New age data refine extent and duration of Paleozoic and Neoproterozoic plutonism at Ganderia–Avalonia boundary, Newfoundland

Abstract

Devonian and younger plutonic suites of the Appalachian orogen in Newfoundland form a curvilinear belt straddling the boundary between Ganderia and Avalonia, and exhibit a general pattern of younging to the southeast. However, many individual plutonic suites within this belt remain undated or have ages that are poorly constrained. This study reports U–Pb sensitive high-resolution ion microprobe (SHRIMP) data from zircon and 40Ar–39Ar data from hornblende, biotite, and muscovite, which collectively improve definition of post-tectonic plutonic events in southeastern Newfoundland, but at the same time raise new questions for further study. U–Pb zircon SHRIMP determinations provide valuable new geochronological information for the Deadman’s Bay Granite (390 ± 4 Ma), the Gander Lake Granite (378 ± 4 Ma), the Terra Nova Granite (384 ± 4 Ma), the Old Woman stock (376 ± 4 Ma), and the easternmost unit (Tolt phase) of the composite Ackley Granite (372 ± 4 Ma). The Pass Island Granite, long considered part of this age group, gave a Silurian age of 423 ± 4 Ma and provides the first confirmation of Silurian granitoid magmatism in Avalonia of Newfoundland. The Berry Hills Granite on the Burin Peninsula, previously designated as Devonian, instead gave a Neoproterozoic age of 570 ± 6 Ma, and a similar age of 575 ± 6 Ma was obtained from granite that is intruded by the ca. 376 Ma Old Woman stock. These results indicate that regional geological context, deformation state, and (or) petrographic–geochemical data are not reliable indicators of emplacement age in this region. 40Ar–39Ar data from the Paleozoic granites provide mixed results in comparison to the U–Pb emplacement ages, and in some cases Ar systematics appear to have been disturbed by later hydrothermal alteration. In other cases, correspondence between the U–Pb and 40Ar–39Ar data sets is consistent with rapid postemplacement exhumation. Granophile-type mineralization (Mo, W, F, base metals) is present within both Ganderia and Avalonia examples, but is generally restricted to the youngest plutons, in which it is associated with high-level, near-roof, or apical settings. This age versus mineralization pattern is consistent with an interpretation in which mineralized environments within older suites have been preferentially eroded as a consequence of greater exhumation. The identification of a Silurian (ca. 423 Ma) granite in an area previously assigned to Avalonia may be important in constraining tectonic models for Ganderia–Avalonia interaction, which has generally been considered Silurian or younger. Given that many other plutonic suites in Avalonia are not well constrained by regional geology and remain undated, more work is needed to better define the extent of such magmatism and evaluate its implications for broader models of orogenic development.