A BIMODAL VOLCANIC-PLUTONIC SYSTEM: THE ZAREMBO ISLAND EXTRUSIVE SUITE AND THE DEER-ETOLIN ISLAND INTRUSIVE COMPLEX

LINDLINE, Jennifer, Natural Resource Management Program, New Mexico Highlands University, Las Vegas, NM 87701, jlindline@hotmail.com; CRAWFORD, William A., and CRAWFORD, Maria L., Geology Department, Bryn Mawr College, Bryn Mawr, PA 19010

The volcanic rocks of southwestern Zarembo Island and the plutonic rocks of Deer and Etolin Islands in central southeastern Alaska were investigated to determine whether a genetic relationship exists between the two rock groups. The Zarembo Island volcanic suite consists of extrusive rocks including agglomerates, ash flow tuffs and basalt and rhyolite lava flows that exhibit features which suggest simultaneous eruptions of both mafic and felsic lavas. A few kilometers to the southeast, the Deer-Etolin Island intrusive complex consists of gabbro-diorite and granite plutons that also show characteristics of contemporaneous mafic and felsic magmatism. These bimodal volcanic and plutonic rocks are correlative in age (approximately 20 m.y.) and similar in major, trace, and rare earth element composition. Both suites show a gap in silica concentration between 60-65 weight percent. Additionally, both suites contain magma mingling and magma mixing textures such as mafic enclaves in felsic members and quartz xenocrysts rimmed by clinopyroxene in mafic members. These characteristics suggest that the Deer-Etolin intrusive complex and the Zarembo Island volcanic suite represent a deeply eroded, shallow level pluton and its eruptive equivalent. The numerous mafic and felsic dikes that intrude the Zarembo Island volcanic suite are likely feeders to higher level volcanic rocks now removed by erosion. The style of volcanism and the bimodal nature of the complex suggest that igneous activity occurred during crustal extension and thinning which accompanied strike-slip tectonic motion in southeastern Alaska during the Tertiary.