MORPHOLOGICAL FEATURES OF MULTIGENERATION BASIC DYKES NEAR NAIN, LABRADOR - CLUES TO ORIGINAL EMPLACEMENT MECHANISMS AND SUBSEQUENT DEFORMATION

The Archean rocks of the Nain Province near Nain, Labrador have been i ntruded by a multi-generation assemblage of basic dykes. The regional setting constrains their intrusion to between similar to 2.5 Ga and si milar to 1.3 Ga; they may be, in fact, products of episodic basic magm atism over this whole 1200 million year span. They are unique among ot her 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 chill ed to intimately mingled contacts against enveloping rocks; their empl acement is interpreted to have occurred under both brittle and ductile crustal conditions. The cuspate borders on the dykes are an enigma; t hey could be either primary features produced at time of emplacement o r be deformation-induced by much later events (or both). Texturally, t he dykes range from subophitic to granoblastic to strongly foliated. F oliated dykes commonly display sigmoidal fabrics indicative of both de xtral and sinistral shear. The dykes have mineral assemblages implying variable recrystallization under amphibolite- and granulite-facies co nditions, but no simple relationship to regional or contact metamorphi c assemblages in enclosing gneisses is evident. It seems highly probab le that some of the granular orthopyroxene-bearing dykes are primary g abbronoritic rocks. The multiplicity of intrusions evident within the basic dyke assemblage and the variable degree of deformation and recry stallization cannot presently be confidently correlated with other eve nts in the surrounding region, but several options are considered. Amo ng the probable regional scenarios that could account for the diversit y and the multigeneration aspect of the assemblage are several discret e episodes of dyke intrusion: (i) Archean dykes related to a basic mag matic event yet unrecognized elsewhere in Labrador; (ii) Paleoproteroz oic dykes equivalent to swarms north and south of Nain; (iii) Paleo- o r Meso-proterozoic dykes related to newly recognized anorogenic pluton s of these ages; and (iv) Mesoproterozoic dykes as leaks from a subcru stal magma reservoir that culminated in the formation of the mafic plu tons within the anorogenic Nain Plutonic Suite.