ECTASIAN AND STENIAN OROGENIES ALONG THE WESTERN MARGIN OF THE LEMHI SUBBASIN OF THE BELT-PURCELL BASIN

In central Idaho and southwestern Montana, a thick section of siliciclastic sediment was deposited in the Lemhi subbasin of the Belt-Purcell Basin—an intracratonic basin that formed at low latitudes within the supercontinent Nuna. While upper Belt sediments were deposited in the main Belt Basin, a thick section of predominantly siliciclastic sediments was deposited in the Lemhi subbasin (mostly between ≤ ca. 1450 and ≤ ca. 1390 Ma, as indicated by dating of detrital zircons). This was followed by:

1. Ectasian folding—F₁ folds formed in metasedimentary strata of the Lemhi subbasin during the East Kootenay orogeny (between ca. 1390 and ca. 1370 Ma),
2. Ectasian bimodal magmatism—mafic and A-type granite plutons intruded F₁-folded host strata at ca. 1370 Ma,
3. Ectasian mineralization—cobaltite-biotite ore (± tourmaline ± xenotime) was deposited in the Blackbird district (with xenotime, dated ca. 1370 to ca. 1315 Ma, and cobaltite, dated ca. 1350 Ma),
4. Stenian folding and metamorphism—F₂ folds formed during a Grenville-age orogeny (between ca.1200 and 1000 Ma), as indicated by dated garnets in western Belt rocks, and by evidence of metamorphic homogenization of Pb-isotopic ratios in metasedimentary rocks of the Lemhi subbasin,
5. Stenian mineralization—cobaltite-biotite lodes of a second generation formed along axial-plane cleavage (S₂) of F₂ folds in the Blackbird district. Such lodes contain cobaltite dated ca. 1130 Ma, and xenotime dated ca.1060 Ma.

Late Calymmian to early Ectasian sedimentation in the Belt-Purcell Basin was punctuated by mafic magmatism, typical of rift-like extensional basins. During the Ectasian East Kootenay orogeny, F₁ folding may have occurred inboard from a zone of tectonic crustal thickening—as represented by high-grade metasedimentary rocks of a metamorphic core complex in southern British Columbia. This core complex is intruded by granite pegmatites, dated ca. 1335 Ma.

The Belt-Purcell Basin and the East Kootenay orogen may have counterparts in Antarctica. The Stenian (Grenville-age) orogeny may have involved a collision between Laurentia and Australia, during the assembly of Rodinia.