Abstract

The Mesoproterozoic Purcell Supergroup (and its equivalent in the United States, the Belt Supergroup) is a thick sedimentary sequence formed in an extensional basin of uncertain age and tectonic setting. The voluminous tholeiitic Moyie sills intrude turbidites of the lower and middle Aldridge Formation, the lowest division of the Purcell Supergroup. Many of the sills were intruded into soft sediment and one intrudes the Sullivan sedimentary exhalative (SEDEX) orebody, so their age approximates that of the sediments and the Sullivan deposit. New U–Pb dates of zircon from four sills are older than previously published U–Pb zircon ages. One sill contained concordant zircons with an age of 1468 ± 2 Ma. Near concordant zircons from the other samples have similar 207Pb/206Pb ages, indicating that all of the sills crystallized at the same time. U–Pb dates of titanites from two of these sills yielded concordant dates ranging from 1090 to 1030 Ma, indicating that they have undergone a previously unrecognized Grenville-age metamorphism. The U–Pb systematics of abraded zircons from one sill indicate that they have also been affected by this event. The recognition of Grenville-age metamorphism in the Purcell (Belt) basin suggests that the prevalent 1.0–1.1 Ga Rb–Sr and K–Ar mineral and whole-rock dates from a wide variety of Purcell (Belt) igneous and sedimentary rocks are also metamorphic, and are not ages of sedimentation or "hybrid" dates reflecting partial resetting by the ca. 760 Ma Goat River orogeny. On this basis, all sedimentation in the Purcell (Belt) basin is constrained to be older than 1.1 Ga and is probably older than 1.25 Ga.

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