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Mesozoic Rocks
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 » 3 percent K2O and »1,000 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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