U-PB GEOCHRONOLOGY OF GRANOPHYRES FROM THE STILLWATER COMPLEX, MONTANA (USA): ZIRCON CHARACTERIZATION AND PRELIMINARY RESULTS

The ca. 2.7 Ga Stillwater Complex is a mafic-ultramafic layered intrusion located in the Beartooth Mountains of southwest Montana. The intrusion is dominated by mafic-ultramafic cumulate rocks, but also contains volumetrically minor granophyres that were emplaced as late-stage differentiates within the plagioclase-rich Banded series of the complex. The granophyres consist primarily of albite (or oligoclase) + quartz and display a wide variety of textures, including granophyric, graphic, equigranular, and pegmatitic. The granophyres are discordant bodies and typically range in thickness from a few centimeters up to tens of meters, although one large body southeast of Picket Pin Mountain is >200 meters in width. High-U accessory minerals (zircon, titanite, rutile) are present in the granophyres and were separated from whole rock samples for characterization by scanning electron microscopy (SEM) and age determination of single zircon grains by the chemical abrasion pre-treatment technique coupled with isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS).

In this study, four separate granophyres were sampled from different stratigraphic locations in the Banded series (lower, middle, and upper). Two samples are from late-stage differentiate-type bodies, one from a coarse grained unit located at the contact between a late-stage differentiate and a mafic cumulate, and one from a small pegmatitic clot. Zircon grains recovered from the two differentiates and the coarse grained contact are mostly euhedral prisms (ranging in size from 75 to 170 µm), but are highly metamict due to very high U-contents. In contrast, zircon grains from the small pegmatic clot are less well-faceted, but are clear, pale pink, high-quality grains, likely of low to moderate U-content. U-Pb dating of zircon from these granophyres by the CA-ID-TIMS method is in progress. These results will provide precise crystallization ages for these units and will be used to interpret the late-stage crystallization history of evolved magmas within the Stillwater Complex. U-Pb geochronology of other accessory minerals, such as titanite and rutile, will also be reported and will help to constrain the cooling path of the Stillwater Complex.