LATE PROTEROZOIC AGES FOR ROCKS ON MOUNT CHEETDEEKAHYU AND ADMIRALTY ISLAND, ALEXANDER TERRANE, SOUTHEAST ALASKA

Felsic intrusive rocks near Four Winds Mountain and intermediate intrusive rocks on Admiralty Island in northern SE Alaska yield latest Proterozoic U-Pb zircon TIMS ages that are similar to U-Pb zircon ages previously reported for intrusive rocks 400 km to the south. The Four Winds metamorphic-igneous complex consists of greenschist to amphibolite facies quartz-biotite schist, black carbonaceous quartzite, amphibolite, marble, and garnet-bearing orthogneiss. A one-meter thick felsic sill in garnet-quartz-biotite schist, gneiss, and amphibolite on Mt. Cheetdeekahyu yielded a U-Pb zircon minimum age of 455 ± 5 Ma. A xenocrystic zircon from this rock that is concordant at 544 ± 9 Ma records the age of basement; some magmatic zircons from this sill also contain Late Precambrian basement inheritance. At False Point Retreat on Admiralty Island, foliated garnet-bearing meta-tonalite to meta-diorite contains metamorphic minerals indicating the metamorphic grade of the intrusive is consistent with the grade of the host rocks. Zircons from two localities near False Point Retreat yielded U-Pb ages of 547.6 ± 1.3 Ma and 544.2 ± 1.3 Ma. Host rocks include quartz-biotite, graphitic, and calcareous schists, and amphibolite similar in composition and relative abundance to the Four Winds complex. At Kaigani Harbor on Dall Island, garnet-bearing biotite-hornblende granodioritic orthogneiss with relict intrusive textures, hosted by Wales Group schists, yielded a U-Pb zircon age of 554 ± 4 Ma (Gehrels, 1990). Wales Group schists include greenschist to amphibolite facies garnet-quartz-biotite schist, siliceous semischist, graphitic quartzite, amphibolite, and marble similar in composition and metamorphic grade to the Four Winds complex and the host rocks to the False Point Retreat intrusive. At Sunny Harbor on Prince of Wales Island, garnet-bearing quartz dioritic orthogneiss hosted by Wales Group schists yielded a 540-520 Ma U-Pb zircon age, and on Bronough Islands, amphibolite facies garnet-biotite-hornblende orthogneiss has relict intrusive textures and yielded a 540-510 Ma U-Pb zircon age (Gehrels, Saleeby, and Berg, 1987). These remarkably similar ages greatly expand the area of known Precambrian rocks in southeast Alaska and suggest a common basement history for a significant part of the Alexander terrane.