New Mexico Geological Society Annual Spring Meeting — Abstracts

The early Oligocene Organ batholith of the Organ Mountains volcano-plutonic complex is a rare example of a large composite batholith which has been west tilted and exposed in structural proflle by late Cenozoic faulting. The Organ Needle pluton (ONP) represents the oldest and largest exposed volume of three comagmatic intrusive phases in the complex. Present detailed modal, petrographic and whole rock chemical data shows trends within the ONP to be consistent with a hypothesis proposed by previous researchers; that the pluton has vertical chemical and compositional zonations (now zoned west to east). Exposures of the ONP in the Organ Mountains show more silicic rocks at the western margin and higher elevations, e.g., alkali feldspar granite (AFG)-quartz alkali feldspar syenite (QAFS)-quartz syenite, and more intermediate rocks along the eastern margin, e.g., quartz syenite-quartz monzonite-syenite.

Samples from four transects in the Organ Mountains were collected along a 20 km N-S trend in order to test this hypothesis and to establish the extent and continuity of zoning in the ONP. Results show that the pluton is chemically zoned from 73%-77% SiO₂ at the roof, representing the original silicic capto 61%-71% SiO₂ in the structural interior. The interior of the pluton is zoned from 61% SiO₂ at the base to 71% SiO₂ near the silicic cap. Additionally, the south central mountain range hosts a frozen mafic diapir which represents the pluton's less differentiated core; SiO₂ values of these monzodioritic rocks ranges from 55% to 60%. Chemical and petrological trends support models of assimilation/fractional crystallization (AFC), convective fractionation and magma mixing as mechanisms of zonation.