GEOLOGIC MAPPING OF THE LAKE CLARK QUADRANGLE, ALASKA, AND GEOCHRONOLOGIC CONSTRAINTS ON THE EVOLUTION OF THE TLIKAKILA COMPLEX AND THE PALEOGENE MAGMATIC HISTORY OF THE REGION

Geologic mapping of the central Lake Clark quad at 1:24,000 scale has been conducted to determine the relationships in the Tlikakila complex (TC) and surrounding igneous rocks. The TC is a NE-SW trending belt of steeply dipping foliated metamorphic rocks with Triassic protoliths similar to those found in the Taku terrane and in parts of Wrangellia. Mappable units include rocks metamorphosed at greenschist grade, with mafic volcanic rocks dominant, mainly basalts and minor andesite. Also present are gabbro, ultramafic rocks, and a series of metasedimentary rocks including marble, chert pebble conglomerates, chert, calc-silicate rocks, and metapelites. Chlorite and epidote are the dominant metamorphic phases in the mafic rocks, and metapelitic rocks have white mica, biotite, and rare garnet, indicating some reached lower-amphibolite facies. Metamorphism peaked at 177 ± 1 Ma based on ⁴⁰Ar/³⁹Ar dating of biotite from three metasedimentary rocks in the TC. Detrital zircon U-Pb ages were obtained from a chert-pebble conglomerate. The youngest robust peak has a weighted mean of 313 ± 10 Ma (2s), the main peak at 424 Ma, and 75% of the 74 grains analyzed are Proterozoic and Archean.

The Cretaceous-Tertiary history of the region is dominated by magmatism probably related to subduction of the Farallon and Kula (and Resurrection) plates, but appears to have occurred at three intervals. Biotite from granodiorite east of the TC is 99 ± 1 Ma (likely a cooling age) and requires revision of existing maps showing Tertiary intrusions. Hornblende from two granodiorite dikes that cut the TC yield a plateau at 101 ± 1 Ma and another partially reset hornblende yields 97 ±1 Ma. These ages are similar to a previously published biotite K/Ar age of 94 Ma to the east, but ~100 Ma magmatism is uncommon in southern Alaska. The two major episodes of magmatism are Paleogene. The first at 70-60 Ma is well represented by K/Ar and ⁴⁰Ar/³⁹Ar dates from granites east and west of the TC and volcanic rocks throughout the region. The second ranges from 45-35 Ma and is represented by our ⁴⁰Ar/³⁹Ar dates of 43.0 ± 0.2 Ma from biotite from a rhyolite, 42 ± 2 Ma, a reset biotite from an older intrusive rock, and 43 ± 1 Ma, a reset hornblende from the 97 Ma granodiorite dike. This episode is related to the onset of Aleutian arc magmatism.