Abstract

Paleozoic blueschist facies rocks are relatively scarce on Earth due to warmer geothermal gradients at that time and/or later reequilibration. Ferro-magnesiocarpholite (Fe-Mg carpholite), the typical low-temperature blueschist facies index mineral in metapelites, was discovered 30 yr ago and is known only in Tethyan belts metamorphosed <80 m.y. ago. Herein we report the discovery of Paleozoic Fe-Mg carpholite in the ca. 470 Ma blueschists of Motalafjella, Svalbard Caledonides, the oldest known occurrence on Earth. The carpholite-bearing rocks reached pressure-temperature (P-T) conditions of 15–16 kbar and 380–400 °C and followed a nearly isothermal exhumation path. In the cooling Earth perspective, these P-T estimates for Motalafjella blueschists demonstrate the existence of cold subduction-zone gradients (∼7 °C/km) from the middle Paleozoic onward.

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