Event Name : Kookooligit Mountains 1220000 yBP
|Start: 1220000 (± 50000 Years) || Years BP Ar/Ar || |
|Lava flow: ||
|Eruption Product: || basalt ||
From Mukasa and others (2007): "Quaternary basalt cinder cones and lava flows form a large shield volcano, over 500 m high, in the north central part of St. Lawrence Island (Kookooligit Mountains). The overwhelming majority of the volcanic rocks at this locality are alkali-olivine basalt, with only subordinate amounts of olivine tholeiite, basanite and minor nephelinite (Patton and Csejtey, 1971; Moll-Stalcup, 1994, 1996). The alkaline basalts are characterized by a high Mg # (61-64), with MgO = 7.9-12.2 wt %, low silica (SiO2=44.8-46.2 wt %), and high total alkalis (Na2O+K2O = 3.8-7.7 wt %) (Moll-Stalcup, 1996). Only one sample (KB-11M) of alkaline olivine basalt has been dated in our study using the 40Ar/39Ar method obtaining a total gas age of 1.32+/-0.02 Ma, plateau age of 1.22+/-0.02 Ma, and isochron age of 1.22+/-0.05 Ma (Figure 4 and Table 1 [in original text]). Moll-Stalcup (1994, 1996) produced K-Ar data for this shield volcano which suggest that volcanic activity in the Kookooligit Mountains continued until 0.24 Ma. Although yet to be verified, combining our 40Ar/39 Ar data with these published K-Ar ages implies that activity continued in a single shield volcano complex for nearly 1 million years."