PROBING THE CRUST OF AVALONIA: HF AND ND ISOTOPIC COMPOSITIONS OF NEOPROTEROZOIC ARC-RELATED ROCKS FROM NEWFOUNDLAND AND NOVA SCOTIA, CANADA

Avalonia is the largest accreted crustal block in the Appalachian–Caledonian orogen and comprises Neoproterozoic–early Paleozoic magmatic arc sequences that experienced a protracted and episodic Neoproterozoic subduction-related tectonomagmatic history before deposition of an Ediacaran–Early Ordovician sedimentary cover sequence. Zircon crystals were obtained from Neoproterozoic arc-related volcanic and plutonic rocks from Nova Scotia and Newfoundland and previously dated by U–Pb TIMS and from Ediacaran-Cambrian clastic sedimentary sequences dated by U–Pb LA-ICPMS and analyzed in situ for their Hf-isotope composition. The majority of magmatic and detrital zircons are dominated by initial ¹⁷⁶Hf/¹⁷⁷Hf values that are more radiogenic than CHUR with calculated crust formation Hf-T_DM model ages that range from 0.84 Ga to 1.30 Ga and were formed by partial melting of juvenile mantle in a Neoproterozoic continental arc system. Some zircons have crust formation Hf-T_DMmodel ages ca. 1.39 to 3.09 Ga with εHf values of –33.9 to –0.5 and indicate involvement of older lithosphere in their petrogenesis.

Whole rock Sm–Nd isotopic compositions from felsic volcanic rocks are characterized by positive initial εNd values with Mesoproterozoic depleted mantle model ages (1.18–1.49 Ga) consistent with juvenile magma extraction dominated by a mantle component with long-term LREE depletion mixed with an older crustal component with long-term LREE enrichment. The data suggest a ca. 1.0–1.2 Ga igneous tectonomagmatic event formed basement to Neoproterozoic crust-forming magmatic arc in Avalonia. The presence of evolved Hf- and Nd-isotopic signatures suggest that significant older Proterozoic crust is present beneath the western part of Avalonia and is compatible with a scenario in which Avalonia formed in a single Neoproterozoic arc system that formed juvenile mantle-derived crust, coupled with lesser anatectic reworking of older lithosphere, that was built upon the boundary between contrasting continental and oceanic lithosphere.