WILL WE EVER UNDERSTAND AVALONIA?

The complexity of Avalonia seems to increase with each new increment of data. Instead of providing answers, new data tend to lead to more questions. Even in areas which we thought were well known, additional mapping and geochronology have provided unexpected results. Examples include the recognition of widespread Ordovician (490 Ma to 460 Ma) gabbroic and syenitic rocks in northern mainland Nova Scotia, and the as yet poorly understood scattered occurrences of Cryogenian units in several areas of Avalonia. Although widely claimed to be composed of juvenile crust, evidence of older basement in Avalonia has been known for decades, including an abundance of felsic Neoproterozoic magmatic rocks with zircon inheritance going back into the Archean. Recent studies using the Sm-Nd and Lu-Hf isotopic systems have confirmed the antiquity of Avalonian crust in at least some areas. The source of the abundant quartzite and detrital muscovite ubiquitous in the clastic Cambrian cover sequences on Avalonia remains enigmatic. These cover sequences, postulated by some workers to correlate throughout Avalonia and even used by some as part of its definition, show differences as well as similarities and may be diachronous. Paleomagnetic data for Avalonia remain sparse and ambiguous, and may not all be derived from rocks which are truly part of Avalonia. In addition to the complexities within Avalonia itself, its relationship to adjacent Ganderia and Meguma are also problematic and controversial. Understanding Avalonia depends in part on understanding its current neighbours, and the history of their interactions over time.

Although it is important to look at the bigger picture in understanding Avalonia, it is equally important to not lose sight of the details. As exemplified by the parable of the “blind men and the elephant”, it is important not to assess the character of all of Avalonia based on results from individual parts of the area, which may not apply to the whole. Before we can understand the origin and evolution of Avalonia, we must be sure that its components have been correctly identified and described.