Zonation of alkali feldspar compositions in the Tshirege Member of the Bandelier Tuff in Pueblo Canyon, near Los Alamos, New Mexico
Mary E. Caress
In the Pajarito Plateau the Tshirege Member of the Bandelier Tuff is a compositionally zoned ignimbrite sheet, composed of, from base to top, cooling units I, II, III and IV, each formed of multiple pyroclastic flow and surge units. Flow units la and 2a are, respectively, the major components of cooling units I and II. This paper presents the results of electron microprobe analyses of 1200 alkali feldspar phenocrysts from the matrix (<2 mm grain size fraction) of the Tshirege Member in Pueblo Canyon and two additional sites. It documents the vertical compositional variation of alkali feldspars through the four cooling units in their proximal facies, and vertical and lateral compositional zonation within flow units la and 2a along flow direction. Cooling units I and II contain mostly grains of rim composition Or36-44 whereas cooling units III and IV contain grains of rim compositions Or25-44. The overall upward increase in proportion of alkali feldspar phenocrysts of progressively more sodic composition is interrupted by two trend reversals within cooling units III and IV. Flow units la and 2a show pronounced lateral (proximal to distal), and lesser vertical, compositional zonation. The first erupted material in an eruptive pulse traveled farthest, while the last erupted material was emplaced near the source. The observed trends are compatible with a model of laminar transport and en masse deposition. Caldera collapse early during eruption of Cooling Unit III is suggested by the presence of basal lithic breccias, high lithic content compared to the other cooling units, and the abrupt influx of feldspars from deep in the magma body. This altered the dynamics of magma withdrawal and eruption, causing eruption of many smaller pyroclastic flows and surges, and resulted in irregularities in the range of compositions tapped during subsequent eruption pulses.
- Caress, Mary E., 1996, Zonation of alkali feldspar compositions in the Tshirege Member of the Bandelier Tuff in Pueblo Canyon, near Los Alamos, New Mexico, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 275-283.