A Refined Interpretation of the Medicine Bow Orogeny, Southeastern Wyoming: Accretion, Delamination, and Collision

New SHRIMP ages from the southern Sierra Madre of southeastern Wyoming indicate a complex Paleoproterozoic magmatic-tectonic history. The analyzed rocks have been interpreted as part of the Paleoproterozoic Green Mountain arc. The accretion of the arc together with the syn- to post-collisional granitic rocks (e.g., Sierra Madre granite) have been referred to as the ca. 1.78–1.75 Medicine Bow orogeny. The new SHRIMP ages consist of 161 points from 9 samples, from a representative suite of (meta)igneous rocks from the Big Creek gneiss. The oldest ages (ca. 1.78 Ga) are from metagabbro and granite gneiss that we correlate with the Green Mountain Formation and related intrusions. The zircons contained no older, xenocrystic cores. The lack of significant spread in the arc-related ages suggests that the arc accreted shortly after crystallization of these arc plutons, perhaps by slab rollback related to a southward-dipping subduction zone. Two samples of augen gneiss and two samples of Fe-rich mafic rocks (one partially mingled with augen gneiss) yielded ages of ca. 1767 Ma, corresponding to the massive phase of the Sierra Madre granite. This episode of synchronous felsic and mafic magmatism may indicate a delamination event associated with breakoff of the subducted slab associated with the Green Mountain arc. Finally, zircons from a paragneiss leucosome, interpreted as having crystallized from an in-situ anatectic melt, yielded an age of ca. 1750–1745 Ma. Igneous and metamorphic rims of this age were found on three other gneissic samples. This metamorphic and fabric-forming event likely records a collision to the south, possibly the accretion of the Rawah arc. In summary, the Medicine Bow orogeny apparently comprises three tectonic events: 1) accretion of the Green Mountain arc ca. 1.78 Ga; 2) delamination and magmatism ca. 1.77 Ga; and 3) inferred arc collision from the south at ca. 1.75 Ga.