THE MAGMATIC SYSTEM ASSOCIATED WITH THE PLATORO CALDERA COMPLEX, SOUTHERN ROCKY MOUNTAINS VOLCANIC FIELD, COLORADO

The polycyclic Platoro caldera in the Southern Rocky Mountain volcanic field is unique in that at least seven dacitic to rhyolitic ignimbrites erupted from this location between 30.2 and 28.8 Ma. Additionally, resurgent uplift and subsequent erosion exposed post-caldera intrusions that yield zircon U-Pb SHRIMP ages ranging from 29.0 to 27.0 Ma. Amphibole compositions for two post-caldera intrusions constrain crystallization pressures to 250-300 MPa, suggesting magma derivation from depths between 9 and 10 km prior to subvolcanic emplacement. Intercalated with ignimbrites are voluminous andesitic lava flows that may reflect mafic magma recharge between, precursory to, or post-ignimbrite eruption.

Whole rock compositions of the most extensive dacitic ignimbrites from the complex, the Black Mountain, La Jara Canyon, Ojito Creek, Ra Jadero, and Chiquito Peak tuffs, are broadly similar (62-69 wt% SiO₂); however, there are distinct differences in modal mineralogy and mineral chemistries. For example, only the Chiquito Peak and Ra Jadero tuffs host sanidine phenocrysts. Sanidine from the Ra Jadero Tuff are higher in barium overall than that of the Chiquito Peak Tuff but have a lower Or-content (Or_63-66 versus Or_74-78). Anorthite-content in Chiquito Peak and Ra Jadero plagioclase cores range up to An₈₄, something not observed in any other tuffs associated with the Platoro complex. Biotite and clinopyroxene compositions also vary, with distinct differences in titanium, magnesium and manganese compositional ranges among the tuffs.

The overall longevity (~3 Ma) of the compositionally diverse magmatic system supplying the Platoro caldera complex is characterized by pulsed volcanic and plutonic activity. These compositional differences along with the timescales documented suggest these magmas were derived from magma reservoirs at different levels in the crust from a complex trans-crustal magmatic system active beneath the Platoro caldera complex.