Geology topics

Lydia Fox

Publications and source records attributed to Lydia Fox.

2 recordsLinked to original sources

USGS2016

Regional and temporal variability of melts during a Cordilleran magma pulse: Age and chemical evolution of the jurassic arc, eastern mojave desert, California

Intrusive rock sequences in the central and eastern Mojave Desert segment of the Jurassic Cordilleran arc of the western United States record regional and temporal variations in magmas generated during the second prominent pulse of Mesozoic continental arc magmatism. U/Pb zircon ages provide temporal control for describing variations in rock and zircon geochemistry that reflect differences in magma source components. These source signatures are discernible through mixing and fractionation processes associated with magma ascent and emplacement. The oldest well-dated Jurassic rocks defining initiation of the Jurassic pulse are a 183 Ma monzodiorite and a 181 Ma ignimbrite. Early to Middle Jurassic intrusive rocks comprising the main stage of magmatism include two high-K calc-alkalic groups: to the north, the deformed 183–172 Ma Fort Irwin sequence and contemporaneous rocks in the Granite and Clipper Mountains, and to the south, the 167–164 Ma Bullion sequence. A Late Jurassic suite of shoshonitic, alkali-calcic intrusive rocks, the Bristol Mountains sequence, ranges in age from 164 to 161 Ma and was emplaced as the pulse began to wane. Whole-rock and zircon trace-element geochemistry defines a compositionally coherent Jurassic arc with regional and secular variations in melt compositions. The arc evolved through the magma pulse by progressively greater input of old cratonic crust and lithospheric mantle into the arc magma system, synchronous with progressive regional crustal thickening.

California↗

USGS1990

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.

California↗