Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

Lincoln County

stripes

Occurrence number: 
174-00-042
Country: 
United States
Region: 
Oregon
Location: 
Longitude: -123.92, Latitude: 44.33
Carbonatite: 
No

Nepheline syenite and phonolite form flat-lying sheets, dykes and a small stock which are usually intruded into Eocene rocks in the central part of the Oregon Coast Range. The most extensive developments are the 65-95 m thick sills that cap Table and Cannibal Mountains, which were probably originally continuous and extended over 500 km2 (Snavely and Wagner, 1961, p. 158). At Blodgett Peak nepheline syenite forms a small stock. The nepheline syenites contain euhedral phenocrysts of albite (An10) in a groundmass of nepheline, analcime, aegirine-augite, olivine, riebeckite and opaques. At Table Mountain the albite phenocrysts have been extensively altered to sericite and clay minerals. Chemical analyses are available (Snavely and Wagner, 1961, Table 344. 1). A sample of nepheline syenite from Table Mountain has been developed as a USGS rock standard and described in detail by Snavely et al. (1976). Pb isotopic data on a sample from Table Mountain are given by Tatsumoto and Snavely (1969).

Age: 
K-Ar on nepheline syenite from Table Mountain gave 33.6 Ma (Tatsumoto and Snavely, 1969, Fig. 2).
References: 

MACLEOD, N.S. and SNAVELY, P.D. 1973. Volcanic and intrusive rocks of the central part of the Oregon Coast range. Bulletin, Oregon Department of Geology and Mineral Industries, 77: 47-74.
SNAVELY, P.D. and WAGNER, H.C. 1961. Differentiated gabbroic sills and associated alkalic rocks in the central part of the Oregon Coast Range, Oregon. Professional Paper, United States Geological Survey, 424D: 156-61.
SNAVELY, P.D., MACLEOD, N.S., FLANAGAN, F.J., BERMAN, S., NEIMAN, H.G. and BASTRON, H. 1976. Nepheline syenite, STM-1, from Table Mountain, Oregon. Professional Paper, United States Geological Survey, 840: 7-10.
TATSUMATO, M. and SNAVELY, P.D. 1969. Isotopic composition of lead in rocks of the Coast Range, Oregon and Washington. Journal of Geophysical Research, 74: 1087-100

Map: 
Fig. 1_138 Nepheline syenites and phonolites in Lincoln County, Oregon (after MacLeod and Snavely, 1973, Plate 1). Non-alkaline basic sheets and dykes omitted.
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith