Devonian plutons in the eastern Meguma terrane, Nova Scotia, Canada: zircon U-Pb, Lu-Hf and O isotopic compositions, age, and petrogenetic implications

Abstract

Abundant granitic plutons intruded the eastern Meguma terrane of Nova Scotia in the middle- to late Devonian. Less voluminous diorite-tonalite and gabbro intrusions are associated with the granitic plutons along the northern margin of the terrane adjacent to the Cobequid-Chedabucto fault zone. All plutons contain metasedimentary xenoliths, and the mafic plutons show magma mingling textures with their adjacent granitic plutons. New U-Pb zircon data from autocrystic zircon in 13 samples indicate coeval emplacement of mafic and granitic plutons between ca. 382 and 368 Ma. However, the zircon grains contain numerous inherited domains that range in age from Palaeoproterozoic to Devonian. These inherited ages correspond to detrital zircon U-Pb dates from the Cambrian to Ordovician metasedimentary host rocks. Zircon oxygen isotopic data (δ18O) are between +7.4 ± 0.2 ‰ and +9.3 ± 0.3 ‰ indicating significant involvement of the crust as the magma source or contaminant. If the high δ18Ozrc values are a result of contamination, the contaminant was likely the metasedimentary rocks of the Meguma terrane. Hafnium isotopic data from autocrystic zircon have εHf(t) between -6.0 ± 1.5 and +2.1 ± 2.5. The new zircon U-Pb, O and Hf isotopic data from plutons in the eastern Meguma terrane are indistinguishable from published data from the South Mountain Batholith. These data suggest that Devonian magmatism in the Meguma terrane post-dated or was coeval with the orogenic event that caused folding and regional metamorphism, involved the same magma source and/or contaminants throughout the terrane, and did not involve Avalonian crust.