TIME CONSTRAINTS ON THE PRE-VARISCAN MAGMATIC METAMORPHIC EVOLUTION OF THE GOTTHARD AND TAVETSCH UNITS DERIVED FROM SINGLE-ZIRCON U-PB RESULTS

Polymetamorphic mafic-ultramafic rock associations preserving relic HP -HT mineralogy represent the oldest igneous remnants so far identified in the pre-Variscan Helvetic basement of the Central Swiss Alps. In o rder to establish time constraints on the origin of these important tr acers of early basement evolution, three eclogitic metagabbro samples of island-are affinity (Gotthard and Tavetsch units) and one basaltic eclogite of MORB affinity (Gotthard unit) have been dated by conventio nal high-resolution single-zircon U-Pb isotope techniques. Zircons dev oid of inherited cores from the three metagabbro samples yield overlap ping linear U-Pb data patterns characterized by upper intercept and ma ximum apparent Pb-206/U-238 ages of similar to 467-475 Ma. The mid-Ord ovician time interval defined by the results coincides with a previous ly published age of similar to 468 Ma assigned to the eclogite stage i n the Gotthard area, whereas the lower intercept ages reflect substant ial overprint by Variscan and partially, Alpine events. With the excep tion of few core-bearing zircons, pre-Ordovician age components are ab sent from the samples analyzed. Multiple internal zoning revealed by c athodoluminescence suggests at least partial preservation of primary m agmatic mineralogical and isotopic features. The island are-type gabbr os therefore most likely have intruded within or closely before the 46 7-475 Ma time interval defined by the present data. This result is in contrast to previous studies which suggested an extended oceanic evolu tionary history beginning with late Precambrian (similar to 870 Ma) is land-are magmatism. The zircons from the MORB eclogite sample yield a slightly older maximal Pb-206/U-238 age of 481 Ma and a nominally olde r upper intercept age of 535 +/150/-75 Ma. Anhedral crystal morphology and absence of multiple magmatic zoning probably reflect a metamorphi c origin related to an earlier intra-oceanic subduction stage. Conform able petrological information and age distribution patterns suggest th at the Gotthard and Tavetsch basement units have undergone similar ear ly evolutionary histories, characterized by island are-type intrusive activity immediately preceding mid-Ordovician collision and therefore appear to have formed part of the same active plate margin environment during Caledonian times. The limited time interval of similar to 30-3 5 Ma bracketed by the intrusion of island are-type gabbros and by the emplacement of Late-Ordovician (similar to 440 Ma) granitoids which po stdate HP-HT and subsequent granulite facies metamorphism, is indicati ve of a fast coherent evolutionary cycle.