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Geology; October 2005; v. 33; no. 10; p. 761-764; DOI: 10.1130/G21693.1
© 2005 Geological Society of America
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Discovery of Paleozoic Fe-Mg carpholite in Motalafjella, Svalbard Caledonides: A milestone for subduction-zone gradients

P. Agard*,1, L. Labrousse1, S. Elvevold2 and C. Lepvrier3

1 Laboratoire de Tectonique, Université Paris VI, Case 129, T46-0, 2E, 4 place Jussieu, 75252 Paris Cedex 5, France
2 Norwegian Polar Institute, 9296 Tromsø, Norway
3 Laboratoire de Tectonique, Université Paris VI, Case 129, T46-0, 2E, 4 place Jussieu, 75252 Paris Cedex 5, France

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.

Key Words: high-pressure–low-temperature metamorphism • blueschists • caledonides • carpholite • Svalbard






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