TECTONIC EVOLUTION OF THE NORTHERN VOLCANIC ZONE, ABITIBI BELT, QUEBEC

The Archean Abitibi Subprovince has been divided formally into a Northern Volcanic Zone (NVZ). including the entire northern part of the subprovince, and a Southern Volcanic Zone (SVZ) on the basis of distinct volcano-sedimentary successions, related plutonic suites, and precise U-Pb age determinations. The NVZ has been further formally subdivided into (i) a Monocyclic Volcanic Segment (MVS) composed of an extensive subaqueous basalt plain with scattered felsic volcanic complexes (2730-2725 Ma), interstratified with or overlain by linear volcaniclastic sedimentary basins, and (ii) a Polycyclic Volcanic Segment (PVS) comprising a second mafic-felsic volcanic cycle (2722-2711 Ma) and a sedimentary assemblage with local shoshonitic volcanic rocks. A sequence of deformational events (D1 - D6) over a period of 25 Ma in the NVZ is consistent with a major compressional event. North - south shortening was first accommodated by near-vertical east-trending folds and, with continued deformation, was concentrated along major cast-trending fault zones and contact-strain aureoles around synvolcanic intrusions, both with a downdip movement. Subsequent dextral strike-slip movement occurred on southeast-trending faults and major east-trending faults which controlled the emplacement of syntectonic plutons (2703-2690 Ma). This study suggests that the NVZ, which is a coherent geotectonic unit, initially formed as a diffuse volcanic arc, represented by the MVZ, in which the northern part, represented by the PVS, evolved into a mature arc as documented by a second volcanic and sedimentary cycle associated with major plutonic accretion. Volcano-sedimentary evolution and associated plutonism, as well as structural evolution, are best explained by a plate-tectonic model involving oblique convergence.