Morphological features of multigeneration basic dykes near Nain, Labrador: clues to original emplacement mechanisms and subsequent deformation

Abstract

The Archean rocks of the Nain Province near Nain, Labrador have been intruded by a multi-generation assemblage of basic dykes. The regional setting constrains their intrusion to between ∼2.5 Ga and ∼1.3 Ga; they may be, in fact, products of episodic basic magmatism over this whole 1200 million year span. They are unique among other mafic dykes in the Nain Province by their morphological diversity, mineral assemblages, and degree of recrystallization. They vary from straight-walled to cuspate-bordered intrusions, and they exhibit chilled to intimately mingled contacts against enveloping rocks; their emplacement is interpreted to have occurred under both brittle and ductile crustal conditions. The cuspate borders on the dykes are an enigma; they could be either primary features produced at time of emplacement or be deformation-induced by much later events (or both). Texturally, the dykes range from subophitic to granoblastic to strongly foliated. Foliated dykes commonly display sigmoidal fabrics indicative of both dextral and sinistral shear. The dykes have mineral assemblages implying variable recrystallization under amphibolite- and granulite-facies conditions, but no simple relationship to regional or contact metamorphic assemblages in enclosing gneisses is evident. It seems highly probable that some of the granular orthopyroxene-bearing dykes are primary gabbronoritic rocks. The multiplicity of intrusions evident within the basic dyke assemblage and the variable degree of deformation and recrystallization cannot presently be confidently correlated with other events in the surrounding region, but several options are considered. Among the probable regional scenarios that could account for the diversity and the multigeneration aspect of the assemblage are several discrete episodes of dyke intrusion: (i) Archean dykes related to a basic magmatic event yet unrecognized elsewhere in Labrador; (ii) Paleoproterozoic dykes equivalent to swarm snorth and south of Nain; (iii) Paleo- or Meso-proterozoic dykes related to newly recognized anorogenic plutons of these ages; and (iv) Mesoproterozoic dykes as leaks from a subscrustal magma reservoir that culminated in the formation of the mafic plutons within the anorogenic Nain Plutonic Suite.