T35 The Jurassic System of North America: Stratigraphy, Tectonics, and Depositional History (Posters)

TECTONIC HISTORY OF THE SNOWCAMP REMNANT OF THE COAST RANGE OPHIOLITE, GAME LAKE PEAK AREA, SW OREGON – OUTLIER OF THE JOSEPHINE OPHIOLITE?

Paper number: T35-9
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Structural analysis, field mapping, and laboratory analyses of rocks in the vicinity of Game Lake Peak (GLP), SW Oregon show that the ultramafic rocks associated with the Snowcamp ophiolite remnant have followed a similar and coeval emplacement history as the Josephine ophiolite (Madstone Cabin thrust, MCT). In the GLP area relatively fresh peridotite structurally overlies >70 m of deformed greenstones, mafic phyllonites, amphibolites and meta-gabbros of a klippe of the Rogue-Chetco arc complex. Structural elements generally parallel those from similar lithologies along the MCT. The greenstones and amphibolites are strongly foliated (shallow northeast dip) and the greenstones are locally mylonitic. Porphyroclasts within the greenstones indicate a gabbroic protolith that has undergone grain-size reduction during faulting. Foliations throughout the mafic rocks are generally parallel (disregarding late, non-cylindrical, open folds that deform all units) suggesting coeval development. A 6-9 m zone of coherent foliated serpentinite marks the base of the peridotite and parallels foliation in the underlying mafic rocks. Stretching lineations and shear sense indicators indicate both NE- and NW- directed motion. The difference in transport direction may be explained by late folding.

Metamorphic hornblende from amphibolite and igneous muscovite from a syn-tectonic, garnet-muscovite tonalite intrusion (similar to intrusions in amphibolite below the MCT) give 40Ar/39Ar ages of 154.2 +/- 2.0 and 149.1 +/- 0.4 Ma, respectively. Metamorphic hornblende-plagioclase pairs and the phengite-content of igneous muscovite give temperature and pressure ranges of 5650 – 7200 C and 2.5 – 6.0 Kb, respectively. These lithologies, ages, P-T conditions, and structural elements are all similar to those from the MCT, strongly indicating that they are correlative, suggesting that either the Coast Range ophiolite has followed a similar tectonic and temporal history as the Josephine ophiolite or that ultramafic rocks near GLP are an outlier of the Josephine ophiolite rather than a remnant of the Coast Range ophiolite.