A COMPARISON OF IGNIMBRITES OF THE SIDEWINDER VOLCANIC SERIES TO EXPOSED PLUTONS, SOUTHERN CALIFORNIA

The relationship between a pluton or batholith and its extrusive counterpart, an ignimbrite, may be a direct one –eruptive products may represent an increment of volcanic material that displays the exact chemistry of the magma chamber during eruption. However, if plutons are the product of a multitude of much smaller pulses, ignimbrites may not reflect the chemistry of the magma which generates batholiths. A prime location to test whether or not these igneous bodies are related is California, where two areas contain both exposed plutons and large volume ignimbrites: the Jurassic Sidewinder volcanic series in the Mojave Desert and the Cretaceous Merced Peak volcanic complex in the Sierra Nevada.

Three ignimbrites within the Sidewinder volcanic series, the tuffs of Black Mountain, Turtle Mountain, and Stoddard Ridge, were chosen for study due to their apparent large volumes (10s to 100s of km3; Schermer and Busby, 1994), varying ages and chemistry, and proximity to broadly contemporaneous plutons. The tuffs are trachyte, dacite, and rhyolite with silica contents ranging from 65 to 74%. The 181 Ma tuff of Black Mountain is relatively phenocryst poor (~10%), and contains quartz, plagioclase feldspar, and biotite. Black Mountain can be classified as a monotonous intermediate tuff due to its limited range in silica (Hildreth, 1981). The 163 Ma tuff of Turtle Mountain ranges from 20-40% total phenocrysts, and is comprised of quartz, plagioclase feldspar, biotite, and oxides. Silica content ranges from 66.2 to 67.7%, thus indicating that this tuff is also a monotonous intermediate. The youngest tuff, the 149 Ma tuff of Stoddard Ridge has a lower total phenocryst percent (~10%), but contains the same minerals as the two older tuffs. The tuff ranges in silica content from 74% near the stratigraphic base to 65.2% near the top, indicating that it is a zoned intermediate.

Emplacement of the tuffs of Turtle Mountain and Stoddard Ridge was contemporaneous with nearby voluminous Jurassic plutonism. It appears that regardless of the manner in which a magma chamber is formed, there must have been, at some time, relatively large bodies of either homogenous or zoned magma massive enough to produce these ignimbrite deposits.