Coolwater culmination: Sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb and isotopic evidence for continental delamination in the Syringa Embayment, Salmon River suture, Idaho

Abstract

During dextral oblique translation along Laurentia in western Idaho, the Blue Mountains superterrane underwent clockwise rotation and impinged into the Syringa embayment at the northern end of the Salmon River suture. Along the suture, the superterrane is juxtaposed directly against western Laurentia, making this central Cordilleran accretionary‐margin segment unusually attenuated. In the embayment, limited orthogonal contraction produced a crustal wedge of oceanic rocks that delaminated Laurentian crust. The wedge is exposed through Laurentian crust in the Coolwater culmination as documented by mapping and by sensitive high‐resolution ion microprobe U‐Pb, Sri, and ɛNd data for gneisses that lie inboard of the suture. The predominant country rock is Mesoproterozoic paragneiss overlying Laurentian basement. An overlying Neoproterozoic (or younger) paragneiss belt in the Syringa embayment establishes the form of the Cordilleran miogeocline and that the embayment is a relict of Rodinia rifting. An underlying Cretaceous paragneiss was derived from arc terranes and suture‐zone orogenic welt but also from Laurentia. The Cretaceous paragneiss and an 86‐Ma orthogneiss that intruded it formed the wedge of oceanic rocks that were inserted into the Laurentian margin between 98 and 73 Ma, splitting supracrustal Laurentian rocks from their basement. Crustal thickening, melting and intrusion within the wedge, and folding to form the Coolwater culmination continued until 61 Ma. The embayment formed a restraining bend at the end of the dextral transpressional suture. Clockwise rotation of the impinging superterrane and overthrusting of Laurentia that produced the crustal wedge in the Coolwater culmination are predicted by oblique collision into the Syringa embayment.

Age and structure of the Crevice pluton: overlapping orogens in west-central Idaho?

“…1). Although the Crevice pluton is located east of the Sr i -0.706 isopleth and was emplaced into continental crust, the interaction of its magmas with oceanic crust (containing Mesozoic zircons) could have been accomplished if accreted terrane rocks were mechanically underplated beneath or tectonically wedged into the Laurentian margin, as described for the Orofino, Idaho area (Strayer et al 1989;Lund et al 2008).…”

Sediment underthrusting within a continental magmatic arc: Coast Mountains batholith, British Columbia

“…Thus, partial underthrusting of accretionary complex rocks [ Ducea et al ., ], relamination of sediment at the plate interface [ Behn et al ., ; Hacker et al ., ], or intraarc burial may also be a viable hypothesis for the burial of these rocks beneath the Coast Mountains batholith. Rocks with similar histories were also described from southern California [ Jacobson et al ., ; Barth et al ., ; Kidder and Ducea , ], south‐central Arizona [ Haxel et al ., ], Idaho [ Lund et al ., ], central Washington [ Matzel et al ., ], southern Alaska [ Bleick et al ., ; Gasser et al ., ], Argentina [ Cristofolini et al ., ], and Tibet [ Kapp et al ., ]. Structurally, the simplest interpretation for these metamorphic rocks is that they were underplated during shallow subduction (Figures b and c).…”

Detrital-zircon geochronology of the Sawtooth metamorphic complex, Idaho: Evidence for metamorphosed lower Paleozoic shelf strata within the Idaho batholith

“…Lewis et al (2010) also showed that quartzites (<2% feldspar) of the Syringa metamorphic succession in northcentral Idaho are likely Neoproterozoic on the basis of detritalzircon ages. These metamorphosed Neoproterozoic rocks in (north) central Idaho are generally composed of a wide range of lithologies, including garnet and kyanitebearing, micaceous, quartzofeldspathic gneiss; amphibolite; schist; calcsilicate gneiss; marble; quartzite; diamictite; and bi modal volcanic strata (e.g., Lund et al, 2003;Lund et al, 2008) are typically metaquartz arenites intercalated with quartziteconglomerate (Lund et al, 2003;Lewis et al, 2010). In contrast, documented occurrences of Cambrian-Ordovician (meta)sedimentary rocks in the Idaho batholith re gion are rare; the nearest ones to the study area are those in northern and eastcentral Idaho (Fig.…”