2 research outputs found
Cambrian successions of the Meguma Terrane, Nova Scotia, and Harlech Dome, North Wales: dispersed fragments of a peri-Gondwanan basin?
<p>The Meguma Terrane of Nova Scotia and the Harlech Dome of North Wales preserve similar sedimentary successions of Cambrian
age. Both successions comprise a thick succession of early Cambrian sandstone turbidites, overlain by early to middle Cambrian
alternating mud-rich and sand-rich units in which manganese is concentrated in two stratigraphic intervals. Above these, both
successions comprise anoxic, organic-rich turbidites, shallowing upward into paler, more bioturbated Tremadocian mudstone
with <em>Rhabdinopora</em>. Within the limited constraints of the available biostratigraphic and geochronological data, major changes in environment
occurred synchronously in the two successions. Both successions show much greater similarity to each other than to adjacent
successions on ‘Avalonia'. A detrital zircon analysis from the Rhinog Formation, low in the Harlech Dome succession, reveals
distinct clusters of ages around 537 Ma and 2.0–2.1 Ga. A close similarity to analyses from the Meguma Terrane suggests proximity
between the two terranes on the margin of Gondwana during the Cambrian Period. We suggest the term Megumia for the palaeogeographical
domain that included the two successions, which was dispersed during subsequent Appalachian and Caledonian movements. These
observations suggest that Megumia may have separated terranes previously regarded as parts of Avalonia.
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Ganderia–Laurentia collision in the Caledonides of Great Britain and Ireland
<p>During terrane convergence, an influx of clastic sediment from an upper plate onto a lower plate is an early indication of
terrane juxtaposition. In the Caledonides of Great Britain and Ireland, units accreted to Laurentia during the early Palaeozoic
Era include peri-Gondwanan terrane assemblages that earlier separated from West Gondwana. However, the Southern Uplands Terrane
contains detrital zircon populations apparently derived entirely from Laurentia, characterized by a large, asymmetric Mesoproterozoic
peak and a scarcity of zircon at 600 Ma and 2.1 Ga. In contrast, Cambrian and Ordovician rocks from the Lake District and
the Leinster Massif of Ireland show abundant grains with these ages, together with a range of Mesoproterozoic zircon. These
characteristics are shared with the Monian terrane of Anglesey and with Ganderia in the Appalachians, indicating probable
derivation from Amazonia in West Gondwana. Silurian sandstones from the Lake District show an influx of Laurentia-derived
zircon, and lack the peri-Gondwanan signal. This indicates that in the Caledonides, Ganderia was not accreted to the Laurentian
margin until <em>c</em>. 430 Ma, in contrast to the Ordovician accretion of Ganderian fragments recorded in the Appalachians, suggesting that the
configuration of the closing Iapetus Ocean varied significantly along the strike of the orogen.
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