DETRITAL ZIRCON DATES AS A CONSTRAINT ON THE AGE OF METASEDIMENTARY ROCKS IN THE HATCHERS PASS AREA, SOUTH-CENTRAL ALASKA

Individual detrital zircons collected from near the mouth of Grubstake Gulch in the Hatcher Pass area of south central Alaska were dated by laser ablation ICP-MS methods. The resulting U/Pb ages show an excellent correlation with mapped units outcropping in the drainage basin. Three groupings of ages are recognized from a total measured population of 34 detrital zircon grains. Group I (16 grains) have ages ranging from 2500 Ma to 151 Ma. Within this distribution there are clusters at 160 to 210 Ma, 350 - 548 and 2422 - 2500. Group II zircons consists of 5 grains with ages from 104 to 87 Ma. These ages overlap published K-Ar dates on metamorphic amphiboles from serpentinite bodies emplaced into the schist. The final population of 13 grains (Group III) have ages ranging from 79 to 59 Ma. Our interpretation of these data are that Group I zircons represent zircons derived exclusively from an extensive metasedimentary unit (the Hatcher Pass schist) present in the Grubstake Gulch drainage basin and indicate a depositional age of the schist-protolith as Late Jurassic. The depositional age is slightly younger than the putative Middle Jurassic metamorphic age assigned to this unit, and indicates the Hatcher Pass schist contains a component derived from the erosion of the Talkeetna arc. Furthermore the Paleozoic and older ages suggest a minor detrital component from cratonic North America. Trace element data from Group II zircons contain subtle differences to indicate that the zircons may be derived from the serpentinites. In this model the serpeninite-protoliths were emplaced and metamorphosed rapidly during the mid-Cretaceous, possibly during the amphibolite-grade metamorphism recorded by the Hatcher Pass schist. The Group III zircons closely match the age of the Late Cretaceous to early Tertiary plutons present in the area, and are interpreted as being derived from them.