13-13 Petrology of Cenomanian Porphyry Intrusions from the Eastern Beartooth Mountains and Complications for Pre-Laramide Montana

Session: Toe to Toe: Cordilleran Systems from Trench to Retroarc Domains

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Abstract:

The Laramide orogeny is expressed by high-angle, deep-seated reverse faults that uplift large blocks of Archean and Proterozoic basement, including the Beartooth Mountains of Montana and Wyoming. This thick-skinned deformation is widely interpreted as a consequence of eastward-propagating flat-slab subduction of the Farallon plate. However, recent studies of the northern Laramide province suggest a more complex and dynamic pre-Laramide tectonic environment than previously understood. Here we provide detailed geochemical, petrographic, and geochronological data on a set of small pre-Laramide Cretaceous intrusions from the eastern Beartooth Mountains and demonstrate a model for this magmatic system. These intrusions are plagioclase-phyric sodic trachyte porphyries with highly unusual geochemistry, including extreme Sr enrichment, positive Eu anomalies, and depletion in Th and middle rare earth elements, suggesting anatexis of a feldspathic crustal source rock with fractionation of titanite and an amphibole-rich restitic assemblage. Zircon rims from each intrusion yield predominantly Cenomanian ages, in contrast to frequently Archean cores. These data complicate the tectonic interpretation of mid-Cretaceous Montana, when subduction-related arc volcanism was located far west of the Beartooths. We propose that lithospheric delamination in advance of the flattening Farallon slab, potentially triggered by metasomatic weakening of the sub-continental lithospheric mantle (SCLM), induced anatexis of feldspar- and amphibole-bearing Archean lower crust similar to the amphibolites of the Long Lake igneous complex. Structural weaknesses produced or reactivated by this delamination event (including the destruction of the SCLM itself) may in turn have played a role in the subsequent thick-skinned crustal response to the stresses imposed by the Farallon flat slab throughout the Laramide orogeny.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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