Richard S. D'Lemos

Neoproterozoic palaeogeography of the Cadomia and Avalon terranes: constraints from detrital zircon U–Pb ages

Detrital zircons from three Neoproterozoic sandstone units from the Cadomia terrane of northern France and the Channel Islands yield ages in three broad groups: late Neoproterozoic (650–600 Ma), early Palaeoproterozoic (2.4–2.0 Ga) and Archaean (>2.5 Ga). The lack of Mesoproterozoic zircon crystals, combined with the high abundance of grains between 2.20 and 2.00 Ga, corresponds closely to the ages of exposed rocks in the West Africa Craton, and thus it is suggested that Cadomia was in close proximity to West Africa by c. 580 Ma. In contrast, the main age groups of detrital zircon from the Neoproterozoic Avalon terrane are Mesoproterozoic and there is a distinct gap of ages between 2.40 and 2.05 Ga. These significant differences suggest that the two terranes were in different locations relative to major Gondwanan cratons in latest Neoproterozoic time.

Significance of a hiatus in down-temperature fabric development within syn-tectonic quartz diorite complexes, Channel Islands, UK

Detailed fabric development histories have been deduced for Neoproterozoic foliated quartz diorite complexes of the Channel Islands, UK. The plutons were emplaced during regional (D2) deformation. Magmatic fabrics, formed at low crystal contents (pre-rheologically critical melt percent) are only locally preserved due to overprinting by solid-state deformation which occurred at moderate temperatures (400–550°C). This is demonstrated by brittle microcracking of plagioclase and amphibole with only local ductile bending and dynamic recrystallization; marginal recrystallization and myrmekite development on alkali-feldspar; ductile deformation of quartz to form types 2–3 ribbons; quartz recrystallization by sub-grain rotation associated with a-slip but not c-slip: biotite recrystallization within micro-shears; and variable amphibole recrystallization. The deformation histories display a distinct division between magmatic state and moderate temperature solid-state deformation features with little evidence for intervening high temperature solid-state fabrics. This contrasts with criteria widely considered diagnostic of syn-tectonic plutons which envisage development of a continuum of magmatic through high temperature solid-state deformation fabrics. However, we consider the plutons syn-tectonic and attribute the hiatus to contrasting strain and pluton cooling rates, length and nature of deformation events and host rock ambient temperatures. Intermediate magmas, emplaced at moderate to shallow levels in the crust may be more prone to develop such apparently punctuated deformation histories than SiO2,-richer granitoids.

The cadomian granites of Mancellia, northeast Armorican massif of France: relationship to the St. Malo migmatite belt, petrogenesis and tectonic setting

Abstract The North Armorican Shear Zone (NASZ) divides the Cadomian belt of the Armorican Massif of France into two major terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT). The NACT is a pastiche of displaced blocks and terranes (St. Brieuc Terrane, St. Malo Terrane and Mancellian Terrane) which results from the amalgamation of Cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a southerly dipping subduction zone. In the NACT, the Neoproterozoic Brioverian succession was deformed and metamorphosed during the Cadomian orogeny and intruded by syn- to post-tectonic plutonic complexes. The St. Malo migmatite belt is a syn-tectonic suite of anatectic migmatites derived through partial melting of the Brioverian succession. Using the t-test on whole-rock geochemical data, there is no evidence to suggest that a sample of St. Malo diatexites comes from a population with a different mean than some samples of the Brioverian succession sandstones. Resulting homogeneous diatexites/anatectic granites of the core were emplaced syn-kinematically into Cadomian strike-slip shear zones. Thus the age of anatexis of ∼540 Ma is also the date of strike-slip shearing and provides a minimum age for the sinistral transpression which amalgamated the terranes of the NACT. The Mancellian granites, to the southeast of the St. Malo migmatites, were emplaced at ∼540 Ma and are overlain unconformably by Cambrian sediments. Overall geochemistry and a variety of specific geochemical parameters show that the Mancellian granites are similar to the diatexites and anatectic granites of the St. Malo migmatite belt. Using the t-test on whole-rock geochemical data, there is no evidence to suggest that the sample of Mancellian granites comes from a population with a different mean than the sample of St. Malo diatexites. Additionally, both suites of rocks are peraluminous, they have similar distributions of Rb, Sr and Ba, they have K/Rb ratios around 225, and they plot largely within the VAG field using trace element discrimination diagrams for the interpretation of tectonic setting. Samples of the Brioverian succession sandstones, the St. Malo migmatites and the Mancellian granites all exhibit similar primordial mantle-normalized element patterns. Preliminary results of Nd and Sr isotope studies reveal a similar restricted range of values for ∈Nd (between −4.0 and −7.3) and ∈Sr (between −3 and +42), and depleted model ages in the range 1.7 to 1.5 Ga (Tdmur) for both Mancellian granites and St. Malo migmatites, isotopic features which set these rock types apart from other Cadomian granites within the NACT. The St. Malo migmatite belt and the Mancellian granites are coeval at ∼540 Ma and both developed by anatexis. at intermediate crustal depths, of the Brioverian succession, within an inverted, but only moderately overthickened, behindarc basin. In comparison with the St. Malo migmatites, the Mancellian granites represent farther-travelled products of intracrustal melting but their geochemistry still preserves the characteristic signature of the source.

Terrane analysis along a Neoproterozoic active margin of Gondwana: insights from U–Pb zircon geochronology

The tectonic affinities of terranes in accretionary orogens can be evaluated using geochronological techniques. U–Pb zircon data obtained from paragneisses of the Coedana Complex (Anglesey) and the Malverns Complex, southern Britain, indicate that they were deposited during the mid- to late Neoproterozoic and have a comparable Amazonian provenance. Metamorphism of the Coedana gneisses occurred at 666 ± 7 Ma, similar to the age of metamorphism in the Malverns Complex. Anglesey therefore probably evolved in proximity to the Avalonian basement of mainland southern Britain during the mid- to late Neoproterozoic and is not a ‘suspect terrane’ relative to the remainder of Avalonia.

Sm–Nd isotope characteristics of late Cadomian granite magmatism in northern France and the Channel Islands

Sm–Nd isotopic studies of granites within the late Precambrian, Cadomian, orogenic belt of the North Armorican Massif (northwestern France) and Channel Islands reveal differences between arc-related granite magmatism in outboard terranes and intracrustal granite magmatism in inboard terranes. Late Cadomian (c. 570 Ma), arc-related granitoids exhibit a range of e nd ( - 2 to - 6) and Nd model ages (T DM 1.0–1.3 Ga) reflecting variable contamination between late Precambrian mantle derived magmas and ancient (c. 2.0 Ga?) continental crust. The contamination did not involve exposed granitic Icartian basement to anygreat degree, a more likely contaminant being unexposed lower crust of intermediate to acidic granulitic composition, or early Cadomian plutons which were themselves contaminated by lower crust. Voluminous granites of the Mancellian region (c. 550–540 Ma) share common isotopic characteristics (e Nd -4 to -7, T DM 1.5–1.7 Ga) with migmatites and anatectic granites produced by partial melting of metasedimentary sequences within the St Malo region consistent with a common source.

Late Precambrian tectonothermal evolution of the Malverns Complex

The Malverns Complex is a variably deformed and metamorphosed late Precambrian calc-alkaline plutonic suite exposed within the Avalon terrane of central England. Published U–Pb zircon and monazite ages indicate emplacement at c. 680–670 Ma. 40Ar/39Ar mineral cooling ages presented here place constraints on the timing of (1) upper greenschist to low amphibolite facies metamorphism and associated ductile deformation; and (2) late hydrothermal activity and variable thermal reactivation. Hornblende from meta-diorites records 36Ar/40Ar v. 39Ar/40Ar isotope correlation ages of c. 649 Ma and c. 652 Ma. These are significantly younger than the crystallization age of the complex and are interpreted to date cooling following metamorphism. They further provide maximum age constraints for the development of mylonitic fabrics that overprint metamorphic textures. Hornblende from a diorite net-veined by late granite pegmatites that are discordant to the mylonitic fabrics records a significantly younger isotope correlation age of c. 610 Ma. This is interpreted to date a static thermal rejuvenation associated with pegmatite emplacement, and places minimum age constraints on the timing of mylonitization. A plateau age of c. 597 Ma recorded by muscovite from a greisen vein within a granite is interpreted to date closely the hydrothermal alteration of the complex. Detrital muscovite of probable metamorphic origin within a Lower Cambrian sandstone records a plateau age of c. 598 Ma. and may have been derived from local units which were first metamorphosed at c. 650 Ma and later reheated at c. 610–600 Ma. The 40Ar/39Ar ages are consistent with episodic tectonothermal activity within the Malverns Complex during the interval c. 650–600 Ma, and indicate a more complex Precambrian history than previously recognized for the Avalon terrane of southern Britain.

Relationships between syn-tectonic granite fabrics and regional PTtd paths: an example from the Gander-Avalon boundary of NE Newfoundland

Abstract Syn-tectonic granitic plutons and their host rocks not only provide a potential wealth of information about the timing and nature of regional kinematics, but also record information about thermal conditions. Successive syn-tectonic plutons emplaced into the Gander Zone of NE Newfoundland preserve fabric overprinting formed during cooling to regional ambient thermal conditions. Ambient conditions have been approximated for each pluton by careful analysis of microstructures and consideration of potential cooling histories, and have illustrated regional exhumation during a tectonic reversal from Silurian-Devonian sinistral transpression to Devonian dextral transpression. Because the time period of emplacement is relatively short by comparison to orogenic events (typically by more than an order of magnitude), individual plutons may record a ‘snap-shot’ of the regional history. By combining information from a number of plutons emplaced sequentially during a period of regional orogenesis, a picture may be built up defining time-temperature-deformation (Ttd) paths. Such information may compliment regional metamorphic PT studies in helping to establish orogenic PTtd paths.

A precise late Neoproterozoic U-Pb zircon age for the syntectonic Perelle quartz diorite, Guernsey, Channel Islands, UK

A precise U-Pb zircon age of 611.4 +2.0–1.1 Ma has been determined for the foliated quartz diorite at Perelle Bay, Guernsey, UK Channel Islands. The age is based on the analysis of single zircons, half of a single grain, and groups of two and three zircons. The age is nearly 100 Ma younger than that suggested by other workers, based on 207Pb/206Pb dates of large, multi-grain fractions of zircon, and thus rules out this intrusion as the earliest marker of the onset of Cadomian magmatism in the North Armorican Massif. A 611 Ma crystallization age for the Perelle quartz diorite is consistent with previous detailed fabric development studies that suggested magmatic fabrics within the pluton are largely overprinted by similarly oriented solid-state deformation fabrics resulting from syntectonic emplacement during Cadomian orogenesis. The U-Pb age is approximately 8–15 Ma older than 40Ar/39Ar hornblende dates for the pluton, consistent with the emplacement of the intrusion followed by slow cooling in a region of moderate to high ambient crustal temperatures. These new data emphasize the fact that periods of pluton emplacement are considerably briefer than periods of deformation and thus syntectonic intrusions do not date regional deformational events, but only mark events within a period of regional tectonism. The ability to analyse single zircons, and selected portions of single zircons, using ultra-low blank U-Pb analytical techniques documents the feasibility of determining precise emplacement ages of other Neoproterozoic granitoids that contain a significant percentage of xenocrystic zircon.

Time span of plutonism, fabric development, and cooling in a Neoproterozoic magmatic arc segment: U–Pb age constraints from syn-tectonic plutons, Sark, Channel Islands, UK

Abstract New U–Pb zircon and titanite dates from syn-tectonic plutons on the British Channel Island of Sark constrain the time span of plutonism, fabric development, and cooling in this part of the Neoproterozoic Cadomian magmatic arc. The Tintageu leucogneiss is a mylonitic unit that was dated previously at 615.6 +4.2 −2.3 Ma. The Port du Moulin quartz diorite, which intruded the Tintageu unit, contains a high-strain solid-state deformation fabric that is less intense than, but parallel to, fabrics in the leucogneiss and yields a U–Pb zircon date of 613.5 +2.3 −1.5 Ma. The Little Sark quartz diorite also displays solid-state deformation fabrics in addition to relict magmatic textures, and yields a U–Pb zircon date of 611.4 +2.1 −1.3 Ma. The North Sark granodiorite is largely penetratively undeformed, exhibits mainly magmatic fabrics and textures and has a U–Pb zircon date of 608.7 +1.1 −1.0 Ma. Two fractions of titanite from each intrusion are essentially concordant and are identical within error, with mean dates of 606.5±0.4 Ma (Port du Moulin quartz diorite), 606.2±0.6 Ma (Little Sark quartz diorite), 606.4±0.6 Ma (North Sark granodiorite). The new U–Pb data, in combination with previous U–Pb and 40 Ar/ 39 Ar data and previous field studies, confirm the syn-tectonic nature of the Sark plutons and quantify the time span (ca. 7 m.y.) required for intrusion and sufficient crystallization of each body to record incremental strain during waning deformation. Titanite U–Pb and hornblende 40 Ar/ 39 Ar dates mark final cooling about 2 m.y. after intrusion of the last pluton.

U–Pb geochronological constraints on the timing of Brioverian sedimentation and regional deformation in the St. Brieuc region of the Neoproterozoic Cadomian orogen, northern France

During the Neoproterozoic Cadomian orogeny in northern France, supracrustal rocks of the Brioverian Supergroup were deposited in marginal and back arc basins, and were subsequently variably deformed and metamorphosed. New U–Pb analyses of single, and small multigrain fractions of zircon from selected plutons from the Baie de St. Brieuc region provide robust geochronological constraints on the timing of these events. The Jospinet granodiorite forms part of the local basement directly overlain by Brioverian metasediments and basic volcanics, and yields a U–Pb zircon date of 625.9 +3.6/−1.9 (2) Ma. The pre-tectonic Port Moguer tonalite, which has been strongly sheared along with its amphibolite facies country rocks, has a crystallization age of 600.4 0.9 Ma. Emplacement ages of 576.3 +1.5/−1.2 Ma for the syn-tectonic Fort La Latte quartz diorite and 574.6 + 1.8/−1.5 Ma for the late-tectonic St. Quay quartz diorite place limits on termination of deposition and timing of subsequent regional deformation of the Brioverian sequence in the Baie de St. Brieuc region. The new dates constrain the age of Brioverian sedimentation to the interval 626–575 Ma, a range consistent with a previously published Pb–Pb zircon evaporation age of ca. 58822 Ma for Brioverian volcanic rocks (Lanvollen Formation). Deformation within this sector of the Cadomian belt is believed to have occurred shortly before 575 Ma, revising previously published estimates for the age of this major tectonothermal Cadomian event by 10–20 My.