Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Sedimentary Serpentinites of the Ingalls Ophiolite Complex: Further Evidence of a Fracture Zone Setting

James H. Macdonald Jr.¹, Ante N. Mlinarevic², Gregory D. Harper¹, Robert B. Miller², Jonathan S. Miller², and Cynthia E. Schultz^2
1 Earth and Atmos. Science, SUNY Albany, Albany, NY 12222, [email protected]
² Dept. of Geology, San Jose State Univ, San Jose, CA 95192-0102

The Ingalls Ophiolite Complex is one of several Mid to Late Jurassic ophiolites in the North American Cordillera. It is located within the central Cascades of Washington, and has been interpreted to have formed in a fracture zone setting. Sedimentary rocks (Peshastin Fm), consisting mostly of mudstone, occur predominantly within the eastern portion of the complex. The lower part of the mudstone unit is Lower Jurassic, whereas the upper part is Upper Jurassic. Minor graywacke, pebble conglomerate, pebbly mudstone, and chert are interbedded with the mudstone. Abundant ophiolitic breccias are also intercalated with mudstone, and isolated outcrops of diabase or gabbro within the mudstone are interpreted as olistoliths. Detrital serpentinites are associated with ophiolite breccias and also form sheared, fault bounded outcrops. Detrital serpentinites are mostly unsorted breccias with angular to subangular clasts, but well-sorted, cross-bedded, and graded serpentinite sandstone and pebble conglomerate beds with flame structures occur. Some serpentinite breccias contain clasts of amphibolite or diabase. Abundant detrital Cr-spinels occur within the sedimentary serpentinites. Cr-spinels from two sedimentary serpentinites plot within the fields for supra-subduction zone peridotite, and originated from a source area that underwent high degrees of partial melting. Spinels from a third sedimentary serpentinite show the effects of serpentine alteration (Al, Mg, and Cr loss with Fe gain), and it is unknown if alteration occurred before or after deposition; however, the least altered spinels plot with the spinels from the two unaltered samples, suggesting that they had a similar source. These detrital spinels have compositions similar to published and new spinel data for the harzburgite and dunite unit of the Ingalls. This, along with the mostly poor sorting and the association with ophiolite breccia, suggests that the sedimentary serpentinites from the Ingalls were derived locally; most likely as mantle rock was exposed along fault scarps during the Ingalls fracture zone origin. This scenario seems likely owning to the fact that sedimentary serpentinites and ophiolite breccias are found in the modern fracture zones such as the Romanche.

Posted with permission of The Geological Society of America; abstract also online (http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85482.htm). © Copyright 2005 The Geological Society of America (GSA).