Abstract

The tectonostratigraphic record of the Ungava orogen contains evidence for the interaction of divergent, transform, and convergent plate boundaries over a > 0.2 Ga period in the Early Proterozoic. Three principal tectonic domains are recognized: (1) autochthonous basement plutonic and supracrustal rocks of Archean age; (2) autochthonous and allochthonous sedimentary and volcanic units associated with a ca. 1.99–1.92 Ga rift-to-drift margin; and (3) "suspect' ' crustal components of a ca. 2.00 Ga ophiolite and ca. 1.90–1.83 Ga island-arc terrane. Domain 1 is the stratigraphic or structural basement to domains 2 and 3. Field relationships indicate that domain 2 units were accumulated on or near domain 1, whereas no tectonostratigraphic linkages are known between domain 3 and the others. Accumulation of continent-derived sedimentary rocks, continental tholeiites, komatiitic basalts, and mafic lavas equivalent to modern normal mid-oceanic ridge basalts (n-MORB's) in domain 2 is interpreted as recording continental rifting leading to the possible formation of oceanic crust. Pillowed basalts, sheeted dykes, and cumulate rocks suggest that the crustal portion of an ophiolite is preserved within domain 3. The ophiolite is interpreted as a "suspect" fragment of oceanic crust, which may represent an earlier rifted portion of domain 2, possibly juxtaposed by transform plate boundary motions. Within domain 3, calc-alkaline volcanic and plutonic units and sequences of volcanogenic and siliciclastic sedimentary rocks are thought to indicate the development of a magmatic arc and fore-arc basin and thus the establishment of a convergent plate boundary. The most internal lithotectonic packages in domain 3 are predominantly crystalline thrust sheets containing (i) an older intrusive suite, interpreted as the plutonic foundation of the magmatic arc, and (ii) younger plutons, interpreted as the result of a reversal in subduction polarity. The reversal may have followed attempted subduction of domains 1 and 2 beneath the upper-plate arc assemblage of domain 3.

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