OCGC Seminar - Ali Polat

Convergent plate boundary processes in the early earth:
Evidence from west Greenland 

Dr. Ali Polat
Department of Earth and Environmental Sciences, University of Windsor

Accretionary prism

Thursday, December 4th, 2014
11:30 a.m.  

233 Advanced Research Complex (ARC)
University of Ottawa 



The Archean craton of West Greenland is composed of fault-bounded tectonic blocks (terranes) characterized mainly by 3800 to 2800 Ma supracrustal (greenstone) belts and TTG (tonalite, trondhjemite and granodiorite) gneisses. In addition to supracrustal rocks and TTG gneisses, there are layered anorthositic complexes consisting of anorthosite, leucogabbro, gabbro, hornblendite, pyroxenite, peridotite, and dunite in several tectonic blocks. Despite polyphase deformation and amphibolite to granulite facies metamorphism, pillow structures, and cumulate textures and igneous layering are locally well preserved in supracrustal rocks and anorthositic complexes, respectively. The anorthositic complexes were derived from hydrous mantle sources and emplaced into oceanic crust as multiple sills and dykes of magma and crystal mush. The style of deformation and fold interference patterns in 1 to 10 m wide shear zones are comparable to those occur on regional scales. The structural, magmatic and metamorphic characteristics of the tectonic blocks are comparable to those of Phanerozoic convergent plate margins, suggesting that Archean supracrustal-TTG terranes can be viewed as relic convergent margin fragments and that Archean continents grew at subduction zones. The supracrustal belts are composed mainly of tectonically-juxtaposed fragments of juvenile mafic to ultramafic volcanic rocks, peridotites, gabbros, and minor sedimentary and volcaniclastic rocks. Volcanic rocks consist mainly of island arc tholeiitic basalts, picrites, boninites, with minor MORB and andesites. These supracrustal belts are interpreted as accretionary prisms and dismembered oceanic island complexes. Field observations and trace element data suggest that the protoliths of the TTG gneisses were derived mainly from partial melting of amphibolites in thickened arcs and emplaced along thrust faults during multiple tectonothermal events.