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A new regional compilation map and U-Pb ages on a suite of variably deformed, Ordovician, calc-alkaline intrusive igneous rocks requires a reinterpretation of the nature of continental collision and extensional exhumation of deep-seated rocks of the Western Gneiss Region west and northwest of Trondheim. A suite of calc-alkaline plutonic rocks, in the age range 482 to 438 Ma, previously known from the region of Smøla-Hitra-Ørlandet-Froan above the Høybakken extensional detachment fault associated with the Devonian 'Old Red Sandstone' basins, is shown to extend over wide areas below the fault, commonly as strongly foliated and lineated gneisses that had been previously mistaken for parts of the Proterozoic Western Gneiss Region. At Follafoss, a member of this intrusive suite is unconformably overlain by weakly metamorphosed conglomerate and volcanogenic sedimentary rocks of probable Late Ordovician age, suggesting that both the sedimentary rocks and the underlying intrusions correlate with the Støren Nappe in the upper part of the local sequence of Caledonide nappes. U-Pb evidence of metamorphic ages from deep-seated rocks of the Western Gneiss Region include: 1) zircon reaction rims on Proterozoic igneous baddeleyite in the Selnes Gabbro at 401 ؎ 2 Ma; 2) widespread development of zircon overgrowth and metamorphic zircon with omphacite and coesite inclusions from the Hareidlandet eclogite at 402 ؎ 2 Ma; and 3) extreme Devonian thermal resetting and neocrystallization of titanite over a wide area of the Proterozoic basement gneisses ending at 395 ؎ 3 Ma, here fully documented for the first time. At Kjørsvika, west of Trondheimsfjord, a ductilely deformed gneiss of the Ordovician intrusive suite contains igneous titanite dated at 455 Ma that shows little evidence for Devonian thermal resetting. This gneiss lies only 1 to 2 km northwest from a large area of Proterozoic gneisses with 100 percent Devonian reset titanite across a ductilely deformed contact that must represent a phase of extensional detachment (Agdenes detachment) much older than the more brittle detachments (compare Høybakken detachment) associated with some of the present outcrops of the Devonian clastic basins. These and other relationships suggest the following broad sequence of Siluro-Devonian events in the region: A) Early Scandian (430 -410 Ma) thrusting with emplacement of the composite Støren Nappe onto the relatively cool Baltoscandian margin of Baltica during contemporaneous subduction of its distal part, locally

The time scale for the Cambrian through Early Devonian periods is revised using new U–Pb ages from volcanic and pyroclastic rocks in sequences that are stratigraphically well defined. A rhyolite flow from the latest Precambrian, dated at 551.4 ± 5.8 Ma, gives a maximum age for the base of the Cambrian (Phycodes pedum Zone) at Fortune Bay, Newfoundland. The age ranges of several Ordovician and Silurian series are now established to within 2–3 Ma based on stratigraphically controlled samples from Britain. A basal Llandeilo ash has an age of 460.4 ± 2.2 Ma; a basal Caradoc lava has an age of 456.1 ± 1.8 ± Ma; and K-bentonites from the latest Llandovery and latest Ludlow have ages of 430.1 ± 2.4 and 420.2 ± 3.9 Ma, respectively. Using other data from the literature, we interpolate the base of the Cambrian to be near 545 Ma, the base of the Ordovician as ca. 495 Ma, and the base of the Silurian as ca. 443 Ma. While we have a good estimate for the base of the Middle Devonian (at ca. 391 Ma), the base of the Devonian is less precise, probably around 417 Ma.

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