EVIDENCE FROM LOWER CRUSTAL XENOLITHS FOR ONSET OF BREAKUP OF THE COLUMBIA SUPERCONTINENT AT 1.4 GA

Much progress has been made in recent years in understanding the formation, amalgamation, breakup and dispersal of supercontinents—the so-called supercontinent cycle. Columbia - arguably the first true supercontinent - amalgamated ca. 2 b.y. ago, although its’ precise configuration is debated. One key correlation, however, exists in nearly all configurations: the connection between Laurentia and Baltica. Breakup occurred 1.4 to 1.3 b.y. ago, at about the same time as a major, globally significant event of anorgenic granite emplacement (1.47 – 1.37 Ga) that ran along the whole of the SE margin, from Laurentia to Baltica. But what processes are involved in granite emplacement on this scale, over such a relatively short time period, and such a large spatial scale?

A major limitation to our understanding is uncertainty around the degree to which the lower crust was involved. What is the role of the lower crust and what are the implications for growth and modification of Proterozoic lithospheric? Lower crustal xenoliths may provide us with the best insights into these questions. Fortunately, at least three zircon-bearing granulite xenolith suites exist for localities that, collectively, span much of the SE margin of Columbia and appear to have recorded the 1.4 Ga event, i.e. xenoliths from the Kola Peninsula (Elovy Island), upper Michigan and the Colorado Plateau. The xenoliths also appear to have been relatively undisturbed from about 1.4 Ga until the time of their more recent entrainment.

The purpose of this presentation is to compare and contrast the petrology, geochemistry and geochronology of the lower crust of these three regions and discuss the implications for growth and modification of Proterozoic lithosphere. Preliminary conclusions are that: (1) the xenoliths provide evidence for long term enrichment of the lower crust in U and Th relative to Pb beneath parts of Columbia, possibly preserved here (unlike the lower crust of many other cratonic regions) because of the long-term stability of the Columbia supercontinent; (2) delamination of the U-Th-K rich continental keel as a result of the 1.4 Ga anorogenic collapse represents an important source of material recycled into the upper mantle that may contribute an enriched component to later magmatic events, such as the 1 Ga Keweenawan plume event of the Lake Superior region.