Jurassic Plutonic Rocks

Widespread emplacement of Jurassic plutons followed by approximately 50 m.y. the emplacement of scattered Triassic plutons in the Mojave Desert region (Tosdal and others, 1990; Anderson and others, 1991). The most thoroughly studied Jurassic plutonic rocks in the region of the EMNSA are those in the area of the southern Bristol Mountains, southern Providence Mountains, and Colton Hills (Miller and others, 1985; Fox and Miller, 1990); Granite Mountains (Young and others, 1992); and Clipper Mountains (Gerber and others, 1991). Three types of Jurassic plutonic rocks are common: mafic rocks, intermediate to silicic mixed or heterogeneous rocks, and leucocratic monzogranite (pl. 1). These plutons are difficult to date precisely, but appear to be largely160-165 Ma. Except in the Granite Mountains, where Late Jurassic diorite is known (Young and others, 1992), the mafic rocks are generally the oldest and the leucocratic rocks the youngest. Other plutons possibly in the 160-165 Ma age group are in the Old Dad Mountains and Devils Playground area. Compositionally, the mafic rocks include fine- to coarse-grained gabbro, diorite, and monzodiorite; common mafic minerals include clinopyroxene, hornblende, and biotite. Generally, the mafic rocks have SiO2 contents of 49 to 55 percent; are subalkaline and metaluminous; and have relatively high abundances of large-ion lithophile elements (LILE), for example commonly as much as ť 3 percent K2O and ť1,000 ppm Ba. Young and others (1992) concluded from geochemical modelling that diorite evolved from parental magma that was derived from hydrous, REE-enriched subcontinental lithosphere and was contaminated by mafic granulite in the lower crust as it ascended to the upper crust.

The mixed intrusive rocks are by far the most abundant. They are markedly heterogeneous, ranging from fine-grained equigranular to coarsely porphyritic, and continuously from quartz monzodiorite to syenogranite and syenite. A number of phases or subgroups are present, typically bounded by gradational contacts. Chemically, the mixed intrusive rocks have a wide range of SiO2 contents: 50 to 74 percent; they are subalkaline to, less commonly, alkaline; and metaluminous to weakly peraluminous. Some rocks are potassic, with K2O/Na2O as great as 2. Ba abundances are as great as 2,000 to 4,000 ppm in some of the mafic and intermediate rocks.

The leucocratic monzogranite is the most homogeneous of the three rock types. Whereas other plutonic phases grade complexly into one another, the monzogranite generally cleanly cross-cuts as the youngest phase. In the Colton Hills, it comprises medium- to coarse-grained, porphyritic leucocratic biotite monzogranite, locally with minor muscovite. It is subalkaline and generally moderately peraluminous. Trace-element abundances are unremarkable. Many of the Jurassic plutonic rocks in the EMNSA are strongly altered. In the southern Providence Mountains and the southern Bristol Mountains, the rocks have undergone widespread albitization, characterized by replacement of potassium feldspar by albite (Miller and others, 1985; Fox, 1989; Fox and Miller, 1990). Intense albitization is present as white zones in otherwise normally mesotype rocks. Less intense albitization produces mottled patches or spots. Chemically, albitization is characterized by doubling of Na2O content and nearly complete loss of K2O: typically K2O decreases from 6 percent to <1 percent. Accompanying changes in Fe, Mg, and Ca abundances depend on the extent of chloritization of mafic phases. Aluminum, Ti, Zr, Y, and REE are generally immobile on a hand-specimen scale during alteration. Alteration was probably mainly caused by repeated intrusion of magma into the shallow crust, creating large, long-lived hydrothermal systems. Late Jurassic plutons in the EMNSA are known in the Granite and Ivanpah Mountains, but may be more widely present. Diorite in the Granite Mountains is ~155 Ma (Young and others, 1992). Other than its younger age, the diorite is similar to diorite in the Providence and Bristol Mountains. The Ivanpah Granite of Beckerman and others (1982) is 150 to 145 Ma (J.D. Walker, 1992, oral commun.). It consists of biotite monzogranite that is strongly porphyritic. The pluton is moderately peraluminous and potassic. Late Jurassic Dikes

In the southern Providence Mountains, swarms of Middle to Late Jurassic intermediate to silicic dikes intrude Jurassic plutons (Miller and others, 1985). The dikes range from dacite porphyry to aphanitic rhyodacite to aplite. Similar intermediate dikes in the Colton Hills are intruded by Cretaceous plutons (Fox and Miller, 1990). Dikes in the Colton Hills have a minimum age of 146 Ma by K-Ar on biotite (D.M. Miller, 1984, unpubl. data). Similar dikes are known in a few other places within the EMNSA, such as the Cowhole Mountains. Possibly related swarms of Late Jurassic mafic or intermediate to silicic dikes are widespread in eastern California and southwestern Arizona (Chen and Moore, 1979; Hopson, 1988; Powell, 1981; Karish and others, 1987; Haxel and others, 1988; Tosdal and others, 1990). Some of the Jurassic dikes that crop out in the Providence Mountains were correlated by James (1989) with the approximately 150 Ma Independence dike swarm of eastern California. James (1989) suggested that the more than 500 km long dike swarm may be related to continental-scale arcnormal extension, changes in plate motions, or a combination of oblique subduction with leftlateral shear. The similar age of the Ivanpah Granite raises the possibility that plutons were also emplaced at the time of dike intrusion.

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