Alkaline Rocks and Carbonatites of the World

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

Granite Mountain (Fourche Mountain Region, Little Rock, Pulaski County)

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Occurrence number: 
174-00-121
Country: 
United States
Region: 
Arkansas
Location: 
Longitude: -92.28, Latitude: 34.67
Carbonatite: 
No

The Granite Mountain complex outcrops over an area of about 11x5 km but there are a number of isolated bodies separated from the main complex by overlying sediments. Most of the margin of the main complex is against overlapping clays, silts, sands etc. of the Tertiary Saline Formation, but Palaeozoic quartzites and schists into which the alkaline rocks are intruded are also exposed on the eastern side, and as a 4 km long lens in the central part of the intrusion. The principal rock type of Granite Mountain, particularly on the ridges and higher slopes, is pulaskite, for which this is the type locality (Williams, 1891, p. 56). It is a nepheline-bearing syenite with a porphyritic or trachytic texture of alkali feldspar, a little interstitial nepheline, arfvedsonite, sodic pyroxene and biotite, with accessory analcime, sodalite and fluorite. Some varieties contain little or no nepheline, and there is a finer grained tinguaitic border facies in places. A nepheline syenite with up to 25% nepheline, biotite and diopside zoned to aegirine-augite, together with some aegirine, occurs as an isolated mass to the south and locally along the margin of the complex. Dykes and plugs of nepheline syenite are relatively abundant and in the north and northeast particularly quartz syenite also occurs. In the southwestern central part of the complex are rocks consisting of up to 75% titanaugite in a turbid base which Williams (1891, p. 106) referred to as fourchites, but are probably monchiquites. Contact metamorphism and metasomatism of Palaeozoic sediments is apparent in places with the production of alkali feldspar, biotite and aegirine.

Economic: 
Weathering of nepheline syenite and pulaskite has produced extensive bauxite deposits, those of this area and Saline County (No. 122) to the south constituting the Arkansas bauxite region, the biggest producer in the U.S.A. Full account of the various deposits, the history of mining and production statistics will be found in Gordon et al. (1958, pp. 71-255). The pulaskite, in particular, has been quarried for building stone and used in many public buildings, e. g. in Little Rock.
Age: 
Biotite from nepheline syenite gave 87 ± 4 and 91 ± 5 Ma by K-Ar and 86 ± 3 Ma for both samples by Rb-Sr (Zartman et al., 1967).
References: 

GORDON, M., TRACEY, J.I. and ELLIS, M.W. 1958. Geology of the Arkansas bauxite region. Professional Paper, United States Geological Survey, 299: 1-268.
WILLIAMS, J.F. 1891. The igneous rocks of Arkansas. Annual Report of the Geological Survey of Arkansas for 1890: 1-457.
ZARTMAN, R.E., BROCK, M.R., HEYL, A.V. and THOMAS, H.H. 1967. K-Ar and Rb-Sr ages of some alkalic intrusive rocks from central and eastern United States. American Journal of Science, 265: 848-70

Map: 
Fig. 1_169 Distribution of alkaline igneous rocks and carbonatites in central Arkansas. and Fig. 1_173 Granite Mountain (after Gordon et al., 1958, Plate 1).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith