Swakane Terrane III: Geology By trane in trane's Diary Wed Jul 11, 2012 at 02:05:13 AM EST Tags: swakane terrane, camping, hiking, geology, rocks, foliation, leucogranite, mystery terrane, banded gneiss, questions (all tags)
	I'm fascinated by white streaks in rocks, such as these ones from the Skokomish river. How did they get like that? The Swakane Terrane has white-banded rocks on a meter scale.  USGS calls it "one of the region's most significant geologic mysteries."

Examination of the rocks, such as this one or this one, show bands of granite separated by bands of white. How did the layers form? From a geologic field trip report: A new sample of qtz-pl-bt gneiss from here (07NC22) produced zircon with LA-ICPMS spot ages of 68-1675 Ma. The section in which the above passage appears refers to the rocks in this picture and this picture, I believe. Why are the layers horizontal? This paper provides one theory: the rocks were buried and melted (at over 650 C). Presumably when melted the rocks formed layers, but why? How does this happen? It's hard to penetrate the author's meaning in sentences such as: "With increasing melt volume, melt migrated through dikes that consistently crosscut the generally subhorizontal anisotropy in the Swakane Terrane." This paper about stromatic migmatite in southern Norway says: Relict textures indicate that the leucosome portions were initially occupied by layers of granitic composition relatively rich in K-feldspar, whereas the mesosomes are the representatives of those metagraywacke layers which were relatively rich in plagioclase. An almost isochemical transformation of a paragneiss into the investigated stromatic migmatite is established. So maybe the layers formed by some sort of metamorphism of the rock under high temperature and pressure conditions; banded parts of the rock turned white? But why should the original chemicals have been in bands... And how did the rock get buried, and why is it exposed today? This paper about southern Italy has a picture of a rock that looks very similar to the rocks in the Swakane Terrane. If the whole terrane was buried, what buried it? Was it forced under the approaching North American tectonic plate, and then later resurfaced with horizontal bands that had formed while it was subjected to high temperatures and pressures? Why would it resurface in such large chunks? If the rocks were formed from 73 million years to 1.6 billion years ago, shouldn't there be an iridium layer (formed 65 MA) over them? Or did they push through it when they resurfaced? Being close to those rocks gives me a feeling of seeing something very mysterious that we can't explain yet, and which involved very large amounts of energy.

Related Links

these ones white-band ed rocks on a meter scale USGS this one this one [2] a geologic field trip report: this picture this picture [2] This paper This paper [2] This paper about southern Italy trane's Diary