Summary
| Location | North Cascades, Metamorphic Core Domain |
| Type | Granite |
| Unit | Golden Horn Batholith |
| Terrane | Chelan Mountains Terrane |
| Period | Eocene…45 Ma |
| Found | Confluence of Early Winters Creek and the Methow River |
The Golden Horn Batholith is a granitic intrusion roughly 20 miles long and 6 miles wide is well exposed along the North Cascades Highway through Rainy Pass and Washington Pass. The rocks weather to a yellowish orange and are the inspiration for the batholith’s name and of course the Golden Horn peak itself. There are 3 types of granite in the batholith based on the feldspar distribution. This specimen is the Alkali Feldspar variety in that the predominant feldspar is orthoclase. As this rock cooled and crystalized, the orthoclase exsolved blobs of albite to exhibit a zebra-like texture, which is visible in the attached thin section image. This happens when crystallization occurs very slowly and at lower temperatures.
Petrography
Alkali feldspar: 65%; subhedral to anhedral crystals with perthitic exsolution and quartz inclusions
Plagioclase feldspar: 5%; anhedral to subhedral crystals less than up to .5 mm in size.
Quartz: 30%; euhedral crystals
The Golden Horn Batholith intrusion into the surrounding rocks occurred approximately 45 million years ago. The batholith intruded during the Eocene extensional event when the crust was being stretched and extended. The forces responsible for extension are possibly due to changes in oceanic plate movement relative to the North American plate and could be why Golden Horn is chemically different than older plutons to the west. At the time of the batholith’s emplacement the Washington Pass area is believed to be 2 miles below its present location. This was followed by millions of years of regional uplift, and erosive forces, including glacial activity in recent periods.
| 22,000 – 18,000 years BP | Alpine glaciers advance |
| 17,000 – 13,500 years BP | Continental Cordilleran Ice Sheet advance/retreat |
| 11,000 - 9500 years BP | Alpine glaciers advance (higher valleys) |
| 7000 years to present | Fluvial erosion in the Methow River Watershed |
The forces of glacial advance and retreat and subsequent fluvial erosion further exposed the plutons resulting in the topography (and river rocks!) we see today. This specimen likely traveled several miles within the Early Winters drainage to its “found” location on the floor of the upper Methow Valley. True to its name, this specimen has weathered to a nice rusty gold.