New Mexico Geological Society Annual Spring Meeting — Abstracts

The 32 Ma Organ Cauldron of south central New Mexico exhibits continuous outcrop exposure of: remnants of the cauldron magma chamber (the Organ batholith), cauldron floor rocks, two large volume silicic intra-cauldron ash-flow tuffs, and compositionally gradational post caldera lavas. The largest volume phase of the batholith is the compositionally zoned Organ Needle pluton (55% to 75% SiO₂). The most mafic part of the pluton is a monzondiorite with preliminary Nd and (⁸⁷Sr/⁸⁶Sr) m values of 2.96 and 0.7053, respectively. These values are consistent with a parental magma source in the subcontinental mantle lithosphere. Mass balance calculations suggest that compositional variation within the pluton is primarily a result of fractional crystallization. The presence of mantle derived from magma in the upper crust suggests extensional tectonics, and may perhaps contribute to the age bracketing of the southern Rio Grande rift, within which the Cauldron is located.

Chemical and isotopic data as well as field relationships suggest that the tuff of Squaw Mountain, a 1.4 km thick ash flow unit located at the top of the cauldron tuff stratigraphy, represents magma extruded from the same magma chamber that formed the Organ Needle pluton. Rapid solidification of the Organ Needle pluton was the result of degassing of the magma chamber during the eruption which produced the tuff of Squaw Mountain. Relicts of the degassing event include numerous and widespread miarolitic cavities.

Detailed chemical studies of the tuff of Squaw Mountain show that the bulk of the ash flow is virtually identical in whole rock chemistry to the most silicic part of the Organ . Needle pluton, an alkali feldspar granite (75% SiO₂). strong stratigraphically vertical gradients in major and trace element chemistry at the top of the tuff are probably genetically related to the gradational contact between the quartz syenite and alkali feldspar granite facies of the Organ Needle pluton.

Other characteristics of the organ Cauldron include: a scarcity of fallout deposits in association with the ash flows; strong depositional thinning of major ash flows toward the inferred cauldron margin, except where interrupted by syn-cauldron normal faulting; fracture or shear zones located in the tuff of Squaw Mountain subparallel to eutaxitic foliation giving the appearance of the presence of multiple cooling units; and near margin listric faults that offset the cauldron fill and are concave toward the cauldron margin.