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Constraining a Precambrian Wilson Cycle lifespan : An example from the ca. 1.8 Ga Nagssugtoqidian Orogen, Southeastern Greenland

Nicoli, G. ; Thomassot, E. ; Schannor, M. ; Vezinet, A. ; Jovovic, I., Lithos

Constraining a Precambrian Wilson Cycle lifespan : An example from the ca. 1.8 Ga Nagssugtoqidian Orogen, Southeastern Greenland

Nicoli, G. ; Thomassot, E. ; Schannor, M. ; Vezinet, A. ; Jovovic, I.

Lithos, 2017, 296-299, 1-16

Abstract :

In the Phanerozoic, plate tectonic processes involve the fragmentation of the continental mass, extension and spreading of oceanic domains, subduction of the oceanic lithosphere and lateral shortening that culminate with continental collision ( i.e. Wilson cycle). Unlike modern orogenic settings and despite the collection of evi- dence in the geological record, we lack information to identify such a sequence of events in the Precambrian. This is why it is particularly difficult to track plate tectonics back to 2.0 Ga and beyond. In this study, we aim to show that a multidisciplinary approach on a selected set of samples from a given orogeny can be used to place constraints on crustal evolution within a P-T-t-d-X space. We combine field geology, petrological observations, thermodynamic modelling (Theriak-Domino) and radiogenic (U-Pb, Lu-Hf) and stable isotopes (δ18O) to quantify the duration of the different steps of a Wilson cycle. For the purpose of this study, we focus on the Proterozoic Nagssugtoqidian Orogenic Belt (NOB), in the Tasiilaq area, South-East Greenland. Our study reveals that the Nagssugtoqidian Orogen was the result of a complete three stages juvenile crust production (Xjuv)–recycling/reworking sequence : (I) During the 2.60–2.95 Ga period, the Neoarchean Skjoldungen Orogen remobilised basement lithologies formed at TDM 2.91 Ga with progressive increase of the discharge of reworked material (X juv from 75% to 50% ;δ 18O : 4 – 8.5‰). (II) After a period of crustal stabilization(2.35– 2.60 Ga), discrete juvenile material inputs (δ18 O : 5 – 6 ‰ ) at T DM 2.35 Ga argue for the formation of an oceanic lithosphere and sea floor spreading over a period of 0.2 Ga (Xjuv from b 25% to 70%). Lateral shortening is set to have started at ca. 2.05 Ga with the accretion of volcanic/magmatic arcs (i.e Ammassalik Intrusive Complex) and by subduction of small oceanic domains (M1 : 520 ± 60 °C at 6.6 ± 1.4 kbar). (III) Continental collision between the North Atlantic Craton and the Rae Craton occurred at 1.84 – 1.89 Ga. Crustal thickening of 25 km was accompanied by regional metamorphism M2 (690 ± 20 °C at 6.25 ± 0.25 kbar) and remobilization of pre-existing supracrustal lithologies (Xjuv 40% ; δ 18 O : 5 – 10.5 ‰). Rates and durations obtained for sea floor spreading (175 ± 25 Ma), subduction (125 ± 75 Ma) and continental collision (ca. 60 Ma) are similar to those observed in Phanerozoic Wilson Cycle but differ from what was estimated for Archean terrains. Therefore, timespans of the different steps of a Wilson cycle might have progressively changed over time as a response to the progressive cratonization of the lithosphere.

Voir en ligne : https://doi.org/10.1016/j.lithos.20...




publié lundi 13 novembre 2017