Late Ordovician Magmatism Research Articles

The first evidence of Late Ordovician magmatism of the October Revolution Island (Severnaya Zemlya archipelago, Russian High Arctic): geochronology, geochemistry and geodynamic settings

The first evidence of Late Ordovician magmatism of the October Revolution Island (Severnaya Zemlya archipelago, Russian High Arctic): geochronology, geochemistry and geodynamic settings

Pre-Variscan tectonic events and Late Ordovician magmatism in the Central Pyrenees: U–Pb age and Hf in zircon isotopic signature from subvolcanic sills in the Pallaresa massif

Late Ordovician magmatism is well-represented in all massifs of the Eastern Pyrenees and the Catalan Coastal Ranges but has not been reported in the central parts of the Pyrenean Axial Zone. This work shows the first geochronological evidence for Late Ordovician magmatism in the Pallaresa massif. This massif is a large E–W trending antiformal structure cored by Paleozoic rocks with a Variscan tectonothermal overprint. The lower Paleozoic sequence shows an Upper Ordovician succession lying unconformably on older Cambrian–Ordovician beds. Despite Variscan overprint, the Cambrian–Ordovician rocks record evidence of a pre-Variscan penetrative fabric which is not present in the rest of the overlying Paleozoic succession. In the eastern part of the Pallaresa massif, rhyodacitic to dacitic subvolcanic rock forming sills intercalated within the Cambrian–Ordovician succession close to the base of the Upper Ordovician rocks have been identified and sampled. These volcanic rocks have a Variscan penetrative foliation and lack the additional pre-Variscan deformation. U–Pb zircon dating of one these subvolcanic levels by U–Pb CA-ID-TIMS, has provided an age of 453.6 ± 1.5 Ma (Sandbian) and the zircon has given ɛHf450 values between − 2.4 and − 5.9, suggesting a mixed source. These new data strengthen the Paleozoic correlation of the Central and Eastern Pyrenees indicating that the Late Ordovician magmatism in the Pyrenees was associated with a pre-Variscan event responsible for the Upper Ordovician angular unconformity and the presence of a penetrative pre-Variscan deformation affecting the underlying Cambrian–Ordovician sequence. This new data, coupled with the occurrence of Zn–Pb SEDEX o Mississippi Valley type deposits associated with Upper Ordovician rocks, suggest the evidence of extensional tectonic activity during Late Ordovician times.

The Anglo-Brabant Massif: Persistent but enigmatic palaeo-relief at the heart of western Europe

The surface geology of central England and Belgium obscures a large ‘basement’ massif with a complex history and stronger crust and lithosphere than surrounding regions. The nucleus was forged by subduction-related magmatism at the Gondwana margin in Ediacaran time. Partitioning into a platform, in the English Midlands, and a basin stretching to Belgium, in the east, was already evident in Cambrian/earliest Ordovician time. The accretion of the Monian Composite Terrane during the Penobscotian deformation phase preceded late Tremadocian rifting, and Floian separation, of the Avalonia Terrane from the Gondwana margin. Late Ordovician magmatism in a belt from the Lake District to Belgium records subduction beneath Avalonia of part of the Tornquist Sea. This ‘Western Pacific-style’ oceanic basin closed in latest Ordovician time, uniting Avalonia and Baltica. Closure of the Iapetus Ocean in early Silurian time was soon followed by closure of the Rheic Ocean, recorded by subduction along the southern margin of the massif. The causes of late Caledonian deformation are poorly understood and controversial. Partitioned behaviour of the massif persisted into late Palaeozoic time. Late Devonian and Carboniferous sequences show strong onlap onto the massif, which was little affected by crustal extension. Compressional deformation during the Variscan Orogeny also appears slight, and was focussed in the west where a wedge-shaped mountain foreland uplift was driven by orogenic indentation, splitting the massif from the Welsh Massif along the reactivated Malvern Line. Permian to Mesozoic sequences exhibit persistent but variable degrees of onlap onto the massif.

Volcanological and environmental controls on the Snowdon mineralization, North Wales, UK: A failed volcanogenic massive sulfide system in the Avalon Zone of the British Caledonides

The Snowdon caldera of North Wales is host to base metal sulfide-bearing veins and stockworks, mineralized breccias, disseminated sulfides, and localized zones of semi-massive to massive sulfide, with subordinate magnetite-rich veins. The late Ordovician host volcanic sequence accumulated in a shallow marine, back-arc environment in the Welsh Basin, which forms part of the Avalon Zone of the British and Irish Caledonides. New field evidence, sulfur isotopes, and U-Pb dating indicate that the Snowdon mineralization is genetically and temporally related to Late Ordovician magmatism and caldera formation. It is interpreted to represent volcanogenic pipe-style sulfide mineralization, resulting from focused hydrothermal fluids moving along caldera-related faults and simultaneous dispersal of fluids through the volcaniclastic pile. Sulfur isotope data suggest that, whilst a limited contribution of magmatic S cannot be ruled out, thermochemical reduction of contemporaneous Ordovician seawater sulfate was the dominant mechanism for sulfide production in the Snowdon system, resulting in a mean value of about 12‰ in both the host volcanic strata and the mineralized veins. Despite the tectonic setting being prospective for VMS deposits, strata-bound sulfide accumulations are absent in the caldera. This is attributed to the shallow water depths, which promoted boiling and the formation of sub-seafloor vein-type mineralization. Furthermore, the tectonic instability of the caldera and the high energy, shallow marine environment would have limited preservation of any seafloor deposits. The new U-Pb dates for the base (454.26±0.35Ma) and top (454.42±0.45Ma) of the host volcanic rocks, indicate that the Snowdon magmatic activity was short lived, which is likely to have limited the duration and areal extent of the ore-forming system. The absence of massive sulfide mineralization is consistent with the general paucity of economic VMS deposits in the Avalon Zone. Despite the highly prospective geological setting this study further illustrates the importance of volcanic facies mapping and associated paleo-environmental interpretations in VMS exploration.

U–Pb zircon dating and nature of metavolcanics and metarkoses from the Monte Grighini Unit: new insights on Late Ordovician magmatism in the Variscan belt in Sardinia, Italy

In the external units of the Sardinian Variscides Nappe Zone, volcanic and volcanoclastic successions of Middle Ordovician age follow Lower Paleozoic calc- alkaline magmatism developed at the northern Gondwana margin. We present geochemical and zircon U-Pb isotopic data for the Truzzulla Formation, a low-to-medium-grade metamorphic volcanic-volcanoclastic succession belong- ing to the Monte Grighini Unit, the deepest unit in the Nappe Zone. Geochemical and radiometric data allow us to define a Late Ordovician (Katian) magmatic (volcanic) event of calc-alkaline affinity. These new data, in con- junction with previously published data, indicate that in the Sardinian Variscides, the age of Lower Paleozoic Andean- type calc-alkaline magmatism spans from Middle to Late Ordovician. Moreover, the age distribution of calc-alkaline volcanics and volcanoclastic rocks in the Nappe Zone is consistent with a diachronous development of Middle-Late Ordovician Andean-type magmatic arc through the portion of the northern Gondwanian margin now represented by the Sardinian Variscides. This reconstruction of the Sardinian Variscides reflects the complex magmatic and tectonic evolution of the northern margin of Gondwana in the Lower Paleozoic.

Geochemical characterization and isotopic age of Caradocian magmatism in the northeastern Iberian Peninsula: Insights into the Late Ordovician evolution of the northern Gondwana margin

This paper presents new geochronological, geochemical and isotopic data for Late Ordovician magmatism (455 ± 2 Ma) from the Variscan massifs in the northeastern Iberian Peninsula. This magmatism is mainly composed of aluminous and metaluminous plutonic rocks and a sequence of calc-alkaline ignimbrites and epiclastic volcanic rocks, which were deformed and metamorphosed during the Variscan orogeny. The metaplutonic rocks are located in the lower part of a Neoproterozoic/Early Cambrian metasedimentary sequence in the Canigó massif (Eastern Pyrenees). Aluminous orthogneisses have isotopic signatures that indicate a crustal origin, whereas metaluminous orthogneisses originate from a mixture of juvenile and crustal melts. The metavolcanic rocks define a calc-alkaline ignimbritic sequence of crustal origin located in the lower part of the Late Ordovician sequence in the Les Gavarres massif (Catalan Coastal Ranges). T DM ages for all the studied Late Ordovician rocks range from Mesoproterozoic to Paleoproterozoic (1.3 Ga, 1.5 Ga, 1.7 Ga and 2.2 Ga), suggesting a Neoproterozoic crust with Paleoproterozoic components and an arc signature, as the most probable source for the crustal melts. The isotopic similarities between the studied samples and other Early Ordovician magmatic rocks of the Central and Western Iberian Massif suggest repeated melt extractions from a common basement as the source for the Early and Late Ordovician magmatism in the Iberian realm. However, the studied massifs are more closely linked to other massifs from the Mediterranean realm. The most probable tectonic setting for the emplacement of the Late Ordovician volcanic and plutonic bodies is that of an extensional regime, which postdates a previous Ordovician contractional event and the opening of the Rheic Ocean.

New insights into the Late Ordovician magmatism in the Eastern Pyrenees: U–Pb SHRIMP zircon data from the Canigó massif

New geochronological data from the Canigó massif (Eastern Pyrenees) using U–Pb SIMS on zircon provide evidence of the existence of a Late Ordovician (456–446 Ma) plutonic event that emplaced granitic and dioritic bodies into a Late Neoproterozoic–Early Paleozoic metasedimentary series. These are the first geochronological data documenting this magmatic episode in the Pyrenees, which is coeval with synsedimentary volcanism and normal fault development in rocks of the Upper Ordovician. This Late Ordovician magmatic event postdates a ?Middle Ordovician compressional episode recorded in the pre-Upper Ordovician sequence, and is distinct from an Early Ordovician magmatic event that gave rise to the protoliths of large gneissic bodies cropping out in the Pyrenees. This evolution argues against a continuous extensional regime related to the opening of the Rheic or the Rheic and Paleotethys oceans in this segment of the Northern Gondwana margin during the Ordovician and Silurian.

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.

U–Pb chronology and duration of late Ordovician magmatism in the English Lake District

U–Pb zircon ages are presented from igneous rocks in the English Lake District for which a wide range of ages had been obtained previously by other methods. The Little Stand and Crinkle tuffs, from the Borrowdale Volcanic Group, have ages of 451.6 ± 1.4 and 452.8 ± 0.7 Ma (2σ), respectively, the Threlkeld Microgranite 451.0 ± 1.1 Ma (2σ) and the Carrock Microgranite 452.4 ± 4.1 Ma (2σ). These dates are identical to the previously published dates for the Ennerdale and Eskdale granites and together are considered to be a good estimate for both the age and length of Caradoc magmatism in the Lake District. This is the first of three magmatic events recognized in the Lake District, the other two occurring during Ashgill and Early Devonian times. Regional comparisons with the Leinster Terrane in SE Ireland and the largely concealed English Midlands basement emphasize the magnitude and significance of the Caradoc event along the Eastern Avalonia margin.