THE CHARACTERIZATION AND ANALYSIS OF QUATERNARY BASALTS FROM NYE CO. NV

Three Quaternary lava flows from Nye Co. NV, were analyzed petrographically and geochemically to compare possible relationships among these flows, and basalts from Lunar Crater Volcanic Field. Two of these flows are from the Golden Gate Range on the east side of the Grant Range; one can be classified as a trachyandesite the other a basaltic andesite. The third flow, in the Mud Springs Basin on the west side of the Grant Range, is classified as a basalt. The trachyandesite contains sparse phenocrysts of plagioclase, usually showing inclusion-rich cores, clinopyroxene, orthopyroxene, and microphenocrysts of Fe-Ti oxide set in a fine-grained groundmass. The basaltic andesite contained phenocrysts of olivine, yellow-brown clinopyroxene (some showing hourglass structure), simply twinned plagioclase usually showing a thin outer zone, and microphenocrysts of Fe-Ti oxide set in a plagioclase-lathe-dominated groundmass showing well-developed flow textures. The size and quantity of phenocrysts for this flow tend to decrease from the top of the flow to the bottom third of the flow where size and quantity of phenocrysts then increase again to the base of the flow. The Mud Springs flow is distinctively different than the others, vesicular, and contains large phenocrysts (up to 5 mm) of olivine, simply twinned plagioclase in a fine-grained groundmass. This flow is also characterized by the presence of numerous xenocrysts of quartz, often found with reaction rims. Geochemically each flow appears to be distinct from one another when plotted on the MgO-based variation diagrams. When the geochemical data are plotted against relative position in the individual flows, the Mud Springs basalt appears to be essentially geochemically homogeneous from top to base, confirming field-based observations that this flow represents a single flow unit. In contrast the basaltic andesite appears to be composed of three distinct flow units, again confirming field observations based on textural criteria. Comparison of the data presented with the Lunar Crater basalts indicate for some compositions, e.g. Al2O3 the Lunar Crater samples span the range between the basalt and the basaltic andesite; for other compositions, e.g. Fe2O3 the Lunar Crater basalts are quite distinct from any of the other lava flows sampled.