Mesozoic Volcanic and Hypabyssal Rocks

Several ranges within the EMNSA contain volcanic and volcaniclastic rocks, intercalated sedimentary rocks, and related hypabyssal rocks of Triassic and (or) Jurassic age. Stratigraphic sequences, in varying degrees of preservation, are exposed in four areas: the Mescal Range, the Old Dad Mountain-Cow Hole Mountains-Soda Mountains area, the New York Mountains, and the Providence Mountains. In a few other areas, metamorphosed or hydrothermally altered Triassic and (or) Jurassic volcanic, hypabyssal, and sedimentary rocks are present as small pendants in Jurassic or Cretaceous plutons, or as slivers in fault zones. Among these small relicts, Jurassic rocks are probably more common than Triassic rocks. Owing to common metamorphism or alteration, and to lack of study, little is known about the petrology and geochemistry of Triassic and Jurassic volcanic rocks in the EMNSA. A sequence of diverse volcanic and sedimentary rocks, more than 3 km thick, in Old Dad Mountain and the Cow Hole Mountains consists of interbedded intermediate to silicic lava flows and flow breccias, quartz arenite, sandstone and siltstone, sedimentary breccia and megabreccia, silicic ignimbrite, and other minor rock types (Busby-Spera, 1988; Busby- Spera and others, 1989; Marzolf, 1983; 1988; 1991). U-Pb zircon ages of some of the volcanic rocks indicate that this sequence is approximately 170 Ma, which is Middle Jurassic according to the geologic time scale of Harland and others (1989). A generally similar sequence of rocks is present in the Soda Mountains, near the west edge of the EMNSA (Grose, 1959). The quartz arenite units in the Mescal Range, Old Dad Mountain, and the Cowhole Mountains are in part eolian. Until recently, these quartz arenites were generally correlated with the Early Jurassic (Peterson and Pipiringos, 1979) eolian Aztec Sandstone of the southern Great Basin and Navajo Sandstone of the Colorado Plateau. However, the U-Pb ages cited above indicate that the Jurassic quartz arenites in these ranges, and probably others in the EMNSA, may correlate with the Carmel Formation or the Entrada Sandstone of the Colorado Plateau. Poor age constraints for the Colorado Plateau units permit either correlation. In the Providence Mountains, intermediate to silicic volcanic, volcaniclastic, and hypabyssal rocks, in part intensely altered, have been mapped by Miller and others (1985) and Goldfarb and others (1988). The hypabyssal rocks typically have granitic textures. These igneous rocks in part overlie the Moenkopi Formation, and are probably Triassic and (or) Jurassic in age (Walker, 1987). In some places, the volcanic rocks contain intercalated conglomerate and siltstone.

In the New York Mountains, a sequence of metamorphosed volcanic rocks approximately 250 m thick overlies the Moenkopi Formation, and is in turn overlain by a metasedimentary unit approximately 70 m thick (Burchfiel and Davis, 1977). The volcanic rocks are silicic in composition, include breccia or agglomerate, and contain subordinate intercalated metasiltstone and, near the base of the unit, metaconglomerate. The metasedimentary unit comprises siltstone, conglomerate, and tuffaceous sandstone and siltstone. The conglomerate beds contain clasts derived from the underlying volcanic unit. These two units could be either Triassic or Jurassic in age; the latter is perhaps more likely. The metavolcanic rocks are generally schistose or mylonitic; this fabric probably is largely inherited from original welded tuff textures. Metasedimentary lithologies range from argillite to schist. Both the volcanic and sedimentary units in the New York Mountains contain metamorphic biotite. In the Mescal Range, a unit of crossbedded arenitic sandstone approximately 250 m thick contains dinosaur tracks, the only dinosaur tracks known in California (Reynolds, 1983). It is overlain by a sequence, approximately 200 m thick, of basaltic, dacitic, and rhyolitic flow breccias and lava flows (Hewett, 1956; Fleck and others, in press). These volcanic rocks have not been studied in detail, but are dated by K-Ar and Rb-Sr as Early Cretaceous, about 117 Ma (Fleck and others, in press), and therefore differ in age as well as composition from Jurassic volcanic sequences in other parts of the EMNSA. Jurassic and Cretaceous Plutonic Rocks

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