Teresa Jeffries

The importance of along-margin terrane transport in northern Gondwana: insights from detrital zircon parentage in Neoproterozoic rocks from Iberia and Brittany

Abstract Detrital zircons from late Neoproterozoic rocks of the peri-Gondwanan Cadomian belt of SW Iberia and north Armorican Domain of Brittany record Neoproterozoic (ca. 860–550 Ma), Palaeoproterozoic (ca. 2300–1800) and Archaean (ca 3300–2600 Ma) U–Pb ages. The absence of Mesoproterozoic zircons suggests that these terranes evolved in a peri-W African realm. This is in contrast to other western European terranes that preserve Mesoproterozoic zircons and are likely to have evolved in a peri-Amazonian realm. Such a contrast in detrital zircon populations, coupled with the presence of Mesoproterozoic zircons in the Ordovician Armorican quartzite, deposited in a peri-African platform, is interpreted to record along-margin terrane transport. The change in provenance suggests that subduction was replaced by transform faults that juxtaposed Amazonia-derived terranes against W Africa-derived terranes to form the Avalonia and Armorica microcontinents. Subsequent extension along the margin resulted in the birth of the Rheic Ocean and the outboard drift of Avalonia.

Application of a frequency quintupled Nd:YAG source (λ=213 nm) for laser ablation inductively coupled plasma mass spectrometric analysis of minerals

This paper reports the use a frequency quintupled Nd:YAG laser (λ=213 nm) for laser ablation of minerals for ICP-MS analysis. The fifth harmonic was produced by in-house modification to an existing frequency quadrupled Nd:YAG laser (λ=266 nm), using commercially available optical components. The maximum pulse energy of the 213 nm output is 7.5 mJ per pulse, based on an output of 350 mJ per pulse at 1064 nm. The pulse energy is controlled and attenuated by a wave plate and MgF2 polariser combination. The laser sampling system was coupled to an enhanced sensitivity ICP-MS system. Comparison of time resolved signals for extended analyses of several materials using 213 and 266 nm for ablation demonstrates that (1) 213 nm laser ablation greatly reduces the incidence of catastrophic ablation of strongly cleaved minerals in thin section, owing to higher absorption; (2) in each case, 213 nm laser ablation produces a longer, flatter, higher intensity signal than conventional 266 nm laser ablation, suggesting a larger volume of transportable particulate is produced; and (3) inter-element fractionation is reduced during analysis using the 213 nm laser ablation system. This study is the first reported use of frequency quintupled laser ablation microprobe (LAM)–ICP-MS.

Terrane accretion and dispersal in the northern Gondwana margin. An Early Paleozoic analogue of a long-lived active margin

If reconstruction of major events in ancient orogenic belts is achieved in sufficient detail, the tectonic evolution of these belts can offer valuable information to widen our perspective of processes currently at work in modern orogens. Here, we illustrate this possibility taking the western European Cadomian–Avalonian belt as an example. This research is based mainly on the study and interpretation of U–Pb ages of more than 300 detrital zircons from Neoproterozoic and Early Paleozoic sedimentary rocks from Iberia and Brittany. Analyses have been performed using the laser ablation–ICP–MS technique. The U–Pb data record contrasting detrital zircon age spectra for various terranes of western Europe. The differences provide information on the processes involved in the genesis of the western European Precambrian terranes along the northern margin of Neoproterozoic Gondwana during arc construction and subduction, and their dispersal and re-amalgamation along the margin to form the Avalonia and Armorica microcontinents. The U–Pb ages reported here also support the alleged change from subduction to transform activity that led to the final break-up of the margin, the birth of the Rheic Ocean and the drift of Avalonia. We contend that the active northern margin of Gondwana evolved through several stages that match the different types of active margins recognised in modern settings.

Anomalously Metal-Rich Fluids Form Hydrothermal Ore Deposits

Hydrothermal ore deposits form when metals, often as sulfides, precipitate in abundance from aqueous solutions in Earths crust. Much of our knowledge of the fluids involved comes from studies of fluid inclusions trapped in silicates or carbonates that are believed to represent aliquots of the same solutions that precipitated the ores. We used laser ablation inductively coupled plasma mass spectrometry to test this paradigm by analysis of fluid inclusions in sphalerite from two contrasting zinc-lead ore systems. Metal contents in these inclusions are up to two orders of magnitude greater than those in quartz-hosted inclusions and are much higher than previously thought, suggesting that ore formation is linked to influx of anomalously metal-rich fluids into systems dominated by barren fluids for much of their life.

Contiguous rather than discrete Paleozoic histories for the Avalon and Meguma terranes based on detrital zircon data

Upper Ordovician–Lower Devonian strata of the Meguma terrane in the Canadian Appalachians contain zircon populations, including an important Mesoproterozoic zircon population (1.0–1.4 Ga), similar to those in coeval strata of Avalonia, and strongly suggest contiguous rather than discrete histories for these terranes throughout the Paleozoic. That these terranes were juxtaposed throughout the early Paleozoic is indicated by the absence of a Cambrian–Ordovician accretionary event, the lack of intervening suture-zone ophio litic units, and the similarity of Avalonian and Meguma basement Nd isotope signatures in early Paleozoic igneous suites. As Avalonia had accreted to Laurentia-Baltica by the Early Silurian, these data suggest that the Meguma terrane, like Avalonia, resided along the same (northern) margin of the Rheic Ocean at that time. These conclusions have implications for reconstructions of the northern Gondwanan margin in the early Paleozoic and imply that the Silurian–Devonian Acadian orogeny in Maritime Canada occurred in an Andean-type setting and was not related to collision of the Meguma terrane with the Laurentian margin.

U–Pb (LA–ICP-MS) dating of detrital zircons from Cambrian clastic rocks in Avalonia: erosion of a Neoproterozoic arc along the northern Gondwanan margin

Most Neoproterozoic and Early Palaeozoic tectonic syntheses place Avalonia and related peri-Gondwanan terranes facing an open ocean along the northern margin of Gondwana, thereby providing important constraints for palaeocontinental reconstructions during that time interval. However, the precise location of Avalonia along the margin and its position relative to other peri-Gondwanan terranes is controversial. We present laser ablation–inductively coupled plasma mass spectrometry U–Pb data for detrital zircons from Cambrian clastic rocks in two localities in Avalonia: the Antigonish Highlands of Nova Scotia (62 analyses) and the British Midlands (55 analyses). The data from both samples are very similar, and taken together indicate an overwhelming dominance of Neoproterozoic (c. 580–680 Ma) or Early Cambrian source rocks with minor older Neoproterozoic clusters at c. 710 Ma or of Mesoproterozoic age, three Palaeoproterozoic zircons and one Archaean zircon. The zircons can all be derived from local Avalonian sources. The Neoproterozoic zircons are attributed to erosion of the underlying Avalonian arc. Mesoproterozoic and Palaeoproterozoic zircons of similar ages are also found in Avalonian Neoproterozoic clastic rocks and their presence in the Cambrian clastic rocks could represent recycling of Neoproterozoic strata and do not necessarily imply the presence of Mesoproterozoic or Palaeoproterozoic basement rocks within their respective drainage basins. Comparison with the data from the Neoproterozoic arc-related clastic sequences suggests significant differences between their respective drainage systems. Whereas the Neoproterozoic data require extensive drainage systems, the Cambrian data can be attributed to localized drainage systems. The change in drainage patterns could reflect rifting and isolation of Avalonia from Amazonia between c. 585 and 540 Ma. Alternatively, it might reflect the creation of topographical barriers along the northern Gondwanan margin, in a manner analogous to the Cenozoic rise of the Andes or the creation of the Basin-and-Range topography in the Western USA.

Advances in U-Pb geochronology using a frequency quintupled Nd:YAG based laser ablation system (λ= 213 nm) and quadrupole based ICP-MS

The analytical potential of a 213 nm laser system coupled to a quadrupole based ICP-MS for U-Pb dating of zircon is investigated. The study was performed on zircons from a Neoproterozoic granitoid of the Cadomian belt of Brittany. Seven zircon fractions of the same sample including two single grains were dated by ID-TIMS to obtain a reference age and error to evaluate the accuracy and precision of LA-ICP-MS U-Pb results. The best estimate for the age of crystallisation obtained by ID-TIMS is 603 ± 2 Ma. LA-ICP-MS analyses were conducted in two sessions, one using only Ar as carrier gas and one using Ar + He. In both cases the age results are within analytical uncertainty of the ID-TIMS age (601 ± 9 Ma for the Ar analyses and 600 ± 2 Ma for the He analyses). It is demonstrated that the use of He carrier gas significantly improves reproducibility and precision of the elemental U-Pb ratios and allows the collection of data with a 2σ relative precision < ca. 1.5% for 206Pb/238U ratios and < ca. 2.5% for 207Pb/206Pb ratios at ca. 25 × 35 µm sample resolution. It is also demonstrated that the various sources of uncertainty associated with LA-ICP-MS U-Pb dating do not preclude the possibility of obtaining precise and accurate age results if an appropriate methodology is applied.

Geochemistry and U–Pb protolith ages of eclogitic rocks of the Asís Lithodeme, Piaxtla Suite, Acatlán Complex, southern Mexico: tectonothermal activity along the southern margin of the Rheic Ocean

Recent data indicating that the Piaxtla Suite (Acatlán Complex, southern Mexico) underwent eclogite-facies metamorphism and exhumation during the Devono-Carboniferous suggest an origin within the Rheic Ocean rather than the Iapetus Ocean. The Asís Lithodeme (Piaxtla Suite) consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks. U–Pb (zircon) data indicate that the metasediments were deposited after c. 700 Ma and before intrusion of c. 470–420 Ma quartz-augen granite. The metasedimentary rocks contain abundant Mesoproterozoic detrital zircons (c. 1050–1250 Ma) and a few zircons in the range of c. 900–992 and c. 1330–1662 Ma. Their geochemical and Sm–Nd isotopic signature is typical of rift-related, passive margin sediments derived from an ancient cratonic source, which is interpreted to be the adjacent Mesoproterozoic Oaxacan Complex. Megacrystic granites were derived by partial melting of a c. 1 Ga crustal source, similar to the Oaxacan Complex. Amphibolitic layers exhibit a continental tholeiitic geochemistry, with a c. 0.8–1.1 Ga source (TDM age), and are inferred to have originated in a rift-related environment by melting of lithospheric mantle in the Ordovician. This rifting may be related to the Early Ordovician drift of peri-Gondwanan terranes (e.g. Avalonia) from Gondwana and the origin of the Rheic Ocean.

Detrital Zircon Data from the Eastern Mixteca Terrane, Southern Mexico: Evidence for an Ordovician—Mississippian Continental Rise and a Permo-Triassic Clastic Wedge Adjacent to Oaxaquia

The eastern part of the Mixteca terrane of southern Mexico is underlain by the Petlalcingo Group (part of the Acatlán Complex), and has been interpreted as either a Lower Paleozoic passive margin, or a trench/forearc sequence deposited in either the Iapetus or Rheic oceans. The group, from bottom to top, consists of: (1) the Magdalena Migmatite protolith (metapsammites, metapelites, calsilicates, and marbles), which grades up into (2) the meta-psammitic Chazumba Formation; overthrust by (3) the Cosoltepec Formation (phyllites and quartzites with minor mafic meta-volcanic horizons). The group is unconformably overlain by the Pennsylvanian—Middle Permian Tecomate Formation, which is overthrust by the ∼288 Ma Totoltepec pluton and unconformably overlain by Middle Jurassic rocks. In contrast to previous inferences that the protoliths of the units (1) to (3) were early Paleozoic in age, detrital zircon LA-ICPMS ages combined with published data constrain depositional ages as follows: (i) Magdalena Migmatite protolith: post-303 Ma-pre-171 Ma (Permian—Early Jurassic); (ii) Chazumba Formation: post—239 Ma—pre-174 Ma (Middle Triassic—Early Jurassic); and (iii) Cosoltepec Formation: post—455 Ma—pre-310 Ma (uppermost Ordovician-Mississippian). Given the different ages and depositional environments of the Cosoltepec Formation versus the Chazumba Formation and Magdalena protolith, we recommend redefining the Chazumba and Magdalena as lithodemes grouped in the Petlalcingo Suite and excluding the Cosoltepec Formation. Detrital zircons in all three units show a population peak at ∼850-1200 Ma, suggesting derivation from the adjacent ∼1 Ga Oaxacan Complex. A ∼470-640 Ma peak is limited to the Cosoltepec Formation whose source may be found in ∼470 Ma plutons in the Acatlan Complex, beneath the Yucatan Peninsula, and in the Brasiliano orogens of South America. The inferred turbiditic protolith of the Chazumba Formation and Magdalena protolith suggests that it represents a clastic wedge deposited in front of S-verging Permo-Triassic thrusts on the western margin of Pangea. The mainly oceanic affinity of the basalts in the Cosoltepec Formation suggests deposition of sedimentary protoliths in a continental rise fringing Oaxaquia. These data are more consistent with deposition of the Cosoltepec Formation in the Rheic Ocean than in the Iapetus Ocean.

The basal tectonic mélange of the Cabo Ortegal Complex (NW Iberian Massif): a key unit in the suture of Pangea

Recent field work and mapping in the lower units of the Cabo Ortegal Complex provided new data about the tectonic melange that appears in the lowest structural position: the Somozas Melange. This melange unit with average thickness of 1800 m is restricted to the eastern part of the complex, and is located at the advancing front of the allochthonous complexes of NW Iberia. Three rock units are involved in the melange: 1) an ophiolitic melange consisting of igneous rocks mixed with serpentinites; 2) a metasedimentary unit with phyllites and phyllonites, with scarce conglomerates, marbles and quartzites; 3) high-T metamorphic rocks with varied types of amphibolites and orthogneisses. Two granitic rocks within the ophiolitic melange were dated using U-Pb zircon geochronology at 527 ± 2 Ma and 499 ± 1 Ma. Two different series of igneous rocks can be distinguished in this melange. The first series consists of gabbros, diorites, granitoids and basalts-basaltic andesites with calc-alkaline affinities. The second series contains common basaltic rocks, diabasic dikes and gabbros with chemical compositions typical of island-arc tholeiites. Both igneous series shared a common geographic setting, but the island-arc tholeiites are younger than the calc-alkaline igneous rocks. The two igneous series were probably generated in a mature volcanic arc located along the periphery of Gondwana. In the metasedimentary unit, a conglomerate from a large tectonic block included in serpentinites yielded age populations of detrital zircons suggesting that the sediments were deposited along the periphery of the West-African Craton. This conglomerate contains a large number of zircons (n = 24) with ages ranging 630-464 Ma, probably representing the chronology of the Pan-African event, including the magmatic activity in the volcanic arc where the igneous lithologies involved in the melange were generated. The maximum age of sedimentation for this conglomerate is estimated as latest Cambrian – earliest Ordovician, and constraints the end of the magmatic activity in the volcanic-arc. Within the unit of high-T rocks, an orthogneiss yields a U-Pb protolith age of 485 ± 6 Ma, which is similar to other ages of igneous rocks in the basal allochthonous terrane in NW Iberia. The three rock assemblages forming part of the Somozas Melange may be linked to the evolution of a mature peri-Gondwanan volcanic arc. This volcanic arc was affected by pronounced extension which caused the opening of intra-arc basins, culminating the rifting and subsequent drift of the external parts of the arc during the opening of the Rheic Ocean. This opening started during a time interval constrained by the peak activity in a mature volcanic arc (c. 527-499 Ma) and the generation of intraarc basins around the Cambrian-Ordovician boundary. Tectonic melanges including high-P rocks have been classically related to subduction zone environments. Regional relationships in NW Iberia and the nature of the rock units involved in the Somozas Melange, suggest that two different subduction zones generated during oblique convergence and collision between Gondwana and Laurussia were active during the final stages of the assembly of Pangea. The first related to the underthrusting beneath Laurussia of the most external Gondwana margin (c. 370 Ma). The second subduction zone was a new one which accreted later remnants of a peri-Gondwanan arc and sediments of the continental margin below a layer of exhumed high pressure rocks. This oblique collision finished the closure of the Rheic Ocean and contributes to define the new oceanic domain located to the East of Pangea, the Palaeotethys.