Allard Stock (La Plata)

Within the La Plata Mountains of southwest Colorado are a number of syenitic stocks (see Eckel, 1949, Plate 2), only one of which, the Allard stock, has been described in any detail (Werle et al., 1984). The latter is emplaced in Permian to Jurassic sedimentary rocks and late Cretaceous diorite-monzonite sills and laccoliths of porphyry, which are all extensively altered. The earliest rocks of the stock are hydrothermally altered grey syenites which were followed by mafic syenites, within which occurs a body consisting of fine-scale rhythmically layered rocks of aegirine-augite, alkali feldspar and accessories. Although the only other peralkaline rocks of the stock are certain mafic pegmatites, nearly all rock types are exceptionally potassic with K2O:Na2O ratios up to 10:1. Much of the potassic nature of the stock is ascribed to metasomatism, associated with which is extensive mineralization, but even the least altered igneous rocks are potassic. A further syenitic intrusion lies a few km to the southeast of the Allard stock (Eckel, 1949, Plate2).