HR: 0830h
AN: V31C-0950    [PDF]
TI: Tertiary Magmatism in the Lake Clark Region of Southern Alaska: A Comparison of Coeval(?)Plutonic and Volcanic Rocks Adjacent to the Tlikakila Complex
AU: * Foley, C
EM: geologygirl23@hotmail.com
AF: New Mexico State University, Department of Geological Sciences, MSC 3AB, P.O. Box 3001, Las Cruces, NM 88003 United States
AU: Amato, J M
EM: amato@nmsu.edu
AF: New Mexico State University, Department of Geological Sciences, MSC 3AB, P.O. Box 3001, Las Cruces, NM 88003 United States
AB: The Cretaceous-Tertiary phase of magmatism in the Lake Clark region is related to the subduction of the Kula plate beneath southern Alaska. Rock types in the Lake Clark region include calc-alkaline intermediate to felsic volcanic rocks, and granitic plutons. These rocks are cut by the Tlikakila shear zone complex, which is a linear belt of metamorphosed volcanic and sedimentary rocks with an unknown displacement history. Exhumation of the plutonic rocks may have occurred along this shear zone. Existing geochronology from Lake Clark Tertiary igneous rocks is sparse. A few K/Ar whole rock dates on volcanic rocks range from 74-39 Ma and K/Ar biotite dates range from 63-59 Ma, and plutonic rocks in the same area yield biotite K/Ar dates between 72-56 Ma (Thrupp, 1987; Eakins et. al., 1978). These ages indicate that at least some of the plutonic and volcanic rocks are coeval. Determining the genetic relationships between the volcanic and plutonic rocks and the implications for exhumation of the plutonic rocks along the Tlikakila complex will be investigated using geochemistry and $^{40}$Ar/$^{39}$Ar geochronology. Preliminary geochemical data from eight samples analyzed thus far suggest that the volcanic and plutonic rocks are not necessarily comagmatic. The SiO$_{2}$ values of plutonic rocks range between 63%-77%. The SiO$_{2}$ values of the volcanic rocks range between 58%-69% indicating andesitic to dacitic compositions. Both rock types are calc-alkaline. K$_{2}$O in the plutonic rocks range from 2.0%-4.7%. The K$_{2}$O values for the volcanic rocks range from 1.8%-3.1%. The Na$_{2}$O values for the plutonic rocks range from 3.1%-5.3%. Na$_{2}$O values for the volcanic rocks range 3.7%-4.2%. The MgO values for the plutonic rocks range from 0%-2.6%. The MgO values for volcanic rocks range from 1.3%-2.5%. The trace element data indicate relatively low levels of incompatible elements. For the plutonic rocks Zr ranges from 134-222 ppm. The volcanic Zr range is 92-191 ppm. The Nb levels in the plutonic rocks range from 12-17 ppm. The Nb levels in the volcanic rocks range from 3-10 ppm. The Y levels in the plutonic rocks range from 25-70 ppm. The Y levels for the volcanic rocks range from 10-29 ppm. These rocks are enriched in the large ion lithophile group elements. The plutonic levels of Ba range from 888-1036 ppm. The volcanic levels of Ba range from 729-1259 ppm. Further work on geochronology is pending.
DE: 1035 Geochronology
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting