Fault-controlled emplacement of arc-related magmas along the Neoproterozoic northern Gondwanan margin: An example from the Antigonish Highlands, Nova Scotia

Abstract

In the Antigonish Highlands, Nova Scotia, Neoproterozoic (ca. 620–605 Ma) magmatism is a local representative of regional arc-related tectonothermal activity that typifies Avalonia and related terranes located along the northern Gondwanan margin. Three distinct types of coeval magma are represented by volcanic rocks of the Georgeville Group and by syn- to post-tectonic plutons (e.g. Greendale Complex); (i) calc-alkalic subduction-related basaltic andesites and andesites, (ii) crustally-derived rhyodacites and rhyolites, and (iii) Fe–Ti rich continental tholeiites derived from the melting of subcontinental lithosphere. These magmas were probably generated in an extensional arc regime with deep-seated faults providing conduits for rising magmas. The Georgeville Group was polydeformed by thrusts, isoclinal folds and upright folds associated with movement along NE-trending faults very soon after it was deposited. A 85-km-long 1–2 km wide magnetic anomaly transects Georgeville Group strata and connects outcrops of Fe–Ti rich continental tholeiites and ultramafic cumulate rocks of the Greendale Complex. The N–S orientation of this anomaly is compatible with coeval sinistral shear along NE-trending faults, and its magnetic signature suggests that it is a shallow (<1 km) complex of narrow dykes that fed the tholeiites. This suggests that continental tholeiites were emplaced in a local sinistral transtensional basin within the arc along the northern Gondwanan margin. Deformation of the basin-fill volcanic and sedimentary rocks was accompanied by a switch to dextral shear during basin inversion. Regional studies indicate that the kinematic reversal and the cessation of magmatism is a local expression of the Neoproterozoic–Early Paleozoic termination of subduction and generation of a transform fault along the northern Gondwanan margin, possibly by ridge–trench collision, analogous to the Cenozoic evolution of the San Andreas Fault system in the western United States.