Chichagof Island

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Occurrence number:

174-00-011

Country:

United States

Region:

Alaska

Location:

Longitude: -135, Latitude: 57.75

Carbonatite:

In the area south-southeast of Tenakee nepheline and sodalite syenites occur in a 220 km2 complex which is composed predominantly of hornblende and biotite syenites, monzonite, granodiorite, granite, trondhjemite and syenodiorite. Loney et al. (1975, p. 27 and Plate 2) have provisionally divided the complex into six plutons, two of which (op. cit. Table 3, nos 3 and 4) contain nepheline and sodalite syenites. Other minerals reported include cancrinite, sodic pyroxene, biotite, amphibole and fluorite. Detailed field relationships are not known.

Age:

K-Ar ages on biotite from syenite gave 119 ±5 Ma and on hornblende 247 ±10 Ma; hornblende from quartz diorite gave 275 ±11 Ma and a hornblende from syenite 406 ±16 Ma (Lanphere et al., 1965, p. B110). But Loney et al. (1975, p. 27) report that clasts of syenite are found in Silurian and Devonian sedimentary rocks, and conclude that the complex is Silurian or older.

References:

LANPHERE, M.A., LONEY, R.A. and BREW, D.A. 1965. Potassium-argon ages of plutonic rocks, Tenakee area, Chichagof Island, southeastern Alaska. Professional Paper, United States Geological Survey, 525-B: 108-11.
LONEY, R.A., BREW, D.A., MUFFLER, L.J.P. and POMEROY, J.S. 1975. Reconnaissance geology of Chichagof, Baranof, and Kruzof Islands, southeastern Alaska. Professional Paper, United States Geological Survey, 792: 1-105

Location:

All locations > North America > United States > Alaska

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