U-Pb zircon ages of Kangamiut dykes and detrital zircons in metasediments in the Palaeoproterozoic Nagssugtoqidian Orogen (West Greenland) - Clues tothe pre-collisional history of the orogen

In the southern marginal zone of the Palaeoproterozoic Nagssugtoqidian orog en and its foreland (West Greenland), Archaean gneisses were intruded by th e syn-kinematic Kangamiut dyke swarm. Leucodioritic cores of two dykes have yielded U-Pb zircon ages of 2036 +/- 5 Ma and 2046 +/- 8 Ma (2 sigma). Dyk e emplacement and associated crustal movements thus occurred well before th e main plutonic and tectonic activity within the orogen (similar to 1900-18 00 Ma). In the central Nagssugtoqidian orogen, pelitic to semi-pelitic metasediment ary rocks were intruded by 1920-1900 Ma quartz diorites and tonalites of th e Arfersiorfik association, which have chemical and isotopic compositions c omparable with igneous suites formed in are environments. The supracrustal and intrusive rocks are separated from Archaean gneisses by decollements of high-grade mylonite and annealed marble, and are allochthonous units. Detr ital zircons in two samples of metasedimentary rocks from these allochthons are mostly large, poorly rounded, 2100-1950 Ma old with only a few Archaea n grains. Deposition of the metasediments must have taken place between sim ilar to 1950 and similar to 1920 Ma, before the intrusion of rocks of the A rfersiorfik association. The zircons may have been shed from presently unre cognised pre-Arfersiorfik association igneous complexes tares?), whose empl acement coincided with intrusion of the similar to 2040 Ma Kangamiut dykes into deforming Archaean crust in what later became the southern margin of t he orogen. Quartz-rich psammitic metasedimentary rocks are also present in the central Nagssugtoqidian orogen. Detrital zircons from one sample yielded a few 215 0-2250 Ma grains, but are predominantly Archaean. They include a high propo rtion of 3300-3400 Ma grains (the oldest material yet found in the orogen). Thrust-bounded panels of metasedimentary rocks in Archaean gneisses occur at the boundary between the southern and central parts of the orogen coinci ding with the northern limit of the Kangamiut dykes. Of the detrital zircon s in one metasediment sample from these panels a few yielded ages around 21 00-2000 Ma, some were Archaean, and >50% were similar to 2400 Ma. Depositio n of these two sediment suites took place after similar to 2100 Ma. The det rital zircon population shows that the sources for these sediments were dif ferent from that of the above-mentioned allochthonous rocks. Neither the 33 00-3400 Ma nor the similar to 2400 Ma detrital grains can be matched with a ny rocks in Greenland, indicating a distal source for these sediments. (C) 1999 Elsevier Science B.V. All rights reserved.