Chapter 7: Jurassic granitoids and related rocks of the southern Bristol Mountains, southern Providence Mountains, and Colton Hills, Mojave Desert, California

Jurassic plutons in the east-central Mojave Desert region are markedly different from older and younger Mesozoic plutons in the region. They form a chemically and texturally heterogeneous group that ranges in composition from diorite to syenogranite; some phases are alkalic. Igneous rocks in the southern Bristol Mountains, southern Providence Mountains, and Colton Hills are subdivided into five broadly defined groups on the basis of field relations and geochemistry: mafic intrusive rocks; mixed intrusive rocks, consisting of subequigranular and porphyritic subgroups; felsic intrusive rocks; metavolcanic and hypabyssal rocks; and dikes. There is a general trend from older, more mafic and heterogeneous rocks to younger, more felsic and homogeneous plutonic phases. Extreme spatial variations in composition and texture and other field relations indicate that the plutons were intruded during a relatively short time span. Field relations also indicate that the plutons were intruded at upper crustal levels. The plutons were affected by extensive late- or early post-magmatic sodium metasomatism (albitization), which resulted from the introduction and circulation of predominantly meteoric fluids. We propose that the plutons in this region were derived from compositionally heterogeneous but genetically related magmas generated from an upper mantle/lower crustal amphibolitic source. Similar Jurassic rocks are found elsewhere in the southern Cordillera, indicating that the genesis of these unusual rocks is a regional phenomenon.