THE ORIGIN AND TECTONIC EVOLUTION OF THE ANNIEOPSQUOTCH OPHIOLITE BELT, SW NEWFOUNDLAND

The Annieopsquotch ophiolite belt (481-478 Ma) comprises several isolated ophiolite complexes that lie immediately west of the Red Indian line, which marks the major Iapetus suture in the Newfoundland Appalachians. Traditionally, the ophiolite belt was regarded as normal oceanic crust (MORB), which was obducted onto the Laurentian margin. Our work on the Annieopsquotch and Star Lake ophiolites indicate that their formation comprises two distinct magmatic phases. The first phase of magmatism is represented by boninitic cumulates, preserved as large (tens of meters) enclaves in gabbros. The gabbros form part of the dominant second phase, which constitutes a typical gabbro-sheeted dyke-basalt sequence with a supra-subduction zone signature. We interpret that the boninites formed immediately before the supra-subduction sequence. The boundary between the Annieopsquotch ophiolite belt and the Laurentian Dashwoods block is the Lloyds River Fault. Structural relationships indicate it accommodated oblique sinistral underthrusting of the ophiolite belt underneath the Dashwoods Block, at amphibolite facies conditions. Geochronology shows the fault was active at least at c. 468 Ma (40Ar/39Ar hornblende), and was intruded by sheets of arc plutons at c. 463 Ma (U/Pb zircon). The relict boninitic substrate combined with the absence of an associated coeval arc suggests that the Annieopsquotch ophiolite belt began its evolution during initiation of subduction at c. 481 Ma. This was followed by the formation an extensional infant arc, represented by the tholeiitic gabbro-sheeted dyke-pillow basalt sequence. The infant arc sequence was underthrust beneath the peri-Laurentian Dashwoods block c. 10 Ma after its formation, either due to convergence initiated by the collision between Dashwoods and Laurentia or by interaction with outboard, younger arc terranes present in the Annieopsquotch accretionary tract.