GEOCHRONOLOGIC DETERMINATION AND PETROGENETIC CHARACTERIZATION OF SELECTED UNITS IN THE TRICKLE MOUNTAIN AREA OF THE SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO

The contributions of this research establish the timing and characterization of eight previously undefined mafic to intermediate composition units in the northeastern region of the San Juan Volcanic Field (SJVF), southwestern Colorado. All of the units in the investigation encompass the immediate surrounding vicinity of Trickle Mountain.  This region includes two of the larger occurrences, the Trickle Mountain trachydacite (Tttm) and the trachyandesite of Hill 9519.  These units represent a variety of occurrences including isolated plugs, fault injections, flows, shallow intrusions and erosional remnants.   Most of these units were erupted at the onset of the silicic ashflow phase of development in the SJVF.  Results of WDXRF whole rock analyses document a broad compositional range from basalts and basaltic-trachyandesites to trachydacites. 

Mineral analyses and petrographic thin section observations indicate these eight units exhibit variable phenocryst abundances ranging from 4% to 32% (by volume).  Results of microprobe analyses of feldspars indicate these units exhibit a broad range of compositions from An _(82-32).  All of the units that were selected for microprobe analyses of pyroxene contain augite Wo_(40-45)-En_(40-45)-Fs_(10-18)-Ac_(2-4). One of the units, the trachyandesite (Tca), contains individual phenocrysts that exhibit orthopyroxene cores Wo₍₃₎-En_(62-71)-Fs_(27-35)-Ac_(0-2) enclosed by clinopyroxene rims Wo_(40-42)-En_(41-42)-Fs_(15-18)-Ac_(0-2).  Microprobe analyses of amphibole from selected units are consistent with magnesiohastingsite NaCa₂(Mg,Fe)₄ Fe³⁺[Si₆Al₂O₂₂](OH)₂. The only exception is unit Tca, in which some phenocrysts contain Tschermakite rims.

Detailed ⁴⁰Ar/³⁹Ar geochronology was used to constrain the ages of these eight units, which range from 32.89 ± 0.07Ma to 27.77 ± 0.11Ma.  These geochronologically well-defined units are interspersed among tuffs within the northeastern region of the SJVF that include the Conejos Formation, Sapinero Mesa Tuff, The Tuff of Saguache Creek and the Fish Canyon Tuff.  The resulting geochronology made appropriate stratigraphic placement for each of the units, as well as a stratigraphic revision of previously positioned units in the Trickle Mountain area of the SJVF possible.