CHEMICAL AND PETROGRAPHIC CHARACTERISTICS OF CRYSTALS FROM THE EAST FORK MEMBER, VALLES CALDERA, NEW MEXICO, USA

The most recent volcanism in the Valles caldera is represented by the El Cajete pumice, Battleship Rock ignimbrite, and Banco Bonito lava flow, as well as the VC-1 rhyolite lava, which are collectively known as the East Fork Member of the Valles Rhyolite. The triggering event that caused these recent eruptions at 55 to 40 ka after a 460 ka lull in volcanism has been suggested to be the intrusion of mafic magma at depth, thus causing the re-melting of a silicic intrusive body.

Plagioclase feldspars are commonly resorbed with distinct overgrowth rims. Transect analyses were performed by electron microprobe to examine compositional profiles in the crystals. Preliminary data shows multiple feldspar crystal populations. One plagioclase population has cores that range from An₂₀ to An₂₅ with rims that range from An₃₀ to An₃₅; rim overgrowth contacts are sharp. Another plagioclase population is characterized by normal zonation from a high calcium core (An₅₀) to a low calcium rim (An₃₀). A third plagioclase population exhibits patchy zonation ranging from An₃₅ to An₅₀. A fourth population of small, euhedral plagioclase grains (~An₅₀) occurs in some samples. Finally, some plagioclase grains are overgrown with alkali feldspar rims in an antirapikivi texture. In contrast to the feldspars, biotites and amphiboles are generally euhedral and appear to represent a single population. There are no systematic compositional variations. For example, the biotite crystals do not show any systematic variation in magnesium and titanium in core to rim analyses. However, many biotites have euhedral hornblende overgrowths, indicating an increase in temperature prior to eruption. In addition, Ti contents in quartz show significant core-to-rim variability. These observations point to a complex thermal history of the magma prior to eruption, and possibly multiple origins for crystals.