Shonkin Sag

Shonkin Sag is the most studied of the Highwood Mountains laccoliths, because it was considered to be an outstanding example of the differentiation in place of an undersaturated alkaline magma. The fullest general account is in Hurlbut and Griggs (1939), and the mineralogy, geochemistry and petrogenesis have been studied in exceptional detail by Nash and Wilkinson (1970 and 1971). The intrusion is apparently nearly circular in plan, varies from about 40 to 70 m in thickness and has almost horizontal contacts with sandstones. Complete vertical sections are exposed in cliffs. The principal part of the intrusion is clearly seen to be layered and extending out from this layered section are numerous sills. Detailed descriptions of the form of the intrusion are given by Hurlbut and Griggs (1939, p. 1051). Five principal rock types have been distinguished which are, from bottom to top: lower chilled margin, shonkinite, syenite, soda syenite and upper chilled margin. The chilled margin rocks contain phenocrysts of salite, olivine, often mantled by biotite, pseudoleucite composed of alkali feldspar and zeolite after nepheline, and sparse phlogopite in a groundmass of the same minerals plus sanidine, biotite and accessories. The lower chilled margin grades over a few metres into shonkinite of green rimmed salite, sanidine and variable olivine, biotite, accessories and rare pseudoleucite. The syenite differs from the shonkinite only in the proportions of the minerals; there is a decrease in olivine, biotite and pyroxene, which now has more extensive aegirine-rich rims, and an increase in sanidine. In the lower part of the syenite is a distinctive pegmatitic syenite which has been particularly described by Barksdale (1937). The soda syenite, considered to be the most evolved rock, occurs as bands and veinlets in the upper part of the intrusion and has a higher Na2O:K2O ratio, increased proportion of aegirine, both as rims and discrete crystals, iron-rich biotite, abundant sanidine and accessories including a little arfvedsonite and melanite. An interpretation of the differentiation at Shonkin Sag in terms of three separate injections of magma was made by Barksdale (1937), and in terms of liquid immiscibility by Kendrick and Edmond (1981). Data on Sr isotopes through the intrusion are given by Powell and Bell (1970).