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Mesozoic Rocks

Cretaceous Plutonic Rocks

Most Cretaceous plutonic rocks of the EMNSA belong to the Early and Late Cretaceous Teutonia batholith (Beckerman and others, 1982). Beckerman and others (1982) considered the Teutonia batholith to be Jurassic and Cretaceous, based chiefly upon K-Ar cooling ages that provide minimum emplacement ages. They divided the batholith into seven plutons or units, with a large area of granitic rocks in the Halloran Hills area (DeWitt and others, 1984) undivided and undescribed (fig. 5). One pluton is Jurassic, the Ivanpah Granite (pl. 1). The other six plutons, which constitute most of the eastern batholith, are Cretaceous. Preliminary U-Pb zircon ages for major plutons of the batholith range from 93 to 100 Ma (determined by E. DeWitt, 1990). “Teutonia batholith” thus is hereby redefined to exclude the coincidentally spatially associated Jurassic Ivanpah Granite; this revised usage is followed in the summary below.

The six major plutons that constitute the eastern Teutonia batholith crop out chiefly in the New York Mountains, Mid Hills, and the Cima Dome-Wildcat Butte-Marl Mountains area (pl. 1). Five of the six plutons are fairly large, with exposed areas »50 to 200 km2. These plutons are intermediate to felsic in composition. The sixth, mafic pluton forms a subcircular outcrop area »2 km in diameter; correlative bodies are yet smaller. Similar mafic to felsic rock units have been discerned in the Halloran Hills area (E. DeWitt and H.G. Wilshire, 1992, oral commun.)

The five relatively large plutons of the Teutonia batholith principally range in composition from quartz monzodiorite to syenogranite, with granodiorite and monzogranite as the principal compositional types; monzodiorite is a minor phase of one pluton. Despite this compositional range, granite constitutes most of the exposed rock. Quartz-poor modal compositions (quartz monzodiorite, quartz monzonite, quartz syenite) are present only in the Rock Spring Monzodiorite of Beckerman and others (1982). Other rocks are medium- to coarse-grained; some plutons or facies within plutons are equigranular, whereas others have alkali feldspar phenocrysts. Biotite is ubiquitous; hornblende is common to absent; the Kessler Springs pluton locally contains a little primary muscovite. Three of the five plutons are leucocratic--Teutonia Adamellite, Mid Hills Adamellite, and Kessler Springs Adamellite of Beckerman and others (1982)--with color indices less than 5 .

The sixth, small pluton (Black Canyon Hornblende Gabbro of Beckerman and others (1982), unit Kbc, pl. 1) comprises compositionally and texturally variable hornblende-rich mesotype to melanocratic gabbro. Magnetite content is high: average is 6.5 volume percent. This pluton intrudes two of the larger granitic plutons. Probable correlative bodies include one in Cedar Canyon and another, not shown on the map, near Wildcat Butte on Cima Dome. Chemically, the six plutons of the Teutonia batholith form a broadly calcalkaline series (Beckerman and others, 1982). The hornblende gabbro contains 43 to 49 percent SiO2; the other five plutons range from 68 to 77 percent SiO2. The granitoid plutons generally straddle the boundary between metaluminous and peraluminous compositions. Moderately or strongly peraluminous granites are absent. Abundances of Ba, Sr, and Rb (the only trace elements analyzed) are generally normal and unremarkable for granitic rocks.

Geobarometric data indicate that the Rock Spring Monzodiorite of Beckerman and others (1982) was emplaced at pressures of <1 to 3 kb (Anderson and others, 1988; 1991), corresponding to upper-crustal depths of approximately <3 to 10 km. According to J.L. Anderson, (oral commun., 1990) present exposures provide a tilted view of the batholith: the shallowest plutons, emplaced at pressures of approximately 0.5 kb, are on the north and the deepest plutons, emplaced at approximately 3 kb, are to the south. However, these pressure data conflict with geologic evidence that the roof of the batholith is exposed in the south (Goldfarb and others, 1988) as a shallowly south-dipping surface in the Providence and Marl Mountains; the south should therefore be the shallowest part of the batholith. The Teutonia batholith is among the shallowest of the Mesozoic plutonic complexes in the Mojave Desert region studied by Anderson and others (1988, 1991): pressure estimates for ten other complexes range from 2 to 9 kb.

Small, shallow-level stocks that lie northeast of the Teutonia batholith appear to represent outliers of the magmatic belt in earliest Late Cretaceous time. Magmatic alteration, intrusive breccia, and felsite dikes in the Colosseum mine area of the Clark Mountains (described in a later section) are about 100 Ma (Sharp, 1984). Similar alteration and breccia, as well as small hypabyssal bodies of biotite granodiorite, lie about 5 km northeast of the EMNSA in the New York Mountains, near Crescent Peak (Miller and Wooden, 1993). This granodiorite yielded a 94.4 ± 2.4 Ma K-Ar biotite date.

Latest Cretaceous plutons in the EMNSA range from 75 to 70 Ma (Howard and others, 1987; Fox and Miller, 1990, citing J.L. Wooden, oral commun., 1987). Known ~70-Ma Late Cretaceous plutons crop out in the Granite Mountains and at Homer Mountain; probable Late Cretaceous plutons crop out in the Fenner Hills and near Bobcat Hill. In the Granite Mountains, a suite of Cretaceous igneous rocks includes a granodiorite pluton, a zoned pluton, and granite, aplite and pegmatite dikes (Howard and others, 1987). The granodiorite pluton makes up most of the western Granite Mountains (pl. 1), and a larger zoned pluton makes up the southeastern part (Howard and others, 1987) and part of the adjacent Providence Mountains (Miller and others, 1985). Magmatic biotite from the zoned pluton yielded K-Ar dates, possibly emplacement ages, of 70.9 to 74.5 Ma (Miller and others, 1985; Howard and others, 1987). In general, latest Cretaceous plutonic rocks are silicic, slightly to strongly peraluminous granites.

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