Formation of Anorthositic Rocks within the Blair River Inlier of Northern Cape Breton Island, Nova Scotia (Canada)

J Gregory Shellnutt,D Fraser Keppie,J Duncan Keppie,Jaroslav Dostal,Sarah M Roeske

doi:10.2113/2020/8825465

Abstract

Abstract Rocks from the Blair River inlier of Northern Cape Breton Island (Nova Scotia, Canada) have been correlated with either the Grenville basement of eastern Laurentia or the accreted Avalon terrane. Additional zircon U-Pb dates of spatially associated anorthositic dykes (425.1±2.2 Ma) and a metagabbro (423.8±2.5 Ma) from the Fox Back Ridge intrusion of the Blair River inlier reveal Late Silurian emplacement ages. Their contemporaneity suggests that they may be members of a larger intrusive complex. The anorthositic rocks have high Eu/Eu∗ values (&gt;2.5), and bulk compositions are similar to the mineral compositions of labradorite (An50-70) and andesine (An30-50). The metagabbro is compositionally similar to alkali basalt and does not seem to have been affected by crustal contamination (Nb/U&gt;24; Th/NbPM≤1.1) although it was metamorphosed. The high Tb/YbN (1.8-1.9) ratios suggest that the parental magma of the metagabbro was derived from a garnet-bearing peridotite. Fractional crystallization and mass balance calculations indicate that the anorthositic rocks can be derived by mineral accumulation from a mafic parental magma similar in composition to the metagabbro of this study. The Late Silurian ages suggest that the rocks were emplaced into the Avalon terrane after the closure of the Iapetus Ocean but before Early Devonian (415-410 Ma) sinistral transpression.

Highlights

The eastern margin of North America records a complex middle Proterozoic to middle Paleozoic geological history
They are distinct from the outboard Appalachian terranes (i.e., Avalonia, Meguma, Carolinia) composed of Neoproterozoic and Paleozoic units that accreted to Laurentia during Paleozoic orogenesis [6, 7]
Our results provide new limits on the emplacement age of anorthositic rocks within the Blair River inlier, which, in turn, have direct implications for the location of the boundary between the cratonic margin of Laurentia and the accreted Avalon terrane

Summary

Introduction

The eastern margin of North America records a complex middle Proterozoic to middle Paleozoic geological history. The inliers are composed of middle to late Proterozoic rocks that were either formed or deformed during the Grenvillian orogeny and dated at 1200-980 Ma [4]. These inliers include rocks of easternmost Laurentia that are elsewhere typically included in the Appalachian orogen [5]. Laurentian basement inliers in the Appalachian orogen include distinctive lithologies such as anorthosites, charnockites, and granulites. These rock types are common in the Grenville Province but are mostly missing from the Appalachian outboard terranes

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Results

Conclusion