OCGC Seminar - The Greatest Story Ever Told: A Wilson Cycle Explained by Mafic and Ultramafic Rocks, Northern Appalachians, USA


Professor Ian Honsberger, 
Department of Earth Sciences,
Carleton University

Thursday, November 3rd 
11:30 a.m. 
3120 HP 



Whole-rock compositions, isotopes, and mineral chemistry of metamorphosed mafic and ultramafic rocks in the Vermont Appalachians coupled with petrologic and tectonic models constrain the opening and closing of the Iapetus Ocean during the Neoproterozoic to Early Paleozoic. The story begins with rift-related magmatism in the Neoproterozoic during the breakup of the ancient supercontinent Rodinia. Snapshots of the developing basin are preserved in mafic rocks from west to east across Vermont. In the west, early continental rifting is marked by whole-rock enrichments in TiO2 and light rare-earth elements (LREE) in transitional to tholeiitic basalts that are embedded within a swath of rift clastics. Towards the east, the regional terrane is comprised of slope-rise deposits and deep ocean sediments that host tholeiitic basalts more depleted in TiO2 and LREE, with mafic rocks associated with serpentinized ultramafics displaying REE patterns similar to mid-ocean ridge basalts. The ultramafics form a lineament throughout central Vermont and mark the edge of an Ordovician (i.e. Taconian) subduction zone terrane. Age-corrected whole-rock εNd (t) values range from 3.46 to 9.2 in mafic rocks throughout the subduction terrane. The wide range in εNd (t) values may result from imbrication of geochemically distinct mafic fragments formed progressively throughout the rifting history, as thrust faults are pervasive throughout the terrane. An alternative hypothesis is that the mafic rocks were formed in the same geochemical system and reflect mixing of mantle plume-like melts with MORB-like melts and/or contamination of depleted melts by enriched lithospheric mantle.

Subduction of the mafic rocks, marking the closure of Iapetus, is confirmed by the presence of blueschist and eclogite in northern Vermont and by barroisite-bearing mafic rocks in central Vermont. Phase equilibria modeling suggests that different mafic fragments reached different depths within the subduction zone, with maximum depths up to ~75 Km – 80 Km (~2.0 GPa – 2.2 GPa) for eclogite in northern Vermont. Furthermore, P-T estimates of ultramafics are consistent with serpentinization at ~2.0 GPa (~75 Km). Reaction analysis of the ultramafics suggests that carbonation followed serpentinization and was prevalent during exhumation. Field relationships imply that ultramafics may have been integral to the exhumation of deeply rooted, high pressure mafic rocks. Tectonic models involving exhumation of slab fragments within a hydrated and carbonated, buoyant, low viscosity channel in the mantle wedge may explain the present day geology.