Spanish Peaks

Within the dyke swarm that radiates from the stocks at Spanish Peaks are alkaline types including fourchite, teschenite, shonkinite and mela- and microsyenite. K-feldspar is present in most of these dyke rocks together with analcime, and pyroxenes which vary from diopside through to aegirine. Arfvedsonite occurs in some varieties and riebeckite rims hornblende in others. At Bradford Lake northeast of Walsenburg are sills 10 and 13 m thick, separated by 6 m of shale, composed of a coarse rock of anorthoclase, some rimming labradorite, about 10% interstitial analcime, barkevikite and accessories. The same rock type constitutes several plugs in the neighbourhood, and the prominent Huerfano Butte lying further north consists of an olivine gabbro very rich in biotite which again contains analcime. Chemical analyses of most of these rock types are available in Johnson (1961) as is a detailed geological map (op. cit., Plate 1). A discussion of petrogenesis in the light of REE data will be found in Bor-ming Jan et al. (1979).