A GEOCHEMICAL COMPARISON OF VOLUMINOUS IGNIMBRITES FROM THE SIDEWINDER VOLCANIC SERIES TO EXPOSED CONTEMPORANEOUS PLUTONS, SOUTHERN CALIFORNIA

The relationship between a pluton 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 prior to eruption. However, if plutons are the product of a multitude of much smaller emplacement pulses, ignimbrites may not reflect the chemistry of the magma which generates batholiths. A prime location to test relationships between igneous bodies is the Sidewinder volcanic series (SWV), Mojave Desert, southern California.

There are four major SWV tuffs, the tuffs of Black Mountain, Sidewinder Mountain, Turtle Mountain, and Stoddard Ridge. Three of these tuffs, the tuff of Black Mountain, Turtle Mountain and Stoddard Ridge, were chosen for study based on their apparent large volumes (10's to 100's of km3; Schermer and Busby, 1994), varying ages and chemistry, and proximity to broadly contemporaneous plutons. SHRIMP-RG U-Pb zircon analyses indicate that each tuff represents a chronologically distinct Jurassic pulse of magma, and supports the laterally extensive and voluminous nature of these ignimbrites. The later two tuffs (Turtle Mountain, Stoddard Ridge) correspond in time to nearby exposed plutons. The tuffs range from 63 to 72% silica, but each tuff has distinct geochemistry and petrography and some geochemical zoning with stratigraphic height. This geochemical data can be used to compare the tuffs of Turtle Mountain and Stoddard Ridge to nearby exposed plutons- the Middle Jurassic Goat Basin and Bullion Suites (Mayo et al., 1998; Howard, 2002), and Late Jurassic calc-alkaline series plutons. The tuffs and contemporaneous plutons are collinear in most elements. The tuff of Turtle Mountain is comparable to the granodiorite and homogeneous monzogranite facies of the Bullion suite, and the tuff of Stoddard Ridge is equivalent to granites from the White Tank (E.F. Palmer, unpublished data) and Rattlesnake Mountain plutons.