J. Brendan Murphy

Sm-Nd isotopic systematics as tectonic tracers: an example from West Avalonia in the Canadian Appalachians

Abstract The tectonothermal evolution of West Avalonia in mainland Nova Scotia, Appalachian orogen, provides an excellent example of the application of Sm–Nd isotopic analyses to tectonic studies. Regional syntheses indicate that West Avalonia originated in the Neoproterozoic as one of a number of terranes (collectively known as “peri-Gondwanan terranes”) along the Gondwanan margin. These terranes are characterized by Late Neoproterozoic voluminous arc-related magmatism. West Avalonia became detached from Gondwana in the Early Ordovician, was accreted to Laurentia by the Silurian and was deformed by post-accretionary strike-slip tectonics in the Late Devonian and Carboniferous, associated with the amalgamation of Pangea. All stages of this evolution can be recognized in the Sm–Nd isotopic signature in various Neoproterozoic and Paleozoic volcanic and sedimentary successions in mainland Nova Scotia. Crustally derived felsic volcanic rocks of Late Neoproterozoic (ca. 615 Ma), Early Cambrian and Early Silurian ages define an envelope that provides gross constraints on the Sm–Nd isotopic composition of the underlying basement source. e Nd values range from +0.78 to 2.92 in the Neoproterozoic rocks, from +2.0 to +5.0 in the Cambrian rocks, and from −0.1 to +2.5 in the Silurian rocks. T DM model ages for each of these suites are remarkably similar (they typically range from 0.9 to 1.2 Ga), suggesting that the Sm–Nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source. This Sm–Nd envelope provides a reference by which other suites may be compared and interpreted. For example, turbidites in a Late Neoproterozoic volcanic arc basin have Sm–Nd compositions that lie within this envelope, suggesting that they are derived from the coeval volcanic rocks that flank the basin. The genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre-date the main arc phase. Two felsic complexes have compositions that lie within the envelope and have similar model ages. The presence of this signature in early Avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca. 1.0 to 1.2 Ga that formed juvenile “proto-Avalonian” crust. By implication, the main arc phase in Avalonia recycled this crust. Sm–Nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre-dated Avalonian arc activity. Instead, the data indicate derivation from two distinct sources (ancient Gondwanan basement and juvenile Avalonian arc), suggesting deposition in a western Pacific-type back arc environment. This re-interpretation has important implications for Neoproterozoic reconstructions. The Sm–Nd isotopic composition of West Avalonian basement contrasts with that of other peri-Gondwanan terranes. For example, similarly aged felsic rocks in the Cadomian terrane of NW France are characterized by more negative e Nd values and older T DM model ages (1.0–1.9 Ga). These data, together with detrital U–Pb data, suggest that West Avalonia was developed along the Amazonian margin, whereas Cadomia was developed along the West African margin of Gondwana. Sm–Nd isotopic analyses of Early Silurian clastic rocks in mainland Nova Scotia show fundamental differences with unconformably underlying Neoproterozoic and Early Paleozoic Avalonian successions. These data have e Nd values ranging from −5.7 to −6.1 with T DM model ages of about 1.4 to 1.7 Ga, indicating that the clastic rocks cannot have been derived from underlying successions. A variety of geologic data and regional constraints suggests that they may have been derived from the Caledonide orogen of western Europe, thereby providing a minimum age for the accretion of West Avalonia to Laurentia–Baltica. Sm–Nd isotopic analyses from Late Devonian–Early Carboniferous continental clastic rocks deposited along the southern flank of Avalonia in mainland Nova Scotia also show little influence from Avalonian rocks, and are compatible with derivation from the Meguma terrane immediately to the south. These data are consistent with regional synthesis, which indicate uplift of the Meguma terrane relative to West Avalonia during dextral transpression.