MIDDLE CAMBRIAN TO EARLY ORDOVICIAN ARC – BACK-ARC DEVELOPMENT ON THE LEADING EDGE OF GANDERIA, NEWFOUNDLAND APPALACHIANS

Evolution of many modern intra-oceanic and continental arc systems is exemplified by cycles of arc construction, rifting and separation of remnant and active arcs by a back-arc basin cored by oceanic crust. Synthesis of recently obtained geochronological, geochemical, isotopic and stratigraphic data is enabling detailed resolution of the evolution of the Cambro-Ordovician Penobscot arc system that developed on the leading edge of Ganderia, a peri-Gondwanan microcontinent. The Cambrian to Lower Ordovician Penobscot arc is characterized by continuous migration of the magmatic front, and the development of multiple volcanically active rift basins and multiple phases of polymetallic VMS formation (c. 513, 505, 496, 491 and 487 Ma). The rift basins display a variety of characteristics ranging from bimodal calc-alkaline magmatism to felsic-dominated incipient rift magmatism to tholeiitic/boninitic supra-subduction zone ophiolites. Zircon inheritance, Sm/Nd isotopic evidence and limited exposure of Ediacaran basement indicate that the Ganderian crust was highly attenuated and/or fragmented but formed the basement to the Penobscot arc throughout its history. Comparison to modern analogues suggests that part of the Penobscot arc developed in a similar tectonic setting as the volcanically active Havre Trough and Taupo Volcanic Zone. The Penobscot arc magmatism was terminated by an orogenic episode marked by the obduction of back-arc ophiolites onto the Ganderian passive margin.