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Can you identify this glacial boulder?

The boulder is a fine-grained quartzite with a vesicular texture, and the vesicles are lined with calcite and a thin layer of green clay. Its shape is oddly reminiscent of a tooth. It sits in a till plain far from any bedrock that looks like it. I'm trying to use its unique mineralogy and texture to "fingerprint" its source region and understand the glacial transport path.

 

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By Rathi Answered 7 months ago

That description is fascinating. A vesicular quartzite is unusual; vesicles typically form in volcanic rocks. This suggests your boulder might be a metamorphosed vesicular volcanic rock, like a meta-rhyolite. The calcite and green clay (likely celadonite or chlorite) fillings are classic low-temperature alteration products. The "tooth-like" shape is likely from glacial plucking and abrasion. I would start by looking at bedrock maps upstream (ice-flow direction) for Silurian-Devonian aged volcanic units that have undergone low-grade metamorphism. A thin section to confirm the quartz is a recrystallized matrix, not a detrital sandstone, would be the next critical step.

 

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