The structural and metamorphic history of the Back River volcanic complex, Nunavut and Northwest Territories

The Back River volcanic complex represents an Archean volcanic centre located in the eastern Slave Province, Northwest Territories and Nunavut. Detailed structural analysis of the metavolcanic Back Group and conformably overlying Beechey Lake Group metasedimentary succession has identified four generations of ductile structures, which are the products of three Archean deformations (D₁—D₃). The oldest structures (D ₁) comprise horizontally plunging, upright to overturned F₁ folds and axial planar S₁ slaty cleavage. These structures are overprinted by tight to isoclinal, strongly asymmetrical F₂ folds and S₂ axial planar foliations. The steep plunge of F₂ results in the development of locally complex F₁/F₂ fold interference geometries. The intensity of D₂ was sufficient to largely transpose D₁ structures, producing a regional northwest-southeast structural trend. D₃ was less intense, producing regionally penetrative east-west trending mesoscopic structures without significant reorientation of older structures. The overprinting of tight F₂ folds by D ₃ resulted in the development of two orientations of S₃ crenulation cleavage on the alternating F₂ fold limbs.; The Back River volcanic complex is exposed in the core of a regional structural dome, a product of D₁/D₂ fold interference, mantled by the overlying Beechey Lake Group. The competency contrast between the Back and Beechey Lake groups resulted in the development of shear zones along the margin of the volcanic complex during the later stages of D ₂. These marginal high strain zones have the same D₂ kinematic framework as the F₂ folding developed in the overlying Beechey Lake Group, and are interpreted as a product of flexural slip along the volcano/sediment interface during D₂ folding.; Deformation was accompanied by two periods of low pressure, moderate to high temperature metamorphism. The first period of metamorphism (M ₁) represented contact metamorphism associated with the intrusion of the syn-kinematic Tarantula Plutonic Suite during the late stages of D ₂. This event produced narrow lower amphibolite facies aureoles around the plutons. M₁ was overprinted by a second metamorphism (M ₂) which also produced lower amphibolite facies assemblages, and coincided with D₃. M₂ was more regional in extent, and the interaction between M₁ and M₂ produced an irregular isograd pattern characterized by diachronous porphyroblast development.; The interaction between D₂ and M₁ produced biotite porphyroblasts with regionally consistent internal S₁ foliations. These internal foliations are used as passive markers to assess the amount of porphyroblast rotation during the development of S₃ differentiated crenulation cleavage within metapelite beds. The analysis of biotite porphyroblast internal foliations indicates that the sense and amount of porphyroblast rotation during crenulation cleavage development is different for porphyroblasts within the crenulation septa and microlithons.