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.
Highlights
The Middle Paleozoic Acadian Orogeny of the Appalachians was largely confined to the current northeastern margin of North America[1]
Www.nature.com/scientificreports terranes is problematic due to the fact that the Meguma terrane is primary comprised of sedimentary rocks but was thrust over Avalonia and may cover a sizeable portion of it[16]
The geochronology results of this study show that the majority of detrital zircons have ages between ~420 Ma and ~660 Ma with minor groups at ~1100 Ma and ~2200 Ma
Summary
The Middle Paleozoic Acadian Orogeny of the Appalachians was largely confined to the current northeastern margin of North America[1]. The metapelites are most abundant but some were disaggregated and it is common to observe xenocrysts of garnet and aluminosilicate (silliminatized kyanite and sillimanite) minerals in the dyke Isotopic evidence from both the metaigneous and metasedimentary xenoliths indicates they are distinct from the overlying Meguma terrane[16]. The Sr-Nd isotopes of Meguma rocks are unradiogenic (87Sr/86Sri = 0.7113 to 0.7177; εNd(t) = −8.8 to −11.3) and their middle Proterozoic Nd model ages (TCHUR = 1358 Ma) are significantly older than the early Cambrian (TCHUR = 544 Ma) ages of the metaigneous granulites[16]. The isotopic data and Nd model ages of the metaigneous granulites are indistinguishable from Avalonia magmatic rocks suggesting the Avalon terrane is the structural basement to Meguma[10,16]
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