PLAGIOCLASE ZONING AND THE SIZE, SHAPE, AND EVOLUTION OF GRANITIC MAGMA CHAMBERS: EVIDENCE FROM THE SNAKE CREEK PLUTON, NEVADA

The middle Jurassic Snake Creek pluton of eastern Nevada outcrops continuously over about 50 sq km. It is compositionally zoned, ranging from tonalite to granodiorite to granite. Bulk rock CaO varies from 4.5 wt. % in tonalite to 0.55 wt.% in granite. A 9-point Rb-Sr isochron gives an age of 154 million years in agreement with U-Pb zircon ages (Lee et al, 1981). O isotopic compositions, major and trace elements vary systematically across the intrusion. To further understand the origin of this zonation, plagioclase from 18 samples was analyzed by electron microprobe. The An-content of plagioclase decreases with whole rock CaO in the pluton. There are 3 distinct groups based on plagioclase compositions which divide the pluton into 3 portions: An50 - An20 (Type I, near the eastern margin), An45 - An10 (Type II, center), and < An20 (Type III, western portion of pluton). Or-content in the plagioclase varies from Or5 to Or1. Fe decreases with An, ranging from 0.012 to 0.00 apfu. Grain profiles and element maps of Si, Ca, Na, K for plagioclase, show oscillatory zonation in Type I plagioclases, oscillatory zonation in the cores and normal zonation in the rims of Type II, and little to no zonation in sodic Type III plagioclase grains. The textures and compositions of plagioclase suggest that initially, Ca-rich plagioclase crystallized both at the margin and the interior of a magma chamber. Complete crystallization of cumulates at the margin expelled residual liquid and plagioclase crystallization stopped at An20 (Type I). Within the interior of the chamber, plagioclase crystallization continued and zoned to An10 (Type II). The most evolved magma crystallized plagioclase (Type III) with the composition of the rims of the Type II plagioclase. We conclude that such a systematic variation of plagioclase could not be produced by sequential emplacement and crystallization of a series of magmas over millions of years. The age, smooth variation in O-isotope, and major and trace element compositions are in accord with this conclusion. Instead, we suggest that the Snake Creek intrusion formed from a single body of magma evolving by fractional crystallization to form marginal cumulates and displaced residual liquids.