OCGC Seminar - Dr. Julian Pearce

   

When Did Subduction Start and How Did it Evolve?
Evidence from the Volcanic Record

 

Dr. Julian Pearce  
School of Earth and Ocean Sciences, Cardiff University

 

Wednesday, April 29th, 2015
11:30 a.m.  

Room O-23
560 Rochester St.
Geological Survey of Canada

 

Abstract

Methods of identification of volcanic arc lavas may utilize: (1) the selective enrichment of the mantle wedge by 'subduction-mobile' elements; (2) the distinctive preconditioning of mantle along its flow path to the arc front; (3) the combination of fluid-flux and decompression melting; and (4) the effects of fluids on crystallization of the resulting magma. It should then be a simple matter uniquely to recognise volcanic arc lavas in the Geological Record and so document past subduction zones. Essentially, this is generally true in the oceans, but not on the continents. Even in recent, fresh lavas and with a full battery of element and isotope tools at our disposal, there can be debate over whether an arc-like geochemical signature results from active subduction, an older, inherited subduction component in the lithosphere, or crustal contamination. In the Archean, metamorphism, deformation, a different thermal regime and potential non-uniformitarian tectonic scenarios make the fingerprinting of arc lavas particularly problematic. Here, I present new, high-resolution immobile element fingerprinting methods, based primarily on Th-Nb fractionation, which may provide an improved insight into Archean settings. In the Pilbara and Superior Provinces, for example, lavas with high Th/Nb (negative Nb anomalies) are common throughout the lava sequence, many also following a basalt-andesite-dacite-rhyolite (BADR) sequence resembling that of present-day arcs. By combining the Th/Nb proxy with selected fractionation indices, it may however be seen that, in both cases, much of the apparent subduction signal is anorogenic. Nonetheless, the youngest units in both cases do have geochemical signatures indicative of subduction and that is supported by apparent inherited subduction components in post-subduction magmas. Based on this very small sample, a viable hypothesis for the Archean is that there were long periods of plume activity followed by short-lived subduction events. The ultimate challenge was the ‘world’s oldest’ volcanic rocks at Nuvvuagittuq where high-temperature metamorphism has mobilized a number of normally immobile elements, including the key element, Th. There, more detailed investigation into element mobility does support proposals that the youngest unit contains a subduction component. Superficially, this indicates that subduction may have taken place throughout Earth history, though not necessarily continuously. However, further work raises questions, particularly for the oldest rocks, over whether it was plate tectonic subduction or some other process that drove crustal materials to mantle depths.