ASSEMBLY OF PROTEROZOIC LAURENTIA IN A GLOBAL CONTEXT AND INTERACTIONS WITH ITS NEIGHBOURS

There is increasing evidence for cyclicity influencing many aspects of Earth evolution. Quantification of the periodicity provides important constraints for repeated formation of supercontinents. A compilation of global igneous and metamorphic ages provides more precise information than the available detrital zircon record, suggesting two major long-term periodicities at least back to about 3800 Ma. Major "lull's" in igneous activity coincide with the intervals of maximum supercontinent stability and provide independent evidence which may be compared with geological models for plate motion. Extrapolation of global cyclic igneous and metamorphic activity suggests that there may have been a pre-Nuna supercontinent or supercraton, consistent with previous suggestions.

Laurentia is a keystone for two supercontinent cycles, Nuna and Rodinia. Increasing use of major data compilations such as the IGCP 509 and 648 databases, DateView and StratDB, provide evolving datasets which may be used to test multiple scenarios. Structured data facilitate visualization of changing geology, episodes of active igneous or metamorphic activity and the distribution of potential provenance areas. Multiple isotopic and geochemical datasets can now be combined to better constrain possible scenarios in contrast to geochronology or geology alone. Evolving computational techniques and software improvements permit better, faster and more effective assessment and testing of the models.

Despite the large increase in available published data, Paul Hoffman's seminal contribution to the Palaeoproterozoic evolution of Laurentia in the form of a series of "United Plates", still remains much as originally proposed. The principal challenge is to associate this 'core' with potential adjacent blocks, especially Siberia, Australia, and China along its present-day northern and western margins. Detrital zircon data with both age and initial Hf isotope information have the potential to significantly improve testing of nearest neighbour concepts but it is also essential to produce plate reconstruction models which account for all extant crustal blocks and are internally consistent from the present day back in time through Pangaea/Gondwana, Rodinia and Nuna.