Complex 3670-3500 Ma Orogenic Episodes Superimposed on Juvenile Crust Accreted between 3850 and 3690 Ma, Itsaq Gneiss Complex, Southern West Greenland

• Main Authors: Friend, Clark R L, Nutman, Allen Phillip
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Research Online 2005
• Subjects: 3670; 3500; ma; orogenic; episodes; 3690; 3850; complex; between; itsaq; accreted; superimposed; crust; greenland; west; southern; gneiss; juvenile; GeoQUEST; Life Sciences; Physical Sciences and Mathematics; Social and Behavioral Sciences; Greenland; Nuuk
• Online Access: https://ro.uow.edu.au/scipapers/917; https://ro.uow.edu.au/cgi/viewcontent.cgi?article=1956&context=scipapers

The Itsaq Gneiss Complex of the Nuuk region, southern West Greenland, is dominated by 3850-3690 Ma tonalites intruded into and intercalated with lesser amounts of different ≥3850-3700 Ma supracrustal units. Published wholerock Sr and Nd isotopic studies demonstrate that the tonalites are juvenile crustal additions from a depleted mantle source. From our field studies and SHRIMP U/Pb zircon dating, we argue that this juvenile crustal accretion was spread over ca. 170 m.yr. (≥3850-3690 Ma). Following 3850-3690 Ma juvenile crustal accretion events, the evolution of the Itsaq Gneiss Complex continued with numerous crustal-reworking events between 3670 and 3500 Ma. Examples of these reworking events are as follows: (1) there is intrusion of several generations of geochemically diverse granites (sensu stricto) with subordinate gabbros and diorites; (2) there are superimposed, multiple episodes of amphibolite to granulite facies metamorphism (illustrated with Akilia Island samples by detailed dating of metamorphic zircon overgrowths in several rocks), (3) part of the complex around Amiitsoq and the hills to the north contains the youngest tonalitic gneisses at ca. 3660 Ma, indistinguishable in age from the oldest recognized granites (sensu stricto) elsewhere; and (4) the hills north of Amiitsoq also contain tectonized remnants of volcanosedimentary basins that were still being filled between 3650 and 3600 Ma (from dating youngest detrital zircons) and that had been inverted and metamorphosed by ca. 3570 Ma (dating of oldest in situ metamorphic overgrowths). These 3670-3500 Ma events are interpreted as reflecting a complex orogen (tentatively collisional and/ or strike slip) superimposed on the products of the earlier (3850-3690 Ma) juvenile crustal accretion regimes. Implications for early crustal evolution studies are discussed. © 2005 by the University of Chicago. All rights reserved.