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9 months ago in Field Geology , Metamorphic Petrology By Aditi Sharma
How can a metamorphosed igneous body be younger than a metasediment from field evidence?
I'm mapping a high-grade gneiss terrane where a granitic orthogneiss is interlayered with pelitic paragneiss. Both have the same amphibolite-facies assemblage. The standard interpretation might assume the orthogneiss is older basement, but what if it's an intrusive syntectonic granite? What unambiguous cross-cutting relationships or contact features survive high-grade metamorphism to prove the igneous body intruded the sediments?
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By Raunaq Answered 8 months ago
At high grades, the evidence becomes subtle but not erased. You must look for relict intrusive textures. I've mapped terrains where, despite strong foliation, you can find a zone where the granitic gneiss contains abundant, rotated xenoliths of the pelitic gneiss, clearly showing incorporation. Sometimes, a chilled margin is preserved as a finer-grained, more mafic band (an amphibolite selvage) at the orthogneiss boundary. Most definitively, look for discordance: does the internal foliation in the orthogneiss wrap around or cut the regional foliation in the paragneiss? If so, and if you can date metamorphic zircons in the orthogneiss that are younger than detrital zircons in the paragneiss, you have a robust case for a younger intrusive age.
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