The ITSAQ Gneiss Complex of Greenland: Episodic 3900 to 3660 Ma Juvenile Crust Formation and Recycling in the 3660 to 3600 Ma Isukasian Orogeny

• Published in: American Journal of Science
• Main Authors: Nutman, Allen P., Bennett, Victoria, Friend, Clark, Hidaka, H., Keewook, Yi, Lee, Seung Ryeol, Kamiichi, Tomoyuki
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Yale University
• Subjects: Greenland
• Online Access: http://hdl.handle.net/1885/32777; https://doi.org/10.2475/09.2013.03; https://openresearch-repository.anu.edu.au/bitstream/1885/32777/5/Bennett_V_2013_The_Itsaq_gneiss.pdf.jpg; https://openresearch-repository.anu.edu.au/bitstream/1885/32777/7/01_Nutman_The_ITSAQ_Gneiss_Complex_of_2013.pdf.jpg

From the 3000 km2 Eoarchean Itsaq Gneiss Complex (IGC) of Greenland, zircon U-Pb dating of numerous meta-granitoid and orthogneiss samples is integrated with geologic observations, whole rock geochemistry and a strategic subset of zircon Hf and whole rock Nd isotopic measurements. This shows that there are multiple episodes of TTG suite formation from ∼3890 to 3660 Ma, characterized by zircon initial εHf≈0 and whole rock initial εNd of > +2. These rocks mostly have geochemical signatures of partial melting of eclogitized mafic sources, with a subset of high magnesian, low silica rocks indicating fusion by fluid fluxing of upper mantle sources. The TTG suites are accompanied by slightly older gabbros, basalts and andesites, which have geochemical signatures pointing to magmas originating from fluid fluxing of upper mantle sources. The data show the formation of juvenile crust domains in several discrete events from ∼3900 to 3660 Ma, probably at convergent plate boundaries in an environment analogous, but not identical to, modern island arcs. In the Isua area, a northern ∼3700 Ma terrane formed distal from a predominantly ∼3800 Ma terrane. These terranes were juxtaposed between 3680 and 3660 Ma - respectively the age of the youngest rocks unique to the northern terrane and the lithologically distinctive ultramafic-granitic Inaluk dykes common to both terranes. This shows the assembly of different domains of juvenile rocks to form a more expansive domain of "continental" crust. A rare occurrence of high-pressure granulite is dated at ∼3660 Ma, demonstrating that assembly involved tectonic crustal thickening. This continental crust was then reworked in the 3660 to 3600 Ma Isukasian orogeny. In the northern part of the Isua area, 3660 to 3600 Ma granites were emplaced into ∼3700 Ma tonalites. The earliest granites are nebulous, and sigmoidal schlieric inclusions within them demonstrate ductile extension. Younger granite sheets were emplaced into extensional ductile-brittle fractures. These granite-tonalite ...