Greg Dunning

Did the Manicouagan impact trigger end-of-Triassic mass extinction?

We use U-Pb zircon dating to test whether the bolide impact that created the Manicouagan crater of Quebec also triggered mass extinction at the Triassic/Jurassic boundary. The age of the impact is provided by zircons from the impact melt rock on the crater floor; we show that the zircons yield a U-Pb age of 214 ±1 Ma. The age of the Triassic/Jurassic boundary is provided by zircons from the North Mountain Basalt of the Newark Supergroup of Nova Scotia; the zircons yield a U-Pb age of 202 ±1 Ma. This should be the age of the end-of-Triassic mass extinction that paleontology and sedimentation rates suggest occurred less than 1 m.y. before extrusion of the North Mountain Basalt. Although the Manicouagan impact could thus not have triggered the mass extinction at the Triassic/Jurassic boundary (impact likely having preceded extinction by 12 ±2 m.y.), the impact may possibly have triggered an earlier mass extinction at the Carnian/Norian boundary in the Late Triassic.

Variscan collisional magmatism and deformation in NW Iberia: constraints from U–Pb geochronology of granitoids

U–Pb geochronology of Variscan granitoid rocks from the West Asturian Leonese Zone of the NW Iberian belt documents the episodic nature of magmatism in this section of the western European Variscides. Each magmatic episode is characterized by granitoids with distinct features and has a duration on the scale of several millions of years. The ages of these granitoids place new constraints on the age and duration of magmatic and tectonic events, that are consistent with previous structural studies and proposed models for the tectonic evolution and migration of deformation in the NW Iberian Variscan belt. Granitoid rocks in this zone belong to two main magmatic episodes (syn‐ and post‐tectonic relative to the Variscan Orogeny) and are broadly representative of the granitoid types found in the NW Iberian Variscan belt. The syntectonic association is formed by: (i) tonalite–granodiorite–monzogranite intrusions emplaced synchronously with the main phase of crustal deformation (D2) at c. 325 Ma and (ii) younger leucogranite intrusions emplaced synchronously with syn‐convergence extensional structures at c. 320–310 Ma. The post‐tectonic association is composed of: (i) volumetrically dominant tonalite–granodiorite–monzogranite intrusions (and associated minor mafic‐intermediate rocks) emplaced at c. 295–290 Ma; and (ii) scarce leucogranite intrusions emplaced at c. 290–285 Ma.

New U–Pb ages for Early Ordovician magmatism in Central Spain

Felsic orthogneisses occur widely in the Ollo de Sapo Domain of the Central Iberian Zone, most of which were previously considered to represent a Precambrian basement to the Palaeozoic sequences of this domain. However, new U–Pb dating across the Berzosa‐Riaza shear zone (Sierra de Guadarrama, Central Spain) indicates an Early Ordovician age for the most representative types of orthogneisses in the eastern part of the Ollo de Sapo Domain. Dated rocks include the volcaniclastic Cardoso gneiss (480±2 Ma); from the low–medium‐grade hanging wall; the Riaza gneiss (468−8+16 Ma, mylonitic granite) in the Berzosa‐Riaza shear zone; three types of ‘leucogneiss’ (Buitrago gneiss; 488−8+10 Ma, megacrystic granite; 482−11+14 Ma, aplitic vein; and 482−8+9 Ma, gneissic leucogranite); and the La Morcuera granitic augen gneiss (477±4 Ma) in the high‐grade footwall. The new age of the Cardoso gneiss brackets to the Mid–Late Arenig, the so‐called Early Ordovician Sardic unconformity, characteristic of the Central Iberian Zone. These new ages suggest that the broadly coeval volcanism and plutonism were closely associated with the Sardic events, and that these orthogneisses were part of a felsic magmatic belt which extended along the Ollo de Sapo Domain of the Central Iberian Zone. This magmatic belt is interpreted to have been active during the Early Ordovician break‐up of the peri‐Gondwanan margin of the Iapetus Ocean.

U/Pb zircon and baddeleyite ages for the Palisades and Gettysburg sills of the northeastern United States: Implications for the age of the Triassic/Jurassic boundary

Zircons extracted from the Palisades and Gettysburg sills and baddeleyite from the Palisades sill yield consistent U-Pb ages of 201 ±1 Ma. These sills are likely related to the lowermost basalt flows of the Late Triassic-Early Jurassic Newark Supergroup rift basins of the eastern margin of North America (because of the geochemical similarity of their high-Ti quartz tholeiites and because drilling suggests that the Palisades sill directly fed some of the lowermost flows). Because the lowermost flows of the Newark Supergroup are paleonto-logically assigned to the lowermost Hettangian, the 201 ±1 Ma age of the sills should be slightly younger than the age of the Triassic/Jurassic boundary. Our data support the age of 204 ±4 Ma assigned the Triassic/Jurassic boundary and suggest that recent K-Ar dating for the Hettangian flows of the Newark Supergroup, which yielded an average age of 190 ±3 Ma, is about 5% low.

Timing of peak metamorphism and deformation along the Appalachian margin of Laurentia in Newfoundland: Silurian, not Ordovician

U/Pb and Ar/Ar isotopic age data from the Corner Brook Lake region of the eastern Appalachian Humber zone in western Newfoundland indicate that regional deformation and peak amphibolite-facies metamorphism are Early Silurian. A lower limit on deformation is provided by a U/Pb zircon age of 434 +2/-3 Ma for a pegmatite that is affected by the regional foliation and is interpreted to be syntectonic. Monazite and rutile from a garnet-kyanite-staurolite schist, which records peak-metamorphic conditions and in which porphyroblasts have overgrown the regional foliation, gave U/Pb ages of 430 ±2 Ma and 437 ±6 Ma, respectively. Ar/Ar cooling ages for hornblende from amphibolites and muscovite from psammitic and pelitic schists range from 430 to 420 Ma. A Silurian age for deformation and metamorphism of the Laurentian margin is coincident with the timing of similar events along the Newfound-land Gondwana margin and suggests that the Silurian was a period of major continent-continent collision.

U-Pb age constraints on Variscan magmatism and Ni-Cu-PGE metallogeny in the Ossa-Morena Zone (SW Iberia)

New U–Pb zircon ages from the Santa Olalla Igneous Complex have been obtained, which improve the knowledge of the precise timing of Variscan magmatism in the Ossa–Morena Zone, SW Iberia. This complex has a special relevance as it hosts the most important Ni–Cu–platinum group element (PGE) mineralization in Europe: the Aguablanca deposit. U–Pb zircon ages have been obtained for seven samples belonging to the Santa Olalla Igneous Complex and spatially related granites. With the exception of the Cala granite (352 ± 4 Ma), which represents an older intrusion, the bulk of samples yield ages that cluster around 340 ± 3 Ma: the Santa Olalla tonalite (341.5 ± 3 Ma), the Sultana hornblende tonalite (341 ± 3 Ma), a mingling area at the contact between the Aguablanca and Santa Olalla stocks (341 ± 1.5 Ma), the Garrote granite (339 ± 3 Ma), the Teuler granite (338 ± 2 Ma), and dioritic dykes from the Aguablanca stock (338.6 ± 0.8 Ma). The Bodonal–Cala porphyry, which has also been dated (530 ± 3 Ma), comprises a group of sub-volcanic rhyolitic intrusions belonging to the Bodonal–Cala volcano-sedimentary complex, which hosts the igneous rocks. The knowledge that emplacement of the Aguablanca deposit was related to episodic transtensional tectonic stages during the Variscan orogeny will be fundamental in future mineral exploration in the Ossa–Morena Zone.

High-pressure, high-temperature rocks from the base of thick continental crust: Geology and age constraints from the Manicouagan Imbricate Zone, eastern Grenville Province

The Grenville Province of the Canadian Shield is composed of crustal material with Laurentian affinities, involved in continental collision during the Grenvillian Orogeny between 1.19 and 0.98 Ga. In the eastern Grenville Province, the Manicouagan Imbricate Zone (MIZ) is composed of Paleoproterozoic (Labradorian, ∼1.6 Ga) and Mesoproterozoic (Pinwarian, ∼1.4 Ga) crustal segments that were metamorphosed under high-pressure, high-temperature (high-P-T) conditions (P∼1800 MPa and 800°<T<900°C) and intruded by synmetamorphic gabbroic stocks and dykes and postmetamorphic granite, during a ∼1.0 Ga crustal shortening event in the Grenvillian Orogeny. The gabbroic dykes display high-P mineral assemblages and a withinplate tholeiite signature attesting to the intrusion of asthenospheric melts during the high-P metamorphism. The structural configuration of MIZ is that of a thrust stack (Lelukuau terrane) overlain by an extensional assembly of slices (Tshenukutish terrane). To the north, Lelukuau terrane overlies Archean basement and Paleoproterozoic supracrustal rocks of the Gagnon terrane along a thrust contact. Tshenukutish terrane is tectonically overlain to the south by crustal slices composed of pre-Grenvillian and early Grenvillian lithologic units that are correlative with those in MIZ, but which were at midcrustal levels during the Grenvillian Orogeny (low-P segment of the eastern Grenville Province), and were intruded by anorthosite and granitoids farther south. A likely tectonothermal evolution of MIZ at ∼1.0 Ga involves crustal shortening by imbrication in a postsubduction phase of continental collision, followed by (convective?) removal of the underlying thickened lithospheric mantle and the rise of hot asthenospheric material close to the base of the crust. Exhumation of MIZ likely occurred from the basal levels of the crust and was achieved by (1) NW directed thrusting over a crustal-scale ramp (Archean basement of the Gagnon terrane) with coeval extension at the higher levels of the pile (stage 1 extension) and (2) a second extension event (stage 2 extension) coeval with emplacement of postmetamorphic granite at the boundary between MIZ and the low-P segment.

Mid–Late Ordovician magmatism and metamorphism along the Gander margin in central Newfoundland

Fieldwork coupled with U–Pb dating has clarified the relationships between the Exploits and Gander-derived rocks in central Newfoundland during the Mid-Ordovician opening of the Exploits back-arc basin, reinforcing correlation across the peri-Gondwanan segments of the Appalachian–Caledonian orogen. We have separated a new back-arc-related unit, the Red Cross Group, from the arc-related Victoria Lake Supergroup. This group includes 466 ± 3 Ma felsic volcanic rock interlayered with alkali basalt–enriched mid-ocean ridge basalt, 457 ± 6 Ma gabbro, Llanvirn–Caradoc normal mid-ocean ridge basalt–island arc tholeite, limestone, black shale and mélange. Locally the Red Cross Group is in parautochthonous contact with low-grade Gander psammite. This contact was overrun by the Meelpaeg Metamorphic Nappe during the Devonian. The nappe contains Gander metapsammite (Meelpaeg Subzone) and the coticule-bearing Howley Waters Complex (Exploits), which hosts 467 ± 3 Ma felsic porphyry. The Gander and Exploits metasediments in the nappe are stitched by Arenig–Llanvirn granite (467 ± 6 Ma and 468 +3.5/−3 Ma) and Caradoc intrusions (458 ± 3 Ma and 454 ± 2 Ma). Metamorphic monazite (462 ± 1 Ma) and titanite (460 ± 3 Ma) from the nearby Mount Cormack Subzone (Gander) indicate the coeval formation of a low-pressure–high-temperature metamorphic dome. The widespread presence of Caradoc magmatism suggests that the width of the back-arc basin remained relatively narrow.

Tectonomagmatic evolution of the Early Ordovician suprasubduction-zone ophiolites of the Trondheim Region, Mid-Norwegian Caledonides

Abstract The Trondheim Region ophiolites in the Mid-Norwegian Caledonides represent variably tectonized ophiolite fragments. We present high-precision thermal-ionization mass spectrometry and secondary-ion mass spectrometry (SIMS) U–Pb zircon dates, whole-rock geochemical and Sm–Nd data and Lu–Hf zircon analyses that permit the timing and nature of various stages in the evolution of the ophiolite to be elucidated. Plagiogranite intrusions dated at 487 and 480 Ma have relatively juvenile Nd and Hf isotopic compositions (ϵNd(t)=6.3, ϵHf(t)=8.2–12.4). Geochemical data indicate a subduction-zone influence, suggesting formation in an oceanic back-arc setting. At 481 Ma, a granitoid body with a relatively strong unradiogenic Nd and Hf isotopic composition (ϵNd(t)=−2.6 to −4.0, ϵHf(t)=3.8–6.4) and subduction-zone geochemical signature intruded the ophiolite. We interpret this stage to reflect the formation or migration of an oceanic arc above a subduction zone influenced by continentally derived sediments. At c. 475–465 Ma, a greenstone-dominated conglomerate and volcaniclastic sequence was deposited on the eroded ophiolite, indicating obduction between about 480 and 475 Ma. At c. 468–467 Ma, the deformed ophiolite and its sedimentary cover was intruded by trondhjemite dykes and shoshonitic volcanic rocks with intermediate Nd and Hf isotopic compositions (ϵNd(t)=3.0–3.9, ϵHf(t)=4.4–10.2). We interpret this magmatism to reflect subduction-polarity reversal and establishment of a magmatic arc at the continental margin shortly after obduction. Supplementary material: Whole-rock geochemistry, Sm–Nd isotopic data, SHRIMP U–Pb zircon, TIMS U–Pb zircon and Lu–Hf isotopic data are available at http://www.geolsoc.org.uk/SUP18689.

Provenance of turbiditic cover to the Caledonian Solund–Stavfjord ophiolite from U-Pb single zircon dating

U-Pb dating has been carried out on single zircon and titanite grains in greywacke intercalated with lavas of the late Ordovician (443 ±3 Ma) Solund-Stavfjord ophiolite. The ages range from 2495 Ma to 462 Ma indicating that detritus was provided by a Precambrian continental region as well as an Ordovician Caledonian terrane. The Caledonian component of the detritus was probably derived from the early Ordovician ophiolitic terrane of west Norway, and the data therefore link two generations of Caledonian ophiolites. The mapped relationships of the ophiolitic terranes suggest that the late Ordovician–early Silurian evolution of the Caledonides was dominated by rifting and the formation of marginal basin(s).