LATE NEOPROTEROZOIC TO CARBONIFEROUS GENESIS OF A-TYPE MAGMAS IN AVALONIA OF NORTHERN NOVA SCOTIA: REPEATED PARTIAL MELTING OF ANHYDROUS LOWER CRUST IN CONTRASTING TECTONIC ENVIRONMENTS

Avalonian rocks in northern mainland Nova Scotia are characterized by voluminous 640-600 Ma calc-alkalic to tholeiitic mafic to felsic magmas produced in a volcanic arc. However, after the cessation of arc activity, repeated episodes of felsic magmatism between ca. 580 Ma and 350 Ma are all dominated by A-type geochemical characteristics. Sm-Nd isotopic data, combined with zircon saturation temperature estimates, indicate that these magmas were formed by high temperature (800-1050^oC) melting of the same anhydrous crustal source. Regional tectonic considerations indicate that A-type felsic magmatism was produced (i) at 580 Ma in a San Andreas-type strike slip setting, (ii) at 495 Ma as Avalonia rifted off Gondwana, (iii) at 465 and 455 in an ensialic island arc environment and (iv) at 360-350 Ma during post-collisional, intra-continental strike-slip activity as Avalonia was translated dextrally along the Laurentian margin. These results attest to the importance of crustal source, rather than tectonic setting, in the generation of these A-type magmas and are an example of how additional insights are provided by comparing the geochemical and isotopic characteristics of igneous suites of different ages within the same terrane. They also suggest that the shallow crustal rocks in northern mainland Nova Scotia were not significantly detached from their lower crustal source between ca. 620 Ma and 350 Ma, a time interval that includes the separation of Avalonia from Gondwana, its drift and accretion to Laurentia as well as post-accretionary strike-slip displacement.