Lithogeochemical and Sm-Nd and U-Pb isotope data from the Silurian–Lower Devonian Arisaig Group clastic rocks, Avalon terrane, Nova Scotia: A record of terrane accretion in the Appalachian-Caledonide orogen

Abstract

The Silurian-Lower Devonian Arisaig Group, in the Avalon terrane of Nova Scotia, consists of a thick (~1900 m) sequence of unmetamorphosed fossiliferous siliciclastic strata deposited during terrane accretionary events in the Appalachian-Caledonide orogen. Geochemistry and Sm-Nd and U-Pb (detrital zircon) isotope data of Arisaig Group rocks contrast with the underlying Avalonian rocks, indicating that they were not derived from Avalonian basement. All sedimentary rocks are characterized by strongly negative ε Nd(t) values (from-4.8 to-9.3, t = 430 Ma) and T DM ages older than 1.5 Ga; the overall trend from the base to the top of the group is toward increasingly negative ε Nd values. The fact that some Silurian sedimentary rocks contain detrital zircons of similar age suggests that basin formation was broadly coeval with active volcanism in the orogen. These samples also contain abundant Neoproterozoic-Early Cambrian zircons (ca. 620-520 Ma) and lesser abundances at ca. 1200-900 and 2200-1500 Ma. Archean zircons are very minor. The sample of Lower Devonian strata contains Late Silurian and Early Ordovician zircons and, in comparison to the Silurian samples, less abundant Cambrian (ca. 520-510 Ma) and Neoproterozoic (610-550 Ma; 834 Ma) zircons and subordinate Mesoproterozoic (1000-1200 Ma), Mesoproterozoic (1400-1600 Ma), and Paleoproterozoic (2000-2100 Ma) zircons. There are no Archean zircons. A comparison between the U-Pb detrital-zircon data and

Geochemical signature of Ordovician Mn-rich sedimentary rocks on the Avalonian shelf

“…For instance, the Nd isotopic signature of crustal melts emplaced into the Palaeozoic sedimentary rocks of the Meguma Zone are consistent with melting of relatively juvenile arc-type crust (Keppie et al 1997), which is known from Avalonia, but not from the West African Craton (Keppie et al 1997;Murphy et al 2004b;Waldron et al 2009). Furthermore, Greenough et al (1999) presented projected U-Pb intercept ages predominantly falling in the range from 850 to 1050 Ma for metamorphic zircon from basement xenoliths.…”

“…The lack of lithologies typically related to oceanic sutures between Meguma and Avalonia, however, favors the interpretation that Meguma and Avalonia were on the same side of the Rheic Ocean (cf. Murphy et al 2004b).…”

“…Actually, the presence or absence of lithologies typical for oceanic sutures may provide a better constraint for the position of both the Rheic Suture and Meguma before the closure of the Rheic Ocean. In this sense, the absence of lithologies typical for oceanic sutures such as high-pressure rocks, ophiolites, and island arc remnants between Meguma and Avalonia (e.g., Murphy et al 2004b) suggests that Meguma had accompanied Avalonia after the opening of the Rheic Ocean.…”

Carbon isotope chemostratigraphy in Arctic Canada: Sea-level forcing of carbonate platform weathering and implications for Hirnantian global correlation

“…This may explain why in the circum-Iapetus region through the early Hirnantian, the positive y 13 C excursions are suppressed until later Hirnantian time when sea level was much lower in that area. In addition, differences in water chemistry between the circum-Iapetus basins and other world's oceans may have been maintained as a result of the relatively weak connection between them during Hirnantian time as a result of the progressive closure of the Iapetus and docking of Baltica with Avalonia (Van Staal et al, 1998;Cocks and Torsvik, 2002;Murphy et al, 2004). This restriction may also explain the survival of some endemic, relict benthic faunas through the Hirnantian in eastern Laurentia, Avalonia and Baltica (e.g., Owen and Robertson, 1995).…”

Does the Meguma Terrane Extend into SW England?

“…Ga (Murphy et al 1996b(Murphy et al , 2004c. Also common among Avalonian detrital zircon populations are Mesoproterozoic ages (e.g.…”