Omineca-Selwyn plutonic belt 

(mid-Cretaceous) 

(Eastern Canadian Cordillera, interior and northern Alaska, and northern Russian Northeast, units 
eKpf, Kpf)

Consists chiefly of granodiorite, granite, quartz syenite and minor syenite plutons of Early to mid-
Cretaceous age (110-90 Ma) that form an extensive belt of discrete intrusions. This belt includes the 
easternmost magmatic rocks in the Canadian Cordillera, and the northernmost plutonic rocks in interior 
and northern Alaska and in the northern Russian Northeast. Many plutons exhibit S-type character. 
Extrusive equivalents (such as the South Fork volcanic rocks in the Yukon Territory) are rare. Commonly, 
the plutons have high initial strontium ratios (about 0.710) indicating partial derivation from old cratonic 
crust (Armstrong, 1988: Woodsworth and others, 1991). The spatial location of the belt about 200 km west 
of the eastern limit of Cordilleran deformation and chemistry suggests an anatectic origin of partial melting 
of cratonic crust during thickening caused by Cretaceous contraction. Armstrong (1988), however, 
interprets the belt as the rear part of a single broad mid-Cretaceous, Cordillera-wide, subduction-related 
arc.

In interior and northern Alaska, the Omineca-Selwyn belt consists of large plutons of mid-Cretaceous 
granite and lesser syenite and granodiorite that occur in an elongate trend that extends from east-central to 
west-central Alaska, the Seward Peninsula, and St. Lawrence Island. The belt includes the Ruby geanticline 
plutons of central Alaska. Granitic plutons tend to be K-rich, locally ultrapotassic Na-depleted, and weakly 
to moderately peraluminous, are locally part of the ilmenite series, and exhibit S-type characteristics 
(Miller, 1989, 1994). Plutons are interpreted as the product of melting or contamination of continental 
crust.

In the northern Russian Northeast, belt consists of a suite of Early Cretaceous granitic rocks that are 
overlain by volcanic rocks of Okhotsk-Chukotka volcanic-plutonic belt. Omineca-Selwyn belt interpreted 
as a suite of anatectic granitic rocks that formed in the late stages of, and immediately after major terrane 
collision and compression along a broad segment of the North America Cordillera and extending into the 
northern Russian Northeast. REFERENCES: Sosunov and others, 1982; Sosunov, 1984; Foster and 
others, 1987, 1994; Armstrong, 1988; Miller, 1989, 1994; Woodsworth and others, 1991.