Uranium-Lead Geochronology of Granophyres from the Archean Stillwater Complex, Montana (USA): Characterization of Uranium-Bearing Accessory Minerals (Zircon, Titanite, Rutile) and Preliminary Dating Results Wall, Corey J.
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 latestage 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 centimetres up to tens of metres. 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 by U-Pb geochronology using isotope dilution thermal ionization mass spectrometry (ID-TIMS). The chemical abrasion pre-treatment technique (annealing and leaching) was employed for determining the age of single grains of zircon. In this study, four separate granophyres were sampled from different stratigraphic levels in the Banded Series (lower, middle, and upper). Three samples yielded mostly euhedral prisms of zircon (ranging from 100-300 μm) that are highly metamict due to very high U-contents (87.3 to 1438.2 ppm). The U-Pb systematics of these zircon grains have been strongly disturbed since crystallization due to self-irradiation (metamictization) and subsequent fluid infiltration events, which resulted in substantial Pb-loss. In contrast, zircon grains from a pegmatic core to a gabbroic pegmatoid that cross-cuts the layered cumulates of the Lower Banded Series are clear, interstitial, pale-pink, high-quality grains, of low to moderate U-content (76.2 to 237.1 ppm). A U-Pb concordia age of 2709.6 ± 0.8 Ma (2σ) was determined from this sample and is interpreted as a minimum age for the crystallization of the Stillwater Complex. This age is consistent with previously published U-Pb zircon geochronology of basal sill and dikes that yielded ages mostly in the range from 2710 to 2712 Ma and suggests that the Stillwater Complex was emplaced within a few million years.
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