Navajo Country

The Navajo volcanic field in the central Colorado Plateau lies within an area of about 250x180 km, but consists of a number of fields, or clusters of centres, namely Tuba, Wildcat Peak, Monument Valley, Chuska and Redrock Valleys, Lukachukai Mountains, Wheatfields, Washington Pass, Zilditloi and Black Rock (Williams, 1936, Fig. 1). Williams also pinpoints the Carrizo laccolith in the centre of the field, but this is not alkaline. The Hopi Buttes lying to the southwest may be considered part of the same province, but the rocks are sodic, in contrast to the markedly potassic Navajo rocks, and are conveniently described separately (No. 63). In recent years a considerable amount of petrological research has been directed towards the Navajo rocks principally because of their abundant suites of xenoliths, some of mantle origin (for references see McGetchin et al., 1977). Many papers have concentrated on a single centre, the names of which such as 'Ship Rock', 'Wildcat Peak' (west of Fig. 145), 'The Beast', 'Buell Park', 'Moses Rock', 'Green Knobs', 'Agathla' and 'The Thumb' are freely used in the literature but are often not easy to locate. Incomplete maps will be found in McGetchin et al. (1977, Fig. 2) and Ehrenberg (1982, Fig. 1). The fullest general account of the area and its petrography is still that of Williams (1936). The Navajo field consists principally of volcanic necks and diatremes with a paucity of flows. Much of the activity was explosive, as indicated by the abundance of brecciated rocks and tuffs in the necks, many of which comprise over 90% of such rock types. Radial dykes are present around some necks and are particularly well developed around Wildcat Peak and Ship Rock. A general account of individual fields and centres will be found in Williams (1936) and many diatremes have been mapped, examples of which will be found in McGetchin et al. (1977) and references cited therein. Much of the brecciated material is derived from the surrounding rocks, together with igneous and metamorphic rocks from greater depths and others of mantle origin including garnet lherzolite (Ehrenberg, 1982). For detailed accounts of xenolith suites and further References: see O'Hara and Mercy (1966), McGetchin et al. (1977) and Ehrenberg (1982); Smith and Ehrenberg (1984) have described zoned minerals in garnet peridotite nodules from The Thumb. The principal rock type of the lavas and dykes in the Navajo Country is generally referred to as minette, but at some localities monchiquites grading into olivine leucitites occur and small bodies of alnoite and peridotite may be found. Pyroxene phenocrysts are ubiquitous, but biotite and olivine phenocrysts are also found in a groundmass of pyroxene, sanidine, apatite and opaques, with biotite and leucite or analcime present in some rocks. The pyroxene is a diopsidic augite and in some lavas sanidine forms large poikilitic plates. The detailed mineral chemistry of minettes from two localities, including REE data, is described by Jones and Smith (1983). Monchiquites are feldspar-free, but rich in analcime. At Wildcat Peak dykes bearing melilite and nepheline are found; the former referred to by Williams (1936, p. 153) as alnoite, the latter as nepheline monchiquite. The modal mineralogy and rock chemistry of numerous lamprophyre dykes are given by Laughlin et al. (1986), who discuss the timing of dyke emplacement in relation to petrogenesis and uplift of the Colorado Plateau. Sr isotope and Rb values for a range of rocks are given by Powell and Bell (1970). Carbonate veins and dykes, thought to be carbonatites, have been described from the Cane Valley diatreme by McGetchin and Nikhanj (1973).