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Long-lived association between Avalonia and the Meguma terrane deduced from zircon geochronology of metasedimentary granulites - PubMed

  • ️Tue Jan 01 2019

Long-lived association between Avalonia and the Meguma terrane deduced from zircon geochronology of metasedimentary granulites

J Gregory Shellnutt et al. Sci Rep. 2019.

Abstract

The Acadian Orogeny of the Northern Appalachians was caused by accretion of the peri-Gondwanan terranes Avalonia and Meguma to the eastern margin of Laurentia during the Devonian. The lithotectonic relationship between Avalonia and Meguma prior to accretion is uncertain. Radioisotopic dating of detrital zircons from metasedimentary granulite xenoliths from the structural basement to the Meguma terrane indicates that Avalonia and Meguma were proximal and likely contiguous as they transited the Rheic Ocean. The zircon ages range from the Cryogenian to Late Silurian with a minor Paleoproterozoic peak. Mesoproterozoic zircons are also identified and, coupled with the Ordovician to Silurian zircons, distinguish the rocks from those of the Meguma terrane. Furthermore, three distinct metamorphic events are identified at 399.0 ± 2.1 Ma, 376.9 ± 1.6 Ma, and 353.8 ± 3.3 Ma. We conclude that the granulite facies metamorphism experienced by the metasedimentary rocks occurred 10 to 20 million years after deposition of their protoliths during the initial stages of the Acadian Orogeny whereas the younger events are related to syn- and post-collisional episodes. The implication is that Avalonia and the Meguma terrane jointly transited from Gondwana.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1

Simplified geological map of mainland Nova Scotia and western Cape Breton showing the distribution of Late Devonian and earlier rock units of Avalonia and Meguma and the location of the Pope’s Harbour dyke. MB = Musquodoboit Batholith. Inset shows the Early Mesozoic reconstruction of Pangea and the locations of Meguma, Avalonia, and other peri-Gondwana terranes.

Figure 2
Figure 2

Frequency distribution of detrital zircon ages from the metasedimentary granulite xenoliths of the Pope’s Harbour dyke. (a) Total range of detrital zircons ages of the metasedimentary xenoliths compare to the major detrital zircon populations of the Meguma terrane. (b) Cryogenian to Early Devonian distribution of detrital zircons showing the major tectonothermal episodes of Avalonia.

Figure 3
Figure 3

Concordia plots of the metamorphic zircons identified within metasedimentary granulite xenoliths of the Pope’s Harbour dyke. (a) Early Carboniferous metamorphic zircons. (b) Late Devonian metamorphic. (c) Early Devonian metamorphic zircons. Concordia plots of the metamorphic zircons identified within metasedimentary granulite xenolith.

Figure 4
Figure 4

Palinspastic reconstructions during Early Palaeozoic time (510–370 Ma) period. The model uses the tropical latitudes of Laurentia, Avalonia, Siberia, Baltica, and minor terranes which later formed Laurussia from previous models. The evolutionary history of Ganderia (dark blue), Avalonia (violet), and Meguma (purple) terrane along with the evolution of Laurussia and Gondwana are confined with current study. AM = Amazon, AV = Avalonia, BT = Baltica, CG = Congo, EAV = Eastern Avalonia, LU = Laurentia, GRN = Greenland, SIB = Siberia. (a) The Ganderia, Avalonia, and Meguma terranes are reconstructed to surround the South America craton around 510 Ma following the breaking up of Pannotia. (b) The separation of Ganderia terrane from South America craton as the Iapetus Sea opened. The separation of Avalonia, Eastern Avalonia terrane, and Meguma terrane from the edge of Amazon craton as the Rheic Ocean opened at 480 Ma. (c) The closure of Iapetus sea and continuous spreading of Rheic ocean bring Ganderia, Avalonia, Eastern Avalonia terrane closer to Laurentia, but reginal extension between Avalonia and Meguma occurred at 440 Ma. (d) Formation of Laurussia during 420 Ma as the Ganderia, Avalonia, and Meguma terranes amalgamated into Laurentia, and the Eastern Avalonia terrane amalgamated into Baltica. (e) Further closing of Rheic Ocean around 420 Ma caused the Meguma terrane to be thrust over Avalonia. (f) The closure of Rheic Ocean brought Laurussia and Gondwana closer to form the supercontinent Pangea around 370 Ma. Based on our reconstruction, Eastern Avalonia is the only terrane that is close to the West Africa craton during the Early Palaeozoic time period. Although the Carolinian terrane also originated from the edge of West Africa, the rifting and amalgamation of Carolinian terrane occurred during the Late Mississippian, which is much later than for Avalonia.

Figure 5
Figure 5

Tectonic evolution showing the possible relationship between Avalonia and Meguma during the Late Silurian to Late Devonian. (a) Late Silurian emplacement of the Rockville Notch Group (440 Ma to 420 Ma). (b) Initial stages of the Acadian Orogeny and peak granulite facies metamorphism (M1) in the underlying Avalonia rocks (410 Ma to 400 Ma). (c) Middle Devonian silicic magmatism (378 Ma) and the high temperature M2 deformation episode. (d) Late Devonian dyke emplacement and entrainment of the granulite xenoliths (~370 Ma). SCLM = subcontinental lithospheric mantle. OLM = oceanic lithospheric mantle. RNG = Rockville Notch Group.

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