Zircon geochronology and ca. 400 Ma exhumation of Norwegian ultrahigh-pressure rocks: an ion microprobe and chemical abrasion study

摘要

Understanding the formation and exhumation of the remarkable ultrahigh-pressure (UHP) rocks of the Western Gneiss Region, Norway, hinges oil precise determination of the time of eclogite recrystallization. We conducted detailed thermal ionization mass spectrometry, chemical abrasion analysis and sensitive high-resolution ion-microprobe analysis of zircons from four ultrahigh- and high-pressure (HP) rocks. Ion-microprobe analyses from the Flatraket eclogite yielded a broad range of apparently concordant Caledonian ages, suggesting long-term growth. In contrast. higher precision thermal ionization mass spectrometry analysis of zircon subject to combined thermal annealing and multi-step chemical abrasion yielded moderate Pb loss front the first (lowest temperature) abrasion step.. possible minor Pb loss or minor growth at 400 Ma from the second step and a 407-404 Ma cluster of slightly discordant - Pb-206/(238) U ages, most likely free from Ph loss, from the remaining abrasion steps. We interpret the latter to reflect zircon crystallization at similar to405-400 Ma with minor discordance from inherited cores. Zircon crystallization occurred at eclogite-facies, possibly post-peak conditions, based on compositions of garnet inclusions in zircon as well as nearly flat HREE profiles and lack of Eu anomalies in zircon fractions subjected to chemical abrasion. These ages are significantly younger than the 425 Ma age often cited for western Norway eclogite recrysiallization, implying faster of exhumation (>2.5-8.5 km/Myr), and coeval formation of eclogites across the UHP portion of the Western Gneiss Region. (C) 2004 Published by Elsevier B.V.