YOUNG AGE AND RAPID EXHUMATION OF NORWEGIAN ULTRAHIGH-PRESSURE ROCKS: AN ION MICROPROBE AND CHEMICAL ABRASION STUDY

Understanding the formation and exhumation of the ultrahigh-pressure (UHP) rocks of western Norway hinges on precise determination of the time of eclogite recrystallization. Our study consists of SHRIMP analysis, in conjunction with CL imagery, of zircon from four UHP and high-pressure (HP) eclogites; and detailed TIMS analysis of zircon from two samples subjected to combined thermal annealing and multi-step chemical abrasion (CA).

SHRIMP analyses from the Otnheim and Langenes eclogites yield Caledonian spot ages of ca. 400 Ma from zircon rims. CL imagery and Th/U ratios from the Langenes eclogite indicate formation of rims by recrystallization of inherited zircon.

SHRIMP analysis of the Flatraket eclogite yielded a broad range of apparently concordant Caledonian ages. CA analyses of two fractions yielded moderate Pb loss from the first (lowest T) steps; possible minor Pb loss or minor growth at 400 Ma from the second steps; and a 407–404 Ma cluster of slightly discordant ²⁰⁶Pb/²³⁸U ages, most likely free from Pb loss, from the remaining steps. We interpret the latter to reflect recrystallization of inherited zircon, with possible new growth, at ca. 400–395 Ma. Zircon recrystallization under HP/UHP conditions is inferred from three lines of evidence: i) zircon occurs as inclusions in garnet, omphacite, breunnerite, dolomite, and quartz, as well as in symplectites after phengite and omphacite; ii) association of zircon with rutile implies zircon formation during HP breakdown of Zr-ilmenite; and iii) chondrite-normalized ICP–MS analyses of the CA steps reveal small Eu anomalies and shallow HREE profiles, indicating zircon recrystallization in the presence of garnet.

CA analysis of the Verpeneset eclogite yielded distinctly discordant step ages from two steps comprising >90% of the sample, with ²⁰⁶Pb/²³⁸U ages of 408 and 414 Ma. CL imagery indicates incomplete recrystallization of inherited zircon, in keeping with steep HREE profiles determined from chondrite-normalized ICP–MS analyses.

Our zircon age of ca. 400–395 Ma is significantly younger than the 425 Ma age often cited for western Norway eclogite recrystallization, implying faster rates of exhumation (ca. 15 km/m.y.) and weakening the link between UHP metamorphism and ophiolite emplacement at 430–425 Ma.