TIMESCALES OF EMPLACEMENT AND DIFFERENTIATION OF SHALLOW LEVEL PLUTONS, THE ORGAN NEEDLE BATHOLITH, NEW MEXICO

To assess the timescales of shallow level pluton emplacement and crystallization, we present new single-grain CA-TIMS U-Pb zircon data from the epizonal Organ Needle pluton, New Mexico. The plutonic complex and associated ash flow tuffs and lava flows are interpreted as the intrusive and extrusive record of an Eocene caldera complex. Prior work on the pluton developed a detailed model of the magmatic history based on detailed mapping and whole rock and mineral geochemistry (Seager and McCurry, 1988; Verplanck et al., 1995; 1999). The comprehensive geochemical data and extensive exposure of both volcanic and plutonic rocks make this an ideal place to study the evolution of a shallow level plutonic-volcanic system.

The main phases of the complex consist of a central equigranular syenite (ε_Nd ~ -2) and a deep seated inequigranular syenite (ε_Nd ~ -5). The top and sides of the pluton (ε_Nd ~ -5) are chemically distinct from the central equigranular syenite, and have been interpreted to represent higher SiO₂ magmas genetically related to the magma(s) from which the inequigranular syenite crystallized (ε_Nd ~ -5). The highest SiO₂ rocks at the top of the pluton are alkali feldspar granites (ε_Nd ~ -5). The associated volcanic rocks include three major ash flow tuff units and overlying lava flows.

New single grain U-Pb dates from the equigranular and inequigranular syenites yield ²⁰⁶Pb/²³⁸U dates of ~36.1–36.3 Ma, consistent with ~200,000 years of igneous crystallization. Two samples from an alkali feldspar granite and adjacent syenite from the top of the pluton yield ²⁰⁶Pb/²³⁸U dates of ~35.2-35.1 Ma, indicating that these rocks were emplaced in a separate magmatic event. The youngest lavas (West Side lavas) yield an intermediate weighted mean ²⁰⁶Pb/²³⁸U date of ~35.6 Ma. The lavas directly overlie the Squaw Mountain tuff, implying that the tuff is significantly older than the alkali feldspar granite. These results call into question previous models that interpreted the alkali feldspar granite as the unerupted remains of the magma from which the Squaw Mountain tuff was derived. Taken together the data indicate that the Organ Needle plutonic complex was assembled over ~1 Ma, with individual magmatic pulses crystallizing over tens to hundreds of thousands of years.