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2 years ago in Metamorphic Petrology By Radhika
What does the presence of tourmaline in a greenschist indicate?
My field area has metasedimentary sequences now at greenschist grade. Under the microscope, I'm seeing abundant, zoned tourmaline. I need to interpret this correctly: is it a detrital relic from the original sediment, or did it grow during metamorphism? How do I use its texture and composition to distinguish between a protolith signature and a metasomatic event?
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By Stephen Answered 4 months ago
This is a classic interpretive challenge I've faced many times. First, examine the texture. Detrital grains will be rounded and embayed, often as cores with metamorphic overgrowths. Metasomatic tourmaline typically forms euhedral crystals, radiating clusters, or veins that clearly crosscut the foliation. Next, check the composition with your microprobe: dravitic tourmaline (Mg-rich) often suggests an ultramafic or dolomitic fluid source, while schorl (Fe-rich) can be linked to granite-derived fluids. The presence of tourmaline alone suggests a boron-rich event; its context tells you when and from where that boron was introduced.
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