DETRITAL ZIRCON GEOCHRONOLOGY AND REGIONAL CORRELATION OF METASEDIMENTARY ROCKS IN THE COAST MOUNTAINS, SOUTHEASTERN ALASKA

U-Pb ages have been determined for 55 detrital zircon grains from a me tasedimentary sequence along the west flank of the Coast Mountains in southeastern Alaska. These rocks belong to the Port Houghton assemblag e, which consists of upper Paleozoic pelitic and psammitic schist, met aconglomerate, metabasalt, and marble. The Port Houghton assemblage re sts unconformably(?) on metamorphosed and deformed mid-Paleozoic are-t ype volcanics (Endicott Arm assemblage), which gradationally overlie u pper Proterozoic(?)- lower Paleozoic continental margin strata (Tracy Arm assemblage). Three main clusters of ages are present: 330-365 Ma ( 19 grains), 1710-2000 Ma (27 grains), and 2450-2680 Ma (6 grains). Add itional grains are approximately 2334, 2364, and 3324 Ma. Comparison o f these ages with detrital zircon ages in other Cordilleran assemblage s supports previous interpretations that metasedimentary rocks in the Coast Mountains (i) form a southwestern continuation of the Yukon-Tana na terrane of eastern Alaska and Yukon, (ii) are not correlative with strata of the Alexander terrane, and (iii) contain detritus that was p robably shed from cratonal rocks in the Canadian Shield to the east. S everal scenarios exist to explain the occurrence of these continental margin rocks west (outboard) of are-type and ocean-floor assemblages s uch as the Stikine, Cache Creek, Quesnel, and Slide Mountain terranes.