CONTRASTING AVALONIAN AND GANDERIAN CRUST USING LU–HF-ISOTOPIC COMPOSITION AND MODEL AGES OF NEOPROTEROZOIC MAGMATIC ZIRCON GRAINS

Avalonia comprises numerous Neoproterozoic magmatic arc sequences that represent protracted and episodic subduction-related magmatism before deposition of an Ediacaran–Ordovician cover sequence of siliciclastic rocks. The nature of the basement on which these arcs were constructed is characterized using zircon grains from arc-related magmatic rocks in New England and Atlantic Canada and analyzed in situ for their Lu–Hf-isotope composition. The majority of Neoproterozoic zircon grains from 8 samples from the Caledonian Highlands of southern New Brunswick are characterized by positive initial εHf values with calculated crust formation Hf–T_DM ages between c. 1.0–1.3 Ga, consistent with juvenile extraction. The results suggest origin by partial melting of juvenile mantle in a Neoproterozoic continental arc. Samples from Nova Scotia (9), southeastern New England (9), and Newfoundland (2) are dominated by initial ¹⁷⁶Hf/¹⁷⁷Hf values that are more radiogenic than CHUR and calculated crust formation Hf–T_DM model ages that range from 0.8 to 1.2 Ga. Some zircons from these areas, however, have calculated crust formation model ages c. 1.4–1.6 Ga and 2.1–2.4 with initial εHf values of –25 to –1 that clearly indicate recycling of older lithosphere in their petrogenesis. These data contrast with those from Ganderia which show typically positive initial εHf values and Hf–T_DM model ages that imply magmatism was derived by melting of crustal sources with diverse ages ranging from c. 1.0 to 1.8 Ga.

The positive distribution of initial εHf values along with the pattern of crust formation Hf–T_DM model ages are compatible with a c. 1.0–1.3 Ga igneous tectonomagmatic event that formed basement to Neoproterozoic magmatic arcs in Avalonia. The occurrence of evolved initial ¹⁷⁶Hf/¹⁷⁷Hf values and Paleoproterozoic Hf–T_DM model ages, however, indicates that significant older Proterozoic crust is present locally beneath parts of Avalonia, implying that crustal evolution during the Ediacaran was more complex than simple recycling of pre-existing Mesoproterozoic crust. We suggest that Avalonia formed in a series of composite Neoproterozoic arcs formed on largely juvenile basement dominated by c. 1.1 Ga mantle-derived material, coupled with lesser anatectic reworking of significantly older (c. 1.5 and 2.2 Ga) crustal components.