CORRELATION OF ASH-FLOW SHEETS, SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO

Two ash-flow sheets, the tuff of Pass Creek (PCT) and the tuff of Sheep Creek (SCT), have been previously mapped in the northeastern San Juan Volcanic Field (SJVF), southwestern Colorado, without attempts at regional correlation. In this study, improved stratigraphic understanding in this hitherto poorly characterized region of the SJVF is achieved through characterization of lithology, petrography, modal mineralogy, mineral chemistry, and major and trace element compositions of the tuffs in question and comparison of these data to possible correlative units. Mineral and rock compositional data enable understanding pre-eruptive and post-emplacement evolution of these units.

The PCT immediately overlies the Carpenter Ridge Tuff (CRT) and, therefore, samples surrounding the hypothesized contact were characterized to determine if compositional trends were continuous across the contact. Useful compositional trends include decreasing celsian content in sanidine phenocrysts, increasing BaO and MgO in biotite grains, and increasing mole% ulvospinel in titanomagnetite grains. Major and trace element concentrations from whole-rock analyses indicate slight compositional trend (SiO2, FeO, BaO decrease and Al2O3, MgO, TiO2 increase) that are continuous across the contact, consistent with deposits from a stratified magma chamber. Amphibole and plagioclase thermobarometry suggest pre-eruptive conditions supporting such a chamber. The absence of compositional discontinuities indicates that the PCT is an upper member of the 27.4 Ma CRT, locally modified by vapor phase crystallization.

The SCT contains the identical phenocryst assemblage (sn, pl, bi, hb, cpx, Fe-Ti oxides) as the 26.8 Ma Nelson Mountain Tuff (NMT) with nearly identical mineral and rock compositions and similar chemical variations with stratigraphic height. Particularly diagnostic features of both tuffs include reverse zoning in plagioclase (cores=An15; rims=An18) and high MnO concentrations in diopside (7-9 mol%). Major and trace element analyses of whole-rock and pumice clasts indicate both tuffs are high-silica rhyolites (75-77 wt% SiO2). Amphibole, plagioclase, and Fe-Ti oxide thermobarometry indicate similar pre-eruptive conditions of origin for the SCT and NMT. We conclude that the SCT is outflow NMT.