NEW JURASSIC AGES DEFINE MAGMATIC AND SEDIMENTARY BELTS IN SOUTHEAST ALASKA: IMPLICATIONS FOR WRANGELLIA, THE ALEXANDER TERRANE AND THE GRAVINA-NUTZOTIN BELT

New isotopic and fossil ages constrain the Jurassic evolution of the Wrangellia (WR) and Alexander (AX) terranes and the timing of their juxtaposition in southeast Alaska (SEAK). The distribution of Jurassic ages defines four magmatic belts and three sedimentary belts. U-Pb and Ar-Ar isotopic ages indicate magmatic belts including 1) Early to Middle Jurassic (~193 Ma to ~160 Ma) diorite-tonalite in a western WR belt (WWR) on Chichagof and Baranof Islands correlative with the Bonanza and Talkeetna arcs, 2) Early Jurassic (~200 Ma to ~175 Ma) alkaline granite and lamprophyre dikes in AX correlative with the Bokan Mountain complex, 3) Early Jurassic andesite (~195 Ma) overlying Late Triassic rocks of the AX Triassic metallogenic belt (ATMB), and 4) Late Jurassic to Late Cretaceous (~150 Ma to ~78 Ma) intermediate composition to mafic intrusive and extrusive rocks of the Gravina arc on AX and an eastern WR belt (EWR) at Berners Bay and east of Chilkat Inlet and the Chilkat River. AX lies entirely between WWR and EWR, which are always separated by faults and at least 110 kilometers.

Bivalve, ammonite, belemnite, and radiolarian fossil ages and detrital zircon ages define sedimentary belts including 1) Lower Jurassic to Upper Cretaceous deposits of the Baranof-Chichagof accretionary complex on WWR, 2) Lower Jurassic to Upper Cretaceous deposits in local basins in the ATMB, and 3) Upper Jurassic to Upper Cretaceous deposits of the Gravina-Nutzotin belt that overlie the east margin of WWR, the east margin of AX, and the west margin of EWR. Although contacts between WWR, AX and EWR are obscured by Cenozoic strike-slip faults, ages and magmatic, sedimentary, and structural characteristics distinguish discrete belts in the Jurassic. Overlapping magmatic activity and sedimentary deposition indicate AX-EWR proximity in the Cretaceous. The position of EWR east of AX and the overlapping Gravina deposits suggest EWR was accreted to North America prior to accretion of AX. WWR was juxtaposed with the west margin AX in the Cenozoic by translational faulting. It is possible that another component of WR may have initially been adjacent to western AX and was removed by translation prior to WWR-AX juxtaposition. Our data suggest distinct parts of WR were juxtaposed with western North America and AX in SEAK at different times in the Mesozoic and Cenozoic.