242-12 Petrogenetic Reconstruction of the Quad Creek Metanorite in the Eastern Beartooth Mountains, MT, USA as a Tool for Examining the Archean Evolution of the Northern Wyoming Province

Abstract:

The Quad Creek Metanorite (QCMN) provides a critical temporal and petrogenetic reference point for resolving the late Archean geologic history of the eastern Beartooth Mountains, northern Wyoming Province. This investigation integrates field relations, petrography, mineral chemistry, whole-rock geochemistry, and U-Pb zircon geochronology to constrain petrogenetic processes and tectonic environment responsible for the formation of QCMN. The QCMN is an undeformed km-scale plutonic body that intrudes the adjacent, volumetrically dominant gneissic granitic rocks of the 2.79-2.83 Ga Long Lake Magmatic Complex (LLMC) and exhibits sharply cross-cutting contacts and a meter-scale chilled margin. The QCMN preserves a cumulate texture and an original igneous mineral assemblage consisting of chadacrystic orthopyroxene (X_Mg=0.70-0.84), oikocrystic plagioclase (An_55-65), biotite (X_Mg=0.70-0.85) and magnetite. Quartz as well as a portion of the clino-amphibole present may represent late magmatic phases. The metamorphic overprint involves local hydration and is expressed by development of clino-amphibole (magnesio-ferri-hornblende, X_Mg=0.78-0.97) along orthopyroxene-plagioclase interfaces and by partial modification of magmatic plagioclase to more sodic compositions (An_33-52). The highly magnesian (16-22 wt% MgO), low Ti (0.2-0.7 wt% TiO₂) whole-rock geochemistry of QCMN samples is consistent with the volcanic equivalent of boninites. Based on magma geothermobarometry, the QCMN parent melt separated from the mantle at conditions of ~1250-1400°C and 1.1-1.2 GPa. Multiple mineral chemical thermobarometers yield (sub)solidus (re)crystallization conditions of ~750-800°C and 0.7 GPa, conditions that are the similar to analogous thermobarometry in surrounding rocks of the LLMC. Separated zircon grains display oscillatory zoning patterns and lack discrete rim overgrowths indicating a magmatic origin. ²⁰⁷Pb/²⁰⁶Pb ages calculated for these zircons display a range of ~2.78-2.81 Ga with a weighted average age of 2.79 Ga. Field and geochronologic constraints demonstrate that the QCMN crystallized ~2.79 Ga, late in the magmatic cycle that produced the LLMC, followed by metamorphic overprinting. The boninitic whole-rock chemical affinity as well as the calc-alkaline chemical trends of the adjacent LLMC suggest that the QCMN was emplaced and metamorphosed in a setting similar to modern subduction zones, a tectonic setting previously invoked for the waning stages of the Archean in the study area.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-17863

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