldspar, mafic minerals and metamorphic fragments which indicate a mixed continental and volcanic arc provenance. Rocks of the Oregon Mountain area chemically resemble other well known and well studied ophiolites. Major element variations in the diabase-spilite zone are interpretable in terms of sea water interacton with mid-ocean ridge tholeiite. Increases in SiO₂, Na₂O, H₂O and FeO and decreases in CaO and MgO relative to mid-ocean ridge basalts were observed in most specimens. Less consistent changes occur in K₂O, Al₂O₃, TiO₂ and MnO. Titanium and zirconium abundances suggest affinities to mid-ocean ridge basalts, but there is some evidence of mobilization of these elements. Rare earth element distributions in rocks from the diabasespilite zone appear to have been generally unchanged during metamorphism although modification of Ce abundance has evidently occurred in one specimen. The distribution patterns are typically flat (La/Lu = 1 - 1.5) with low abundances (av. La = 15x chronditic abundance) indicating that basaltic rocks from the Oregon Mountain area have affinities with mid-ocean ridge basalts and arc tholeiites. Theoretical models indicate that a 15 to 20 percent melt of mantle source rock with a REE abundances about 2 to 3 times those of chondrites is a reasonable mechanism to produce the observed REE distributions. The ophiolite exposed in the Oregon Mountain area apparently formed as oceanic crust in a marginal basin environment during Jurassic time prior to the Nevadan Orogeny. The marginal basin probably lay between the Rogue-Galice island arc and the western continental margin. Nevadan deformation telescoped the arc-marginal basin terrain and caused accretion of the Jurassic rocks onto the continental margin.
Rights Statement

Relationships

Parents:

Items

Thumbnail Title Date Uploaded Visibility Actions
file details Vail_Scott_Garret_1977_Plate_I.tif 2017-08-08 Public Download